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Syllabus
Introduction to bio-medical instrumentation system, overview of anatomy and physiological
systems of the body. Sources of bio-electric potential: Resting and action potential,
propagation of action potentials. Bioelectric potentials examples (ECG, EEG, EMG, ERG,
EOG, EGG, etc introduction only.) Electrode theory: Nernst relation. Bio potential
electrodes: Microelectrodes, skin surface electrodes, needle electrodes. Instrumentation for
clinical laboratory: Bio potential amplifiers-instrumentation amplifiers, carrier amplifiers,
isolation amplifiers, chopper amplifiers.
Biomedical Engineering
Biomedical Engineering is the application of engineering principles and design concepts to
medicine and biology
The biomedical engineering provides electrical, electronic, electro-optical, and computer
engineering support to clinical and biomedical applications.
Biomedical Engineering improves the field of healthcare diagnosis, monitoring and therapy.
Introduction to Biomedical Instrumentation System
Major Components of Medical Instrumentation System
1. Energy Source 2. Measurand 3. Sensor / Transducer 4. Signal
Conditioning 5. Output Display
Auxiliary Components
A calibration signal
1. Energy Source
Used to energize the whole instrumentation system
Examples: Different sources used are electric, light, infrared, mechanical and ultrasound
2. Measurand
The physical quantity, property, or condition that the system measures is called measurand.
Examples:
Internal (Blood Pressure)
On the Body Surface (Electrocardiogram)
Emanate from the body (Infrared Radiation)
Derived from Tissue Sample (such as Blood or a Biopsy)
3. Sensor / Transducer
The transducer is defined as a device that converts one form of energy to another.
A sensor converts a physical measurand to an electric output.
4. Signal Conditioning
Simple signal conditioners may only amplify and filter the signal or merely match the
impedance of the sensor to the display.
Often sensor outputs are converted to digital form and then processed by specialized digital
circuits or a microcomputer.
For example, signal filtering may reduce undesirable sensor signals.
It may also average repetitive signals to reduce noise, or it may convert information from the
time domain to the frequency domain.
5. Output Display
The results of the measurement process must be displayed in a form that the human operator
can perceive.
The best form for the display may be:
a. Numerical
b. Graphical,
c. Dispalcement,
d. CRT
e. Visual / Hearing
The processed signal after conditioning passed through
1. Alarm System: Indicate when measurand goes beyond a preset limit.
2. Data Storage: To maintain the data for future reference
3. Data Transmission: Used to transmit the information obtained from one location to another.
Auxiliary Components
1. Calibration signal: with the help of this, measurand should be applied to the sensor input or
as early in the signal-processing chain as possible.
2. Control and Feedback Signal: Required to bring out the measurand, to adjust the sensor and
signal conditioner, and to direct the flow of output for display, storage or transmission. The
control and feedback may be automatic or manual.
Measurement in biomedical instrumentation can be divided in to two
1. VIVO
• Measurement is made on or within the human body
• E.g. Device inserted in to the blood stream to measure PH of blood
2. VITRO
• Measurement is performed outside of the body.
E.g. Measurement of blood PH from blood samples
Overview of Anatomy and Physiological Systems of the Body
Anatomy –The study of the structure and shape of the body and body parts & their
relationships to one another. The term anatomy comes from the Greek words meaning to cut
(tomy) apart ( ana) .
Gross anatomy( macroscopic anatomy) – the study of large, easily observable structures (by
naked eye), such as the heart or bone.
Microscopic anatomy (cytology, histology) – the study of very small structures, where a
magnifying lens or microscope is needed.
Physiology – the study of how the body and its parts work or function
Physiology can be classified in to
1. Cell Physiology: Study of cells
2. Patho Physiology: Pathological Functions
3. Circulatory Physiology: Study of blood circulation
4. Respiratory Physiology: Study of breathing organs
Major Subsystems of the body
1. The Cardiovascular System
Complex closed hydraulic system performs service of transportation of oxygen, CO2,
numerous chemical compounds and the blood cells to maintain optimum environment for
cellular function.
The heart is the power source which provides energy to move the blood through the body and
remove waste products.
2. The Respiratory System
The respiratory system supplies the blood with oxygen and removes carbon dioxide waste
that cells produce.
Nasal Cavity: Passes air through nose
Mouth: Passes air through
Pharynx: The throat. Cone shaped passageway leading to trachea.
Trachea: Windpipe. Main tube connecting nose/mouth to lungs.
Epiglottis: Flap that covers the entrance to the trachea.
Lungs: Main organ of the respiratory system.
Bronchi: Two tubes inside of lungs that air passes through to the bronchioles.
Bronchioles: Small branching out tubes divided into alveoli.
Alveoli: Tiny air sacs that do the oxidation and the exhale of carbon dioxide.
Capillaries: Blood vessels that are imbedded in the walls of the alveoli. While in the
capillaries the blood discharges carbon dioxide into the alveoli and takes up oxygen from the
air in the alveoli.
Cilia: Hair like structures that remove dust and dirt from the air.
Respiratory System Structure
Working
Air that flows from the mouth or nasal cavity travels through the pharynx and moves down to
the trachea.
Then the air moves to the bronchi tubes as they enter the lungs.
The primary organs of the respiratory system are the lungs, which function to take in oxygen
and expel carbon dioxide as we breathe.
The gas exchange process is performed by the lungs and respiratory system. Air, a mix of
oxygen and other gases, is inhaled.
In the throat, the trachea, or windpipe, filters the air. The trachea branches into two bronchi,
tubes that lead to the lungs.
Once in the lungs, oxygen is moved into the bloodstream. Blood carries the oxygen through
the body to where it is needed.
Red blood cells collect carbon dioxide from the body’s cells and transports it back to the lungs.
An exchange of oxygen and carbon dioxide takes place in the alveoli, small structures within
the lungs. The carbon dioxide, a waste gas, is exhaled and the cycle begins again with the
next breath.
The diaphragm is a dome-shaped muscle below the lungs that controls breathing. The diaphragm flattens out and pulls forward, drawing air into the lungs for inhalation. During
exhalation the diaphragm expands to force air out of the lungs.
Adults normally take 12 to 20 breaths per minute. Strenuous exercise drives the breath rate up
to an average of 45 breaths per minute.
3. Nervous System
The nervous system includes the brain, spinal cord, and a complex network of neurons. This
system is responsible for sending, receiving, and interpreting information from all parts of the
body.
The nervous system monitors and coordinates internal organ function and responds to
changes in the external environment.
This system can be divided into two parts: the central nervous system and the peripheral
nervous system.
The central nervous system (CNS)
It is the processing center for the nervous system. It receives information from and sends
information to the peripheral nervous system. The two main organs of the CNS are the
brain and spinal cord. The brain processes and interprets sensory information sent from the
spinal cord. Both the brain and spinal cord are protected by a three-layered covering
of connective tissue called the meninges.
The Peripheral nervous system (PNS)
The primary role of the PNS is to connect the CNS to the organs, limbs and skin.
The nerves that make up the peripheral nervous system are actually the axons or bundles of
axons from neuron cells.
The peripheral nervous system is divided into two parts:
1. The somatic nervous system
The somatic system is the part of the peripheral nervous system responsible for carrying
sensory and motor information to and from the central nervous system.
This system contains two major types of neurons:
Sensory neurons (or afferent neurons) that carry information from the nerves to the
central nervous system.
Motor neurons (or efferent neurons) that carry information from the brain and spinal cord
to muscle fibers throughout the body.
2. Autonomic Nervous systems
The autonomic system is the part of the peripheral nervous system responsible for regulating
involuntary (reflex/Un intentional)body functions, such as blood flow, heartbeat, digestion
and breathing.
This system is further divided into two branches:
The sympathetic system regulates the flight-or-fight responses.
The "fight or flight response" is our body's primitive, automatic, inborn response that
prepares the body to "fight" or "flee" from perceived attack, harm or threat to our survival.
Parasympathetic system helps maintain normal body functions and conserves physical
resources
The brain is the control center of the body. Covering the brain is a protective layer of
connective tissue known as the meninges.
There are three main brain divisions: the forebrain, the brainstem, and the hindbrain.
The forebrain is responsible for a variety of functions including receiving and processing
sensory information, thinking, perceiving, producing and understanding language, and
controlling motor function.
The forebrain contains structures, such as the thalamus and hypothalamus. It also contains the
largest part of the brain, the cerebrum.
Most of the actual information processing in the brain takes place in the cerebral cortex. The
cerebral cortex is the thin layer of gray matter that covers the brain.
It is divided into four cortex lobes: frontal lobes, parietal lobes, occipital lobes, and temporal
lobes. These lobes are responsible for various functions in the body that include everything
from sensory perception to decision-making and problem solving.
Below the cortex is the brain's white matter, which is composed of nerve cell axons that
extend from the neuron cell bodies of gray matter. White matter nerve fiber tracts connect the
cerebrum with different areas of the brain and spinal cord.
The midbrain and the hindbrain together make up the brainstem. The midbrain is the portion
of the brainstem that connects the hindbrain and the forebrain. This region of the brain is
involved in auditory and visual responses as well as motor function.
The hindbrain extends from the spinal cord and contains structures such as
the pons and cerebellum. These regions assist in maintaining balance and equilibrium,
movement coordination, and the conduction of sensory information. The hindbrain also
contains the medulla oblongata which is responsible for controlling such autonomic functions
as breathing, heart rate, and digestion.
Sources of Bioelectric Potential
The systems in the human body generate their on monitoring signals when they carry out
their functions.
These signals provide useful information about their function.
These signals are bioelectric potentials associated with nerve conduction, brain activity,
heartbeat, muscle activity and so on.
Bioelectric potentials are actually ionic voltages produced as a result of electro chemical
activity of certain cell.
Transducers are used to convert these ionic potentials in to electrical signals
Resting and Action potentials
Resting Potential [Polarization]
Certain types of cells within the body, such as nerve and muscle cells are encased in a semi
permeable membrane. This membrane permits some substances to pass through while others
are kept out. Surrounding the cells of the body are the body fluids. These fluids are
conductive solutions containing charged atoms known as ions.
The principle ions are sodium (Na+) Potassium (K+) and chloride (Cl-). The membrane of
excitable cells permits entry of Potassium (K+) and chloride(C-) ions but blocks the entry of
sodium (Na+) ions. So inside the cell is more negative than outside cell. This membrane
potential is called Resting potentials. This potential is measured from inside the cell with
respect to body fluids. So resting potential of a cell is negative.
This resting potential ranging from -60mv to -100 mv.
Cell in the resting state is called polarized cell.
Action Potential [Depolarization]
When a section of a cell membrane is excited by the flow of ionic current or by some form of
externally applied energy,, the membrane allows some Na+ and try to reach some balance of
potential inside and outside. Same time the some K+ goes outside but not rapidly like
sodium.
As a result, the cell has slightly Positive potential on the inside Due to the imbalance of the
Potassium ions. This potential is known as “action potential” and is approximately +20 mV.
A cell that has been excited and that displays an action potential is said to be depolarized and
process from resting to action potential is called depolarization.
Characteristics of Resting and Action Potential
Propagation of action potentials
When a cell is exited and generates an action potentials ionic currents to flow. This process
excite neighboring cells or adjacent area of the same cell
The rate at which an action potential moves down a fiber or is propagate from cell to cell is
called the propagation rate.
In nerve fiber the propagation rate is also called the nerve conduction rate, or conduction
velocity. Velocity range in nerves is from 20 to 140 meters per second. In heart muscle, the
rate is slower, average 0.2 to 0.4 m/sec.
The Bioelectric Potentials
The Electrocardiogram(ECG)
The Electroencephalogram(EEG)
The Electromyogram(EMG)
The Electroretinogram(ERG)
The Electro-oculogram(EOG)
The Electrogastrogram(EGG)
1. The Electrocardiogram(ECG)
The bio-potentials generated by the muscles of the heart result in the electrocardiogram
(ECG). German word EKG. The Electrical activity of the heart is recorded by
electrocardiogram (ECG).
P wave corresponds to Atrial depolarization of SA node
Atrial repolarization record is masked by the larger QRS complex
QRS complex corresponds to ventricular depolarization
T wave corresponds to ventricular repolarization
2. The Electroencephalogram(EEG)
The recorded representation of bioelectric potential by the neuronal activity of the brain is
called the electroencephalogram.
The waveform varies greatly with the location of the measuring electrodes on the surface of
the scalp.
Frequency Range Signal Type Activity
Below 3.5 Hz Delta Deep sleep
From 3.5 Hz to about
8 Hz
Theta Fall aslpeep
From about 8 Hz to
about 13 Hz
Alpha Drowsy person
Above 13 Hz Beta Paradoxial sleep,
Rapid eye
movement(REM)
3. Electromyogram [EMG]: The bioelectric potentials associated with muscle activity
constitute the electromyogram. Can be measure on the surface of the body or by penetrating
the skin using needle electrodes.
4. Eelctoretinogram [ERG]: A record of the complex
pattern of the bioelectric potentials obtained from the retina
of the eye. This is usually a response to a visual stimulas.
The two components that are most often measured are the
a- and b-waves. The a-wave is the first large negative
component, followed by the b-wave which is corneal
positive and usually larger in amplitude.
When light falls on photo receptors outer portion of photoreceptor becomes positive and
inner part becomes negative
‘a’ wave - Reflects the potential of photoreceptors in outer retina
‘b’ wave - Reflects the function of the inner layers of the retina.
5. Electro-oculogram [EOG]: A measure of the variation in the corneal-retinal potential as
affected by the position and movement of eye.
The clinical electro-oculogram (EOG) makes an indirect measurement of the minimum
amplitude of the standing potential in the dark and then again at its peak after the light rise.
6. Electrogastrogram [EGG]: Electrogastrography is a non-invasive technique for recording
gastric myoelectrical activity using cutaneous electrodes placed on the abdominal skin over
the stomach. The surface recording obtained using electrography is called the
electrogastrogram.
Electrode Theory
• To measure bioelectric potentials, a transducer is required. Electrical signals produced by
various body activities are used in monitoring / diagnosis.
• In order to measure and record potentials and, hence, currents in the body, it is necessary to
provide some interface between the body and the electronic measuring apparatus. Bio-
potential electrodes carry out this interface function.
• A transducer consists of two electrodes, which measure ionic potential difference between
two points.
• The designation of the Bio potential waveform ends with “Gram”. The name of the
instrument bio potential normally ends with “Graph”. Propagation of action potential through
different body tissues produces final waveform recorded by electrodes
• Electrical activity is explained by differences in ion concentrations within the body (sodium,
Na+; chloride, Cl–; potassium, K+). A potential difference (voltage) occurs between 2 points
with different ionic concentrations
• Propagation of action potential through different body tissues produces final waveform
recorded by electrodes
• Electrical activity is explained by differences in ion concentrations within the body (sodium,
Na+; chloride, Cl–; potassium, K+). A potential difference (voltage) occurs between 2 points
with different ionic concentrations
Nernst Relation
It can be shown that an electric potential E will exist between the solutions on either side of
the membrane, based upon the relative activity of the permeable ions in each of these
solutions. This relationship is known as the Nernst equation.
The relationship between the ionic concentration (activity) and the electrode potential is
given by the Nernst equation:
When no electric current flows between an electrode and the solution of its ions or across an
ionpermeable membrane, the potential observed should be the half-cell potential or the
Nernst potential, respectively. If, however, there is a current, these potentials can be altered.
Equivalent circuit for bio-potential electrode
Where Rd and Cd are components that represent the impedance associated with the electrode-
electrolyte interface and polarization at this interface.
Rs is the series resistance associated with electrode materials.
The battery Ehc represents the half-cell potential
Polarizable and Non-Polarizable Electrodes
1. Polarizable Electrodes
No charge crosses the electrode-electrolyte interface when a current is applied. (e.g
Platinum electrode)
2. Non-Polarizable Electrode
Current passes freely across the electrode-electrolyte interface. (e.g. Ag/AgCl Electrode)
Equivalent circuit for bio-potential with two electrode
Bio Potential Electrodes
Bio-potential electrodes transduce ionic conduction to electronic conduction so that bio-
potential signals can be obtained
They generally consist of metal contacts packaged so that they can be easily attached to the
skin or other body tissues
Classification of Electrodes
1. Micro Electrodes--- Bio electric potential near or within a single cell
Metal Type—Tip must be tungsten or stainless steel
Micro pipette---It is a glass micropipet with size of 1 micron, It is filled with electrolyte
2. Skin surface electrode —Measure ECG,EEG,EMG
3. Needle electrode ---Penetrate the skin to record EEG
1. Micro Electrodes: To measure potential across the cell membrane.
Smaller in size with respect to the cell dimension
Avoids causing serious injury
Doesn’t change the cell’s behavior
Tip diameter ranging from approximately 0.05 to 10µm
Formed from
Solid-metal needles
Or a Metal contained within or on the surface of a glass needle.
A glass micropipette having a lumen filled with an electrolytic solution.
a. Two types
Metal
Micropipette
1. Metal Electrodes
Fine needle of strong metal
• Stainless Steel
• Platinum-iridium alloy
• Tungsten
• Compound tungsten carbide
• Insulated with an appropriate insulator up to its tip.
• Usually produced by electrolytic etching, using an electrochemical cell in which the metal
needle is the anode.
• The electric current etches the needle as it is slowly withdrawn from the electrolytic solution.
• The etched metal needle is then supported in a larger metallic shaft that is then insulated.
2. Micropipette Electrodes
• Prepared from glass capillaries
• Glass Micropipette with the tip drawn out to the desired size (usually about 1µm) in diameter.
• The central region of a piece of capillary tubing is heated to the softening point
• It is then rapidly stretched to produce the constriction
• Is then broken apart at the constriction to produce a pipette structure.
• Filled with an electrolyte solution frequently 3M KCL
• A Cap containing a metal electrode is then sealed to the pipette.
2. Surface electrodes
These are placed in contact with the skin of the subject
a. Immersion electrodes
Early stages immersion electrodes were used.
A bucket of saline water is used
An improvement of immersion electrode is the plate electrode.
Another old type electrode is suction type
b. Suction type
• Modification of metal-plate electrode that require no straps or adhesives for holding it in
place
• Frequently used in precordial (chest) leads
• Can be placed at a particular location
• Consists of a hollow metallic cylindrical electrode that makes contact with the skin at its
base.
• A Lead wire is attached to the metal cylinder
• A rubber suction bulb fits over its other base.
• Electrolyte gel is placed over the contacting surface.
• The bulb is squeezed and placed on the chest wall and then the bulb is released and applies
suction against the skin, holding the electrode assembly in place.
• Suction & pressure of the contact surface against the skin creates irritation
• Small contacting area with a large overall size
c. Metal-Plate Electrodes
Historically, one of the most frequently used forms of bio-potential sensing electrodes is the
metal-plate electrode.
In its simplest form, it consists of a metallic conductor in contact with the skin.
An electrolyte soaked pad or gel is used to establish and maintain the contact.
d. Floating electrodes
Conductive paste reduces effect of electrode slippage and resulting motion artifact.
• In practice the electrode is filled with electrolyte gel and then attached to the skin surface by
means of a double-sided adhesive tape ring.
• The electrode element can be a disk made of a metal such as silver coated with AgCl.
3. Needle electrodes
Unipolar electrode---Single wire inside a needle
Bipolar electrode---Two wires inside a needle
Mostly used for contacting with internal body tissues
(a) Insulated needle electrode.
(b) Coaxial needle electrode.
(c) Bipolar coaxial electrode.
(d) Fine -wire electrode connected to hypodermic needle, before being inserted .
(e) Coiled fine -wire electrode in place
Coaxial needle electrode
A shielded percutaneous electrode consists of a small-gage hypodermic needle that has been
modified by running an insulated fine wire down the center of its lumen and filling the
remainder of the lumen with an insulating material such as resin.
When the resin has set, the tip of the needle is filed to its original bevel, exposing an oblique
cross section of the central wire, which serves as the active electrode.
The needle itself is connected to ground through a shield of a coaxial cable, thereby
extending the coaxial structure to its very lip.
Bipolar coaxial needle electrode
Two wires are placed within the lumen of the needle
Connected differentially to be sensitive only to the electrical activity in the immediate
vicinity of the needle.
Bio potential Amplifiers
These are very important part of modern medical instrumentation. We need to amplify bio-
potentials which are generated in the body at low levels with high source impedance.
Bio-potentials amplifiers are required to increase signal strength while maintaining fidelity
To take a weak bio-potential and increase its amplitude so that it can be processed, recorded
or displayed
To amplify voltage, power and current.
In some cases a biopotential amplifier is used to isolate the load from the source current gain
only
Characteristics
1. high input impedance
2. low output impedance
3. The biopotential amplifier must be sensitive to important frequency components of the
biosignal.
4. Biopotential amplifiers have a gain of 1000 or greater.
5. most biopotential amplifiers are differential
6. High common mode rejection ratio.
Commonly used Bio-potential Amplifier is Differential amplifier. But it has some limitations.
These limitations overcome with the availability of improved version of differential amplifier
called Instrumentation Amplifier
1. Instrumentation Amplifier
Advantages
• Extreme high i/p impedence
• Very high CMRR
• Low power consumption
• Available in single IC
• High slew rate
• Low bias & offset currents
Structure consists of 3 op-amps and seven resistors
Two buffer amplifiers A1 and A2 connected to a differential amplifier A3.
In the above circuit op-amp A3 with four equal resistors R form a differential amplifier with
gain 1.
Rvar = Variable resistance, used to balance out any common mode voltage.
Rg = used to set the gain using the formula
2. Carrier Amplifier
Structure consists of
1. Carrier Oscillator: used to energize the transducer with an alternating carrier voltage.
2. Strain gauge transducer:
The information signal from the body electrodes reaches the transducer where it is amplitude
modulated using carrier signal from the carrier oscillator.
The transducer changes the amplitude of carrier signal with respect to the physiological
variable being measured.
The output of transducer is amplitude modulated signal.
3. Amplifier
Amplifier used is Multistage Capacitance coupled Amplifier
The modulated signal from the transducer is given to this amplifier.
The first stage produces amplification of AM signal.
Second stage responds to signal frequency components of carrier signal only.
Further amplified in the third stage.
4. Rectifier
Output from the amplifier is converted into unidirectional signal using a rectifier.
5. Phase sensitive Detector
The signal is demodulated and extracts the amplified information signal.
6. Direct Writing Recorder
The voltage produced by the detector stage is then fed to the driver stage of the recording
system.
Features of carrier amplifier
Used to obtain zero frequency response of dc amplifier.
Inherent stability of capacitance coupled amplifier.
3. Chopper Amplifier
Used to avoid the drift problem occurs in the direct coupled amplifier.
This circuit uses a chopping device which converts slowly varying dc to an alternating form
with amplitude proportional to the input direct current and phase depends on the polarity of
original signal.
The ac voltage is then amplified by an ac amplifier whose output is then rectified back to get
an amplified direct current.
Figure given below shows single ended chopper stabilizer amplifier.
Aim of this circuit is to avoid the dc offset voltage present in the output signal.
For that purpose first convert the dc signal into ac using a chopper.
1. Low Pass Filter
The low frequency components derived from the input signal by passing it through low pass
filter R2C2 and R2
2. Chopper
The output of LPF is then chopped using a transistor switch w.r.t a carrier signal from the
oscillator.
3. Demodulator
The original signal recovered in demodulator which is again applied to second stage of
amplification.
4. Low Pass Filter
Before given to the amplifier low frequency components again derived using LPF
5. Second Stage of Amplification (A2)
At the input of A2 an HPF C1R1 is used to derive the high frequency signals.
This is to reduce the dc offset and drift of second amplifier A2.
Advantage of Chopper Amplifier
Insensitivity to component changes due to ageing, temperature change, power supply
variation, or other environmental factors.
Small offset voltage
Used to amplify small dc signals of few microvolts.
4. Isolation Amplifier
1. Transformer Isolation
2. Optical Isolation
3. Capacitive Isolation
Syllabus
Heart and cardiovascular system (brief discussion), electro conduction system of the heart.
Electrocardiography, ECG machine block diagram, ECG lead configurations, ECG recording
system, Einthoven triangle, analysis of ECG signals. Measurement of blood pressure: Direct,
indirect and relative methods of blood pressure measurement, auscultatory method,
oscillometric and ultrasonic non- invasive pressure measurements. Measurement of blood
flow: Electromagnetic blood flow meters and ultrasonic blood flow meters.
I. Heart and cardiovascular system
Cardio vascular system can be viewed as closed hydraulic system with 4 chamber pump.
It is mainly used for transportation of oxygen, Carbon dioxide, numerous chemical
compounds and the blood cells.
In some part of the system diameter of the arteries are changed to control pressure.
Heart is an isolated two stage synchronized chamber
1. The first stage is to collect blood from the system and pump it in to 2nd stage.
2. The second stage then pump these blood to the system
Structure of Heart
Heart wall consists of three layers
1. Pericardium: Outer most layer, keeps outer surface moist, prevents friction
2. Myocardium: Middle layer, Main muscle of heart, made up of short cylindrical fibres
3. Endocardium: Inner layer of heart, Provides smooth lining for blood flow
Heart has 4 valves
1. Tricuspid/Right Atrio-Ventricular valve
Between Right A and V, Prevents blood flow from right V to A
2. Bicuspid/ left Atrio-Ventricular valve
Between left A and V, Prevents blood flow from left V to A
3. Pulmonary valve
At right ventricle, It has 3 cusps
4. Aortic Valve
Between left ventricle and aorta, It has 3 cusps.
Blood Flow inside the heart
One of the two stage pump (Right side) collect fluid from the system and pump it
through oxygenation system(Lungs).
Other side pump receives blood from oxygenation system (Lungs) and pump blood to
main hydraulic system.
Blood act as communication and supply network for all parts of the body
The blood is carried out to the various parts of the body through blood vessels. There are
three types of blood vessels
1) Arteries--- Thick, Carries oxygenated blood
2) Veins--- Thin, De-oxygenated blood
3) Capillaries---Smallest, Last level of blood vessels, 800000 km of capillaries
Heart pumps blood through the pulmonary circulation to the lungs and through the
systemic circulation to the other parts of the body.
1) Pulmonary circulation
2) Systemic circulation
In pulmonary circulation, venous blood (de-oxygenated) flows from right ventricle
through pulmonary artery to lungs.
The arterial (oxygenated) blood flows to left atrium through pulmonary veins.
In systemic circulation blood flows from left auricle to left ventricle and it is pumped
to aorta and its branches
II. Electro Conduction System of the Heart
The heart is able to create it's own electrical impulses and control the route the
impulses take via a specialised conduction pathway.
The conducting system of the heart consists of cardiac muscle cells and
conducting fibers that are specialized for initiating impulses and conducting them rapidly
through the heart.
They initiate the normal cardiac cycle and coordinate the contractions of cardiac
chambers. Both atria contract together, as do the ventricles, but atrial contraction occurs first.
The conducting system provides the heart its automatic rhythmic beat. For the
heart to pump efficiently and the systemic and pulmonary circulations to operate in
synchrony, the events in the cardiac cycle must be coordinated.
The conduction pathway made up of 5 elements:
1. The sino-atrial (SA) node
2. The atrio-ventricular (AV) node
3. The bundle of His
4. The left and right bundle branches
5. The Purkinje fibres
Electrical impulses from your heart muscle (the myocardium) cause your heart to
beat (contract). This electrical signal begins in the sinoatrial (SA) node, located at the top of
the right atrium.
The SA node is sometimes called the heart's "natural pacemaker." When an
electrical impulse is released from this natural pacemaker, it causes the atria to contract.
The electrical stimulus from the SA node eventually reaches the AV node and is
delayed briefly so that the contracting atria have enough time to pump all the blood into the
ventricles.
The impulse leaves the AV node through bundle of His. The fibers in this bundle
known as Purkinje fibers after a short distance split into two branches to initiate action
potentials simultaneously in the two ventricles.
III. Electrocardiography
Bio electric potentials generated by heart muscles are called Electro Cardio Gram.
It is sometimes called EKG(Electro Kardio Gram)
Electrocardiography (ECG) is an interpretation of the electrical activity of the heart over a
period of time.
The recording produced by this noninvasive procedure is termed as electrocardiogram (also
ECG or EKG).
IV. ECG machine block diagram
Lead Selector Switch
The potentials picked up by the patient electrodes are taken to the lead selector
switch. Here the electrodes are selected two by two according to the lead
program.
Preamplifier
The signal from lead selector switch given to preamplifier. A preamplifier is an
electronic amplifier which prepares an electronic signal for further amplification
or processing. It is usually 3 or 4 stage dual input unbalanced output differential
amplifier.
Power Amplifier
It is a push pull differential amplifier.
Single ended output from preamplifier is given to power amplifier.
Another input is from frequency selective network.
Output is single ended
The output of the power amplifier is fed to the pen motor which deflects the
writing arm of the paper.
Frequency selective network is an R-C network, which provides necessary
damping of the pen.
Auxiliary Circuits
The auxiliary circuits provide a 1 mV calibration signal and automatic blocking
of the amplifier during change in the position of the lead switch. It also includes a
speed control circuit for the chart driver motor.
V. ECG lead configurations
To record ECG 12 electrodes connected to the body of the patient. Electrodes connected to
ECG machine using wires called leads. Leads are electrodes which measure the difference in
electrical potential between either:
1. Two different points on the body (bipolar leads)
2. One point on the body and a virtual reference point with zero electrical potential, located in
the center of the heart (unipolar leads).
Classification
1. Standard Limb Leads or Bipolar Leads or Einthoven Leads : I, Il & III
2. Augmented Limb Leads (Unipolar): aVR, aVL & aVF
3. Precordial Leads: V1- V6
1. Standard Limb Leads or Bipolar Leads or Einthoven Leads : I, Il & III
The Standard Limb Leads are used to display a graph of the potential difference recorded
between two limbs at a time. In these leads, one limb carries a positive electrode and the
other limb, a negative one.
The three limb electrodes, I, II and III form a triangle (Einthoven’s Equilateral Triangle), at
the right arm (RA), left arm (LA) and left leg (LL).
Lead I has a positive electrode on the left arm and a negative electrode on the right arm.
Lead I is a bipolar, indirect lead.
Lead II has a positive electrode on the right arm and a negative electrode on the left foot.
Lead II is a bipolar, indirect lead.
Lead III has a positive pole on the left foot and a negative pole on the left hand. Lead III is a
bipolar, indirect lead.
Einthoven’s Equilateral Triangle
The three limb electrodes, I, II and III form a triangle (Einthoven’s Equilateral Triangle), at
the right arm (RA), left arm (LA) and left leg (LL).
2. Augmented Limb Leads (Unipolar): aVR, aVL & aVF
3. Precordial Leads: V1- V6
VI. Analysis of ECG Signals
The recording of electrical activity associated with the functioning of the heart is known as
electrocardiogram.
It’s a quasi periodical, rhythmically repeating signal synchronized by the function of the
heart, which act as a generator of bioelectric events.
This generated signal can be described by means of an electric dipole. i.e. pole consisting of
positive and negative pair of charge.
The dipole generates a field vector changing periodically in time and space and its effects are
measured on the surface.
The waveform thus recorded standardized in terms of amplitude and phase relationship and
any deviation from this reflect the presence of abnormality.
The “P” wave is called base line or isopotential line.
P wave ----- De polarization of Auricles.
Combined QRS wave --- Re-polarization of atria and depolarization of ventricles
T wave ----- Ventricular re polarization.
1. PR Interval
Time taken by the impulse to travel from atria to AV node.
It’s the autrio-ventricular conduction time.
Measured from the onset of the P wave to onset of the QRS complex.
Duration is from 0.12 to 0.20 sec.
Prolonged pR interval>0.20 secs is first degree heart block.
2. QRS Complex
Represents the time taken by the heart impulse to travel first through the intro-ventricular
system and then through the free walls of the ventricles. ventricular contraction
Measured from the onset of Q wave to end of S wave.
Duration is between 0.05 and 0.10 secs.
Amplitude is 1 mv
Since the ventricles contain greater muscle mass than the atria, the QRS complex is larger
than the P wave.
VII. Blood Flow Meters
• Blood flow is the one of the important physiological parameter and the most difficult to
measure accurately.
• The average velocities of blood flow vary over a wide range depending on diameter of blood
vessel.
• There are many techniques for measuring the blood flow and velocity.
• They are categorized into
1. Invasive (surgical).
2. Non invasive (through the skin).
Need for blood flowmeter
• Inspection for block in blood flow.
• Testing artificial blood vessels during organ transplantation.
Most widely used blood flow meters are:
Electromagnetic blood flow meter
Ultrasonic blood flow meters
ELECTROMAGNETIC BLOOD FLOWMETER
• Measures instantaneous pulsatile flow of blood
• Works based on the principle of electromagnetic induction
• The voltage induced in a conductor moving in a magnetic field is proportional to the velocity
of the conductor
• The conductive blood is the moving conductor
Structure
• A permanent magnet or electromagnet positioned around the blood vessel generates a
magnetic field perpendicular to the direction of the flow of the blood.
• The blood stream which is a conductor cuts the magnetic field and voltage is induced in the
blood stream.
• Voltage induced in the moving blood column is measured with stationary electrodes located
on opposite sides of the blood vessel and perpendicular to the direction of the magnetic field.
• This method requires that the blood vessel be exposed so that the flow head or the measuring
probe can be put across it.
• The magnitude of the voltage picked up is directly proportional to the strength of the
magnetic field, diameter of the blood vessel and the velocity of blood flow.
Types of Electromagnetic Flow Meters
Basically, all modern flowmeters consist of a generator of AC, a probe assembly, a series of
capacitance coupled amplifiers, a demodulator, a DC amplifier and a suitable recording
device.
Basing shape of the energizing current waveform for the electromagnet 2 types of EM
Flowmeters are :
A. Sine wave flometer.
B. Square wave flowmeter.
SINE WAVE FLOWMETERS
Probe magnet is energized with a sine wave and the induced voltage will also be sinusoidal.
Since the flow of blood acts as a secondary terminal of a transformer w.r.t probe magnet, an
additional artifact voltage induced is called transformer voltage.
This voltage is 90° out of phase with the original signal corresponding to flow of blood .
A method for eliminating transformer voltage by using a gated amplifier(amplify signal only
during flow induced voltage is maximum).
This type of instrument is known as 'gated sine wave flowmeter',
SQUARE WAVE FLOWMETER
Transducer
• consist of an electromagnet, a pair of electrodes.
• Electrodes may be in contact with either flowing blood or outer surface of the blood vessel
Preamplifier
• The induced voltage pick up by the electrodes, is given to a low noise differential amplifier
through a capacitive coupling
• Must have a very high CMRR and input impedance
Gating circuit
• It helps to remove spurious voltages generated during magnet current reversal
• The gating action is controlled by the circuit which provides an excitation current to the
electromagnet
Band pass filter
• It is an active RC band pass amplifier , which selectively pass through it the amplified
square wave signal
• Peak response is kept for 4(X)Hz
Detector
• A phase sensitive detector is used to recover the signal
• It also helps in the rejection of interfering voltages at frequencies below the carrier
frequency
Low pass filter and output stage
• Demodulated signal is given to an RC LPF, which provides a uniform frequency response
and a linear phase shift
• Followed by an integrator provide output corresponding to the mean flow
Magnet current drive
• The square wave input to the power amplifier stage which supplies current to the
electromagnet is fed from a free running multivibrator
Zero flow reference line
• it establish the signal corresponding to zero-flow before measurement.
BLOOD PRESSURE MEASUREMENTS
The systolic pressure is the maximum pressure in an artery at the moment when the heart is
beating and pumping blood through the body.
The diastolic pressure is the lowest pressure in an artery in the moments between beats when
the heart is resting.
Both the systolic and diastolic pressure measurements are important
If either one is raised, it means you have high blood pressure (hypertension).
Blood pressure measurement can be classified in to
1. Indirect
2. Direct
Indirect Method
Simple equipment ,Very little discomfort, Less informative and Intermittent
The indirect method is also somewhat subjective, and often fails when the blood pressure is
very low (as would be the case when a patient is in shock).
1. Auscultatory
Auscultatory method uses aneroid sphygmomanometer with a stethoscope.
The auscultatory method comes from the Latin word "listening.
2. Oscillometric
The oscillometric method was first demonstrated in 1876 and involves the observation of
oscillations in the sphygmomanometer cuff pressure which are caused by the oscillations
of blood flow, i.e., the pulse.
3. Ultrasonic
1. AUSCULTATORY METHOD
Blood pressure measurements using
sphygmomanometer
First, a cuff is placed around your arm and
inflated with a pump until the circulation is
cut off.
A small valve slowly deflates the cuff, and
the doctor measuring blood pressure uses a
stethoscope, placed over your arm, to listen
for the sound of blood pulsing through the
arteries.
That first sound of rushing blood refers to
the systolic blood pressure; once the sound fades, the second number indicates the diastolic
pressure.
2. Oscillometric Measurement Method
3. Ultrasonic Doppler Shift Method
ULTRASONIC BLOOD FLOW METERS
There are basically two types of ultrasonic blood flow-velocity meters.
1. Transit time velocity meter and
2. Doppler-shift type.
Doppler-shift Flow-velocity Meters
It is a non-invasivc technique to measure blood velocity in a particular vessel from the
surface of the body.
lt is based on the analysis of echo signals from the erythrocytes in the vascular structures.
Because of the Doppler effect, the frequency of these echo signals changes relative to the
frequency which the probe transmits.
The Doppler frequency shift is a measure of the size and direction of the flow velocity.
The principle is illustrated in Fig.
The incident ultrasound is scattered by the blood cells and the scattered wave is received by
the second transducer. The frequency shift due to the moving scatterers is proportional to the
velocity of the scatterers. Alteration in frequency occurs first as the ultrasound arrives at the
‘scattered and second as it leaves the scattcrer.
The piezo-electric crystal A is electrically excited to generate ultrasonic waves, which enter
the blood. Ultrasound scattered from the moving blood cells excites the receiver crystal.
The electrical signal received at B consists of a large amplitude excitation frequency
component, which is directly coupled from the transmitter to the receiver, plus a very small
amplitude Doppler-shifted component scattered from the blood cells.
The detector produces a sum of the difference of the frequencies at I). The low-pass filter
selects the difference frequency, resulting in audio frequencies at E. Each time the audio
wave crosses the zero axis, a pulse appears at G. The filtered output level at II will be
proportional to the blood velocity. The following two pitfalls are encountered in Doppler
ultrasonic blood flowmeters.
The transit-time ultrasonic flow meter is an instrument designed for measuring the volume
flow rate in clean liquids or gases. It consists of a pair of ultrasonic transducers mounted
along an axis aligned at an angle _ with respect to the fluid-flow axis, as shown in Figure.
Each transducer consists of a transmitter–receiver pair, with the transmitter emitting
ultrasonic energy which travels across to the receiver on the opposite side of the pipe. These
ultrasonic elements are normally piezoelectric oscillators of the same type as used in Doppler
shift flow meters. Fluid flowing in the pipe causes a time difference between the transit times
of the beams travelling upstream and downstream, and measurement of this difference allows
the flow velocity to be calculated. The typical magnitude of this time difference is 100 ns in a
total transit time of 100 μs, and high-precision electronics are therefore needed to measure it.
Types of Electrodes
Essentially, five types of electrodes are typically used:
1. Scalp: silver pads, discs, or cups; stainless steel rods; and chlorided silver wires.
2. Sphenoidal: Alternating insulated silver and bare wire and chlorided tip inserted through
muscle tissue by a needle.
3. Nasopharyngeal: silver rod with silver ball at the tip inserted through the nostrils.
4. Electrocorticographic: cotton wicks soaked in saline solution that rests on the brain
surface (removes artifacts generated in the cerebrum by each heartbeat).
5. Intracerebral: sheaves of Teflon-coated gold or platinum wires cut at various distances
from the sheaf tip and used to electrically stimulate the brain.
Reusable disks
These electrodes can be placed close to the scalp, even in a region with hair because they are
small. A small amount of conducting gel needs to be used under each disk. The electrodes are
held in place by a washable elastic head band. Disks made of tin, silver, and gold are
available. They can be cleaned with soap and water or Cidex.
The cost of each disk and lead is dependent on the type of metal used as a conductor, the
gauge of wire used as a lead, and the type of insulation on the wire lead.
EEG Cups with disks
Different styles of caps are available with different numbers and types of electrodes. Some
caps are available for use with replaceable disks and leads. Gel is injected under each disk
through a hole in the back of the disk.
Since the disks on a region of the scalp covered with hair cannot be placed as close to the
scalp as individual disc electrodes, a greater amount of conducting gel needs to be injected
under each. After its use, more time is required to clean the cap and its electrodes, as well as
the hair of the subject. Depending on the style and longevity of the cap and the electrodes,
their expense can be moderate to high.
Adhesive Gel Electrodes
These are the same disposable silver/silver chloride electrodes used to record ECGs and
EMGs, and they can be used with the same snap leads used for recording those signals.
These electrodes are an inexpensive solution for recording from regions of the scalp without
hair. They cannot be placed close to the scalp in regions with hair, since the adhesive pad
around the electrode would attach to hair and not the scalp.
Subdermal Needles
These are sterilized, single-use needles that are placed under the skin. Needles are available
with permanently attached wire leads, where the whole assembly is discarded, or sockets that
are attached to lead wires with matching plugs. Since they are a sterile single-use item, the
expense of needle electrodes is moderate to high.
Placement of electrodes
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An internationally recognized method that allows EEG electrode placement to be
standardized.
Ensures inter-electrode spacing is equal
Electrode placements proportional to skull size & shape
Covers all brain regions
F = Frontal P = Parietal T = Temporal O= Occipital
Numbering system
Odd=Left Side Even=Right Side Z=Midline
Four Skull Landmarks
•Nasion
• Inion
• Left Pre‐auricular point
•Right Pre‐auricular point
Lead FP1 for example, is in the frontal area and lies on
a circle with other leads. Nineteen electrodes, plus one
for grounding the subject, are used.
Electrodes are placed (using a flexible tape measure)
by measuring the nasion-inion distance and marking points on the (shaved) head 10%, 20%,
20%, 20%. and 10% of this length.
The vertex. C2 electrode is the midpoint. Figure given below shows the complete 10- 20
electrode placement system. Here, 19 electrodes are used on the scalp, plus one for grounding
the subject.
Electrode arrangements may be either unipolar or bipolar. A unipolar arrangement is
composed of a number of scalp leads connected to a common indifference point such as an
earlobe.
Hence, one electrode is common to all channels. For example, FP2 may be measured with
respect to two ear electrodes connected together or summation of scalp electrodes.
A bipolar arrangement is achieved by the interconnection of scalp electrodes. The difference
of voltage between and Fpg may also be measured. Montages are patterns of connections
between electrodes and recording channels. All of these combinations have inputs to a three-
lead differential amplifier and use a third connection for the reference (two ears, forehead, or
nose).
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Measurement of Cz
• Measure the distance from pre‐auricular point to pre‐ auricular
point
•Mark the midpoint (50%) with a vertical line • This cross
represents Cz which has been correctly aligned in the horizontal
& vertical planes
Measurements - T3, C3, Cz, C4, T4
Mark 10%, 20%, 20%, 20%, 20%, 10% =T3,C3, Cz, C4, T4
Measurements - Fpz, Fz, Cz, Pz, Oz
Reapply the tape along the midline from nasion to inion
Mark 10%, 20%, 20%, 20%, 20%, 10% =FpzFz, Cz, Pz, Oz
Measurements - Fp1, F7, T3, T5, O1, Oz
Measure the distance between Fpz & Oz by applying the tape
around the head via T3.
Mark at 10%, 20%, 20%, 20%, 20%, 10% =Fp1F7, T3, T5, O1,
Oz,
Measurement - F3
Measure Fp1 to C3 and mark midpoint
Measure Fz to F7 and mark midpoint
Mark 50% = F3
(Repeat the process using Fp2 to C4 & Fz to F8 to mark
F4)
Measurements M1 & M2
M1 & M2 are the reference electrodes (formally known as A1 & A2)
M1 & M2 are placed on the mastoid (M) process.
These are the bony prominences behind the ears.
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EVOKED POTENTIAL
A special form of electroencephalography is the recording of evoked potentials from
various parts of the nervous system. In this technique the EEG response to some form
of sensory stimulus, such as a flash of a light or an audible click, is measured.
To distinguish the response to the stimulus from ongoing EEG activity, the EEG
signals are time-locked to the stimulus pulses and averaged, so that the evoked
response is reinforced with each presentation of the stimulus, while any activity not
synchronized to the stimulus is averaged out.
Figure given below shows a raw EEG record containing an evoked response from a
single presentation of the stimulus and the effect of averaging 8 and 64 presentations,
respectively.
Averaging of EEG evoked potentials:
(a) raw EEG of single response; (b) average of 8 responses; (c) average of 64 responses.
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Body Plethysmographs
Body plethysmography is a well-established technique of lung function determination.
plethysmographs are commonly used to measure the functional residual capacity (FRC) of
the lungs—the volume in the lungs when the muscles of respiration are relaxed—and total
lung capacity.
Principles of body plethysmography
Apparatus
B, body plethysmograph;
S, shutter which occludes airway;
L, lung;
C, capacitance manometer to record pressure changes in the plethysmograph (which are
proportional to the change in body volume);
P, capacitance manometer to record pressure changes in the mouth (which are equal to
alveolar pressure when there is no airflow);
O, cathode ray oscillograph with x and y axes.
The volume-constant whole-body plethysmograph is a chamber resembling a glass-walled
telephone box in shape and volume (about 700–1000 L). During measurement the box is
closed with an airtight seal, except for a small controlled leak that is used to stabilize the
internal pressure by allowing for equilibration of slow pressure changes, e.g. due to warming-
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up. One pressure transducer serves to measure the pressure inside the box relative to ambient
pressure, another one is placed close to the mouth for recording mouth pressure during a
shutter maneuver. The shutter mechanism can be used to deliberately block the airflow by
transient occlusion. Moreover, respiratory flow rate is recorded by conventional equipment,
such as pneumotachograph, anemometer, or ultrasound measurement, all of which is
calibrated via syringes delivering a defined volume.
Principle of measurement
The principle of measurement of the commonly used plethysmographs relies on detecting
changes in box pressure in combination with either changes of mouth pressure or with flow
rate under defined breathing conditions. These signals are evaluated in order to determine
static lung volumes and airflow resistance.
The basic physical principle exploited by body plethysmography is the law of Boyle-
Mariotte.
Based on Boyle’s law PV = k
Assumes temperature remains constant
When subject breathes in and out against a shutter, changes in pressure and volume occur
For the present purpose this law can be concisely expressed as the statement that for a fixed
amount of gas in a closed compartment the relative changes in the compartment’s volume are
always equal in magnitude but opposite in sign to the relative changes in pressure. Thus one
can infer relative volume changes from pressure changes and, even more, absolute volumes if
the absolute volume changes are known.
Biomedical Engineering Module III S5-ECE, 2017
Gas Exchange and Distribution
Once air is in the lungs, oxygen and carbon dioxide must be exchanged between the air and
the blood in the lungs and between the blood and the cells in the body tissues. In addition, the
gases must be transported between the lungs and the tissue by the blood. A number of tests
have been devised to determine the effectiveness with which these processes are carried out.
Measurements of Gaseous Exchange and Diffusion
The mixing of gases within the lungs, the ventilation of the alveoli, and the exchange of
oxygen and carbon dioxide between the air and blood in the lungs all take place through a
process called diffusion.
Diffusion is the movement of gas molecules from a point of higher pressure to a point of
lower pressure to equalize the pressure difference. This process can occur when the gas is
unequally distributed in a chamber or wherever a pressure difference exists in the gas on two
sides of a membrane permeable to that gas.
Measurements required for determining the amount of diffusion involve the partial pressures
of oxygen and carbon dioxide, Pq2 and Pc02» respectively. There are many methods by
which these measurements can be obtained, including some chemical analysis methods and
measurements of diffusing capacity.
1. Chemical analysis methods. In the original gas analyzers devices, a gas sample of
approximately 0.5 ml is introduced into a reaction chamber by use of a transfer pipet at the
upper end of the reaction chamber capillary. An indicator droplet in this capillary allows the
sample to be balanced against a trapped volume of air in the thermobarometer. Absorbing
fluids for C02 and 02 can be transferred in from side arms without causing any change in the
total volume of the system. The micrometer is adjusted so as to put mercury into the system
in place of the gases being absorbed. The volume of the absorbed gases is read from the
micrometer barrel calibration.
2. Diffusing capacity using CO infrared analyzer. To determine the efficiency of perfusion of
the lungs by blood and the diffusion of gases, the most important tests are those that measure
02, C02, pH, and bicarbonate in arterial blood. In trying to measure the diffusion rate of
oxygen from the alveoli into the blood, it is usually assumed that all alveoli have an equal
concentration of oxygen. Actually, this condition does not exist because of the unequal
distribution of ventilation in the lung; hence, the terms diffusing capacity or transfer factor
(rather than diffusion) arc used to describe the transfer of oxygen from the alveoli into the
pulmonary capillary blood.
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Gas chromatograph. The quantities of various gases in the expired air can also be
determined by means of a gas chromatograph, an instrument in which the gases are separated
as the air passes through a column containing various substances that interact with the gases.
The reactions cause different gases to pass through the column at different rates so that they
leave the column at different times. The quantity of each gas is measured as it emerges. To
identify the gases in the expired air other than oxygen, nitrogen, or COj, a mass spectrometer
is used in conjunction with the gas chromatograph. The mass spectrometer identifies the ions
according to their mass/charge ratio.
Measurements of Gas Distribution
The distribution of oxygen from the lungs to the tissues and carbon dioxide from the tissues
to the lungs takes place in the blood. The process, by which each gas is transported, however,
is quite different. As mentioned earlier, oxygen is carried by the hemoglobin of the red blood
cells. On the other hand, carbon dioxide is carried through chemical processes in which C02
and water combine to produce carbonic acid, which is dissolved in the blood. The amount of
carbonic acid in the blood, in turn, affects the pH of the blood. In assessing the performance
of the blood in its ability to transport respiratory gases, then, measurements of the partial
pressures of oxygen (Pq}) and carbon dioxide (Pco2) ]n the blood, the percent of oxygenation
of the hemoglobin, and the pH of the blood are most useful.
INSTRUMENTS USED FOR CLINICAL LABORATORY
1. SPECTROPHOTOMETERS
A spectrophotometer is an instrument which isolates monochromatic radiation in a more
efficient and versatile manner than colour filters used in filter photometers.
In these instruments, light from the source is made into a parallel beam and passed to a prism
or diffraction grating, where light of different wavelengths is dispersed at different angles.
The amount of light reaching the detector of a spectrophotometer is generally much smaller
than that available for a colorimeter, because of the small spectral band width.
Therefore, a more sensitive detector is required. A photomultiplier or vacuum photocell is
generally employed.
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In this device the simple selection filter of the colorimeter is replaced by a monochromator.
A monochromator uses a diffraction grating G (or a prism) to disperse light from a lamp that
falls through an entrance slit S, into its spectral components.
An exit slit S2 selects a narrow band of the spectrum, which is used to measure the absorption
of a sample in cuvette C.
The narrower the exit slit, the narrower the bandwidth of the light, but also the smaller its
intensity.
A sensitive photodetector D (often a photomultiplier) is therefore required, together with an
amplifier and a meter, which is calibrated in units of transmittance or absorbance. The
wavelength of the light can be changed by rotating the grating.
The spectrophotometer allows the determination of the absorption of samples at various
wavelengths. The light output of the lamp, however, as well as the sensitivity of the
photodetector and the light absorption of the cuvette and solvent, varies when the wavelength
is changed.
This situation requires that, for each wavelength setting, the density reading be set to zero,
with the sample being replaced by a blank cuvette, usually filled with the same solvent as
used for the sample. In double-beam spectrophotometers this procedure is done automatically
by switching the beam between a sample light path and a reference light path, generally with
a mechanical shutter or rotating mirror. By using a computing circuit, the readings from both
paths are compared and only the ratio of the absorbances (or the difference of the densities) is
indicated.
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OXIMETRY
Oximetry refers to the determination of the percentage of oxygen saturation of the circulating
arterial blood. By definition:
Where [Hb02] is the concentration of oxygenated haemoglobin |Hb | is the concentration of
deoxygenated haemoglobin.
In clinical practice, percentage of oxygen saturation in the blood is of great importance. This
saturation being a bio-constant, is an indications of the performance of the most important
cardio-respiratory functions. It is maintained at a fairly constant value to within a few percent
in an healthy organism.
The main application areas of oximetry arc the diagnosis of cardiac and vascular anomalies:
the treatment of post-operative anoxia and the treatment of anoxia resulting from pulmonary
affections.
Oximetry is now considered a standard of care in anaesthesiology and has significantly
reduced anaesthesia-related cardiac deaths.
In Vitro Oximetry
When blood is withdrawn from the subject under anaerobic conditions and measurement for
oxygen saturation is made at a later time in the laboratory, the procedure is referred to as in
vino oximetry.
Transmission Oximetry:
Measurement of the degree of oxygen saturation of the blood can be made by
spectrophotometric method.
In spcctrophotometry the concentration of substances held in solution are measured by
determining the relative light attenuations that the light absorbing substances cause at each of
several wavelengths.
In Vivo Oximetry
In vivo oximetry measures the oxygen saturation of blood while the blood is flowing through
the vascu lar system or it may be flowing through a cuvette directly connected with the
circulatory system by means of a catheter. The blood in this case is unhaemolyzed. Both
techniques, reflection and transmission,are utilized for in vivo oximetry.
EAR OXIMETER
In this case, the pinna of the ear acts as a cuvette. Blood in the ear must be made similar to
arterial blood in composition. This is done by increasing the flow through the ear without
appreciably increasing the metabolism. Maximum vasodilatation is achieved by keeping the
ear warm. It takes about 5 or 10 min for the ear to become fully dilated after the ear unit has
been put up in place and the lamp turned on.
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Figure above explains the basic operation of the instrument. The light source isa tungsten-
iodine lamp that has a high output in the spectrum of interest. A lens system collimates the
light beam and directs it through thin-film interference filters that provide wavelength
selection.
These filters are mounted in the periphery of a wheel rotating at 1300 rpm and thus cut the
light beam sequentially. The filtered light beam then enters a fibre optic bundle that carries it
to the ear.
Another fibre optic bundle carries the light passing through the car back to a detector in the
instrument. A second light path is developed with a beam splitter in the path of the collimated
light beam near the source. This path also passes through the filter wheel and then through a
fibre optic bundle directly to the photodetector. So, the detector receives two light pulses for
each wavelength.
The processor takes the ratio of two pulses as the measured value; so readings are
compensated for any changes in the spectral characteristics of the light source and optical
system.
The current developed at the photodetector is only 0.5 nA or less during a light puIse. This is
amplified in a high gain amplifier and then converted to a 16-bit digital form by an A-D
converter synchronized with the wheel rotation. The 16-bit words are given to a digital signal
averager that performs two fu net ions. First, it averages out the noise content of the signal w
ith a time constant of 1.6sand secondly it servesasa buffer to hold information till it is
required for computation.
Computation of percent oxygen saturation is accomplished by a 24-bit algorithmic-state
machine. It uses serial processing with the program stored in ROM and the necessary
coefficients of the equations stored on a field programmable ROM. The computation circu its
also derive the quantity of total haemoglobin seen within the field of view of the earpiece. If
this quantity is low, the instrumentdisplaysan ‘Off Ear’ indication. From the computational
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Biomedical Engineering Module III S5-ECE, 2017
section,data is transferred in pulse-decimal form to the output circuit board where it is
converted to BCD for the front panel digital display.
The patient related part consists of arterializing blood flow in the pinna by a brisk 15 s rub.
Application of the probe to the ear results in a suitable display in about 30s. A built-in heater
regulated to 41°C maintainsartcrialization. Restandardization is not required when the
instrument is to be used on other patients.
Measurements at eight wavelengths provide a great deal of information, which makes it
possible to account for eight unknowns. This is sufficient to take into consideration the
patient to patient variables and account for the various forms of haemoglobin.
The procedure is simple, requiring only the storage of initial light intensities at each of the
eight wavelengths. However, it is still necessary to arterialize blood flow by warming the ear,
and a large ear probe incorporating fibre optics is necessary to make the system work.
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Types of Electrodes
Essentially, five types of electrodes are typically used:
1. Scalp: silver pads, discs, or cups; stainless steel rods; and chlorided silver wires.
2. Sphenoidal: Alternating insulated silver and bare wire and chlorided tip inserted through
muscle tissue by a needle.
3. Nasopharyngeal: silver rod with silver ball at the tip inserted through the nostrils.
4. Electrocorticographic: cotton wicks soaked in saline solution that rests on the brain
surface (removes artifacts generated in the cerebrum by each heartbeat).
5. Intracerebral: sheaves of Teflon-coated gold or platinum wires cut at various distances
from the sheaf tip and used to electrically stimulate the brain.
Reusable disks
These electrodes can be placed close to the scalp, even in a region with hair because they are
small. A small amount of conducting gel needs to be used under each disk. The electrodes are
held in place by a washable elastic head band. Disks made of tin, silver, and gold are
available. They can be cleaned with soap and water or Cidex.
The cost of each disk and lead is dependent on the type of metal used as a conductor, the
gauge of wire used as a lead, and the type of insulation on the wire lead.
EEG Cups with disks
Different styles of caps are available with different numbers and types of electrodes. Some
caps are available for use with replaceable disks and leads. Gel is injected under each disk
through a hole in the back of the disk.
Since the disks on a region of the scalp covered with hair cannot be placed as close to the
scalp as individual disc electrodes, a greater amount of conducting gel needs to be injected
under each. After its use, more time is required to clean the cap and its electrodes, as well as
the hair of the subject. Depending on the style and longevity of the cap and the electrodes,
their expense can be moderate to high.
Adhesive Gel Electrodes
These are the same disposable silver/silver chloride electrodes used to record ECGs and
EMGs, and they can be used with the same snap leads used for recording those signals.
These electrodes are an inexpensive solution for recording from regions of the scalp without
hair. They cannot be placed close to the scalp in regions with hair, since the adhesive pad
around the electrode would attach to hair and not the scalp.
Subdermal Needles
These are sterilized, single-use needles that are placed under the skin. Needles are available
with permanently attached wire leads, where the whole assembly is discarded, or sockets that
are attached to lead wires with matching plugs. Since they are a sterile single-use item, the
expense of needle electrodes is moderate to high.
Placement of electrodes
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An internationally recognized method that allows EEG electrode placement to be
standardized.
Ensures inter-electrode spacing is equal
Electrode placements proportional to skull size & shape
Covers all brain regions
F = Frontal P = Parietal T = Temporal O= Occipital
Numbering system
Odd=Left Side Even=Right Side Z=Midline
Four Skull Landmarks
•Nasion
• Inion
• Left Pre‐auricular point
•Right Pre‐auricular point
Lead FP1 for example, is in the frontal area and lies on
a circle with other leads. Nineteen electrodes, plus one
for grounding the subject, are used.
Electrodes are placed (using a flexible tape measure)
by measuring the nasion-inion distance and marking points on the (shaved) head 10%, 20%,
20%, 20%. and 10% of this length.
The vertex. C2 electrode is the midpoint. Figure given below shows the complete 10- 20
electrode placement system. Here, 19 electrodes are used on the scalp, plus one for grounding
the subject.
Electrode arrangements may be either unipolar or bipolar. A unipolar arrangement is
composed of a number of scalp leads connected to a common indifference point such as an
earlobe.
Hence, one electrode is common to all channels. For example, FP2 may be measured with
respect to two ear electrodes connected together or summation of scalp electrodes.
A bipolar arrangement is achieved by the interconnection of scalp electrodes. The difference
of voltage between and Fpg may also be measured. Montages are patterns of connections
between electrodes and recording channels. All of these combinations have inputs to a three-
lead differential amplifier and use a third connection for the reference (two ears, forehead, or
nose).
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Measurement of Cz
• Measure the distance from pre‐auricular point to pre‐ auricular
point
•Mark the midpoint (50%) with a vertical line • This cross
represents Cz which has been correctly aligned in the horizontal
& vertical planes
Measurements - T3, C3, Cz, C4, T4
Mark 10%, 20%, 20%, 20%, 20%, 10% =T3,C3, Cz, C4, T4
Measurements - Fpz, Fz, Cz, Pz, Oz
Reapply the tape along the midline from nasion to inion
Mark 10%, 20%, 20%, 20%, 20%, 10% =FpzFz, Cz, Pz, Oz
Measurements - Fp1, F7, T3, T5, O1, Oz
Measure the distance between Fpz & Oz by applying the tape
around the head via T3.
Mark at 10%, 20%, 20%, 20%, 20%, 10% =Fp1F7, T3, T5, O1,
Oz,
Measurement - F3
Measure Fp1 to C3 and mark midpoint
Measure Fz to F7 and mark midpoint
Mark 50% = F3
(Repeat the process using Fp2 to C4 & Fz to F8 to mark
F4)
Measurements M1 & M2
M1 & M2 are the reference electrodes (formally known as A1 & A2)
M1 & M2 are placed on the mastoid (M) process.
These are the bony prominences behind the ears.
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EVOKED POTENTIAL
A special form of electroencephalography is the recording of evoked potentials from
various parts of the nervous system. In this technique the EEG response to some form
of sensory stimulus, such as a flash of a light or an audible click, is measured.
To distinguish the response to the stimulus from ongoing EEG activity, the EEG
signals are time-locked to the stimulus pulses and averaged, so that the evoked
response is reinforced with each presentation of the stimulus, while any activity not
synchronized to the stimulus is averaged out.
Figure given below shows a raw EEG record containing an evoked response from a
single presentation of the stimulus and the effect of averaging 8 and 64 presentations,
respectively.
Averaging of EEG evoked potentials:
(a) raw EEG of single response; (b) average of 8 responses; (c) average of 64 responses.
Syllabus
Therapeutic Equipments: Principle, block schematic diagram, working and applications of :
pacemakers, cardiac defibrillators, heart–lung machine, dialyzers, surgical diathermy
equipment, ventilators
I. Therapeutic Equipments:
Equipments that replace certain critical physiological functionalities, or provide needed pain
therapy.
Pacemakers
A device capable of generating artificial pacing impulses and delivering them to the heart is
known as a pacemaker system (commonly called a pacemaker). It consists of pulse generator
and appropriate electrodes.
The rhythmic beating of the heart is due to the triggering pulses that originate in an
area of specialized tissue in the right atrium of the heart. This area is known as the
sino-atrial node.
In abnormal situations, if this natural pacemaker ceases to function or becomes
unreliable or if the triggering pulse does not reach the heart muscle because of
blocking by the damaged tissues, the natural and normal synchronization of the heart
action gets disturbed.
When monitored, this manifests itself through a decrease in the heart rate and changes
in the electrocardiogram (ECG) waveform.
By giving external electrical stimulation impulses (Fig.1) to the heart muscle, it is
possible to regulate the heart rate. These impulses are given by an electronic
instrument called a 'pacemaker'.
A pacemaker basically consists of two parts:
(i) an electronic unit which generates stimulating impulses of controlled rate and amplitude,
known as pulse generator, and
(ii) the lead which carries the electrical pulses from the pulse generator to the heart. The lead
includes the termination which connects to the pulse generator and the insulated conductors,
which interface with electrodes and terminate within the heart.
Types of Pacemakers
1. Internal Pacemakers:
In this entire system is inside the body.
Permanently implanted in the body whose SA node failed to function properly.
The system is implanted with the pulse generator placed in a surgically formed pocket
below the right or left cavicle.
Internal leads connected to electrodes that directly contact surface of myocardium
Pulse generator must be self contained with a power source capable of continuously
operating the unit for a period of years.
Above fig shows an RC, reference voltage source, and a comparator determines the
basic pacing rate of the pulse generator.
Its output signal feeds into a second RC network, the pulse width circuit, which
determines the stimulating pulse duration.
A third RC network, the rate-limiting circuit, disables the comparator for a preset
interval and thus limits the pacing rate to a maximum of 120 pulses per minute for
most dagfe-component failures.
The output circuit provides a voltage pulse to stimulate the heart. The voltage monitor
circuit senses cell depletion and the rate slowdown circuit and energy compensation
circuit of this event.
The rate slowdown circuit shuts off some of the current to the basic timing network to
cause the rate to slow down 8 ± 3 beats per minute when ceil depletion has occurred.
The energy-compensation circuit causes the pulse duration to increase as the battery
voltage decreases, to maintain nearly constant stimulation energy to the heart.
There is also a feedback loop from the output circuit to the refractory circuit, which
provide a period of time following an output pulse or a sensed R-wave during which
the amplifier will not respond to outside signals.
The sensing circuit detects a spontaneous R wave and resets the oscillator timing
capacitor.
The reversion circuit allows the amplifier to detect a spontaneous R wave in the
presence of low-level continuous wave interference. In the absence of an R wave, this
circuit allows the oscillator to pace at its preset rate ± 1 beat per minute.
2. External Pacemakers
Employed to restart normal rhythm of heart in case of cardiac standstill.
An external pacemaker usually consists of an externally worn pulse generator
connected to electrodes located on or within the myocardium.
Used on patients with temporary heart irregularities.
In this the pulse generator located outside the body and connected to ventricle using a
long thin tube called catheter.
The pacing impulse(80 mA) is applied through metal electrodes placed on the surface
of the body.
Types of Pacing Modes
The noncompetitive method, which uses pulse generators that are either ventricular
programmed or programmed by the atria, is more popular. Ventricular-programmed
pacemakers arc designed to operate either in a demand (R-wave-inhibited) or standby (R-
wave-triggered) mode, whereas atrial-programmed pacers are always synchronized with the
P wave of the ECG
1. Competitive [Fixed Rate or Asynchronous Pacemaker]
The first (and simplest) pulse generators were fixed-rate or asynchronous devices that
produced pulses at a fixed rate and were independent of any natural cardiac activity:
A synchronous pacing is called competitive pacing because the fixed-rate impulses
may occur along with natural pacing impulses generated by the heart and would
therefore be in competition with them in controlling the heartbeat. This competition is
largely eliminated through use of ventricular or atrial-programmed pulse generators.
2. Ventricular Programmed
The problems of shorter battery life and competition for control of the heart led, in
part, to the development of ventricular-programmed (demand or standby) pulse
generators.
Either type of ventricular-programmed pulse generator, when connected to the
ventricles via electrodes, is able to sense the presence (or. absence) of a naturally
occurring R wave.
Above figure shows a functional block diagram of a ventricular synchronous demand
pacer.
The pulse generator has two functions, viz., pacing and sensing. Sensing is accomplished by picking up the ECG signal.
In the case of dual-chamber pacing, the P wave is also sensed.
Once the signal enters the sensing circuit, it is passed through a QRS bandpass filter.
This filter is design to pass signal components in the frequency range of 5-100 Hz,
with a centre frequency of 30 Hz.
This is followed by an amplifier and threshold detector which is designed to operate with a detection sensitivity of 1-2 mV. Sensitivity of this order ensures reliable
detection of cardiac signals sensed on the electrodes which typically have amplitudes
in the 1-30 mV range depending onto 1 electrode surface area and the sensing circuit
loading impedance.
Refractory period (T1) is necessarily incorporated to limit the pulse delivery rate,
particularly in the ' presence of electromagnetic interference. It is meant to prevent
multiple re-triggering of the astable multivibrator following a sensed or paced
contraction.
The free-running multivibrator provides a fixed rate mode with an interval of T2 via
the output driver circuit. The output pulses of a length T3 synchronous with input
signals that fall outside the sensing refractory period T1 are thus delivered at the
stimulating electrodes.
R-wave-inhibited (demand):
The output of an R-wave-inhibited (demand) unit is suppressed (no output pulses are
produced) as long as natural (intrinsic) R waves are present. Thus, its output is held
back or inhibited when the heart is able to pace itself.
However, should standstill occur, or should the intrinsic rate fall below the preset rate
of the pacer (around 70 BPM), the unit will automatically provide an output to pace
the heart after an escape interval at the designated rate.
In this way, ventricular-inhibited pacers are able to pace on demand.
A demand pacer, in the absence of R waves, automatically reverts to a fixed-rate
mode of operation.
R-wave- triggered :
R-wave-triggered pulse generators, like the inhibited units, sense each intrinsic R
wave. However, this pacer emits an impulse with the occurrence of each sensed R
wave. Thus, the unit triggered rather than inhibited by each R wave.
The pacing impulses are transmitted to the myocardium during its absolute refractory
period, so they will have no effect on normal heart activity.
3. Atrial Programmed
In cases of complete heart block where the atria are able to depolarize but the impulse
fails to depolarize the ventricles, atrial synchronous pacing may be used. Here the
pulse generator is connected through wires and elcetrodes to both the atria and the
Ventricles.
The atrial electrode couples atrial impulses to the pulse generator, which then emits
impulses to stimulate the ventricles via the ventricular electrode. In this way, the heart
is paced at the same rate as the natural pacemaker.
When the SA node rate changes because of vagus or sympathetic neuronal control, the
ventricle will change its rate accordingly but not above some maximum rate (about
125 per minute).
CARDIAC DEFIBRILLATORS
NEED FOR A DEFIBRILLATOR
Ventricular fibrillation is a serious cardiac emergency resulting from asynchronous
contraction of the heart muscles. This uncoordinated movement of the ventricle walls
of the heart may result from coronary occlusion, from electric shock or from
abnormalities of body chemistry.
Because of this irregular contraction of the muscle fibres, the ventricles simply quiver
rather than pumping the blood effectively. This results in a steep fall of cardiac output
and can prove fatal if adequate steps are not taken promptly.
In fibrillation, the main problem is that the heart muscle fibres are continuously
stimulated by adjacent cells so that there is no synchronised succession of events that
follow the heart action. Consequently, control over the normal sequence of cell action
cannot be captured by ordinary stimuli.
DEFIBRILLATOR
Ventricular fibrillation can be converted into a more efficient rhythm by applying a
high energy shock to the heart. This sudden surge across the heart causes all muscle
fibres to contract simultaneously. Possibly, the fibres may then respond to normal
physiological pacemaking pulses. The instrument for administering the shock is called
a defibrillator.
Restoration of normal rhythm in fibrillating heart as achieved by direct
The shock can be delivered to the heart by means of electrodes placed on the chest of the
patient (external defibrillation) or the electrodes may be held directly against the heart when
the chest is open (internal defibrillation). Higher voltages are required for external
defibrillation than for internal defibrillation.
DC DEFIBRILLATOR
Basic Principle
In this an energy storage capacitor is charged at a relatively slow rate (in the order of
seconds) from the AC line by means of a step-up transformer and rectifier arrangement or
from a battery and a DC to DC converter arrangement. During defibrillation, the energy
stored in the capacitor is then delivered at a relatively rapid rate (in the order of milliseconds)
to the chest of the subject. For effective defibrillation, it is advantageous to adopt some
shaping of the discharge current pulse. The simplest arrangement involves the discharge of
capacitor energy through the patient's own resistance (R). This yields an exponential
discharge typical of an RC circuit. If the discharge is truncated, so that the ratio of the
duration of the shock to the time constant of decay of the exponential waveform is small, the
pulse of current delivered to the chest has a nearly rectangular shape. For a somewhat larger
ratio, the pulse of current appears nearly trapezoidal. Rectangular and trapezoidal waveforms
have also been found to be effective in the trans-thoracic defibrillation and such waveforms
have been employed in defibrillators designed for clinical use (Schuder et al. 1980).
The basic circuit diagram of a DC defibrillator is shown in Fig.
A variable auto-transformer Tj forms the primary of a high voltage transformer T2.
The output voltage of the transformer is rectified by a diode rectifier and is connected
to a vacuum type high vol tage change-over switch. In position A, the switch is
connected to one end of an oil-filled 16 micro-farad capacitor.
In this position, the capacitor charges to a voltage set by the positioning of the auto-
transformer. When the shock is to be delivered to the patient, a foot switch or a push
button mounted on the handle of the electrode is operated. The high voltage switch
changes over to position 'B' and the capacitor is discharged across the heart through
the electrodes.
In a defibrillator, an enormous voltage (approx. 4000 V) is initially applied to the
patient.
Discharging Pulse of a DC defibrillator
A typical discharge pulse of the defibrillator is shown in Fig. below.
The most common waveform utilized in the RLC circuit employs an under-damped response with a damping lector leas than unity.
This particular waveform is called a Lown' waveform. This waveform is more or leas
of an oscillatory character, with both positive and negative portion. The pulse width in
this waveform is defined as the time that elapses between the start of the impulse and
the moment that the current intensity passes the zero line for the first time and
changes direction.
The pulse duration is usually kept as 5 ms or 2.5 ms.
IMPLANTABLE DEFIBRILLATORS
An implantable defibrillator is continuously monitors a patient's heart rhythm.
If the device detects fibrillation, the capacitors within the device are charged up to
750 V. The capacitors then discharged into the heart which mostly represents a
resistive load of 50 ohm and to bring heart into normal rhythm.
This may require delivery of more than one high energy pulse.
Implantable defibrillator systems have three main system components: the
defibrillator itself (AID), the lead system, and the programmer recorder/monitor
(PRM). The AID houses the power source, sensing, defibrillation, pacing, and
telemetric communication system. The leads system provides physical and electrical
connection between the defibrillator and the heart tissue. The PRM communicates
with the implanted AID and allows the physician to view status information and
modify the function of the device as needed.
Programmer Recorder/Monitor (PRM): The PRM is an external device that provides
a bidirectional communications link to an implanted AID. This telemetry link is
established from a coil which is contained within the wand of the PRM, to a coil
which is contained within the implanted device. This telemetry channel may be used
to retrieve real-time and stored intracardiac ECG, therapy history, battery status, and
other information pertaining to device function. A number of combinations of
programmable therapy and detection options are available and it is not unusual to alter
these prescriptions dozens of times over the life of the implant.
Leads: Until recently, the defibrillating high energy pulse was delivered to the heart
via a 6 cmx9 cm titanium mesh patch with electrodes placed directly on the external
surface of the heart. Sensing was provided through leads screwed in the heart. This
approach required an invasive surgical approach to provide access to the heart. The
modem implantable defibrillators make use of a single transvenous lead with the
multiple electrodes inserted into the right ventricle for ventricular pacing and
defibrillation.
Pulse Generator: Major sub-systems of the implanted pulse generator are shown in
Fig. 26.8. It has a microprocessor which controls overall system functions. An 8-bit
device is sufficient for most systems. ROM provides non-volatile memory for system
start-up tasks and some program space,whereas RAM is required for storage of
operating parameters, and storage of electro-cardiogram data. The system control part
includes support circuitry for the microprocessor like a telemetry interface, typically
implemented with a UART-like (universal asynchronous receiver/transmitter)
interface and general purpose timers.
The power supply to the circuit comes from lithium Silver Vanadium oxide (Li SVO)
batteries. Digital circuits operate from 3 V or lower supplies whereas analog circuits
typically require precision nanoampere current source inputs. Separate voltage
supplies are generated for pacing (approximately 5 V) and control of the charging
circuit (10-15 V),
High power circuits convert the 3-6 V battery voltage to the 750 V necessary for a
defibrillation pulse, store the energy in high voltage capacitors for timed delivery, and
finally switch the high voltage to cardiac tissue or discharge the high voltage
internally if the cardiac arrhythmia self- terminates. The major components of these
circuits are the battery, the DC to DC converter, the output storage capacitors, and the
high power output switches.
Commercially available implantable defibrillators all utilize lithium SVO cells, with
the most common configuration being two connected in series to form an
approximately 6 V battery. Unlike 2.8 V lithium iodide (LI) pacemaker cells which
develop high internal impedance as they discharge (up to 20,000 Q over their useful
life), SVO cells are characterized by low internal impedance (less than 1ft) over their
useful life. The output voltage of SVO is higher than LI ranging from 3.2 V for a
fresh cell to approximately 2.5 V when nearly depleted.
DC to DC converter used to convert the 6 volt battery voltage to 750 V is of classical
configuration. They are operated at as high a frequency (in the range of 30-60 KHz)
as practical to facilitate the use of the smallest possible core.
The storage capacitors are typically aluminum electrolytics because of the high
volumetric efficiency and working voltage required. Most designs utilize at least two
such capacitors in series
Syllabus
Medical Imaging systems (Basic Principle only): X-ray imaging - Properties and production
of X-rays, X-ray machine, applications of X-rays in medicine. Computed Tomography:
Principle, image reconstruction, scanning system and applications. Ultrasonic imaging
systems: Basic pulse echo system, propagation of ultrasonic through tissues and reflections,
display types, A-Scan, B-Scan, M-Scan, applications, real-time ultrasonic imaging systems
and probes.
I. X-ray imaging:
Equipments that replace certain critical physiological functionalities, or provide needed pain
therapy.
Properties of X-rays
The X-rays in the medical diagnostic region have wavelength of the order of 10-l0m. They
propagate with a speed of 3 x 1010 cm/ s and are unaffected by electric and magnetic fields.
They have short wavelength and extremely high energy.
X-rays are able to penetrate through materials which readily absorb and reflect visible light.
X-rays are absorbed when passing through matter. The extent of absorption depends upon the
density of the matter.
X-rays produce secondary radiation in all matter through which they pass. This secondary
radiation is composed of scattered radiation, characteristic radiation and electrons.
X-rays produce ionization in gases and influence the electric properties of liquids and solids.
The ionizing property is made use of in the construction of radiation-measuring instruments.
X-rays also produce fluorescence in certain materials to help them emit light. Fluoroscopic
screens and intensifying screens have been constructed on the basis of this property. X-rays
affect photographic film in the same way as ordinary visible light.
Production of X-rays
X-rays are produced whenever electrons collide at very high speed with matter and are thus
suddenly stopped. The energy possessed by the electrons appears from the site of the
collision as a parcel of energy in the form of highly penetrating electromagnetic waves (X-
rays) of many different wavelengths, which together form a continuous spectrum.
X-rays are produced specially constructed glass tube, which basically comprises,
(i) a source for the production electrons,
(ii) a energy source to accelerate the electrons,
(iii) a free electron path,
(iv) a mean t focusing the electron beam and
(v) a device to stop the electrons.
Stationary mode tubes and rotating anode tubes are the two main types of X-ray tubes:
Stationary Anode Tube
Fig. shows the basic components of a stationary anode X-ray tube. The normal tube is a
vacuum diode in which electrons are generated by thermionic emission from the filament of
the tube.
The electron stream is electrostatically focused on a target on the anode by means of a
suitably shaped cathode cup.
The kinetic energy of the electrons impinging on the target is converted into X-rays. Most
electrons emitted by the hot element become current carriers across the tube.
Some X-ray tubes function as a triode with a bias voltage applied between the filament and
the cathode cup.
The cathode block, which contains the filament, is usually made from nickel or from a form
of stainless steel. The filament is a closely wound helix of tungsten wire, about 0.2 mm thick,
the helix diameter being about 1.0-1.5 mm.
The target is normally comprised of a small tablet of tungsten about 15mm wide, 20mm long
and 3mm thick soldered into a block of copper. Tungsten is chosen since it com bines a high
atomic number (74)—making it comparatively efficient in the production of X-rays. It has a
high melting point (3400°C) enabling it to withstand the heavy thermal loads.
Copper being an excellent thermal conductor, performs the vital fu nction of carrying the heat
rapidly away from the tungsten target. The heat flows through the anode to the outside of the
tube, where it is normally removed by convection. Generally, an oil environment is provided
for convection current cooling.
In addition, the electrodes have open high voltages on them and must be shielded. The tube
will emit X-rays in all directions and protection needs to be provided except where the useful
beam emerges from the tube.
In order to contain the cooling oil and meet the above-mentioned requirements, a metal
container is provided for completely surrounding the tube. Such a container is known as a
‘shield’.
Rotating Anode Tube
The construction of a rotating-anode x-ray tube
The filament is constructed from a spiral of tungsten wire (melting point 3410 °C), which is
set in a nickel block. This block supports the filament and is shaped to create an electric field
that focuses the electrons into a slit beam.
The anode has a bevelled edge, which is at a steep angle to the direction of the electron beam.
The exit window accepts x-rays that are approximately at right angles to the electron beam so
that the x-ray source as viewed from the receptor appears to be approximately square even
though the electron beam impinging on the target is slit-shaped.
The choice of the anode angle will depend upon the application, with the angle being varied
according to the requirements of field and focal spot sizes and tube output. For general-
purpose units, an angle of about 17° is appropriate.
Most of the energy in the electron beam is deposited in the target in the form of heat. The use
of a slit source of electrons helps by spreading out the target area and this idea can be
extended by using a rotating anode, so that the electron beam impinges on the bevelled edge
of a rotating disc and the target area is spread out over the periphery of the disc.
A rotation speed of about 3000 RPM and an anode diameter of 10 cm are used in general-
purpose units.
X-RAY MACHINE
Block diagram of basic x-ray machine.
Parts
1. X-Ray tube 2. High Tension Supply 3. Collimator
4. Patient Table. 5. Grid. 6. Radiographic film
1. X ray Tube – For details refer stationary anode tube and rotating anode tube given above.
It is an important component of x-ray machine which is inaccessible as it is contained in a
protective housing. It is a vacuum tube.
There are two primary parts.
1) the cathode
2) the Anode.
2. Operating Console
It is an apparatus in X-Ray machine that allows to control the x-ray tube current and voltage.
The Console Controls are: -
1. Voltage compensator. 2. kV Meter. 3. mA Meter. 4. Exposure time.
1. Voltage Compensator
Because of variations in power distribution to the hospital and in power consumption by the
various sections of the hospital, the voltage to the x-ray unit may vary by 5%, which will
result in large variations in x-ray output.
High Tension Supply with Rectifier
Power supply system consists of Autotransformer
The power supplied to x-ray machine is delivered to a special transformer called an
Autotransformer. It works on the principle of electromagnetic induction but is very different
from conventional transformer.
It has only one winding and one core. The single winding has number of connections, or
electric taps. The purpose to use the Autotransformer is to overcome induction losses. Its
value ranges from 0 to 400V.
Used for producing high voltage which is applied to the tube’s anode and cathode and
comprises a high voltage step-up transformer followed by a recifier.
Self Rectification Circuit for High Voltage Generation
The high voltage is produced using a step-up transformer whose primary is connected to
auto-transformer. The secondary of the H.T. transformer can be directly connected to the
anode of the x-ray tube which will conduct only during the half cycles when the cathode is
negative with respect to anode or target.
The current through the tube follow, the H.T. pathway and is measured by a mA meter.
A kV selector switch enables to change voltage between exposures. The voltage is measured
with the help of a kV meter.
The exposure switch controls the timer and thus the duration of the application of kV.
To compensate for mains voltage variations, a voltage compensator is used in the circuit.
The filament is heated with 6 to 12 V of ac supply at a current of 3 to 5 amperes. The
filament temperature determines the tube current or mA, and, therefore, the filament
temperature control has an attached mA selector. The filament current is controlled by using,
in the primary side of the filament transformer, a variable choke or a rheostat. The rheostat
provides a stepwise control of mA and is most commonly used in modem machines.
These machines have maximum tube currents of about 20 mA and a voltage of about 100 kV.
When self-rectification is used, it is necessary to use a parallel combination of a diode and
resistance, in series with the primary of the H.T. transformer for suppressing higher inverse
voltage likely to appear during the non-conducting half-cycle of the x-ray tube. This helps to
reduce the cost and complexity of the x-ray machines.
A preferred method of providing high voltage dc to the anode of the x-ray tube is by using a
bridge rectifier using four valve tubes or solid state rectifiers. This results in a much more
efficient system than with half wave of self-rectification methods.
The kV meter is connected across primary of the H.T. transformer. It actually measures volts
whereas it is calibrated in kV by using an appropriate multiplication factor of turns-ratio of
the transformer.
In order to obtain the load voltage which varies with the tube current, a suitable kV meter
compensation is provided in the circuit. The kV meter compensator is ganged to the mA
selector mechanically. Therefore, the mA is selected first and the kV setting is made
afterwards during operation of the machine.
Moving coil meters are used for making current I (mA) measurements, for shorter exposures,
a mAs meter is used which measures the product I of mA and time in seconds.
The exposure time is generally controlled by using some form of timing arrangement coupled
with a contactor which supplies the H.T. to the anode of the x-ray tube only during that time.
Collimator: The Collimator is attached to the x-ray tube below the glass window where the
useful beam is emitted. Lead shutters are used to restrict the beam. Its purpose is to minimize
field of view, to avoid un necessary exposure by using lead plates.
Grid: By virtue of function and material, collimator and grid are same but they have different
location. It is made up of lead. It is located just after patient. It is used to destroy scattered
radiation from the body.
Radiographic Film: Two types of x-ray photon are responsible for density, contrast and
image on a radiograph. Those that pass through the patient without interacting and those that
are scattered in the patient through compton interaction. Together these x-rays that exit from
the patient and intersect the film are called Remnant x-rays.
APPLICATIONS OF X-RAYS IN MEDICINE
1. X-rays are used in medicine for medical analysis. Dentists use them to find complications,
cavities and impacted teeth. Soft body tissues are transparent to the waves. Bones and teeth
block the rays and show up as white on the black background.
2. A mammogram:
(Also called a mammography exam) is a safe, low-dose x-ray of the breast. A high-quality
mammogram is the most effective tool for detecting breast cancer early. Early detection of
breast cancer may allow more treatment options. Using a low-dose x-ray, the mammogram
machine takes a snapshot of the inside of a woman’s breast. The machine holds and
compresses the breasts so that images at different angles can be taken. Doctors and nurses
examine these snapshots, looking for signs of abnormalities such as lumps, which could be
tumors.
3. Skeletal system:
A standard radiograph is usually the first course of action when a patient is suffering from a
suspected bone injury. The excellent natural contrast provided by bone produces clear images
with good resolution. Two views at right angles to each other are generally required and can
lead to the diagnosis of fractures, dislocations, spinal injuries and so on. Other abnormalities,
ranging from tumours and cysts in the spine to arthritis (figure 2.23), can also indicated.
4. The chest:
A standard chest X-ray is the commonest means of detecting lung cancer and other
abnormalities (figure 2.24). Difficulties sometimes arise due to liie inevitable obstruction of
the heart.
5. Circulatory system:
An artificial contrast medium, typically an organic iodine compound, is injected into the
blood vessel to be examined. The structure and effective flow diameter of both arteries and
veins can be examined, allowing the diagnosis of blood vessel blockages and heart disease
6. Dental studies:
Most dental practices now have small X-ray units, to investigate problems with the
overcrowding or uneven growth of teeth, particularly in juveniles, or with the growth of
wisdom teeth. Surgery or orthodontal treatment may then be recommended.
7. Foreign bodies:
It is amazing what people, particularly children, will swallow! A standard radiograph can
help to identify the shape and position of such objects to assist with their removal.
COMPUTED TOMOGRAPY
Limitations of X-rays
1. The super-imposition of the three-dimensional information onto a single plane makes
diagnosis confusing and often difficult.
2. The photographic film usually used for making radiographs has a limited dynamic range
and, therefore, only objects that have large variations in X-ray absorption relative to their
surroundings will cause sufficient contrast differences on the film to be distinguished by the
eye. Thus, whilst details of bony structures can be clearly seen, it is difficult to discern the
shape and composition of soft tissue organs accurately.
3. In such situations, growths and abnormalities within tissue only show a very small contrast
difference on the film and consequently, it is extremely difficult to detect them, even after
using various injected contrast media.
4. The problem becomes even more serious while carrying out studies of the brain due to its
overall shielding of the soft tissue by the dense bone of the skull.
Basic Principle of CT
In computed tomography (CT), the picture is made by viewing the patient via X-ray imaging
from numerous angles, by mathematically reconstructing the detailed structures and
displaying the reconstructed image on a video monitor.
Computed tomography differs from conventional X-ray techniques in that the pictures
displayed are not photographs but are reconstructed from a large number of absorption
profiles taken at regular angular intervals around a slice, with each profile being made up
from a parallel set of absorption values through the object.
In computed tomography, X-rays from a finely collimated source arc made to pass through a
slice of the object or patient from a variety of directions. For directions along which the path
length through-tissue is longer, fewer X-rays are transmitted as compared to directions where
there is less tissue attenuating the X-ray beam. In addition to the length of the tissue
traversed, structures in the patient such as bone may attenuate X-rays more than a similar
volume of less dense soft tissue.
In principle, computed tomography involves the determination of attenuation characteristics
for each small volume of tissue in the patient slice, which constitute the transmitted radiation
intensity recorded from various irradiation directions. It is these calculated tissue attenuation
characteristics that actually compose the CT image.
If a slice of heterogeneous tissue is irradiated given below, and we divide the slice into
volume elements or voxels with each voxel having its own attenuation coefficient, it is
obvious that the sum of the voxel attenuation coefficients for each X-ray beam direction can
be determined from the experimentally measured beam intensities for a given voxel width.
However, each individual voxel attenuation coefficient remains unknown. Computed
tomography uses the knowledge of the attenuation coefficient sums derived from X-ray
intensity measurements made at all the various irradiation directions to calculate the
attenuation coefficients of each individual voxel to form the CT image.
X-rays incident on patient from different directions. They are attenuated by different
amounts, as indicated by the different transmitted X-ray intensities
Block Diagram of the CT System
Figure below shows a block diagram of the system. The X-ray source and detectors are
mounted opposite each other in a rigid gantry with the patient lying in between, and by
moving one or both of these around and across the relevant sections, which is how the
measurements are made.
The X-ray tube and the detector are rigidly coupled to each other. The system executes
translational and rotational movement and transradiates the patient from various angular
projections. With the aid of collimators, pencil thin beam of X-ray is produced.
A detector converts the X-radiation into an electrical signal. Measuring electronics then
amplify the electrical signals and convert them into digital values. A computer then processes
these values and computes them into a matrix-line density distribution pattern which is
reproduced on a video monitor as a pattern of gray shade.
In one system which employs 18 traverses in the 20s scanning cycle, 324,000 (18 x 30 x 600)
X-ray transmission readings arc taken and stored by the computer. These arc obtained by
integrating the outputs of the 30 detectors with approximately 600 position pulses.
The position pulses are derived from a glass graticule that lies between a light emitting diode
and photo-diode assembly that moves with the detectors. The detectors arc usually sodium-
iodide crystals, which are thallium-doped to prevent an after-glow. The detectors absorb the
X-ray photons and emit the energy as visible light. This is converted to electrons by a photo-
multiplier tube and then amplified. Analog outputs from these tubes go through signal
conditioning circuitry that amplifies, clips and shapes the signals.
A relatively simple analog-to-digital converter then prepares the signals for the computer.
Simultaneously, a separate reference detector continuously measures the intensity of the
primary X-ray beam .The set of readings thus produced enables the computer to compensate
for fluctuations of X-ray intensity. Also, the reference readings taken at the end of each
traverse are used to continually calibrate the detection system and the necessary correction is
carried out.
After the initial pre-processing, the final image is put onto the system disc. This allows for
direct viewing on the operator’s console. The picture is reconstructed in either a 320 x 320
matrix of 0.73 mm squares giving higher spatial resolution or in a 160x 160 matrix of 1.5 mm
squares which results in higher precision, lower noise image and better discrimination
between tissues of similar density.
Each picture clement that makes up the image matrix has a CT number, say between -1000
and + 1000, and therefore, takes up one computer word. A complete picture occupies
approximately 100 K words, and u pto eight such pictures can be stored on the system disc.
There is a precise linear relationship between the CT numbers and the actual X-ray
absorption values, and the scale is defined by air at -1000 and by water at 0.
Image Reconstruction
The formation of a CT image is a distinct three phase process.
1. The scanning phase produces data, but not an image.
2. The reconstruction phase processes the acquired data and forms a digital image.
3. Digital-to analog conversion phase: The visible and displayed analog image (shades of
gray) is produced by the digital-to analog conversion phase.
1. The scanning phase
During the scanning phase a fan-shaped x-ray
beam is scanned around the body. The amount
of x-radiation that penetrates the body along
each individual ray (pathway) through the body
is measured by the detectors that intercept the
x-ray beam after it passes through the body.
The projection of the fan-shaped x-ray
beam from one specific x-ray tube focal spot
position produces one view. Many
views projected from around the patient's body
are required in order to acquire the necessary
data to reconstruct an image.Each view produces one "profile" or line of data as shown here.
The complete scan produces a complete data set that contains sufficient information for
the reconstruction of an image. In principle, one scan produces data for one slice image.
2. Image Reconstruction Phase
Image reconstruction is the phase in which the scan data set is processed to produce an
image. The image is digital and consist of a matrix of pixels.
Filtered back projection is the reconstruction method used in CT.
"Filtered" refers to the use of the digital image processing algorithms that are used to improve
image quality or change certain image quality characteristics, such as detail and noise.
"Back projection" is the actual process used to produce or "reconstruct" the image.
Back projection Principle
We start with one scan view through a body section
(like a head) that contains two objects. As we know,
the data produced is not a complete image, but
a profile of the x-ray attenuation by the objects.
Let's now take this profile and attempt to draw an
image by "back projecting" the profile onto our image
surface.
We have now rotated the x-ray beam around the
body by 900 and obtained another view. If we
now back project this profile onto our image area we
see the beginnings of an image showing the two
object. Two views does not give us a high-quality
image. Several hundred views are used to produce
clinical CT images. A part of the reconstruction
process is the calculation of CT number values for
each image pixel.
Image Reconstruction Computer, used in CT scanners.
This method enables pictures to be reconstructed within a few seconds. Figure shows a block
diagram image reconstruction computer, used in CT scanners.
SCANNING SYSTEM
Page 12
The purpose of the scanning system is to acquire enough information to reconstruct a picture
for an accurate diagnosis. A sufficient number of independent readings must be taken to
allow picture reconstruction with the required spatial resolution and density discrimination
for diagnostic purposes. The readings are taken in the form of ‘profiles’.
When a plane parallel X-ray beam is passing through a required section, a profile is defined
as the intensity of the emergent beam plotted along a line perpendicular to the X-ray beam.
This profile represents a plot of the total absorption along each of the parallel X-ray beams. It
thus follows that the higher the number of profiles obtained, the better is the resulting picture.
Types of Scanning Systems
1. First Generation—Parallel Beam Geometry: In the
basic scanning process, a collimated X-ray beam passes
through the body and its attenuation is detected by a
sensor that moves on a gantry along with the X-ray tube.
The tube and detector move in a straight line, sampling
the data 180 times. At the end of the travel, a 1° tilt is
made and a new linear scan begins. This assembly travels
180°around the patient’s position. This arrangement is
known as Traverse and Index' and was used in the
earliest commercial system. This procedure results in 32,400 independent measurements of
attenuation, which are sufficient for the systems computer to produce an image. Obviously,
this is a fairly slow procedure and requires a typical scan time of 5 minutes. It is essential for
the patient to keep still during the entire scan period and for this reason, the early scanners
were limited in their use to only brain studies.
2. Second Generation—Fan Beam, Multiple Detectors: An
improved version of the traverse-index arrangement
consists in using a bank of detectors and a fan beam of X-
rays. This system effectively takes several profiles with
each traverse and thus permits greater index angles. For
example, by using a 10° fan beam, it is possible to take 10
profiles, at 1° intervals, with each traverse and then index
through 10° before taking the next set of profiles.
Therefore, a fu II set of 180 profiles can be obtained with
18 traverses. This method has permitted a reduction in the
scan time, and at the rate of approximately 1 s for each traverse, it has led to the systems
operating in the 18-20 s range.
3. Third Generation—Fan Beam, Rotating
Detectors: The main obstacle for a further
increase in speed with the conventional
computer tomographs arises from the
mechanically unfavorable multiple alterations
between the translational and rotational
movement of the measuring system. But using a fan-shaped beam and an array of detectors,
larger steps can be taken and the scanning process speeded up linear scanning movement can
be avoided by using a sufficiently wide fan-shaped X-ray beam which encompasses the
whole object cross-section, and a multiple detector system mechanically tied to the tube
which permits a simultaneous measurement of the whole absorption profile in one projection
direction (Fig. 20.6(c)). Also, on account of the largeness of the measuring system consisting
of X-ray tube and detectors, the rotational movement must not be stepwise but continuous.
4. Ultrafast Electron Beam CT Scanner
Comparison of Electron Beam Tomography and conventional CT
Electron Beam Tomography Conventional CT
In this electron beam sweeps back and
forth through a magnetic field. The
impact of electron beam on a semi
circular tungsten array underneath the
patient generates X-rays and the X ray
detectors are mounted on a semi circular array above the patient.
In this X ray tube and X ray detector are
mounted across each other on a circular
frame and rotate around the patient.
Light weight Heavy moving parts weighing 250 kg
Takes only 50ms with electron beam tomography.
Takes 1 sec or more to take all the Snapshots
Schematic of ultrafast electron beam CT scanner
The detector array consists of two continuous ranges of 216° with 432 channels each.
Luminascent crystals coupled to silicon photo-diodes are used.
The scanning electron beam emitted by an electron gun is accelerated by 130-140 kV,
electromagnetically focused and deflected over a target in a typical time of 50-100 ms.
It was originally designed for cardiac examinations. The unit was equipped for this purpose
with four anode rings and two detector rings which enabled eight contiguous slices, an area of
approximately 8x8 mm. to be scanned without movement of the patient.
5. Spiral /Helical Scanning. This is a scanning technique in which the X-ray tube rotates
continuously around the patient while the patient is continuously translated through the fan
beam. The focal spot therefore, traces a helix around the patient. The projection data thus
obtained allow for the reconstruction of multiple contiguous images. This operation is often
referred to as helix, spiral, volume, or three-dimensional CT scanning. This technique has
been developed for acquiring images with faster scan times and to obtain fast multiple scans
for three-dimensional imaging to obtain and evaluate the Volume at different locations.
Figure illustrates the spiral scanning technique, which causes the focal spot to follow a spiral
path around the patient. Multiple images are acquired while the patient is moved through the
gantry in a smooth continuous motion rather than stopping for each image. The projection
data for multiple images covering a volume of the patient can be acquired in a single breath
hold at rates of approximately one slice per second. The reconstruction algorithms are more
complex because they need to account for the spiral or helical path traversed by the X-ray
source around the patient (Kalender,1993).
Detectors used in CT:
For a good image quality, it is important to have a stable system response and in that,
detectors play a significant role. The detectors used in CT systems must have a high
overall efficiency in order to minimize the patient radiation dose, have large dynamic
range, he very stable with time and insensitive to temperature variations within the
gantry.
Figure shows the three types of detectors commonly used in CT scanners. Fan-beam
rotational scanners mostly employ xenon gas ionization detectors. The schematic
diagram of the detector shows that X-rays enter the detector through a thin aluminium
window. The aluminium window is a part of a chamber that holds the xenon gas,
which fills the entire chamber. Only one gas volume is present so that all detector
elements arc under identical conditions of pressure and gas purity.
Processing System
A typical data acquisition system is shown in Fig. It
consists of precision pre-amplifiers, current to
voltage convertor, analog integrators, multiplexers
and analog-to-digital convertors. Data transfer rates
of the order of 10 Mbytes/s are required in some
scanners. This can be accomplished with a direct
connection for systems having a fixed detector
array. The third generation slip ring systems make
use of optical transmitters on the rotating gantry to
send data to fixed optical receivers.
Applications of CT
1. Unlike other medical imaging techniques, such as conventional x-ray imaging (radiography),
CT enables direct imaging and differentiation of soft tissue structures, such as liver, lung
tissue, and fat.
2. CT is especially useful in searching for large space occupying lesions, tumors and metastasis
and can not only reveal their presence, but also the size, spatial location and extent of a
tumor.
3. CT imaging of the head and brain can detect tumors, show blood clots and blood vessel
defects, show enlarged ventricles (caused by a build up of cerebrospinal fluid) and image
other abnormalities such as those of the nerves or muscles of the eye.
4. Due to the short scan times of 500 milliseconds to a few seconds, CT can be used for all
anatomic regions, including those susceptible to patient motion and breathing. For
example, in the thorax CT can be used for visualization of nodular structures, infiltrations
of fluid, fibrosis (for example from asbestos fibers), and effusions (filling of an air space
with fluid).
5. CT has been the basis for interventional work like CT guided biopsy and minimally
invasive therapy. CT images are also used as basis for planning radiotherapy cancer
treatment. CT is also often used to follow the course of cancer treatment to determine how
the tumor is responding to treatment.
6. CT imaging provides both good soft tissue resolution (contrast) as well as high spatial
resolution. This enables the use of CT in orthopedic medicine and imaging of bony
structures including prolapses (protrusion) of vertebral discs, imaging of complex joints
like the shoulder or hip as a functional unit and fractures, especially those affecting the
spine. The image postprocessing capabilities of CT - like multiplanar reconstructions and
3-dimensional display (3D) - further enhance the value of CT imaging for surgeons. For
instance, 3-D CT is an invaluable tool for surgical reconstruction following facial trauma.
ULTRASONIC IMAGING SYSTEMS
The term ultrasound refers to acoustical waves above the range of human hearing
(frequencies higher than 20.000 Hz).
Medical ultrasound systems operate at frequencies of up to 10 MHz or more.
An ultrasonic wave is acoustical; i.e., it is a mechanical wave in a gaseous, liquid, or solid
medium. Such mechanical waves consists of alternating areas of higher and lower pressures,
called compression and rarefaction zones, respectively.
Ultrasonic imaging is used in medicine, engineering, geology, and other scientific areas.
Radio signals are electromagnetic waves, while medical ultrasound signals are acoustical.
Comparison of Ultrasound and Radio Waves
Properties of Ultrasound and X-rays
1. Ultrasound rays are Non-invasive, While X-rays are invasive.
2. Ultrasound rays are Externally applied and non-traumatic, also apparently safe at the
acoustical intensities. While X-rays only respond to atomic weight differences and often
require the injection of a more dense contrast medium for visualization of non-bony tissues.
3. Diagnostic ultrasound is applied for obtaining images of almost the entire range of internal
organs in the abdomen
4. These include the kidney, liver, spleen, pancreas, bladder, major blood vessels and of
course, the foetus during pregnancy.
5. It has also been usefully employed to present pictures of the thyroid gland, the eyes, the
breasts and a variety of other superficial structures.
6. In a number of medically meaningful cases, ultrasonic diagnostics has made possible the
detection of cysts, tumours or cancer in these organs.
Ultrasound Waves Electromagnetic Waves
1. The acoustical signal requires a medium
in which to propagate.
1. Electromagnetic signal can propagate in outer space, where no known medium
exists.
If an alternating current (ac) oscillation of,
say, 2500 kHz, were connected to an
appropriate antenna, then an
electromagnetic (radio) wave would be
launched.
But if that same 2500 kHz ac signal were
applied to an ultrasound transducer, then an
acoustical signal would be launched.
7. The main limitation of ultrasound is that it is almost completely reflected at boundaries
with gas and is a serious restriction in investigation of and through gas-containing structures.
8. Ultrasonic waves are sound waves associated with frequencies above the audible range and
generally extend upward from 20 kHz.
Characteristics
1. Ultrasonic waves can be easily focused, i.e., they are directional and beams can be
obtained with very little spreading.
• They are inaudible and are suitable for applications where it is not advantageous
to employ audible frequencies.
• By using high frequency ultrasonic waves which are associated with shorter
wavelengths, it is possible to investigate the properties of very small structures.
• Information obtained by ultrasound, particularly in dynamic studies, cannot be
acquired by any other more convenient technique.
Use of ultrasound in Medical Field
The use of ultrasound in the medical field can be divided into two major areas: the
therapeutic and the diagnostic.
The major difference between the two applications is the ultrasonic power level at which the
equipment operates.
In therapeutic applications, the systems operate at ultrasonic power levels of up to several
watts per square centimeter while the diagnostic equipment operates at power levels of well
below 100 in W/ cm'.
The therapeutic equipment is designed to agitate the tissue to the level where thermal heating
occurs in the tissue, and experimentally has been found to be quite successful in its effects for
the treatment of muscular ailments such as lumbago.
For diagnostic purposes, on the other hand, as long as a sufficient amount of signal has
returned for electronic processing, no additional energy is necessary. Therefore, considerably
lower ultrasonic power levels are employed for diagnostic applications.
BASIC PULSE ECHO SYSTEM
The basic layout of the apparatus based on this principle is shown in Fig. given below.
The pulse-echo technique, basically, consists in transmitting a train of short duration
ultrasonic pulses into the body and detecting the energy reflected by a surface or
boundary separating two media of different specific acoustic impedances.
With this technique, the presence of a discontinuity can be conveniently established
and its position located if the velocity of travel of ultrasound in the medium is known.
Also, it is possible to determine the magnitude of discontinuity and to assess its
physical size.
Basic Principle
The transmitter generates a train of short duration pulses at a repetition frequency determined
by the PRF generator.
These are converted in to corresponding pulses of ultrasonic waves by a piezoelectric crystal
acting as the transmitting transducer.
The echoes from the target or discontinuity are picked up by the same transducer and
amplified suitably for display on a cathode ray tube.
The X plates of the CRT are driven by the time base which starts at the instant when the
transmitter radiates a pulse.
In this way, the position of the echo along the trace is proportional to the time taken for a
pulse to travel from the transmitter to the discontinuity and back again. Knowing the velocity
of ultrasonic waves and the speed of the horizontal movement of trace on the CRT, the
distance of the target from the transmitting end can be estimated.
1. Transducer
The transducer consists of a piezo-electric crystal which generates and detects ultrasonic
pulses.
The piezo-electric materials generally used are barium titanate and lead zirconate titanate.
When the transducer is excited at its resonance frequency, it will continue to vibrate
mechanically for some time after the electrical signal ceases. This effect is known as 'after
ringing' and destroys the precision with which the emission or detection of a signal can be
timed. To reduce it, the transducer must have a good transient response and consequently a
low Q is desirable. To achieve this, the transducers are normally damped.
The probes are designed to achieve the highest sensitivity and penetration, optimum focal
characteristics and the best possible resolution. This requires that the acoustic energy be
transmitted efficiently into the patient.
2. Pulse Repetition Frequency Generator
This unit produces a train of pulses which control the sequence of events in the rest of the
equipment. The PRF is usually kept between 500 Hz to 3 kHz.
Oscillator or some form of the astable multi-vibrator can be used as the PRF Generator.
The width of the output pulse from the PRF generator should be very small, preferably of the
order of a micro-second, to generate short duration ultrasonic pulse.
Generally astable circuit is used to generate train of pulses with the required frequency and
then to use them to trigger a mono-stable multi-vibrator which produces pulses of the
required width. With the short pulse duration and the repetition rate of 1 kHz, only a few
micro-seconds are occupied by the emission of the pulse, and the transducer is free to act as a
receiver for the remainder of the time.
3. Transmitter
The transmiter is driven by a pulse from the PRF generator and is made to trigger an SCR
circuit which discharges capacitor through the piezo-electric crystal in the probe to generate
an ultrasonic signal. The circuit typically employed is shown in Fig. 23.7.
Circuit diagram of a transmitter used in pulse-echo application
Under normal conditions, the SCR is non-conducting. The capacitor C, can charge through
the resistance R to the +V potential.
If a short triggering positive pulse is applied to the gate of the SCR. it will fire and conduct
for a short time. Consequently, the voltage at ‘A’ will fall rapidly resulting in a short duration, high voltage pulse at ‘B*.
This pulse appears across the crystal which generates short duration ultrasonic pulse. For
producing a pulse with a very short duration it is necessary to use an SCR with a fast turn ’on'
time and high switching current capability, which can be able to withstand the required
supply voltage.
4. Receiver:
The function of the receiver is to obtain the signal from the transducer and to extract from it
the best possible representation of an echo pattern.
To avoid significant worsening of the axial resolution, the receiver bandwidth is about twice
the effective transducer bandwidth.
5. Wide Rand Amplifier:
The echo-signals received at the receiving transducer may be as small as a few microvolts.
This is achieved in a wide-band amplifier, which is wide enough to faithfully reproduce the
received echoes and to permit the use of different transducers operating at several different
frequencies.
A desirable gain of wide hand amplifier is of the order of 80-100 dB. It must also have a very
wide dynamic range.
The amplifier must also have a low noise level to receive echoes from deep targets. The
Input amplifier is usually a dual gate MOSFET which is very suitable lor high frequency
signals and provides a high input impedance to the signals from the transducer.
Due to the wide dynamic range of echo-amplitudes that are contained in an ultrasonic image,
a log amplifier is usually utilized. By utilizing a log amplifier, one can see small relative
differences in both low amplitude and high amplitude echoes in the same image.
6. Swept Gain Control:
The receiving amplifier can only accept a limited range of input signals without overloading
and distortion. Abrupt changes in tissue properties that shift the acoustical impedance can
cause the echo amplitudes to vary over a wide dynamic range, perhaps 40 to GO dB.
7. Detector:
Alter the logarithmic amplification, the echo signals are rectified in the detector circuit. The
detector employed could he of the conventional diode-capacitor type with an inductive filter
to have additional filtering of the earner frequency.
In this rectification process, the negative half-cycles in the echo voltage waveforms are
convened into positive half-cycles. This is followed by a demodulation circuit in which the
fundamental frequency signal upon which the echo amplitude information has been riding, is
eliminated.
The output of the demodulator circuit is in the form of an envelope of the echo signal.
8. Video Amplifier:
The signal requires further amplification after its demodulation in detector circuit before it
can be given to the Y- plates of the Cathode ray tube (CRT). The output of the detector
circuit is typically around I V. but for display on the CRT. the signal must be amplified to
about 100 to 1000 V. In addition to this, the amplifier must have a good transient response
with minimum possible overshoot. The most commonly used video amplifier is the RC
coupled type, having an inductance in series with the collector load.
9. Time Delay Unit:
Time base will begin to move the spot across the CRT face at the same moment as the SCR is
fired. If desired, in special cases, the start of the trace can be delayed by the time delay unit so
that the trace can be expanded to obtain better display and examination of a distant echo.
10. Time Base:
The Time base speed is adjusted so that echoes from deepest structures of interest will appear
on the screen before the beam has completely traversed It.
Taking the speed of ultrasound in soft tissue to be about 1500 m/s, a time of 13.3 micro sec.
must be allowed for each centimeter that the reflecting interlace is below the surface.
In many applications, distance markers appropriate to each time-base setting appear directly
on the screen, which greatly simplifies distance measurements.
Several standard circuits are available for generating the sawtooth waveform to provide a
time base suitable for horizontal deflection of the spot on the CRT screen. The horizontal
sweep generator is controlled by the PRF generator as die sweep starts at the moment that the
transmitting pulse is applied to the transducer.
11. Time Marker:
The time marker produces pulses that are a known time apart and, therefore, respond to a
known distance apart in human tissues. These marker pulses are given to the video amplifier
and then to the Y plates for display along with the echoes.
12. Display:
After amplification in the video amplifier, the signal is given to die Y plates of die CRT. CRT
is not only a fast-acting device but also gives a clear presentation of the received echo
signals.
PROPAGATION OF ULTRASONIC THROUGH TISSUES AND REFLECTIONS
Ultrasound waves are vibrations or disturbances consisting of alternating zones of
compression and rarefaction in a physical medium such as gas, liquid, or solid matter.
Wavelength, frequency, and velocity
Frequency is defined as cycles per unit of time.
All waves, including both acoustical and electromagnetic (or ocean waves, for that matter)
possess three related attributes: frequency (F). wavelength (X), and velocity (V).
Frequency: is defined as the number of complete cycles per unit of time. Figure given above
shows one complete cycle.
In terms of alternating current, the cycle consists of one entire positive excursion followed by
one complete negative excursion of the voltage or current. In terms of sound and ultrasound
waves, the cycle consists of one complete zone of compression followed by one complete
zone of rarefaction.
The basic unit of cycles is the hertz (Hz), which equals one cycle per second (1 Hz = 1 cps);
Wavelength: is the distance traveled by one cycle propagating away from the source and is
expressed in meters (m), or subunits centimeters (cm) or millimeters (mm). The wavelength
is also the distance between successive identical features on successive cycles.
one wavelength is expressed in terms of the distance between two positive peaks, between
two negative peaks, or between negative- going crossings of the zero baseline.
Velocity: is the speed of propagation of the wave. In radio signals, the velocity is the speed
of light (c), or 300.000,000 m/s. In human tissue, ultrasound propagates at a much slower
rate. i.e.. around 1500 m/s.
For all forms of wave, the relationship between frequency, wavelength, and velocity is:
V=F (17-1)
Where
V is the velocity of the wave in meters per second (m/s)
Lambda is the frequency in hertz (Hz)
is the wavelength in meters (m)
The period of the wave is die lime required to complete one cycie and can be measured in
terms of either time (T) or angle (one cycle = 2pi radians).
The type of the wave refers to the method of propagation.
The two forms are
1. Longitudinal Propagation
In the longitudinal form, the waves propagate in the same direction as the zones of
compression and rarefaction.
2. Transverse Propagation
In transverse propagation, the waves propagate in a direction orthogonal (at right angles) to
the direction of the zones of compression and rarefaction. Transverse propagation occurs
when the wave propagate along the surface of the medium, as on the surface of a container of
water or the surface of a bone.
In medical ultrasound both forms are seen. While the main mode is longitudinal propagation,
a mode conversion to transverse propagation can occur. Mode conversion is associated with a
significant loss of signal level.
Reflection of Ultrasound
Ultrasound travels freely through fluid and soft tissues. However, ultrasound bounces back (is
reflected back) as echoes when it hits a more solid (dense) surface.
For example, the ultrasound will travel freely though blood in a heart chamber. But, when it
hits a solid valve, a lot of the ultrasound echoes back. Another example is that when
ultrasound travels though bile in a gallbladder it will echo back strongly if it hits a solid
gallstone. So, as ultrasound 'hits' different structures of different density in the body, it sends
back echoes of varying strength.
Ultrasonography (sonography) uses a probe containing multiple acoustic transducers to send
pulses of sound into a material. Whenever a sound wave encounters a material with a
different density (acoustical impedance), part of the sound wave is reflected back to the probe
and is detected as an echo. The time it takes for the echo to travel back to the probe is
measured and used to calculate the depth of the tissue interface causing the echo. The greater
the difference between acoustic impedances, the larger the echo is. If the pulse hits gases or
solids, the density difference is so great that most of the acoustic energy is reflected and it
becomes impossible to see deeper.
Figure given below illustrates the situation for reflection and refraction.
Reflection and refraction of waves
At the boundary between two zones of different density,
some of the wave energy is reflected back into the
original medium, and some propagates into the second
medium but is refracted (i.e.. changes its direction of
travel).
if the incident wave impinges on the surface or boundary at an angle of 90 degrees (i.e.. it is
coincident with the normal line), it will be reflected back on itself. But if the angle is other
than 90 degrees, then the reflected wave will travel away from the surface at the same angle.
Refraction phenomena affect the portion of the incident wave that enters the second medium.
We may infer the behavior of ultrasound waves
Specular reflection, diffuse reflection, and scattering
A single incident ray resulting in a single reflected mi is termed specular reflection.
In medical ultrasound systems this rarely occurs because it requires a flat surface that is large
compared with the wavelength of the signal. In most situations, however, the 1 surface is
rough and thus so produces diffuse reflections.
DISPLAY TYPES OR IMAGING MODES
Various imaging modes or display types found in today's ultrasound systems are:
1. A-mode (Amplitude Mode):
A-scan mode uses a stationary transducer to fire a pulse into tissue. The oscilloscope or hard-
copy readout scans time along the horizontal axis and plots the signal amplitude along the
vertical axis. A large spike at the left corner (unless it is suppressed) represents the transmit
spike. The tissue at the interface with the transducer will produce some near-field scatter
immediately to the right of the transmit spike. Other spikes represent reflections from targets
within the tissue.
This mode displays the amplitude of a sampled voltage signal for a single sound wave as a
function of time. This mode is considered One Dimensional and used to measure the distance
between two objects by dividing the speed of sound by half of the measured time between the
peaks in the A-mode plot.
For A-scan applications, the CRT is usually of the electrostatic deflection type. It is better to
use CRT with post-deflection acceleration of the electron beam so that a very bright trace is
obtained with lower deflecting voltages on the plates. The cathode ray tube should preferably
be a flat face type to eliminate sereen curvature error. A variable persistence scope with
storage facilities would be useful for prolonged viewing.
Applications
1. Echoencephalograph:
In the normal brain, the mid-line surfaces are parallel to the flat areas of the bone near the ear.
When there is a head injury, the brain gets tilted to one side or the other due to bleeding, but
it still retains its normal shape. In such cases, the echoes can be easily obtained but they are
placed at different distances from the probe, when the probe is placed first on one side and
then on the other side of the skull like the figure given below
Echoes received from the brain (a) in the normal brain, the mid-line echoes coincide for each
way (b) in the abnormal case, and there is a shift between the two echoes.
2. Echo-ophthalmoscope
A-mode ultrasonic technique was found to be useful in ophthalmology for the diagnosis of
retinal detachments, intra-ocular tumours, vitreous opacities, orbital tumours, and lens
dislocation. It helps in the measurement of axial length in patients with progressive myopia,
localization of intra-ocular foreign bodies and extraction of nonmagnetic foreign bodies.
Echo-ophthalmoscopy employs a 7.5-15 MHz pencil type transducer. The transmitted pulse
should be of very small width (in nanosec) and range.
2. B-mode (Brightness) imaging is the same as A-mode, except that brightness is used to
represent the amplitude of the sampled signal. B mode imaging is performed by sweeping the
transmitted sound wave over the plane to produce a 2D image.
A-Mode display is very difficult to interpret when many echoes are present simultaneously
and often potentially useful information is wasted. A pictorial display can be conceived as a
means of simultaneously presenting the echo information as well as information about the
position of the probe and the direction of propagation of the sound. This is achieved in the B-
scan display which results from brightness modulation with amplitude of the echoes obtained
for various probe positions and orientations to produce a cross-sectional image of the object
integrated by a storage display from individual scans.
The B-scan mode may use the same lime base as the A-scan but plots the strength of the
returning signal as changes in brightness, i.e., a strong reflection is brighter than the weaker
reflection. When the transducer is mechanically scanned back and forth, successive images
are built up. allowing a two-dimensional (2-D) view of the underlying structure. Again, the
strength of the reflection is graphed by the brightness of the cathode ray tube display.
Figure represents the difference between A-scan and B-scan.
Figure above is a hypothetical body with the probe placed upon its surface. The probe is
positioned in such a way that it transmits a beam obliquely inwards. The beam encounters
three interfaces in its travel.
A-scan representation of this structure consists of vertical peaks (2,3,4) received as echoes at
the receiving crystal in response to the transmitted pulse (1). The same structure in B-scan
appears as light dots whose position is related to the echoing interface within the body.
Types of Scans
Three types of scanning arrangements are utilized for building cross-sectional images using
ultrasound.
1. Linear Scan: The most common scan used for abdominal studies is the linear scan.
A linear scan is when the ultrasonic transducer remains parallel to the surface of the object
being examined and the sound beam is perpendicular to the transducer movement. Only the
location of the transducer is changed but the angle of the beam is held constant.
2. Sector Scan
The most common scan used in echocardiography is the sector scan. The scan is made by
rocking the transducer about a fixed point such that the sound beam covers a sector.
3. Compound scanning is merely a combination of linear and sector scans.
3. M-mode (Motion) display
M-mode (Motion) display refers to scanning a single line in the object and then displaying
the resulting amplitudes successively. This shows the movement of a structure such as heart.
Because of its high pulse frequency (up to 1000 pulses per second), this is useful in assessing
rates and motion and is used extensively in cardiac and foetal cardiac imaging.
M-mode (M for motion) is a technique that uses B-mode information to display the echoes
from a moving organ, such as the myocardium and valve leaflets, from a fixed transducer
position and beam direction on the patient.
The echo data from a single ultrasound beam passing through moving anatomy are acquired
and displayed as a function of time, represented by reflector depth on the vertical axis (beam
path direction) and time on the horizontal axis.
M-mode can provide excellent temporal resolution of motion patterns, allowing the
evaluation of the function of heart valves and other cardiac anatomy.
Only anatomy along a single line through the patient is represented by the M-mode
technique.
Principle of time-motion (M-mode) display
If one of the echo sources is a moving structure, then the echo dots of light from that structure
will also move back and forth. If the dots are made to move with an electronic sweep, from
bottom to the top of the screen at a pre-selected rate of speed, the moving dots will trace out
the motion pattern of the moving structure. This display is known as M-mode display. If a
photographic film is continuously exposed to one sweep cycle of this display, a composite
picture will result, providing a waveform representation of the motion pattern of the moving
structure. Alternately, thermal video printers are used for recording the M-mode information.
REAL-TIME ULTRASONIC IMAGING SYSTEMS
One of the serious limiting factors in B-scanning is the length of time taken to complete a
scan. This results in blurring and distortion of the image due to organ movement, as well as
being tedious for the operator.
Elimination of motion artifact is important in conventional B-scanning but is critical if
rapidly moving areas such as the chambers of the heart are to be made visible.
Rapid scanning techniques have been developed to meet these needs. The approaches used
include fast physical movement of a single transducer.
Alternatively, electronic methods using arrays of transducers which can be triggered in
sequence or in groups, may be utilized. In these systems, electronic manipulation allows the
beam to be swept rapidly through the area of interest.
Finally, instruments in which an array of transducers is combined with mechanized motion,
have also been introduced. These instruments are called real-time scanners as there is
negligible time delay between the input of data and the output of processed data in such
systems.
A very important property of ultrasonography is the short image reconstruction time of about
20 to 100 ms, permitting the real-time scanning and observation of processes in the organs of
the body. Consequently, sonography is well-suited to the fast, interactive screening of even
larger organ regions and the display of dynamic processes, such as cardiology.
The real-time systems, therefore, have the following characteristics:
Possibility of studying structure in motion—this is important for cardiac and foetal structures.
During observation, the scan plane can be easily selected since the echo image appears
instantaneously on the display.
Requirements of Real Time Ultrasonic Imaging Systems
The primary requirements of an ultrasonic imaging system are:
1. High resolution
High resolution or the ability of the system to resolve fine spatial dimensions is a key
performance requirement. A resolution of 1-3 mm in all three spatial dimensions is desirable
for a number of diagnostic studies like the early detection of tumours or other pathological
conditions.
2. Long range
The required depth range varies considerably for different anatomical studies. For example, a
range of 25-30 cm is desirable for abdominal and obstetrical studies. For cardiac studies, the
distance from the chest wall to the posterior heart wall is 15 cm or more. In superficial organs
like the breast, thyroid, carotid and femoral arteries and in infant studies, the range of depth is
3-10 cm.
3. Adequate field of view
The field of view should be large enough to display the entire region under examination and
to provide a useful perspective view of an organ of interest. When viewing small superficial
structures such as the thyroid, a field of approximately 5 cm * 5 cm can encompass the
desired region. For cardiac imaging, sector scans are preferable to rectilinear scans since a
large structure is to be viewed through a small window. An angle ol 60° is adequate to
simultaneously view most of the heart.
4. Sufficiently high frame rate and high detectivity
In real-time imaging systems, the frame rate (the rate at which the image is retreated) should
Ire rapid enough to resolve the important motions and to obtain the image without
undesirable smearing. Most of these requirements arc met with a frame rate of about 30
frames/s.
This also satisfies the requirements for flicker-free display and is compatible with standard
television formats. For some special studies, greater frame rates are often required for the
data acquisition mode. These frames would then be stored and played back at about 30
frames/s to provide a slow motion presentation.
5. Detectivity
It is the ability of an imaging system to effectively capture, process and display the very wide
dynamic range of signals which may, in turn, help to detect ah image, lesion or other
abnormal structure or process. Poor system detectivity manifests itself in lack of fidelity or
picture quality which is often apparent in the visual displays of ultrasonic images.
Syllabus
Magnetic Resonance Imaging – Basic NMR components, Biological effects and advantages
of NMR imaging.
Biomedical Telemetry system: Components of biotelemetry system, application of telemetry
in medicine, single channel telemetry system for ECG and temperature.
Patient Safety: Electric shock hazards, leakage current, safety codes for electro medical
equipments
I. MAGNETIC RESONANCE IMAGING:
Magnetic Resonance Imaging (MR1) or Nuclear Magnetic Resonance (NMR) Tomography
has emerged as a powerful imaging technique in the medical field because of its high
resolution capability and potential for chemical specific imaging.
Comparison of NMR system and XRAY and CT
1. Similar to the X-ray computerized tomography (CT), MRI uses magnetic fields and radio
frequency signals to obtain anatomical information about the human body as cross-sectional
images in any desired direction and can easily discriminate between healthy and diseased
tissue.
2. MRI images are essentially a map of the distribution density of hydrogen nuclei and
parameters reflecting their motion, in cellular water and lipids.
3. The total avoidance of ionizing radiation, its lack of known hazards and the penetration of
bone and air without attenuation make it a particularly attractive non-invasive imaging
technique.
4. CT provides details about the bone and tissue structure of an organ whereas NMR
highlights the liquid-like areas on those organs and can also be used to detect flowing liquids,
like blood.
5. A conventional X-ray scanner can produce an image only at right angles to the axis of the
body, whereas the NMR scanner can produce any desired cross-section, which offers a
distinct advantage to and is a big boon for the radiologist.
Basic Principle
MR1 systems provide highly detailed images of tissue in the body. The systems detect and
process the signals generated when hydrogen atoms, which are abundant in tissue, are placed
in a strong magnetic field and excited by a resonant magnetic excitation pulse.
All materials contains nucleus that have a combination of protons and neutrons. It possesses a
spin and the amount of spin give rise to a magnetic moment. The magnetic moment has a
magnitude and direction. In tissues Magnetic moments of nuclei making up the tissue are
randomly aligned and net magnetization=0.
Random alignment of magnetic moments of the nuclei making up the
tissue, resulting in a zero net magnetization.
When a material is placed in a magnetic field B0, some of the randomly oriented nuclei
experience an external magnetic torque which tends to align the individual parallel or anti-
parallel magnetic moments to the direction of an applied magnetic field. This gives a
magnetic moment that accounts for the nuclear magnetic resonance signal on which the
imaging is based. This moment is in the direction of applied magnetic field Bo. With the
magnetic moments being randomly oriented with respect to one another, the components in
the X-Y plane cancel one another out while the Z components along the direction of the
applied magnetic field add up to produce this magnetic moment M0 shown in Figure given
below.
The application of external magnetic
field causes the nuclear magnetic
moments to align themselves,
producing a net moment in the direction
of the field B0.
NMR Resultant Signal Pick up by the Instrument
When a nucleus with a magnetic moment is placed in an externally applied magnetic
field, the energy of the nucleus is split into lower (moment parallel with the field) and
higher (anti-parallel) energy levels. The energy difference is such that a proton with
specific frequency (energy) is necessary to excite a nucleus from the lower to die
higher state.
The excitation energy E obtained by the application of external RF signal, and is
given by the Planck's equation
E = hωo Where h is Planck's constant. This energy is usually supplied by an RF
magnetic field. ωo = Frequency of appied RF.
The excited proton tends to return or relax to its low-energy state with spontaneous
decay and re-emissions of energy at a later time Y in the form of radio wave photons.
This decay is exponential in nature and produces a “free induction decay” (FID)
signal (Fig. below) that is the fundamental form of the nuclear signal obtainable from
an NMR system.
To summarize, if in a static field, RF waves of
the right frequency are passed through the
sample of interest (or tissue), some of the
parallel protons will absorb energy and be
stimulated or excited to a higher energy in the
anti-parallel direction. Sometime later, the RF
frequency absorbed will be emitted as
electromagnetic energy of the same frequency
as the RF source. The amount of energy
required to flip protons from the parallel to the
anti-parallel orientation is directly related to the
magnetic field strength; stronger fields require more energy or higher frequency
radiation. This is picked up by the instrument and then processed.
BASIC NMR COMPONENTS
The basic components of an NMR
imaging system are shown in Fig. These
are:
1. Magnet: Provides a strong uniform,
steady, magnet field B0.
2. RF transmitter, which delivers radio-
frequency magnetic field to the sample.
3. Gradient system, which produces
time-varying magnetic fields of
controlled spatial non-uniformity;
4. Detection System, which yields the
output signal; and
5. Imager system, including the
computer, which reconstructs and
displays the images.
1. Imager System
The imaging sequencing in the system is provided by a computer. Functions such as
gates and envelopes for the NMR pulses, blanking for the pre-amplifier and RF power
amplifier and voltage waveforms for the gradient magnetic fields are all under
software control.
The computer also performs the various data processing tasks including the Fourier
transformation, image reconstruction, data filtering, image display and storage.
Therefore, the computer must have sufficient memory and speed to handle large
image arrays and data processing, in addition to interfacing facilities.
2. The Magnet:
In magnetic resonance tomography, the base field must be extremely uniform in space
and constant in time as its purpose is to align the nuclear magnets parallel to each
other in the volume to be examined.
Also, the signal-to-noise ratio increases approximately linearly with the magnetic
field strength of the basic field, therefore, it must be as large as possible.
Four factors characterize the performance of the magnets used in MR systems; viz.,
field strength, temporal stability, homogeneity and bore size.
The gross non-homogeneities result in image distortion while the bore diameter limits
the size of the dimension of the specimen that can be imaged.
Such a magnetic field can be produced by means of four different ways, viz.,
permanent magnets, electromagnets, resistive magnets and super-conducting magnets.
Permanent Magnet: In case of the permanent magnet, the patient is placed in the gap
between a pair of permanently magnetized pole faces. Permanent magnet materials
normally used in MRI scanners include high carbon iron alloys such as alnico or
neodymium iron.. Although permanent magnets have the advantages of producing a
relatively small fringing field and do not require power supplies, they tend to be very
heavy (up to 100 tons) and produce relatively low fields of the order of 0.3 T or less.
Electromagnets: Make use of soft magnetic materials such as pole faces which
become magnetized only when electric current is passed through the coils wound
around them. Electromagnets obviously require external electrical power supply.
Resistive magnets: make use of large current-carrying coils of aluminium strips or
copper tubes. In these magnets, the electrical power requirement increases
proportionately to the square of the field strength which becomes prohibitively high as
the field strength increases. Moreover, the total power in the coils is converted into
heat which must be dissipated by liquid cooling.
Superconductive magnets. Most of the
modem NMR machines utilize
superconductive magnets. These magnets
utilize the property of certain materials,
which lose their electrical resistance fully
below a specific temperature. The
commonly used superconducting material
is Nb Ti (Niobium Titanium) alloy for
which the transition temperature lies at 9 K
(-264°C). In order to prevent
superconductivity from being destroyed by
an external magnetic field or the current passing through the conductors, these
conductors must be cooled down to temperatures significantly below this point, at
least to half of the transition temperature. Therefore, superconductive magnet coils are
cooled with liquid helium which boils at a temperature of 4.2 K (-269°C).
RF Transmitter System
The system consists of an RF transmitter, RF power amplifier and RF transmitting coils.
1. RF Transmitter System
In order to activate the nuclei so that they emit a useful signal, energy must be
transmitted into the sample. This is what the transmitter does.
The RF transmitter consists of an RF crystal oscillator at the Larmor frequency. The
RF voltage is gated with the pulse envelopes from the computer interface to generate
RF pulses that excite the resonance.
2. RF Power Amplifier
These pulses are amplified to levels varying from 100W to several kW and are fed to
the transmitter coil.
3. RF Transmitting Coils
The coil generates RF field perpendicular to the direction of main magnetic field.
Coils are tuned to the NMR frequency and are usually isolated from the remaining
system using RF shielding cage.
Detection System
Block diagram is shown in above fig.
The function of detection system is to detect the nuclear magnetization and generate
an output signal for processing by the computer.
The receiver coil usually surrounds the sample and acts as an antenna to pick up the
fluctuating nuclear magnetization of the sample and converts it to a fluctuating output
voltage V(i).
NMR signal is given by
Where M(t, x) is the total magnetization in a volume and
Bc(x) the sensitivity of the receiver coil at different points in space.
Bc(x) describes the ratio of the magnetic field produced by the receiver coil to the
current in the coil.
The receiver coil design and placement is such that Bc(x) has the largest possible
transverse component. The longitudinal component of Bc(x) contributes little to the
output voltage and can he ignored.
The RF signals constitute the variable measured in magnetic resonance tomography.
These are extremely weak signals having amplitude in the nV (nano-Volt) range thus
requiring specially designed RF antennas. The sensitivity of an MR scanner therefore
depends on the quality of its RF receiving antenna. For a given sample magnetization,
static magnetic field strengths and sample volume, the signal-to-noise-ratio (SN R)of
the RF signal at the receiver depends in the following manner upon the RF-receiving
antenna.
This implies that the SNR of an MR scan can be improved by maximizing
magnetization to coil volume.
Some of the commonly available coils are:
Body Coils: Constructed on cylindrical coils forms with diameter ranging from 50 to
60 cm entirely surround the patient's body.
Head Coils: Designed only for head imaging, with typical diameter of 28 cm.
Surface coils:
Orbit/ear coil: flat, planar ring-shaped coil with 10 cm diameter;
Neck coil: flexible, rectangular shaped surface coil (10 cm x 20 cm) capable of
adaptation to the individual patient anatomy; and
Spine coil: cylindrical or ring-shaped coil with 15 cm diameter.
Organ-enclosing coils:
Breast coil: cylindrical or ring-shaped coil with 15 cm diameter.
Helmholtz-type coil: a pair of flat ring coils each having 15 cm diameter with
distance between the two coils variable from 12 to 22 cm.
Matching Network
Following the receiver coil is a matching network which couples it to the pre-
amplifier in order to maximize energy transfer into the amplifier. This network
introduces a phase shifty to the phase of the signal.
Pre-amplifier: The pre-amplifier is a low-noise amplifier which amplifies the signal and
feeds it to a quadrature phase detector.
Quadrature phase detector
The detector accepts the RF NMR signal which consists of a distribution of
frequencies centred around or near the transmitted frequency w and shifts the signal
down in frequency by w.
The detector circuit accepts the inputs, the NMR signal V(t) and a reference signal,
and multiplies them, so that the output is the product of the two inputs. The frequency
of the reference signal is the same as that of the irradiating RF pulse. The output of
the phase-sensitive detector consists of the sum of two components, one a narrow
range of frequencies centred at 2w0, and the other, a narrow range centred at zero.
The low pass filter following the phase-sensitive detector removes all components
except those centred at zero from the signal.
ADC
It is necessary to convert the complex (two-channel) signal to two strings of digital
numbers by analog-to-digital converters. The A-D converter output is passed, in serial
data form to the computer for processing.
Gradient System for Spatial Coding:
Spatial distribution information can be obtained by using the fact that the resonance
frequency depends on the magnetic field strength. By varying the field in a known
manner through the specimen volume, it is possible to select the region of the
specimen from which the information is derived on the basis of the frequency of the
signal. The strength of the signal at each frequency can be interpreted as the density of
the hydrogen nuclei in the plane within the object where the magnetic field
corresponds to that frequency.
. The imaging methods differ mainly in the nature of the gradient time dependence
(static, continuously time-depended or pulsed), and in the type of NMR pulse
sequence employed.
Spatial information and therefore images obtained by super-imposing a linear
magnetic field gradient on the uniform magnetic field applied to the object to be
imaged. When this is done, the resonance frequencies of the processing nuclei will
depend primarily on the positions along the direction of the magnetic gradient.
This produces a one-dimensional projection of the structure of the three- dimensional
object. By taking a series of these projections at different gradient orientations, a two
or even three-dimensional image can be produced.
In NMR systems, for spatially resolving the signals emitted by the object, the initially
homogeneous magnetic field B0 is overlaid in all three spatial dimensions, X, Y, Z
with small linear magnetic fields-gradient fields G.
These gradient fields are produced with die aid of current carrying coils and can be
switched on or off as desired, both during the application of the RF energy and also in
any phase of the measuring procedure.
A block diagram of gradient control system is shown in Fig. given below. The hardware can
be broken down into four sub-system.
Block diagram of gradient control system. Each X,Yand Z coil pair has its own control
circuit.
1. Serial Parallel Computer
The first sub-system includes the interface between the computer and the gradient
control system. Its primary function is to allow the independent positioning of the
three planes (X, Y and Z).
2. The digital oscillator
Consists of a 555 timer followed by shift registers A digital oscillator facilitates
varying itis output frequency over an extremely wide range through the use of a single
control
The 8-bit input from the interface circuit is used directly to one attenuator while the
same 8-bits are inverted to control the second attenuator. The output of the attenuators
is then voltage-amplified by two op amps prior to the driven circuits.
Current control used to adjust the static field gradients be available for setting the DC
levels upon which the alternating gradients are superimposed .
An op amp serves the differential voltage drop across a dummy load and produces an
output which is then DC coupled to the drivers.
The high current drivers use a conventional design with a single op amp providing the
input to a driver and a complimentary pair of power transistors to provide a sufficient
current to the gradient coil.
In typical scanners, gradient coils have an electric resistance of about l Ohm and an
inductance of 1 mH. The gradient fields are required to be switched from 0 to 10 mT/
m in about 0.5 ms. The current switches from O to about 100 A in this interval. The
power dissipation during the switching interval is about 20 kW. This places very
strong demands on the power supply and it is often necessary to use water cooling to
prevent overheating of the gradient coils.
With well-designed coils, errors resulting from non-linear gradients will perhaps not
be evident in a medical image since the image will remain clear and will not contain
rigidly shaped objects or those with sharp edges for close comparison. But these
gradient coils are usually designed to optimize linearity in the central region. Away
from the centre, gradient linearity becomes progressively worse. Without restoration,
the image will not give accurate information on the outer regions. Therefore, non-
linear field gradients result in a geometrical distortion of the image reconstructed from
projections.
Imager System:
The imager system includes the computer for image processing, display system and control
console. The timing and control of RF and gradient pulse sequences for relaxation time
measurements and imaging, in addition to FT image reconstruction and display necessitate
the use of a computer.
The computer is the source of both the voltage waveforms of all gradient pulses and the
envelopes of the RF pulses. A general purpose mini-computer of the type used for a ('AT
scanner is adequate for these purposes.
BIOLOGICAL EFFECTS OF NMR IMAGING
The three aspects of NMR imaging which could cause potential health hazard are:
(i) Heating due to the rf power.
A temperature increase produced in the head of NMR imaging would be about
0.3°C. This does not seem likely to pose a problem.
(ii) Static magnetic field:
No significant effects of the static field with die level used in NMR are known,
but the possible side effects of electromagnetic fields are decrease in cognitive
skills, mitotic delay in slime moulds, delayed wound healing and elevated serum
triglycerides.
(iii) Electric current induction due to rapid change in magnetic field:
It is believed that oscillating magnetic field gradients may induce electric currents
strong enough to cause ventricular fibrillation. However, no damage due to NMR
from exposures has been reported. It is suggested that fields should not vary at a
rate faster than 3 tesla/s.
ADVANTAGES OF NMR IMAGING SYSTEM
1. The NMR provides substantial contrast between soft tissues that are nearly identical.
2. NMR uses no ionizing radiation and has minimal hazards for operators of the
machines and for patients.
3. Unlike CT, NMR imaging requires no moving parts, gantries or sophisticated crystal
detectors.
4. The system scans by superimposing electrically controlled magnetic fields
consequently, scans in any pre-determined orientation are possible.
5. With the new techniques being developed, NMR permits imaging of entire three-
dimensional volumes simultaneously instead of slice by slice, employed in other
imaging systems.
6. In NMR both biochemical (spectroscopy) and spatial information (imaging) can be
obtained without destroying the sample.
BIOMEDICAL TELEMETRY SYSTEM
The term telemetry is derived from the two Greek terms: “tele” and “metron”, which
mean “remote” and “measure”.
In general, a physical variable or quantity under measurement, whether local or
remote, is called a measurand.
Telemetry is a technology that allows the remote measurement and reporting of
information of interest to the system designer or operator.
Literally, biotelemetry is the measurement of biological parameters over a distance.
Elements of Telemetry
1. Transducer or Sensor:
• Converts the physical variable to be telemetered into an electrical quantity.
2. Signal Conditioner-1:
• Converts the electrical output of the transducer (or sensor) into an electrical
signal compatible with the transmitter.
3. Transmitter:
Its purpose is to transmit the information signal coming from the signal conditioner-1 using a
suitable carrier signal to the receiving end.
The transmitter may perform one or more of the following functions:
(i) Modulation: Modulation of a carrier signal by the information signal.
(ii) Amplification: As and if required for the purpose of transmission.
(iii) Signal Conversion: As and if required for the purpose of transmission.
(iv) Multiplexing: If more than one physical variables need to be telemetered
simultaneously from the same location, then either frequency-division multiplexing
(FDM) or time-division multiplexing (TDM) is used.
Receiver: Its purpose is to receive the signal(s) coming from the transmitter
(located at the sending end of the telemetry system) via the signal transmission
medium and recover the information from the same.
It may perform one or more of the following functions:
1. Amplification
2. Demodulation:
3. Reverse Signal Conversion
De-multiplexing
Signal Conditioner-2: Processes the receiver output as necessary to make it suitable
to drive the given end device.
End Device: The element is so called because it appears at the end of the system.
End device may be performing one of the following functions:
1. Analog Indication:
2. Digital Display
Digital Storage
Telemetry Classification Based on Transmission Medium
Divided into 2
1. Wired Telemetry 2. Wireless Telemetry
COMPONENTS OF WIRED BIOTELEMETRY SYSTEM
General Block Diagram
ANALOG SYSTEM [WIRED]
DIGITAL SYSTEM [WIRED]
Elements of Wireless Bio-Telemetry
ELEMENTS OF WIRELESS BIO-TELEMETRY
A typical biotelemetry system comprises:
1. Sensors appropriate for the particular signals to be monitored
2. Battery-powered, Patient worn transmitters
3. A Radio Antenna and Receiver
4. A display unit capable of concurrently presenting information from
multiple patients
1. Wireless Bio-telemetry Transmitter
2. Wireless Bio-telemetry Receiver
Physiological signals are obtained from the subject by means of appropriate
transducers. The signal is then passed through a stage of amplification and processing
circuits that include generation of a subcarrier and a modulation stage for
transmission.
The circuitry which generates the carrier and modulates it constitutes the transmitter.
Equipment capable of receiving the transmitted signal and demodulating it to recover
the information comprise the receiver. By tuning the receiver to the frequency of the
desired RF carrier, that signal can be selected while others are rejected.
The range of the system depends upon a number of factors, including the power and
frequency of the transmitter, relative locations of the transmitting and receiving
antennas, and the sensitivity of the receiver.
The receiver consists of a tuner to select the transmitting frequency, a demodulator to
separate the signal from the carrier wave, and a means of displaying or recording the
signal. The signal can also be stored in the modulated state by the use of a tape
recorder, as shown in the block diagram.
The biotelemetry system use two modulators. The physiological signals are used to
modulate a low-frequency carrier called sub-carrier in the audio frequency range. The
RF carrier is then modulated by the sub-carrier and transmitted. The double
modulation gives better interference free performance in transmission and enables the
reception of low frequency biological signals. The sub modulator can be FM or PWM
but the final modulator is practically always FM system.
If several physiological signals are to be transmitted simultaneously, each signal is
placed on a sub-carrier of a different frequency using FM or AM. The sub-carriers are
added together to give a composite signal in which none of the parts overlap in
frequency. The composite signal modulates the RF carrier by the same or different
method. This process of transmitting many channels of data on a single RF carrier is
called frequency division multiplexing (FDM). This is more efficient and less
expensive than employing a separate transmitter for each channel.
At the receiver, a multiplexed RF carrier is first demodulated to recover each of the
separate sub-carriers which are then demodulated to retrieve the original physiological
signals. Both FM/AM (sub-carrier is frequency modulated and RF carrier is amplitude
modulated) and FM/FM systems are used in biotelemetry, the later more extensively.
The modulation system is selected based on the size, complexity, noise transmission and other operational problems.
APPLICATION OF TELEMETRY IN MEDICINE
1. Telemetry of ECGs from Extended Coronary Care patients
To make monitoring possible for the cardiac patients, some hospitals have extended
coronary-care units equipped with patient-monitoring systems that include telemetry.
In this arrangement, each patient has ECG electrodes taped securely to his chest.
The electrodes are connected to a small transmitter unit that also contains the signal-
conditioning equipment. The transmitter unit is fastened to a special belt worn around
the patient’s waist.
Batteries for powering the signal conditioning equipment and transmitter are also
included in the transmitter package. These batteries must be replaced periodically.
The output of each receiver is connected to one of the ECG channels of the patient
monitor.
2. Telemetry for ECG Measurements During Exercise
For certain cardiac abnormalities, such as ischemic coronary artery disease, diagnostic
procedures require measurement of the electrocardiogram while the patient is
exercising, usually on a treadmill or a set of steps. Although such measurements can
be made with direct-wire connections from the patient to nearby instrumentation, the
connecting cables are frequently in the way and may interfere with the performance of
the patient. For this reason, telemetry is often used in conjunction with exercise ECG
measurements.
The transmitter unit used for this purpose is similar to that described earlier for
extended coronary care and is normally worn on the belt. Care must be taken to
ensure that the electrodes and all wires are securely fastened to the patient, to prevent
their swinging during the movement of the patient.
3. Telemetry for Emergency Patient Monitoring
In many areas ambulances and emergency rescue teams are equipped with equipment
to allow electrocardiograms and other physiological data to be transmitted to a nearby
hospital for interpretation.
Two-way voice transmission is normally used in conjunction with the telemetry to
facilitate identification of the tele-metered information and to provide instructions for
treatment.
Through the use of such equipment, ECGs can be interpreted and treatment begun
before the patient arrives at the hospital of this type requires a much more powerful
transmitter than the two applications previously described. Often the data must be
transmitted many miles and sometimes from a moving vehicle.
To be effective, the system must be capable of providing reliable reception and
reproduction of the transmitted signals regardless of conditions.
4. Telephone Links
One application involves the transmission of ECGs from heart patients and (particularly)
pacemaker recipients. In this case the patient has a transmitter unit that can be coupled to an
ordinary telephone. The transmitted signal is received by telephone in the doctor’s office or
in the hospital. Tests can be scheduled at regular intervals for diagnosing the status and
potential problems indicated by the ECGs.
SINGLE CHANNEL TELEMETRY SYSTEM FOR ECG AND TEMPERATURE
In a majority of the situations requiring monitoring of the patients by wireless telemetry, the
parameter which is most commonly studied is the electrocardiogram. Therefore, we shall first
deal with a single channel telemetry system suitable for the transmission of an
electrocardiogram.
ECG Telemetry System
Figure shows the block diagram of a single channel telemetry system suitable for the
transmission of an electrocardiogram.
There are two main parts:
The Telemetry Transmitter which consists of an ECG amplifier, a sub-carrier
oscillator and a UHF transmitter along with dry cell batteries.
Telemetry Receiver consists of a high frequency unit and a demodulator to which an
electrocardiograph can be connected to record, a cardioscope to display and a
magnetic tape recorder to store the ECG. A heart rate meter with an alarm facility can
be provided to continuously monitor the beat-to-beat heart rate of the subject.
Block diagram of a single channel telemetry system
For distortion-free transmission of ECG, the following requirements must meet.
• The subject should be able to carry on with his normal activities while carrying the instru-
ments without the slightest discomfort. He should be able to forget their presence after some
minutes of application.
• Motion artefacts and muscle potential interference should be kept minimum.
• The battery life should be long enough so that a complete experimental procedure may be
carried out.
• While monitoring paced patients for ECG through telemetry, it is necessary to reduce
pacemaker pulses. The amplitude of pacemaker pulses can be as large as 80 mV compared to
1-2 mV, which is typical of the ECG.
Components
1. Transmitter
A block diagram of the transmitter is shown in Fig. The ECG signal, picked up by
three pre-gelled electrodes attached to the patient's chest, is amplified and used to
frequency modulate a 1 kHz sub-carrier that in turn frequency-modulates the UHF
carrier. The resulting signal is radiated by one of the electrode leads (RL), which
serves as the antenna. The input circuitry is protected against large amplitude pulses
that may result during defibrillation.
ECG input amplifier is ac coupled to the succeeding stages The coupling capacitor
not only eliminates dc voltage that results from the contact potentials at the patient-
electrode interface, it also determines the low-frequency cut-off of the system which
is usually 0.4 Hz.
The subcarrier oscillator is a current-controlled multi-vibrator which provides ±320
Hz deviation from the 1 kHz centre frequency for a full range ECG signal.
The sub-carrier filter removes V square-wave harmonic and results in a sinusoid for
modulating the RF carrier.
The carrier is generated in a crystal-controlled oscillator operating at 115 MHz.
2. Receiver
The receiver uses an omni-directional receiving antenna which is a quarter-wave
monopole, mounted vertically over the ground plane of the receiver top cover. This
arrangement works well to pick up the randomly polarized signals transmitted by
moving patients.
Receiver comprises of an RF amplifier, which provides a low noise figure, RF
filtering and image frequency rejection. In addition to this, the RF amplifier also
suppresses local oscillator radiation to -60 dBm to minimize the possibility of cross-
coupling where several receivers are used in one central station.
The local oscillator employs a crystal (115 MHz) similar to the one in the transmitter
and x 4 multiplier and a tuned amplifier.
The mixer is followed by an 8-pole crystal filter that determines the receiver
selectivity.
The IF amplifier provides the requisite gain stages and operates an AGC amplifier
which reduces the mixer gain under strong signal conditions to avoid overloading at
the IF stages.
The IF amplifier is followed by a discriminator, a quadrature detector. The output of
the discriminator is the 1 kHz sub-carrier. This output is averaged and fed back to the
local oscillator for automatic frequency control.
The 1 kHz sub-carrier is demodulated to convert frequency-to-voltage to recover the
original ECG waveform.
Temperature Telemetry System
Systems for the transmission of alternating potentials representing such parameters as ECG,
EEG and EMG are relatively easy to construct. Telemetry systems which are sufficiently
stable to telemeter direct current outputs from temperature, pressure or other similar
transducers continuously for long periods present greater design problems. In such cases, the
information is conveyed as a modulation of the mark/space ratio of a square wave. A
temperature telemetry system based on this principle is illustrated in the circuit shown in Fig.
Temperature is sensed by a thermistor having a resistance of 100 Ohm (at 20°C)
placed in the emitter of transistor T1.
Transistors T1 and T2 form a multi-vibrator circuit timed by the thermistor, R4, R5
and C1.
R5 is adjusted to give 1:1 mark/space ratio at midscale temperature (35 - 41 °C). The
multi-vibrator produces a square wave output at about 200 Hz.
Its frequency is chosen keeping in view with the available bandwidth, required
response time, the physical size of the multi-vibrator, timing capacitors and the
characteristics of the automatic frequency control circuit of the receiver.
This is fed to the variable capacitance diode D2 via potentiometer R3. D2 is placed in
the tuned circuit of a RF oscillator [L1C1] constituted by T3.
Transistor T3 forms a conventional 102 MHz oscillator circuit, whose frequency is
stabilized against supply voltage variations by the Zener diode D3 between its base
and the collector supply potential.
T4 is an untuned buffer stage between the oscillator and the aerial. The aerial is
normally taped to the collar or harness carrying the transmitter.
On the receiver side, a vertical dipole aerial is used which feeds a FM tuner, and
whose output, a 200 Hz square wave, drives the demodulator.
In the demodulator, the square wave is amplified, positive dc restored and fed to a
meter where it is integrated by the mechanical inertia of the meter movement.
Alternatively, it is filtered with a simple RC filter to eliminate high ripple content and
obtain a smooth record on a paper. A domestic FM tuner can be used for this purpose.
Temperature measurements in this scheme were made with a thermistor probe.
Patient Safety
Electric shock burns and fire hazards result from the careless use of electricity. When
electricity is relied upon to support life with devices like external pacemakers,
respirators, etc. power failure is a continuous threat.
Shock resulting from electric power is a common experience. Disruption of
physiologic function by leakage current applied internally remains sometimes hidden
and mysterious. While faulty electric cords and appliances contribute to the former,
lack of concept and faulty design are responsible for the latter.
Electric current can flow through the human body either accidentally or intentionally.
Electrical currents are administered intentionally in the following cases:
(i) for the measurement of respiration rate by impedance method, a small current at high
frequency is made to flow between the electrodes applied on the surface of the body,
(ii) high frequency currents are also passed through the body for therapeutic and surgical
purposes,
(iii) when recording signals like EGG and EEG, the amplifiers used in the preamplifier
stage may deliver small currents themselves to the patient. These are due to bias
currents.
Accidental transmission of electrical current can take place because of a defect in the
equipment; excessive leakage currents due to defect in design; operational error (human
error) and simultaneous use of other equipment on the patient which may produce potentials
on the patient circuit.
ELECTRIC SHOCK HAZARDS
Hazards due to electric shock are also associated with equipment other than that used in
hospitals. Some such special situations are as follows:
(i) A patient may not be usually able to react in the normal way. He is either ill, unconscious,
anaesthetized or strapped on the operating table. He may not be able to withdraw himself
from the electrified object, when feeling a tingling in his skin, before any danger of
electrocution occurs.
(ii) The patient or the operator may not realize that a potential hazard exists. This is because
potential differences are small and high frequency and ionizing radiations are not directly
indicated.
(iii) A considerable natural protection and barrier to electric current is provided by human
skin. In certain applications of electromedical equipment, the natural resistance of the skin
may be by-passed. Such situations arise when the tests are carried out on the subject with a
catheter in his heart or on large blood vessels.
(iv) Electro-medical equipment, e.g. pacemakers may be used either temporarily or
permanently to support or replace functions of some organs of the human body. An
interruption in die power supply or failure of the equipment may give rise to hazards, which
may cause permanent injuries or may even prove fatal for the patient.
(v) Several times there are combinations of high power equipment and extremely sensitive
low signal equipment. Each of these devices may be safe in itself, but can become dangerous
when used in conjunction with others.
(vii) The environmental conditions in hospitals, particularly in the operating theatres, cause
an explosion or fire hazards due to die presence of anaesthetic agents, humidity and cleaning
agents, etc.
There are two situations which account for hazards from electric shock.
They are (i) Gross shock and (ii) Micro-current shock.
In the case of gross shock, the current flows through the body of the subject, e.g. as
from arm to arm.
The other case is that of micro-current shock in which the current passes directly
through the heart wall. This is the case when cardiac catheters may be present in the
heart chambers. Here, even very small amounts of currents can produce fatal results.
1. Gross Shock
Gross shock is experienced by the subject by an accidental contact with the electric wiring at
any point on the surface of the body. The majority of electric accidents involve a current
pathway- through the victim from one upper limb to the feet or to the opposite upper limb
and they generally occur through intact skin surfaces. In all these cases, the body acts as a
volume conductor at the mains frequency.
For a physiological effect to take place, body must become part or an electric circuit. Current
must enter the body at one point and leave at some other point. In this process, three
phenomena can occur. These are:
(i) Electrical stimulation of the excitable tissues nerves and muscles
(ii) Resistive heating of tissue
(iii) Electro-chemical burns and tissue damage for direct current and very high
voltages.
The value of electric current, flowing in the body, which causes a given degree of
stimulation, varies from individual to individual. Typical threshold values of current produce
certain responses where the current flows into the body from external contacts (e.g. hand to
hand) and these have been investigated.
For a given voltage present on the surface of the body, the value of current passing through it
would depend upon the contact impedance.
Besides this, it depends on many other factors such usage, sex, condition of skin (dry or wet,
smooth or rough, etc.), frequency of current. duration of current and the applied voltage.
Effects of Electric Current on the Human Body
Threshold of Perception: Bruner (1967) states that the threshold of perception of electric
shock is about 1 mA. At this level a tingling sensation is felt by the subject when there is a
contact with an electrified object through the intact skin. The threshold varies considerably
among individuals and with the measurement conditions. The lowest threshold could be 0.5
mA when the skin is moistened at 50 Hz. Threshold for dc current are2 to 10mA.
Let-go Current: As the magnitude of alternating current is increased the sensation of
tingling gives way to the contraction of muscles. The muscular contractions increase as the
current is increased and finally a value of current is reached at which the subject cannot
release his grip on the current carrying conductor. The maximum current at which the subject
is still capable of releasing a conductor by using muscles directly stimulated by that current is
called "let-go current".
Physical Injury and Pain: At current levels higher than the ‘let-go current’, the Subject
loses the ability to control his own muscle actions and he is unable to release his grip on the
electrical conductor. Such currents are very painful and hard to bear. This type of accident is
called the 'hold-on-type accident, and is caused by currents in the range of20-100mA.
Ventricular Fibrillation: If current comes in contact with intact skin and passes through the
trunk at about 100 mA and above, there is a likelihood of pulling the heart into ventricular
fibrillation. In this condition, the rhythmic action of the heart ceases, pumping action slops
and the pulse disappears. Ventricular fibrillation occurs due to the derangement of function of
the heart muscles rather than any actual physical damage to it. Ventricular fibrillation is a
serious cardiac emergency because once it starts; it practically never stops spontaneously,
even when the current that triggered it is removed.
Sustained Myocardial Contraction: At currents in the range of 1 to 6 A. the entire heart
muscle contracts. Although the heart stops beating while the current is applied, it may revert
to a normal rhythm if the current is discontinued in time, just as in fibrillation. The damage is
reversible if the shock duration is only of a few seconds.
Burns and Physical Injury: At very high currents of the order of 6 amperes and above there
is a danger of temporary respiratory paralysis and also of serious burns. Resistive heating
causes burns, usually on the skin at the entry points, because skin resistance is high. The
brain and other nervous tissue loose all functional excitability when high currents pass
through them.
Gross shock hazards arc usually caused by electrical wiring failures, which allow
personal contact with a live wire or surface at the power line voltage. This type of
hazard is dangerous not only to the patient but also to the medical and attending staff.
The most vulnerable part in the system of electrical safety is the cord and plug. Their
use can result in fatal accidents. Broken plugs, faulty sockets and defective power
cords must be immediately replaced.
The commonly found fluids in medical practice such as blood, urine,
solutions, etc. can conduct enough electricity to cause temporary short circuits if they
are accidentally spilled into normally safe equipment. This hazard is particularly more
in hospital areas that arc normally subject to wet conditions, such as haemodialysis
and physical therapy areas. The cabinets of many electrically operated equipment
have holes and vents for cooling that provide access for spilled conductive fluids,
which can cause potential electric shock hazard.
Micro-current Shock
The threshold of sensation of electric currents differs widely between currents applied arm to
arm and currents applied internally to the body. In the latter case, a far greater percentage of
the current may flow via the arterial system directly through the heart, thereby requiring
much less current to produce ventricular fibrillation. Such situations arc commonly
encountered in hospitals;
For example, The patients in the catheter laboratory or in the operating room, with a catheter
in the heart, would have very little resistance 10 electric currents. A cardiac catheter
connected to an electrical circuit for the measurement of pressure provides a conductive fluid
connection directly to the heart. This makes the patient highly vulnerable to electric shock
because the protection he would have had from layers of intact skin and tissue between his
heart and the outside electrical environment has now been by-passed by the wire of fluid
column within his heart or blood vessels.
Maximum permissible leakage-current through the heart versus frequency
Electrophysiology of Ventricular fibrillation
Current passing through the human body can prove hazardous as it can induce circulatory
arrest. The primary cause of circulatory arrest induced by low voltage electric shock is
ventricular fibrillation.
The shock hazards described above lead to the classification of patients in hospitals into three
categories:
(i) A general patient who is normally not connected to any instrumentation. Such
patients would not ordinarily be present in intensive care wards. They may.
however, come in casual contact with electrical instrumentation.
(ii) The susceptible patients will be all those patients who arc ordinarily connected to
instruments like cardioscopes and other monitoring instruments. They are
decidedly more susceptible to ventricular fibrillation than the general category.
(iii) A critical patient will have a direct electrically conductive path to any part of the
heart. Based on this classification, the type of leakage current associated w nh
each category can be worked out.
LEAKAGE CURRENTS
Currents of extremely small magnitude can be fatal to a patient when a direct,
localized electrical path exists to the heart.
Patients in coronary care units recovering from heart diseases are especially
vulnerable since their hearts are already in an irritable state.
In such cases, the amount of electrical stimulation necessary to induce a life
threatening arrhythmia is greatly reduced. Accidents of this nature can occur in
unpredictable circumstances.
The major source of potentially lethal currents is the Leakage Current
Leakage current by definition is an inherent flow of non-functional current
from the live electrical parts of an instrument to the accessible metal parts.
Leakage currents usually flow through the third wire connection to the
ground. They occur by the presence of a finite amount of insulation
impedance, which consists of two parts: capacitance and resistance.
The magnitude of the leakage current is determined by the value of the
capacitance present therein.
The resistive component of leakage current arises because no substance is a
perfect insulator and some small amount of current w ill always flow through
it.
Types of Leakage Current
Leakage currents are divided according to the current path into the following types:
1. Enclosure Leakage Current:
The enclosure leakage current is the current which flaws, in normal condition, from the
enclosure or part of the enclosure through a person (or an external conductive part other than
the earth connection) in contact with an accessible part of the enclosure to earth or another
part of the enclosure. This current becomes significant when the person touching the
equipment is connected to earth either directly or via a large capacitance.
2. Earth Leakage Current
'The earth leakage current is the current which flows, in normal condition, to earth from the
mains parts of an apparatus via the earth conductor.
3. Patient Leakage Current
The patient leakage current is the current which flows through the patient from or to the
applied part of the patient circuits. This current does not include any functional patient
current.
Consider electrical equipment connected to the power line.
Path of leakage current in a normal case, i.e. the ground wire intact
A leakage current of 100 microA can be assumed to be flowing through the ground wire. If
the chassis of this equipment is connected to the patient who is grounded, then very little of
this current will flow through him.
In the above case the patient offers a resistance of 1000 ohm to the ground and the
ground connection from the instrument has 1 ohm of series resistance. The current
division shall be such that only 0.1 microA of current shall pass through the patient,
the rest will flow to the ground.
If by chance the ground connection breaks, the full leakage current will flow through
the patient. This is a very hazardous situation particularly if the current goes through
internal electrodes in the vicinity of the patient's heart.
Precautions to Minimize Electric Shock Hazards
The following precautions should be observed to prevent hazardous situations:
• In the vicinity of the patient, use only apparatus or appliances with three-wire power cords.
• Provide isolated input circuits on monitoring equipment.
• Have periodic checks of ground wire continuity of all equipment.
• No other apparatus should be put where the patient monitoring equipment is connected.
• Staff should be trained to recognize potentially hazardous conditions.
• Connectors for probes and leads should be standardized so that currents intended for
powering transducers arc not given to the leads applied to pick up physiologic electric
impulses.
• The functional controls should be clearly marked and the operating instructions be perma-
nently and prominently displayed so that they can be easily familiarized.
• Many of the portable medical equipment such as dialysis units, hypothermia units, phys-
iotherapy apparatus, respirators and humidifiers are used with adapter plugs that do not
ensure a proper grounding circuit.
• The operating instructions should give directions on the proper use of the equipment, hi fact,
for electro-medical equipment, the operating instructions should be regarded as an integral
pail of the unit.
• The mechanical construction of the equipment must be such that the patient or operator
cannot he injured by the mechanical system of the equipment, if properly operated.
• The patient equipment grounding point should be connected ind ividu ally to all receptacle
grounds, metal hods and any other conductive services.
SAFETY CODES FOR ELECTROMEDICAL EQUIPMENT
The problem of ensuring a safe environment for the patients as well as for the operators has
been engaging the attention of all concerned in several countries at the national and
international levels.
Various countries laid down codes of practice for equipments used in hospitals.
The International Electro technical Commission (IEC) has brought out a fairly
voluminous document, the IEC 601 (General Requirements for the Safety of Medical
Electrical Equipment), to provide a universal standard for manufacturers of electro-
medical equipment as well as a reference manual on good safety practice.
The IEC comprises over 40 countries: East and West European, Canada, New
Zealand, Japan, Russia as well as USA.
The intention of the common standard is that any electromedical equipment built to
the standard should be completely acceptable in all IEC countries. This standard has
been adopted in many countries. Adopting a common standard implies that
construction of equipment shall be universally acceptable, the leakage and earth
resistance paths will be assessed in identical manners and the mains leads will be
coloured to the same code, etc.
Based on the IEC Document, the Bureau of Indian Standards (BIS) has issued the
IS:8607 standard to cover general and safety requirements of electromedical
equipment. The standard issued in eight parts, covers the following aspects:
PartI General
Part II Protection against electric shock
Part III Protection against mechanical hazards
Part IV Protection against unwanted or excessive radiation
Part V Protection against explosion hazards
Part VI Protection against excessive temperature, fire and other hazards
Part II Construction
Part VIII Behavior and reliability v
This standard applies to all medical electrical equipment except or otherwise stated in the
individual specification for the particular medical equipment for which additional or modified
equipments have been specified.
Individual standards on different electromedical equipment have also been issued by the BIS.
Some of the important standards issued are: Radiofrequency diathermy apparatus (IS.7583),
ectrocardiograph (IS:8048), Cardiac Defibrillators (IS:9286), Diagnostic medical X-ray
equipment 5:7620), and Electromyograph (IS:8885).