Imaging Technology and Equipment Unit 1

8/16/2019 Imaging Technology and Equipment Unit 1

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MAGING TECHNOLOGY AND EQUIPMENT PREPARED BY: ER.SARBESH CHAUDHARY, ELECTRONICS DEPT.,MMP

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UNIT 1

MEDICAL IMAGING SYSTEM

INTRODUCTION

Photographic film was the principal means for storing medical images for many years which has been

replaced by computers which provides new means of storing, processing, transferring and displaying images.

Computerized tomography, computers and digital image processing have revolutionized the way medical

images are produced and manipulated.

Now it is possible to acquire data, perform mathematical operations to produce images, emphasize details

or differences of images, & store and retrieve images from remote site by the means of various

communications system all without film.

In medical imaging system all the imaging systems such as photographic, x-ray, ultrasonic, radionuclide,

television etc. can be thought of as a cameras. All the cameras are limited by resolution, amplitude scale and

noise content.

The photographic images can be enlarged until it becomes quite “grainy”, and this graininess is the bound to

both resolution and noise. The x-ray image is resolution-limited by the dimensions of x-ray source and noise

limited by the beam intensity. The ultrasound images are limited by angular resolution of the transducer and

its ability to separate true signals from false signals and noise. The television image is limited by the electron

storage capacity of the camera sensor.

INFORMATION CONTENT OF AN IMAGE

In most of the cases, the information content of an image is the product of the number of discrete elements

called pixels and number of amplitude levels of each pixel.

Noise whether originating from photographic grain, electric charge or statistical variations of the number of

quanta (light, x-ray, or gamma ray) limits the amplification permitted in a channel. It is better to use

amplitude steps within the channel same as the signal-to-noise ratio (SNR).

RESOLUTION

Spatial resolution is the quantitative measure of the ability of a display system to produce separable imagesof different points of an object with high fidelity. Resolution is expressed in terms of line pairs per millimetre

(lp/mm).

Resolution is defined this way so that objects and spaces between them are counted equally. A single

countable object requires at least one line pair (2 pixels) on each axis so that both, spaces between the

objects and object itself may be resolved.

Systems must be designed with adequate spatial resolution characteristics to guarantee that spatial details

of interest are preserved when a medical image is displayed. Portraying image data on a display with

insufficient resolution will affect the accuracy of the radiological interpretation.


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Ne and zero response above Ne . The Ne is obtained by integrating the square of the MTF amplitudes and is

expressed as,

Ne = ∫ 2( )∞0 The value of Ne is an excellent measure for calculating the noise performance of any imaging system. Thus,

it is being a useful and practical means for describing system performance rather than limiting resolution.

PHOTOGRAPHY

Photography is based on the lattice properties of silver bromide crystals in the film emulsion. Light

photons ejects electron from the bromide atoms and some of these electrons are caught by the silver

atoms which neutralizes it and form metallic silver atoms.

For a single silver atom, the neutralizing electron may be lost thermally and the silver atom again go

back to their lattice structure. Thus, if five or more contiguous silver atoms are freed at the same time

then the probability of the silver atom remaining in the metallic state will be high.

During the chemical processing (developing), the silver ions in the grain accumulates around the metallic

silver atom to form a grain of about 109 metallic silver atom. And the remaining silver bromide is carried

away by the solution (fixer).

Let 25 atoms are required to produce the first 5 silver atom to make a single grain capable of beingdeveloped in the required sensitivity site. Then, we can say that, for a given type of emulsion and

development, the number of grains per unit area and the size of the grain can be varied but the amount

of light required per grain is constant.

Thus, we can say that, fine grained film requires more lights per unit area and coarse-grained film

requires less light per unit area.

Image formed in the film before chemical development is called latent image. There is the possibility of

some metallic silver atoms to return back to their lattice because of slow thermal effect, this is called

latent image fading.

All the films have some range of exposure time such that, constant-light-intensity-time product will

bring the emulsion to a particular value of density. Where density is defined as D=log 1/T, where T is

light transmission on film.

The value of Ne is about 40 lp/mm for most photographic negative. Electron-beam recording on the film

permit the use of 8-mm film to record about 30 times as much as information as light exposure.


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Since each requires 25 photons captured, 2X1010 photons are required to take a picture. The lesser

exposure increases the noise or say decreases the gray scale. The optical defocussing reduces the noise

due to graininess but this increases the cell size which in turn decreases the resolution.

TELEVISION SYSTEM

The television system consists of a TV camera, optional image storage unit, image processing unit and display

monitor. In fluoroscopic system, the television camera is coupled through a lens to the output of the x-ray

image intensifier. The camera detects the light signal, blanks it at the edge of the image frame and adds

synchronising pulses, which helps storage and display devices to get synchronized with the camera scanning.

For the storage of medical images magnetic tape/disks or optical discs are used and small computer are used

to process the image. Because of computer processing, rapid communication and diagnosis from the distant

is possible with the aid of internet and other communication technologies.

Television camera

The television comprises of a sensor, which has a matrix or surface of photodetector and capacitors. The

characteristics and thickness of the photodetector determine the light sensitivity and the capacitance

determines the noise properties and dynamic range of the sensor. The two basic type of geometries like

electron-beam-scanned camera tube and charge coupled device (CCD) are used in TV camera.

Figure below shows the construction of the vidicon camera tube which is an electron-beam-scanned

camera tube. It consists of the following parts:

Indirectly heated cathode (electron gun or electron source). Four grids (G1 to G4).

Face plate made of optically flat screen. Target plate having two sides. The side facing the light is known

as “signal plate”. It has a thin coating of tin oxide (SnO2) which is transparent to light and is good conductor


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of electricity. The back side of the target plate faces electron gun and has a coating of antimony trisulphide

which is a semiconductor or may have a coating of lead oxide (PbO2) which acts as a photoconductors.

Deflection coils, which allows an electron beam to scan the target horizontally and vertically.

Focus coil, mounted outside the tube to sharpen the electron beam. Alignment coil, which keeps the beam

aligned when there is no deflection.

Working or operation:

The electron gun produces the scanning electron beam by heating the cathode and scan the target

containing the photoconducting layer.

The grid G1 surrounds the cathode and controls the beam current. Whereas G2 accelerates the electron.

Focus grid G3 slows down the electrons, so that, the beam should fall perpendicularly on the target at low

speed without causing of secondary emission.

Grid G4 is a wire mesh and works as a muzzle of electron gun. When light falls on the photoconductiveplates its resistance about to 2mΩ. When an optical image is focused on the target, the illuminated pixels

becomes conducting and gets partially discharged. This charged image corresponds to the optical image

focused on it. Therefore, a pattern of positive charges is formed on the side of the target plate facing

electron gun.

When the electron beam falls on the target plate, it leaves electrons on the target plate. This electrons

neutralizes the positive charge on the target plate. This causes an electric current to flow.

The current at any spot of scanning is proportional to the electrons deposited by the beam and is

therefore proportional to the intensity of brightness of the spot. Each element in the target plate is

scanned every 40 ms.

The sensitivity of the vidicon tube coated with antimony trisulphide on the target can be adjusted by

changing the target voltage but it cannot be adjusted if the target plate is coated with lead oxide (PbO2).

The detector in video fluoroscopic X-ray systems, vidicon camera tube is replacing by the charge coupled

device (CCD) which provides improved image quality than the vidicon tube. Each elemental circuit in CCD has

a photodiode, a charging circuit, a capacitor and a charge transfer circuit. It offers high resolution, so for

certain application where 2048X2048 pixel matrices are required, CCD is preferable. Commonly CCD have

about 250X350 pixel matrices. CCD are also used abundantly in dental radiography.

Image processing

During processing, the analog vidicon camera tube information is quantized by sampling and digitized for the

input to computers and CCD images are quantized by pixel-by-pixel information.

Because of the storage facility the medical image can be stored and can be seen continuously which prevents

regular exposure of X-ray to the patient. In fluoroscopy, even if the X-rays are turned off, the last image can

be obtained as a “last image hold”.


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RADIOGRAPHY

Radiography is the photography of internal organ of the body that uses other kinds of radiation rather

than visible light which is called X-ray. X-rays were first discovered by the German physicist named

Wilhelm Konrad Rontgen in November 1895. He called this new kind of ray, the X-ray, where X meansunknown.

X-ray photons are electromagnetic radiations similar as light photons but has 104 times energy than light

photons i.e. 10 to 150 KeV. Because of such high energy X-ray photons are more penetrating. Streams

of X-rays can dissociate molecules by ionization, so, they are also called ionizing radiation.

The modern unit of radiation is gray (Gy), which is defined as absorbed energy of 1joule per Kg. the

modern unit of exposure is sievert (Sv) which is also known as biologically equivalent dose.

A radiological examination is one of the main diagnostic aid available in the medical science. The X-raypicture is called a radiograph, which is a shadow picture produced by x-rays radiating from a point

source. The X-ray picture is usually obtained on photographic film placed in image plane.

It is based on the fact that various anatomical structures of the body have different densities for the X-

rays. When X-rays from a point source penetrate a part of the body, the internal body structure absorb

varying amount of radiation. The radiation that leaves the body has a spatial intensity variation, i.e. an

image of internal structure of the body.

Properties of X-ray

The properties of X-ray which makes them useful in medical examination are as follows:

1. X-rays are able to penetrate through any materials because of its short wavelength (in the range of

0.01 to 10 nanometres) and high energy.

2. Produces ionization in some materials.

3. Produces fluorescence in some materials to help them emit light.

4. It affect photographic film in the same way as ordinary visible light.

Effects of X-ray

Despite the benefits, excessive X-rays may cause harm to the human body. The effects of X-ray are as follows:

1. The children of pregnant patients may suffer from congenital abnormalities as a result of

irradiation.

2. X-rays may also cause abnormal cellular proliferation resulting in cancer development.

3. It may cause reddened skin on the area of exposure.

GENERATION OF X-RAYS

A simple X-ray system consists of a high voltage generator, an x-ray tube, a collimator, the object orpatient, an intensifying screen and the film as shown in the figure below. A simple X-ray generator has a


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line circuit breaker, a variable autotransformer, an exposure timer and contactor, a step-up transformer

and rectifier & a filament control for the tube.

The X-ray tube is a temperature-limited vacuum diode with a heated cathode located opposite to the

target anode. The beam electron strikes the anode and produces X-ray through two mechanisms i.e.

bremsstrahlung and characteristic radiation.

The X-rays are produced by bremsstrahlung mechanism by deceleration of the arriving electrons by

positively charged nuclei of the anode atoms. The X-rays are produced through characteristic radiation

when anode’s innermost electron is knocked out of the orbit by arriving electrons which is then replaced

by the outer shell electrons.

As shown in the figure above the X-ray tube generates the x-ray. The lowest energy x-rays are absorbed

in the anode metal and the glass envelope of the tube. Further an AI filter reduces the low energy x-rays

which do not penetrate through the body which would only increase the patient dose.

High energy x-rays from an AI filter is restricted by the aperture of the collimator and which is then falls

on the patient body part of interest.

The scattered secondary radiation from the body is trapped by the grid whereas primary radiation strike

the screen phosphor. The resulting light then exposes the film which will give us the x-ray of the patient

body part required for the diagnosis.

The intensity of x-rays depends upon the current through the tube and wavelength depends on the target

material and the velocity of the electron holding the target. The X-ray equipment for the diagnostic

purpose uses the target voltage of 30 to 100 kV, current of 300 mA, time period of 0.1 s and more than100kW of power level.


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COMPUTED RADIOGRAPHY

Computed radiography (CR) are rapidly replacing screen-film imaging systems in many countries. CR uses

photostimulable phosphor (PSP) plates which must be transported to a scanner (or reader) and scanned with

a laser beam to convert the stored image to a digital image. These systems rely on picture archiving and communication systems (PACS) for transmission and storage of

the digital images.

CR systems use a PSP plate to store the image resulting from the interaction of x-rays with the phosphor.

This stored image is then stimulated by a laser beam with a very small focus (0.1 mm or less). This stimulation

results in the emission of light which is captured, amplified and digitized to produce the digital array. The

image thus obtained must be of higher resolution than the resolution of display monitor so that magnified

portion (i.e. Region of Interest (ROI)) can be viewed clearly.

The advantages of this technology is that;

It reduced cassette handling for the radiographer in the clinical setting.

The various features of image (like sharpness, contrast, brightness, aspect ratio etc.) obtained by CR can

be emphasized and even the noise level can be reduced using computer processing.

Image fusion can also be achieved through CR in which the image of same body part obtained from CR

and other imaging sources like MRI, CT scan, ultrasound etc. can be superimposed on a single monitor for

the diagnosis.

Figure below shows the block diagram of Computed radiography system, which consists of x-ray system,

reusable PSP detector (film), image reader, image scanner and image recorder. The process of CR image

acquisition is given below;

First, the X-ray system produces x-rays which strikes the phosphor plate through patient. Then latent

image is formed on the phosphor plate where low energy electrons in the phosphors are elevated and

trapped in a metastable energy state.

Plate is placed in an image reader “processor” where it is scanned with a red laser light by the image

scanner. Laser light inputs energy into the plate knocking the electron out of the trap. As the electron

returns to lower energy level it emits the excess energy as visible light.


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These visible light energy is detected by a collection of Photo-multiplier Tubes or photodiodes which

provides amplified analog electrical signal of an image.

The analog electrical signal is converted to a digital image via an analog-to-digital converter (ADC). ADC of

lower bit capacity but faster response must be used to generate clean signals of wide dynamic range.

Now, once the digital image is obtained it can be stored in image recorder and can be viewed in monitorscreen as a high resolution X-ray image.

COMPUTED TOMOGRAPHY

The Computed Tomography (CT) also known as Computerized Axial Tomography (CAT) is the medical-imaging

systems which generates images of internal body structures using complex x-ray and computer-aided

tomographic imaging techniques. It was developed in 1970’s because of the limitation of a traditional X-ray

image.

The idea behind the development of CT evolved for imaging the brain. By using CT we can obtain theinformation about internal organs and body structures which could not be done by conventional X-ray

photograph. CT scan is one of the major developments in medical instrumentation. Actual image dimensions

of CT are generally 256x256 or 512x512 pixels.

Main application of CT scan in medical field is to detect the injury in the human brain and to detect inner

wounds.

Working principle

The x-ray images used to generate the tomographic images are generated first by exposing the patient to a

fan-shaped x-ray beam and then detecting the projected image on a thin semi-circular, digital x-ray detector

as shown in the figure below.

Figure: The patient is exposed to a fan-shaped x-ray beam and the projected image is detected on a thin,

semi-circular digital x-ray detector.


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The patient is placed between the source and detector, and the detector is configured with its geometric

centre located at the x-ray source. Each image is an x-ray projection of a very thin transverse slice of the

body.

To collect the multitude of x-ray projections necessary to generate a tomographic CT image, both the x-raysource and detector are rotated about a patient within a supporting gantry. While the source and detector

rotate, images are collected and stored.

Photodetector arrays used in CT imaging have as many as 1000 detectors in the long dimension along the

semi-circular detector arch; 16 or more detectors are positioned in the shorter dimension tangential to the

arch. The number of detectors in the short dimension determines the number of available image slices.

Detection operation

Early CT imaging systems accomplished x-ray detection using both scintillation crystals and photomultiplier

tubes. The scintillation crystals converted x-rays to light and the photomultiplier tubes converted these lightsignals to a usable electrical signal. Modern CT systems now employ more sophisticated scintillation crystal

materials and solid-state photodetector diodes for this purpose.

The output from each photodiode is a current proportional to the light striking the diode. The signals from

the photodiode array are then routed to the digital acquisition system (DAS) which amplifies and converts

the signals to a digital format using high-resolution analog-to-digital converters (ADCs).

An alternative method routes the signals from every photodiode to an integrator in the DAS. In these

implementations, the integrated current signals are converted to a voltage, sampled at the same time, and

multiplexed into the input of an ADC.

ADC’s with 16-bit resolution or greater are commonly used. The outputs of the ADCs are routed to an image

signal processor over a high-speed bus for further signal processing and image reconstruction.

The output may be displayed on the cathode ray tube or photographed with a camera to produce

a hard copy record.

MAGNETIC RESONANCE IMAGING (MRI)

MRI was discovered in 1947 simultaneously by two physicist Felix Bloch and Edward Mills Purcell. The first

clinical images was obtained in 1977. MRI is a non-invasive technique which uses magnetic field and radiofrequencies rather than ionising radiation as used in X-ray and CT scan for the diagnosis. Magnetic field

strength of an MRI machine is measured in Tesla (T). For clinical diagnosis MRI machine uses magnetic field

strength of 1.5 to 3 T, this produces a strong magnetic field which is much more than the earth magnetic

field i.e. 0.00003T.

Construction

The schematic diagram of a typical MRI machine is shown in the figure below.

It consists of Primary magnet to generate strong static magnetic field.

The gradient coils to generate magnetic field altering the primary magnetic field.

The RF transmitter and RF receiver coils to produce and receive RF pulses.

The computer system i.e image processing system. The monitor to view the MRI image.


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Working principle

The human body contains of 70% of water which is the composed of hydrogen and oxygen atoms. MRI relies

on the magnetic properties of hydrogen atoms to produce an images. Hydrogen nucleus is composed of a

single proton and the elements having odd number of proton like hydrogen have magnetic properties. Such

spinning charged particles like hydrogen proton produces magnetic field which is called magnetic moment.

Normally the hydrogen protons are oriented randomly so there is no magnetic field. When a person is

subjected to the strong magnetic field the hydrogen atoms align parallel or anti-parallel to the primary

magnetic field B0, this is called longitudinal magnetisation. Greater proportion of the hydrogen atom align

parallel to the magnetic field (i.e low energy state) than anti-parallel to the magnetic field (i.e high energy

state), thus, the net magnetic vector M is in the direction of the primary magnetic field B0 oriented along the

z-axis.

The proton spin along the axis of the primary magnetic field which is known as precession and this precession

rate is termed as Larmor frequency. When protons precess together it is called in-phase and when the proton

precess separately it is called out-of-phase. The frequency changes with respect to the magnetic field, which

is given by the Larmor relationship as

ω= γB

where, ω = 2π times the Larmor frequency f .

γ = gyrometric ratio i.e 42

B = magnetic field strength.

so, at magnetic strength of 1.5 T Larmor frequency is 63.9MHz.

There are three gradient coils which are named according to the axis on they work, they are X-gradient coil,

Y-gradient coil and Z-gradient coil. The gradient coils generates secondary magnetic field which is less than

the primary field. The arrangement of these gradient coils gives the MRI the capacity to image directionally

along the Z, X, and Y axis.

Gradient magnets alter the strength of the primary magnetic field thereby changing the precession frequency

between the slices which can then be used for slice selection along X, Y and Z-axis, this is called spatial

encoding of MRI imaging. The Z-gradient runs along the Z-axis to produce axial images, the X-gradient runs


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along the horizontal axis to produce sagittal images and Y-gradient runs along the vertical axis to produce

coronal images.

The RF transmitter coil is used for transmitting radio frequency equal to Larmor frequency to produce

resonance and the RF receiver coil is used to receive the radio frequency from the body due to resonating

hydrogen atom. The RF coils are designed according to the specific body parts to be examined. The RF

transmitter transmits the RF pulse which disturbs the proton alignment.

The parallel atoms when subject to such pulse flips to the anti-parallel state decreasing longitudinal

magnetisation and the protons becomes synchronised and precess in-phase. As a result the net magnetic

vector M from the longitudinal plane turns towards the transverse plane i.e at right angle to the primary

magnetic field, this is known as transverse magnetisation.

After the application of the RF pulse the protons stats to decay to its equilibrium state giving out the energy

to the lattice which is called relaxation. The two types of energy decay are called spin-lattice decay or T1

relaxation and spin-spin decay or T2 relaxation. These time constants T1 and T2 are quite long, ranging from

milliseconds to seconds and they depends upon the type of tissues and surrounding material.

The RF signal received by the RF receiver coil is fed to the computer system, the analog signal from the RF

receiver is then converted to digital signal by the analog-to-digital converter.

During data acquisition the digital signal representing the imaged body part is stored in the image space

called K-space. Then the K-space image is sent to the image processor where the mathematical formula called

Fourier Transformation is applied and the image of the MRI scan is displayed in the monitor.

NUCLEAR MEDICINE

In the diagnostic imaging there are two kinds of modalities, one is the anatomical imaging which provides avery accurate visualization of the internal structures of the human body; and the other is the functional

imaging, which is aimed to measure the physiological processes taking place inside the human body without

influencing them. One of the most widely spread practices used for functional imaging are contained in the

Nuclear Medicine Diagnostics. In nuclear medical diagnostics, vital processes such as blood circulation,

metabolism and vitality of organs and tumours can be displayed as functional images.

The main principle of the Nuclear Medical Diagnostics is to determine physiological processes through the

non-invasive techniques. A radio-nuclide is a radioactive drug used for diagnosis or therapy which is also

known as “tracer” or “radiopharmaceutical”. In a nuclear medicine test, small amounts of

radiopharmaceuticals are introduced into the body by injection, swallowing, or inhalation. This tracer isdistributed, metabolized, and excreted according to their chemical structure. The biological functions can be

displayed as images, numerical data or time-activity curves.

According with the physiological process that is being targeted there are different pharmaceuticals available.

Technetium-99m (Tc-99m) and iodine-123 labelled substances are the most important radio-nuclide used to

examine the brain, liver, lungs, bones, thyroid, kidney, heart etc. The amount of radiopharmaceutical used

is carefully selected to provide the least amount of radiation exposure to the patient but ensure an accurate

test. Radio-nuclides can be obtained from the nuclear reactors or nowadays can also be generated in the

hospital itself with the help of machine called “cyclotron”, which is expensive.

During diagnostic, after some minutes of giving radio-pharmaceuticals to the patient, the radio-nuclide from

the targeted body part starts emitting gamma rays as they starts decaying. A special camera (PET, SPECT or


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gamma camera) is then used to detect those gamma rays emitting from the body and take pictures of your

body.

In nuclear medicine the most used detectors are the scintillation crystals. Normally, radiation from

radioactive materials interacts with matter by causing ionization and/or excitation of atoms and molecules.

In this process energy is released. In the case of the scintillation crystals, part of this energy is released aslight. The camera detects the radiopharmaceutical in the organ, bone or tissue and forms images that provide

data and information about the area in question.

Nuclear medicine differs from an x-ray, ultrasound or other diagnostic test because it determines the

presence of disease based on biological changes rather than changes in anatomy. Nuclear medicine tests

(also known as scans, examinations, or procedures) are safe and painless.

Single Photon Emission Computed Tomography (SPECT) & Positron Emission Tomography (PET)

Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are two

radio-nuclide imaging techniques which provide a means of examining regional cerebral blood flow,

metabolism, and pharmacology of living organisms under both resting and activating conditions. These

molecular imaging techniques rely on radiolabelled molecules called tracers that bind to enzyme sites or

surface receptors. PET utilizes short-lived positron emitting isotopes (O-15, C-11, F-18, Br-76), whereas SPECT

uses lower energy γ-emitting isotopes (I-123, Tc-99m). Both techniques can detect nano-moles of tracer.

The main difference between these two techniques is really in the radioactive materials used as tracers and

the sensitivity of the detection methods. In both procedures the image acquisition takes place by use

of scintillation crystals resulting in 3D images. Some differences between PET and SPECT are given below:

S.no PET SPECT

1. PET traces two gamma rays moving in

180oopposite directions, which can trace the

location of the radio-pharmaceuticals better.

SPECT only traces one single radiation or a

general radiation, not a simultaneous double

one.

2. Radio-pharmaceuticals have short life time so

PET has to be close to the facility that makes the

tracers.

Radio-pharmaceuticals have longer life time so

SPECT should not be close to the facility that

makes the tracers.

3. Radio-isotopes are produced by cyclotron, thus,

it is more expensive to produce.

Radio-isotopes for it are not so expensive to

produce.

4. Better image sensitivity. Poorer image sensitivity due to the use of

collimator (made up of lead) which blocks 99% ofincoming radiation.

5. The photon energy of the detected radiation in

PET is 511 keV.

The photon energy of the detected radiation in

SPECT is 140 keV.

6. PET imaging gives a precise measurement of

the activity distribution within the patient’s

body.

SPECT imaging does not gives a precise

measurement.

7. Higher resolution (2-3mm) Lower resolution (6-8mm)

ULTRASONOGRAPHY


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Ultrasonography is the non-invasive technique used for diagnosis. The ultrasonic energy used for medical

applications does not harm the tissues like the ionizing radiation of x-rays.

The frequency range audible to the human beings is 20 Hz to 20 KHz and the frequencies of sound above 20

KHz are called Ultrasound. Ultrasound waves are longitudinal compression waves. The ultrasonic waves obeythe laws of reflection and refraction. The reflected energy is dependent on the difference between the

densities of the two media and the angle at which the transmitted wave hits the medium.

The depth of penetration of the ultrasonic wave is inversely proportional to the frequency and attenuation.

The depth of penetration increases as the frequency is decreased and vice-versa. The depth at which the

ultrasound energy is attenuated to half of the applied amount is known as half value layer (HVL). The HVL

indicates the amount of energy absorbed by the material. Table below shows the HVL for various materials.

Table: Ultrasound Absorption

An important characteristics of ultrasound frequency used in biomedical instrumentation is Doppler Effect.

If the reflected ultrasonic energy frequency is increased or decreased by the moving interface, the frequency

shift is given by;

∆ =2

Where, f = reflected wave frequency shiftv = interface velocity= transmitted ultrasound wavelength

Thus, we can say that frequency increases if the interface moves towards the transducer and frequency

decreases if the interface moves away from the transducer. This measured frequency shift is proportional to

the velocity. Velocity of the ultrasonic waves depends upon the frequency of the wave and density of the

medium.

Besides its other applications like detecting submarines, seismology, non-destructive testing of metals

(aeroplane wings, bridges etc) it is also vastly used in the medical diagnosis like cardiology, abdominal

imaging, brain studies, eye analysis, obstetrics and gynaecology.

Principle of Ultrasonic Diagnosis

For the diagnostic purpose in ultrasonography an ultrasound machine makes use of a piezo-electric

transducer which has the properties of the ceramics such barium titanate or similar materials.

When stressed these materials produces a voltage across their electrodes and similarly when voltage pulse

is applied the ceramics deforms. If short pulse is applied then the ceramic element rings at its mechanical

resonating frequency generating ultrasonic waves.

With appropriate electronic circuits, the ceramic is pulsed to transmit short burst of ultrasonic energy as aminiature loudspeaker and then switched to act as a microphone to receive signal reflected from the

interfaces of various tissues inside the body.

Type of material Frequency Half-Value-Layer (HVL)

Blood 1.0 35

Muscle 0.8 2.1

Fat 0.8 3.3

Bone 0.8 0.23


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The time delay between the transmitted pulse and its echo is a measure of the depth of the tissue interface.

Fine structures of tissues like blood vessels, muscle sheaths and connective tissue produce extra echo within

uniform tissue structures. The frequency of echo is different for various types of tissues and each change of

tissue type reflection occurs.

Figure below shows simple diagram for ultrasonic scanner which shows how the body structure produce the

echoes which identifies their locations. This type of simple ultrasonic scanner was used in early days to

measure the displacement of the brain midline.

MODES OF ULTRASOUND TRANSMISSION

The ultrasound is transmitted in various forms. The modes of transmissions are as follows;

1) Pulsed ultrasound: It is transmitted in pulsed form for the duration of about 1 microsecond. The returning

echoes are displayed with respect to time. The echoes are proportional to the distance from the source

to the interface. It is used in most of the imaging applications.

2) Continuous Doppler: Here, continuous ultrasound signal is transmitted and a separate receiving

transducer receive the echoes. Using this mode frequency shifts due to moving interfaces are detected

and the average velocity of the target is determined as a function of time. It is used in blood flow

measurements.

3) Pulsed Doppler: In this mode also short pulse of ultrasound are transmitted and returning echoes are

received. The frequency shifts are observed and thus, the velocity and distance of moving target ismeasured.

4) Range-gated Pulsed Doppler: A gating circuit is used in this mode for the measurement of velocity of

target at specific distance from the transducer. Using this mode, the velocity of blood can be measured

as a function of time and also as a function of distance from the vessel wall.

DISPLAY MODES

Depending upon the mode of transmission used ultrasound images are displayed in various modes. The

received information is amplified and displayed in one of the three display modes listed below.

1) A-Mode: This display mode is also known as Amplitude mode. It is the oldest and simplest display mode.

A-mode shows the echo intensity as a voltage-time plot on a x-y plane as shown in the figure below. The


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applications of A-mode are in ophthalmology (eye length, tumors), localization of brain midline, liver

cirrhosis, myocardium infarction.

Figure: A-mode scan

2) M-Mode: This display mode is also known as Motion mode. It is just a repeated A-mode displaying the

echoes from the various tissues in separate rows in conjunction with ECG. It is useful in viewing rates and

motion. The applications of M-mode are cardiac and fetal diagnosis.

3) B-Mode: This display mode is also known as Brightness mode. It is a 2-D image of a stationary organ. The

brightness of the oscilloscope is controlled by the returning echoes. The B-scan transducer is moved with

respect to the body and the vertical deflection of the oscilloscope corresponds to the movement of the

transducer.

END OF UNIT-1

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Imaging Technology and Equipment Unit 1

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