medical electronics

UNIT-IV RADIOLOGICAL EQUIPMENTS

EC1006 - MEDICAL ELECTRONICS / PANIMALAR ENGG. COLLEGE 1

UNIT-IV

Radiological Equipments



UNIT-IV Radiological Equipments

Ionisoing radiation, Diagnostic X-ray equipment, use of radio isotope

in diagnosis, Radiation therapy.

Ionosing Radiation:

Ionosing radiation means, the rays coming out from x-rays or

radioactive materials has the characteristics of ionizing the gases through

which it travels.

Non-Ionosing radiations are radio waves, light and infrared radiations.

Today manmade isotopes are also available along with the X-ray tube

and radium as sources of radiation. This radiation has the ability to

penetrate the materials which are opaque to visible light are used in

numerous techniques in medical diagnosis and research.

The ionizing effects of radiation are also used for treatment of diseases

such as cancer.

There are 3 different types of radiation, each with its own distinct

properties. The properties of 3 types of radiation are defined as below.

Alpha rays are positively charged particles that consist of helium

nuclei travel at the moderate velocity of 5 to 7 percent of the velocity of light.

They have a very small penetration depth which in air is only about 2

inches.

Beta rays are negatively charged electron particles. Their velocity can

vary over a wide range and can almost reach the velocity of light.

Gamma rays and X-rays are both electromagnetic waves that have a

much shorter wavelength than radio waves or visible light. Their

wavelengths can vary between approximately 10-6 and 10-10 cm,

corresponding to a frequency range of between 1010 and 1014 MHZ with the

X-rays at the higher end of this range.



Diagnostic X-rays:

X-rays are electromagnetic radiation (waves) like visible light but it

has very short wavelength in the range of 0.5 °A to 10°A

Types of X-rays:

Based on the penetration power, X-rays are classified into 2 different

types

1. Soft X-rays

2. Hard X-rays

1. Soft X-rays:

� Low penetrating long wavelength X-rays are called as soft X-rays.

� Soft X-rays are produced using Coolidge X-ray tube.



� The applied voltage between anode and cathode is of the order of

50KV.

Properties:

� Low penetrating power

� Long wavelength

� Low frequency

Application:

1. It is mainly used for diagnostic purpose.

2. It is used for X-ray radiography. It helps us to study the internal

structure of the body.

3. It is used for detecting fractures and the presence of foreign matter

like bullet in the human body.

2. Hard X-rays:

� High penetrating short wavelength X-rays are called as Hard X-

rays.

� Soft X-rays are produced using Coolidge X-ray tube.

� The applied voltage between anode and cathode is of the order of

400KV.

Properties:

� High penetrating power

� Short wavelength

� High frequency

Application:

1. It is mainly used for therapeutic purpose.

2. X-ray therapy is widely used for trating certain type of skin

disease such as cancer, tumour etc.

Properties of X-rays:

� X-rays can penetrate through materials which readily absorb

and reflect visible light. (this forms the basis for the use of

radiography).

� X-rays are absorbed when passing through 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 produces ionization in gases and influence the electric

properties of liquid and solids.

� X-rays also produces fluorescence in certain materials to help

them emit light.

Generation of Ionization Radiation:

X-rays are produced whenever electrons collide at very high speed

with matter and 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 in a specially constructed glass tube which

basically comprises

1. A Source for the production of electrons

2. A Energy source to accelerate the electrons

3. A free electron path

4. A means of focusing the electron beam

5. A device to stop the electrons.

The two types of X-ray tubes are,

1. Stationary mode tube

2. Rotating Anode tube

Stationary Anode tube:

An X-ray tube is basically a high vaccum diode in which electrons are

generated by thermionic emission from the filament of the tube.

The electron stream is electrostatically focused on the anode by means

of a suitably shaped cathode cup.



The kinetic energy of the electrons impringing on the target is

converted in to X-rays.

The intensity of X-rays depends on the current through the tube. This

current can be varied by varying the heater current, which in turn controls

the cathode temperature. The wavelength of the X-rays depends on the

target material and the velocity of the electrons hitting the target. It can be

varied by varying the target voltage of the tube.

X-ray equipment for diagnostic purposes uses target voltages in the

range of 30 to 100 k V. while the current is in the range of several hundred

milli amperes. These voltages are obtained from high-voltage transformers

that are often mounted in oil-filled tanks to provide electrical insulation.

When ac voltage is used, the X-ray tube conducts only during one

half-wave and acts as its own rectifier. Otherwise high-voltage diodes (often

in voltage-doubler or multiplier configurations) are used as rectifiers.

For therapeutic X-ray equipment, where even higher radiation

energies are required, linear or circular particle accelerators have been used

to obtain electrons with sufficiently high energy.

When the electrons strike the target, only a small part of their energy

is converted into X-rays; most of it is dissipated as heat. The target,

therefore, is usually made of tungsten, which has a high melting point. It

may also be water or air-cooled, or it may be in the form of a motor-driven

rotating cone to improve the dissipation of heat.

The cathode block which contains the filament is usually made from

nickel of from stainless steel. The filament is a closely wound helix of



tungsten wire of about 0.2 mm thick and the helix diameter of about 1.0 to

1.5 mm.

The target is comprised of small tablet of tungsten about 15 mm wide,

20mm long and 3 mm thick soldered in to a block of copper.

Tungsten material is chosen because it has high malting point

(3400°C) enabling it to withstand heavy thermal loads.

Copper being an excellent thermal conductor performs the virtual

function 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.

The electron beam is concentrated to form a small spot on the target.

The X rays emerge in all directions from this spot, which therefore can

be considered a point source for the radiation.

Block Diagram and operation of an X-ray machine:

X-ray machine generate high energy, high electromagnetic waves (X-

rays) for use in diagnosing and treating disease. To accomplish this, X-ray

machines should have the following major sections, as shown in the figure

below.

1. multitap ac line autotransformer, which allows selection of taps to

compensate for incoming variations. These also permit the operator oose

voltages for specific applications.

2. X-ray tube filament circuit and transformer, which transforms the ac

line to supply power for heating the cathode filament. This power can be

selected by taps to change filament heat (filament mA), which changes X-ray

tube current (tube mA) and, hence, total X-ray delivered to the patient.

3. X-ray tube high-voltage circuit, transformer, bridge rectifier, which

transforms the ac line to supply the high dc voltage for accelerating

electrons from cathode to anode. The high voltage can be selected by taps to

change the kVp (kilovolt peak) and, hence, total X-ray energy delivered to

the patient.



4. Timing circuit, which controls turn-on, turn-off, and length of X-ray

exposure delivered to the patient.

On the X-ray machine three basic controls knobs to control patient X-

ray dose (penetrating quality, quantity, and timing) are provided.

These are interrelated and must be properly chosen to suit the slim or

obese patient. Good photographic results are sometimes difficult to obtain.

These controls are filament heat control (mA) for exposure strength, not

depth; kilovolt control (kV) for penetration depth and contrast; and timing

devices for time exposure length.

It is extremely important to observe X-ray tube heat ratings. Excessive

heat will damage a very expensive tube, and the cost and inconvenience of

replacement are equally high.

X-ray emission from the tube can be improved by using filters,

stationary grids, moving grids (Potter-Buckey diaphragm), cones, cylinders,

diaphragms, collimators, and intensifiers (image intensifier tube to increase

brightness of the photographic image).

The multitap AC line autotransformer has several purposes. One is to

compensate for normal input line variations and secondly the

autotransformer contains switch settings for coarse (10 kV) and fine (1 kV)

high-voltage selection.



The X-ray tube filament circuit consists of selector switch, filament

transformer, and the filament of the X-ray tube. Filament transformer

provides isolation from the high-voltage transformer and an added measure

of safety. Filament current is adjusted by filament resistors during

calibration.

As X-ray tubes age, more filament current is required to achieve constant X-

ray intensity. A filament current meter shows the milliamperes in the X-ray

cathode.

The diode bridge provides full wave rectified DC voltage to the X-ray

tube anode. X-ray tube current meter shows the milliamperes passing

through the tube.

An electronic timer circuit is used to switch on and off the X-ray tube.

Larger X-ray machines have three phase power instead of 120 peaks

per second in single phase.

Block Diagram and operation of a Fluoroscopic machine:

Fluoroscopic machines are X-ray machines that generate soft X-rays

(reduced frequency and Intensity) to produce dynamic visualizations on a

fluoroscope.

Internal body organs are viewed through the use of a contrast medium

that is opaque to X-rays.

Patient dosage should not exceed 10R per minute.

Transmitted X-rays fall upon a fluorescent screen or plate as a

function of varying tissue density.

Fluorescence is the emission of visible light produced when X-rays fall

upon crystals in the coating of the screen.

The major sections of fluoroscope machine are,

1. X-ray machine

2. Fluoroscope image pickup and

3. CRT or closed circuit video system



The X-ray image falling on a fluorescent screen or grid causes a visible

light picture to appear. This is optically focused by a lens on the film of a

motion picture camera.

The film can be played back at a later date.

The visual image is also focused on a phototube lens and made

brighter by an image enhancer.

A video camera converts the light image into an electrical video signal

which is delivered to a CRT and displayed through a closed circuit video

system. This gives a real time or instantaneous visualization.



Difference between Radiography and fluoroscopy

S.No Radiography Fluoroscopy

1

X-ray image is developed by

photosensitive film.

X-ray image is developed by

photoelectric effect and

fluorescence.

2 High geometric resolution in

images can be obtained.

Fair resolution in images can

be obtained.

3

A wide range of contrast can be

obtained.

Contrast can be increased by

introducing electronic image

intensifier.

4

Patient is not exposed to X-rays

during examination of the X-

ray image

Patient is exposed to X-rays

during the examination of the

X-ray image.

5 The patient dose is low. The patient dose in high.

6

Permanent record is available. Permanent record can be made

by inserting video tape

recorder.

7

The image can be obtained for

developing the film and the

examination cannot be made

before developing the film.

Immediately image can be seen

and examination can be

finished with a short time.

8

Movement of organs cannot be

observed.

Movement of organs can be

observed (Real time

experiment).

9

Efficiency is more. Even though efficiency is less

in direct fluoroscopy, with the

modem television system, the

efficiency can be increased.



Precautions to be taken against radiation hazards:

1. Radioactive materials are kept in thick walled lead

containers so that radiation cannot penetrate them.

2. Lead aprons and lead gloves are worn.

3. All radio active samples are handled by a special remote

operated robot.

Radiation monitoring instruments:

1. Pocket dosimeters

2. Pocket type radiation alarm

3. Film dosimeter

4. Film badge holder.

Effects of Radiation:

The effects of cumulative X-ray dosage of ionizing radiation may result

in

1) mutations-genetic changes resulting damage to chromosomes:

2) Physical illness-vomiting, headache. dizziness. loss of hair. and 3)

bums: and 4) death-destruction of vital physiological systems such as

nervous. cardiovascular, respiratory, renal and digestive systems and

tissues.

Ionization Chamber:

It is device used for two major purposes

� Detecting particles in air

� Measurement of ionizing radiation

An ionization chamber is an instrument to measure the number of

ions within a medium (gas, solid or liquid). It consists of a gas filled

enclosure between two conducting electrodes. The electrodes may be in the

form of parallel plates or coaxial cylinders to form a convenient portable

detector. One of the electrodes may be the wall of the vessel itself. When gas

between the electrodes is ionized by any means, such as by alpha particles,

beta particles, X-rays or other radioactive emission, the ions move to the



electrodes of the opposite polarity, thus creating an ionization current which

may be measured by a galvanometer.

Angiography:

Angiography is a medical imaging technique in which an X-ray picture

is taken to visualize the inner opening of blood filled structures, including

arteries, veins and the heart chambers. X-ray or image of blood vessels is

called an angiograph or an angiogram.

Angiograms require the insertion of a catheter into a peripheral artery. The

most common angiogram performed is to visualize the blood in the coronary

arteries. Angiography is also commonly performed to identify vessel

narrowing in patients with retinal vascular disorders.

What is the use of image intensifier?

Some X-rays ate lost by the presence of bucky grid, the density of the

image in the film will be reduced and the image resolution will become low.

Therefore to improve the density and the resolution of the image, image

intensifiers are used.

USE OF RADIOISOTOPES IN DIAGNOSIS:

The exposure time of radiation during radioisotope examination is

much longer when compared to the exposure time of X-ray. So, the radiation

intensity from the isotope must be kept much smaller in order not to exceed

a safe radiation dose.

For this reason, the techniques used for radiation detection and

visualization with radioisotopes differ greatly from those used for X rays.

All nuclear radiation detectors used for medical applications utilizes

the light flashes caused by radiation in a suitable medium.

Scintillation detectors (also called scintillation counters) are used for

gamma ray detection.

Two types of scintillation detectors used for the detection of gamma

rays are as shown in the figure below.



1. Well Counter Scintillation detector:

In this scintillation detector, the crystal has a hole into which a test

tube with the sample is inserted.

In this configuration almost all radiation from the sample passes the

crystal and is counted while a lead shield reduces the background count.

In scintillation detector, a crystal made from thallium activated

sodium iodide is used. The crystal is kept in close contact with the active

surface of a photomultiplier tube. Each radiation quantum passing the

crystal causes an output pulse at the photomultiplier, the amplitude of

which is proportional to the energy of the radiation.

The output from the photomultiplier tube is passed to pulse height

analyzer.

(This is an electronic circuit that passes only pulses within a certain

amplitude range).

The limits of the pulse height analyzer circuit are adjusted in such a

way that only pulses from the radioisotope can pass, whereas pulses with

other energy levels are rejected.



2. Collimator Scintillation detector:

For activity determinations inside the body, a collimated scintillation

detector is used.

In this detector, a lead shield around the scintillation crystal has holes

arranged in such a way that only radiation from a source located at one

particular point in front of the detector can reach the crystal.

Only a very small part of the radiation comes from the source and

passes to the crystal. Therefore the detector is much less sensitive than the

well counter type.

Block Diagram of an instrumentation system for radioisotope

procedures:

The pulses from the photomultiplier tube are amplified and shortened

before they are passed to through the pulse height analyzer. A timer and

gate allow the pulses that occur in a set time interval to be counted by

means of a scaler (decimal counter with readout).

A rate meter (frequency meter) shows the rate of the pulses. Based on

the reading of the rate meter, the detector can be aimed towards the location

of maximal radioactivity and the pulse-height analyzer can be set in the

range where it passes all the pulses from the particular isotope used.

In an automatic system, for the measurement of radioactivity in "in

vitro" samples an automatic sample changer arm (right) selects test tube

containing the samples from a carousel and drops them into a counting

well.

The number of radioactive disintegrations measured over a preselected

time interval is printed out on the printer.

The principle of the collimated scintillation detector can be used to

visualize the spatial distribution of radioisotopes in a body organ.

In a radioisotope scanner, the detector is slowly moved over the area

to be examined in a zigzag fashion. A recording mechanism attached to the

mounting arm of the detector produces a plot of the distribution of the

radioactivity.



X-ray Therapy (OR) Radiation Therapy:

Certain disease and tumours can be treated by the ionization effect of

X-rays.

The use of radiation for the treatment of disease is called as radiation

therapy.

� Soft X-rays are used in the treatment of some skin diseases.

� Deep penetrated tumours are treated with very hard X-rays

� Concentrated and high energy X-rays are used to destroy cancer

cells.