tissue engineering introduction for physicists - lecture two

Tissue Engineering

Medical Imaging

September 2014

Part One

Ali Bakhshinejad

[email protected]

Medical Imaging

● Develop a firm understanding of the fundamentals of

medical imaging, and principles underlying various

modalities and more importantly how use them in tissue

engineering

● Gain a basic understanding of the physical principles

underlying the major modalities, such as X-ray,

computed tomography and MRI.

Medical Imaging

What is the usage in Tissue Engineering?

Ultimate goal is to generate 3-D geometry of the Tissue/Organ

Medical Imaging

What does the human body look like on the inside?

It depends on how we look at it

● The most direct way → is to cut it open

○ A refinement of this procedure might be to use an endoscope

These are invasive techniques, which have the potential to cause damage or trauma to the body

Medical Imaging

Using Medical Imaging techniques means that we do not need to cut the body

or put a physical device into it in order to “see inside”

Various imaging techniques allow us to see inside the body in different ways -

the “signal” is different in each case and can reveal information that the other

methods cannot

Medical Imaging

Source

(e.g. light, x-ray)

Signal

System S Output g

Object

(e.g. tissue,

organ)

DetectionImage

reconstruct

● Excite the object with signal

● Acquire the image with detectors

● Reconstructing images

● Further image processing (i.e. generating 3D geometry)

Medical Imaging

For Example:

Functional Magnetic Resonance Imaging (fMRI) allows us to obtain images of

organ perfusion or blood flow

Positron Emission Tomography (PET) allows to obtain images of metabolism or

receptor binding

Medical Imaging

Again:

What does the human body look like on the inside?

It depends on the measured signal of interest

Medical Imaging

There are different methods of medical imaging measuring different signals

● Projection Radiography

● Computed Tomography

● Nuclear medicine

● Ultrasound Imaging

● Magnetic Resonance Imaging

● ...

Medical Imaging

● Projection Radiography

● Computed Tomography

● Nuclear medicine

● Ultrasound Imaging

● Magnetic Resonance Imaging

} Using ionizing radiation }}

Transmission imaging

Emission imaging

Reflection imaging}

Medical Imaging /Projection Radiography

● Routine diagnostic radiography

● Digital radiography

● Angiography

● Neuroradiology

● Mobile x-ray systems

Medical Imaging/X-Ray

x-ray imaging modality can be divided into two types

Projection radiography

Computed tomography

Whilhem C. Rontgen, the german physicist, father of x-

ray, named the rays coming out of the Crooke’s tube as

x-ray, because he didn’t know what are those rays

Today, we know that x-rays are electromagnetic waves

whose frequencies are much higher than visible light

Medical Imaging / X-Ray

The most common modality in projection radiography

Medical Imaging /Magnetic Resonance Imaging

Medical Imaging /Comparison

Characteristics X-Ray Imaging MRI

soft-tissue contrast Poor Excellent

Spatial resolution Excellent Good

Maximum imaging depth Excellent Excellent

Nonionizing radiation No Yes

Data acquisition Fast Slow

Cost Low High

Medical Imaging/Atom

Atomic Structure

Medical Imaging/Atom

Electron Shells

Medical Imaging/Physics of Radiography

What is Binding energy?

Binding energy of electrons to its nucleus is the amount of energy is needed to

bind the energy to its shell

Binding energy for hydrogen atom is 13.6eV


If radiation (particulate or electromagnetic) transfers energy to an orbiting

electron which is equal to or greater than the electron’s binding energy, then

the electron is ejected from the atom. This process called ionization.


● Not all rays are ionizing

● High energy rays such as x-rays are ionizing which

results in free electron and an atom with positive charge

● Rays with energy higher than 13.6 eV are ionizing rays


Excitation:

If an ionizing particle or ray transfers some energy to a bound electron but less

than the electron’s binding energy, then the electron is raised to a higher

energy state but is not ejected.

Medical Imaging/Physics of Radiography → Ionization

Ionizing can be divided into two broad categories:

● Particulate Radiation: Any subatomic particle (e.g, proton, neutron, or electron) can be

considered to be ionizing radiation if it possess enough kinetic energy to ionize an atom.

● Electromagnetic: Electromagnetic radiation consists of an electric wave and a magnetic

wave traveling together at perpendicular angle to each other.

Radio waves, microwaves, infrared light, x-ray, gamma rays and etc.


Object

X-ray

detector

X-ray

source


X-rays are generated using an x-ray tube. A current, typically 3 to 5 amperes at 6

to 12 volts is passes through a thin tungsten wire, called the filament, contained

within the cathode assembly.

Electrical resistance causes the filament to heat up and discharge electrons in a

cloud around the filament through a process called thermionic emission.

These electrons are now available to be accelerated toward the anode when the

anode voltage is applied, producing the tube current.

The filament’s current directly controls the tube’s current, because the current

controls the filament’s heat, which in turn determines the number of discharged

electrons.

Once the filament’s current is applied, the x-ray tube is primed to produce x-rays.


This is accomplished by applying a high voltage, the tube voltage ( kV) between

the anode and cathode for a brief period of time.

While the tube voltage is being applied, electrons within or near the cathode are

accelerated toward the anode. The focusing cup, a small depression in the

cathode containing the filament, is shaped to help focus the electron beam toward

a particular spot on the anode.

This target, or focal point, of the electron beam is a bevelled edge of the anode

disk, which is coated with a rhenium-alloyed tungsten.


X-Rays are generated by two different processes known as:

● Bremsstrahlung

● Characteristic X-Ray


➔ Characteristic radiation is produced when inner shell electrons of

the anode target are ejected by incident electrons

➔ The resultant vacancies are filled by other shell electrons, and the

energy difference emitted as characteristic radiation

➔ K-shell electrons are ejected only if incident electrons have

energies greater than K-shell binding energy


➔ Characteristic x-ray production starts when a

bombarding electron interacts with an atomic

electron, ejecting it from its electronic shell.

Subsequently, outer-shell electrons fill in the

vacant shell, and in the process emit

characteristic x rays.


➔ L-Shell characteristic x-rays have very low

energies and are absorbed by the glass of the

x-ray tube

➔ Most incident electrons interact with outer

shell electrons and produce heat but not x-ray

➔ (1) Characteristic radiation is produced when

an incoming electron (2) interacts with an

inner shell electron (3) and both are ejected

(4) when one of the electrons from any outer

shell moves to fill the inner shell vacancy, the

excess energy is emitted as characteristic

radiation


➔ Bremsstrahlung (braking) x-rays are produced

when incident electrons interact with electric

fields, which slow them down and change

their direction

➔ Bremsstrahlung x-ray production increases

with the accelerating voltage (kV) and the

atomic number (Z) of the anode

➔ Bremsstrahlung radiation is produced when

an energetic electron (with initial energy E1)

passes close to an atomic nucleus

➔ The attractive force of the positively charged

nucleus causes the electron to change

direction and lose energy


Main risk from ionizing radiation at high doses of x-ray is

cancer production

This is due to damage of Cell’s DNA due to radiation

The low dose of x-ray (like in dental x-ray or chest x-ray) is

not dangerous


X-ray radiation bioeffects

Low dose long term effects: genetic damage

Recommended maximal dose (NCRP, National Council

on Radiation Protection): 5 R per year

Rad: stands for a certain dose of energy absorbed by 1

gram of tissue

Rem: Multiply the dose in rads by a quality factor (Q)

for each type of radiation

Rem = Rad x Q


Filtration removes low-energy photons (long

wavelength or “soft” x-rays) from the beam by

absorbing them and permits higher energy

photons to pass through. This process reduces

the amount of radiation received by a patient

With Restriction, those rays that are not in the

certain area of interest are removed


X-ray interactions during passing through masster

Photons may:

1) Pass through

2) Absorbed (and transfer energy)

3) Scattered (change direction and lose energy)

which cause Compton scatter and

Photoelectric (PE) effects


Photoelectric (PE)

The PE effect occurs when an incident x-ray is

totally absorbed by an inner shell electron,

which is ejected as a photoelectron.

The vacancy is filled by an outer shell electron,

and the energy difference is emitted as

characteristic radiation


Compton scatter

Occurs when incoming x-ray photon interacts

with outer shell electron, x-ray photon loses

energy and changes direction and compton

electron carries away energy lost by scattered

photon

● This electron loses energy by ionizing

other atoms in the tissue, thereby

contributing to the patient dose


● Photoelectric (PE) effect is desirable and provides the contrast in x-ray

images

● Compton scattering is unwanted and causes blur in x-ray Image and is the

limiting factor of resolution

In order to reduce compton scattering anti-scatter grids are used


Object

X-ray

detector

X-ray

source

Linear attenuation coefficient

Linear attenuation coefficient depends on

density of the absorber, atomic number, and

incident photon energy

Mass attenuation coefficient


Photoelectric has important effect in low x-ray energies and compton scattering is

important in high x-ray energies

Medical Imaging /Computed Tomography

Medical Imaging/Computed Tomography (CT)

1st Generation CT: Parallel Projection


2nd Generation


3G


4G


● Horizontal movement of the bed as the x-ray is rotating

● Rapid volumetric data acquisition


CT Number

Consistency across CT scanners is desired. CT Number allow for the display of the image in terms of

the attenuation coefficient wrt water no matter what x-ray tube is used across different scanners

Where K is a constant (Usually = 1000), and are respectively the attenuation coefficients for

the pixel of interest and water

The CT number has a unit of Hounsfield or HU when K = 1000


CT Number

The Hounsfield number of a tissue varies according to the density of the tissue with more dense tissues

having higher numbers. Hounsfield numbers range between -1000 HU and +1000 HU for Air and bone

respectively.

High numbers are shown as white and low numbers as black in radiographs.


Example:


Each projection contain M data point, and there are N rotations

For each projection, the signal intensity depends upon the composite attenuation coefficient of the

tissue corresponding to the particular beam path.

Medical Imaging /X-ray and CT

● Projection radiography → shadows of transmitted x-ray

intensities after absorption and scattering by body. 2D

projection

● CT is 3D x-ray imaging

● Tissue attenuates x-ray depending on their attenuation

coefficients and x-ray energies

● Both x-ray and CT uses transmission of ionizing radiation

through body

● Various organs change the intensity of beam differently

● The beams exiting the body contains shadows of tissue

Medical Imaging/Magnetic Resonance Imaging

MRI is making high contrast cross-sectional images like CT

taking advantage of magnetization of protons in the body

rather than using any ionizing radiation through the body


The five principal components:

1. The main magnet

2. A set of coils to provide a switchable spatial

gradient in the main magnetic field

3. coils for the transmission and reception of

radio-frequency pulses

4. Electronics for programming the timing of

transmission and reception of signals

5. Console


● Gradient coils provide the means to encode spatial information and to

choose slices of body to be images

● (a) axial by applying Gz

● (b) coronal by applying Gy

● (c) sagittal by applying Gx


Magnetic resonance imaging is an imaging modality

which is primarily used to construct pictures of the NMR

(nuclear magnetic resonance) signal from hydrogen

atoms in an object


1. Magnetize your subject in strong magnetic field

2. Transmit radio waves into subject

3. Turn off radio waves transmitter

4. Receive radio waves echoed by subject

5. Manipulate echoes with additional magnetic fields

6. Store measured radio wave data vs. time

- Repeat steps 2 through 6

7. Process raw data to reconstruct images

What is the physical basis?


What is Gyroscope?

device for measuring or maintaining orientation, based

on the principles of angular momentum.

http://youtube.com/v/cquvA_IpEsA

http://youtube.com/v/cquvA_IpEsA


Why Gyroscope?

Atoms with electrons moving around them acting like a

Gyroscope

Each atom has it’s own small magnetic field


f is resonance frequency of a spin

is the gyromagnetic ratio and it’s equal to

265.5 M rad/s/T for hydrogen

is Larmor precession frequency


m

B0

B

B

01

Medical Imaging /Magnetic Resonance Imaging

● Standard MRI

● Echo-planar imaging (EPI)

● Magnetic resonance spectroscopic imaging

● Functional MRI (fMRI)

Medical Imaging

References:

● EE437 Introduction to Biomedical Imaging, Dr. Mahsa Ranji

● Prince, J. L., & Links, J. (2006). Medical Imaging Signals and Systems.

Pearson (1st ed., p. 481).

tissue engineering introduction for physicists - lecture two

Engineering

medical imagingsourcee

various imaging techniques

ray imaging modality

human body

binding energy of electrons

different ways

father of xray

organdetection image