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Medical Imaging
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Textbook and Materials
Rafael C. Gonzalez, Richard E. Woods,
Techniques “Digital Image Processing”, 2nd Edition,
Pearson Education, 2003
Digital Image Processing by Jayaraman,
Veerakumar, 2012
Khandpur R.S, Handbook of Biomedical
Instrumentation, 3/e, Tata McGraw
Hill,New Delhi, 2014
Dr. K. Adalarasu
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Reference
William K. Pratt, “Digital Image Processing” ,
John Willey ,2001
Steve Webb, The physics of medical imaging, Ultrasound &
Adam Hilger, Bristol, England, Philadelphia,
USA, 1988 Thermal Imaging
Jain A.K., “Fundamentals of Digital Image
Processing”, PHI, 1995.

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Diagnostic Ultrasound
 Ultrasound has become increasingly important in
medicine and has taken its place along with X-ray and
nuclear medicine as a diagnostic tool
 Non-invasive character and ability to distinguish
Ultrasound & Thermal interfaces between soft tissues
 X-rays
Imaging  Respond to atomic weight differences
 Often require the injection of a more dense contrast medium
for visualization of non-bony tissues
 Nuclear medicine techniques
 Measure the selective uptake of radioactive isotopes in
specific organs to produce information concerning organ
function
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Diagnostic Ultrasound
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Diagnostic Ultrasound
 Radioactive isotopes and X-rays are, thus, clearly Main limitation of ultrasound, however, is that
invasive it is almost completely reflected at boundaries
 Obtaining images of almost the entire range of with gas
internal organs in the abdomen
Is a serious restriction in investigation of and
 Kidney, liver, spleen, pancreas, bladder, major blood
through gas-containing structures
vessels and of course, the foetus during pregnancy
 To present pictures of the thyroid gland, the eyes, the
breasts and a variety of other superficial structures
 Ultrasonic diagnostics has made possible the
detection of cysts, tumours or cancer in these organs

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Physics of Ultrasound Physics of Ultrasound
 Ultrasonic waves are sound waves associated with
frequencies above the audible range and generally  Information obtained by ultrasound, particularly in
extend upward from 20 kHz dynamic studies, cannot be acquired by any other
more convenient technique
 Ultrasonic waves can be easily focussed, i.e., they
are directional and beams can be obtained with very  Transmission of ultrasonic wave motion
little spreading  Longitudinal, transverse or shear

 They are inaudible and are suitable for applications  Medical ultrasonic diagnostic applications
where it is not advantageous to employ audible  Longitudinal mode of wave propagation is normally used
frequencies  Can be propagated in all types of media, viz. solids, liquids
and gases
 High frequency ultrasonic waves which are
associated with shorter wavelengths
 It is possible to investigate the properties of very small
structures.
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Physics of Ultrasound Characteristic Impedance
 Specific acoustic impedance of a medium is defined as the
Characteristic Impedance product of the density of the medium with the velocity of
sound in the same medium
Wavelength and Frequency
Velocity of Propagation  where z= specific acoustic impedance
 ρ = density of the medium
Absorption of Ultrasonic Energy
 V= velocity of sound in the medium
Beam Width
Resolution

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Characteristic Impedance Characteristic Impedance
 Percent of the incident wave energy which is reflected is This difference is so large that most of the
ultrasonic energy tends to be reflected at the
 Where
 Z1 = acoustic impedance of medium 1
interface
 Z2 = acoustic impedance of medium 2 Coupling medium like olive oil or special jelly is
 Approximate value of acoustic impedance for most of the used
biological materials or organs is the same
 1.6 X 105 g/cm2 s To minimize the energy reflection by providing an
 Greater the difference in acoustic impedance air-free path between ultrasonic transducer and
 Greater the amount of reflected energy skin
 Acoustic impedance of air and tissue
 42.8 g/ cm2s and 1.6 X l05 g/cm2 s respectively

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Wavelength and Frequency Wavelength and Frequency
Ultrasonics Ultrasonic waves are transmitted
As mechanical vibrations
Where V = propagation velocity of sound Pass only through a medium
n = frequency or number of cycles which pass any rf energy would be in the form of
given point in unit time Electromagnetic radiations
λ = wavelength, i.e., distance between any two No medium is necessary for propagation of
corresponding points on consecutive cycles energy and it would, therefore, pass even through
Ultrasonic frequencies employed for medical vacuum
applications range from 1 to 15 MHz
Also corresponds to radio frequencies (rf)

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Velocity of Propagation Velocity of Propagation
 Velocity of propagation of the wave motion
 Density of the medium it is travelling through and the
stiffness of the medium
 Given temperature and pressure
 Density and stiffness of the biological substances are
relatively constant
 Therefore, the sound velocity in them is also constant
 Velocity of sound in a particular medium is important
 Calculating the depth to which the sound wave has
penetrated before being reflected

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Velocity of Propagation Absorption of Ultrasonic Energy
 Velocity of ultrasound in all body tissues is almost  Reduction of amplitude of ultrasonic beam while
constant passing through a medium
 Therefore, the depth of penetration can be read  Can be due to its absorption by the medium
directly from the position of the echo pulse on the  Its deviation from the parallel beam by reflection,
calibrated time axis of the oscilloscope trace refraction, scattering and diffraction etc
 Relative intensity and the attenuation of an ultrasound
beam expressed in decibels (dB)
 Absorption coefficient α is normally quoted in dB/cm
 In soft tissues
 Depends strongly on the frequency
 Lower frequency ultrasonic signal would travel more than
the higher frequency signal
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Absorption of Ultrasonic Energy Beam Width
 Average value of sound absorption in soft tissues is of the  Ultrasonic waves are projected in a medium as a
order of 1 dB/cm/ MHz
beam
 Near and far fields
 Near field, within the first Fresnel zone
 Beam is cylindrical with little spread
 A series of maxima and minima are encountered in this
region
 As one travels out from the transducer which corresponds
to constructive and destructive interference

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Beam Width Beam Width
 Where r and λ are the radius of the transducer and Where n = frequency
the wavelength of the ultrasound respectively V = velocity of sound waves
 Far field D = diameter of the transducer
 Intensity of the beam reduces constantly with distance as
it spreads out due to the finite size of the source
cm diameter transducer
 Angle of divergence within a cone of semi-angle θ about Excited at 1 MHz has a near field of about 10 cm
the central axis in water and a semi-angle of divergence of 3.5
degrees
Beam shape may be modified by the use of
focusing elements in front of the transducer

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Resolution Resolution
 Ability to distinguish between closely related  Axial resolution is determined by the wavelength of
structures the transmitted pulse
 Axial and lateral resolution  This means that the smaller the wavelength, the higher the
frequency and better the axial resolution
 Axial Resolution
 Lateral Resolution
 Minimal axial distance
 Lateral distance, in a plane perpendicular to the beam axis,
 Parallel to the beam axis, at which two reflecting structures at which two reflecting structures can be seen as two
are recognized as separate structures separate structures
 Lateral resolution is determined by the shape/divergence of
the ultrasound beam, produced by the probe

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Generation and Detection of 27

Generation and Detection of 28

Ultrasound Ultrasound
 Piezoelectric effect  Materials with high mechanical Q factor
 Quartz, tourmaline and Rochelle salt  Suitable as transmitters
 Converting electrical energy into mechanical energy and  Low mechanical Q and high sensitivity are
vice versa
 Preferred as receivers
 Natural crystals
 Lead zirconate Titanate (PZT) crystals
 It is difficult to establish the appropriate axis and cut the
crystal in the required form  Much better than quartz crystals upto a frequency of about
15 MHz
 Quartz has generally been replaced by synthetic  High electro-mechanical conversion efficiency and low
piezoelectric materials namely intrinsic losses
 Barium nitrate and lead zirconate titanate  Operate at temperatures up to 100°C or higher and it is
stable over long periods of time
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Generation and Detection of 29

Generation and Detection of 30

Ultrasound Ultrasound
 Polyvinylidene difluoride (PVDF)
 Ferro-electric polymer that has been used effectively in high
frequency transducers
 Three parameters that are important in optimizing
transducers
 Frequency, active element diameter and focusing  Active Element Diameter (AED)
 Transducer face diameter increases
 Higher frequencies (10-15 MHz) are used for
superficial organs, such as the eye, where deep  Beam width decreases and therefore, lateral resolution
improve
penetration is not required

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Generation and Detection of Ultrasound
 Patients’ body the transducer is to be positioned
 Depth in the body to the structures of interest
 Focusing
 Minimizing the beam width and adjusting the focal zone to
give optimum results for a particular examination Basic Pulse-echo
Apparatus
 Acoustic lenses can be used to shape the ultrasonic beam
pattern
 Modern transducers are internally focused and externally
are of flat face

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Basic Pulse-echo Apparatus Basic Pulse-echo Apparatus
 Probe  Pulse-echo technique of using ultrasound for
 Pulse Repetition Frequency Generator diagnostic purposes in medical field
 Transmitter  Transmitter
 Receiver  Generates a train of short duration pulses at a repetition
frequency determined by the PRF generator
 Transmitter-Receiver Matching
 Converted into corresponding pulses of ultrasonic
 Wide Band Amplifier waves by a piezoelectric crystal acting as the
 Swept Gain Control transmitting transducer
 Detector  Echoes from the target or discontinuity are picked up
 Video Amplifier by the same transducer and amplified suitably for
 Time Delay Unit display on a cathode ray tube
 Display
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Basic Pulse-echo Apparatus Basic Pulse-echo Apparatus
 X plates of the CRT are driven by
 Time base which starts at the instant when the transmitter
radiates a pulse
 Probe
 Transducer consists of a piezo-electric crystal which
generates and detects ultrasonic pulses
 Barium titanate and lead zirconate titanate
 Crystal is cut in such a way that it is mechanically resonant of
an increased efficiency of conversion of electrical energy to
acoustic energy
 Transducer is excited at its resonance frequency
 It will continue to vibrate mechanically for some time after the
electrical signal ceases
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Basic Pulse-echo Apparatus

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Basic Pulse-echo Apparatus
 Transducer must have a good transient response
 Consequently a low Q is desirable
 Backing material is made thick enough for complete
absorption of the backward transmitted ultrasonic
waves
 Probes are designed
 To achieve the highest sensitivity and penetration
 Optimum focal characteristics and the best possible
resolution
 Single quarter wavelength design, however
 Provides optimal transmission of ultrasonic energy at a
particular wavelength only
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Basic Pulse-echo Apparatus Pulse Repetition Frequency Generator
 Single quarter wavelength matching layer transducer  Produces a train of pulses which control the
 Concave curvature sequence of events in the rest of the equipment
 Lead to air bubble entrapment or patient contact problems
 PRF is usually kept between 500 Hz to 3 kHz
 Multi-layer matching technology overcomes these
 Blocking oscillator or some form of the astable multi-
problems by interposing two layers between the
vibrator
piezo-electric element and body
 Width of the output pulse
 Two materials are chosen with acoustic impedances
 Order of a micro-second
between the values for ceramic and tissue
 Stepwise transition of impedance from about 30 for
ceramic to about 1.5 for tissue

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Pulse Repetition Frequency Generator

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 Astable – A free-running multivibrator that

Only a few micro-seconds are occupied by the has NO stable states but switches continuously
emission of the pulse between two states this action produces a train of
square wave pulses at a fixed frequency.
Transducer is free to act as a receiver for the
 Monostable – A one-shot multivibrator that has
remainder of the time
only ONE stable state and is triggered externally
Transmitter with it returning back to its first stable state.
Driven by a pulse from the PRF generator  Bistable – A flip-flop that has TWO stable states
Made to trigger an SCR circuit that produces a single pulse either positive or
Which discharges a capacitor through the piezo-electric negative in value.
crystal in the probe to generate an ultrasonic signal

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Basic Pulse-echo Apparatus Basic Pulse-echo Apparatus
 Under normal conditions  Voltage at ‘A’ will fall rapidly resulting in a short
 SCR is non-conducting duration
 Capacitor C1 can charge through the resistance R to the +V potential  High voltage pulse at ‘B’
 Short triggering positive pulse is applied to the gate of the
SCR
 This pulse appears across the crystal which
 It will fire and conduct for a short time
generates short duration ultrasonic pulse
 SCR 2N4203 can be used because
 Its high peak forward blocking voltage (700 V)
 High switching current capability (100 A)
 Fast turn-on time (100 ns)

Circuit diagram of a transmitter

used in pulse-echo application

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Basic Pulse-echo Apparatus Basic Pulse-echo Apparatus
 During reception, the presence of Rs degrades the signal-to-
 Receiver noise ratio due to signal attenuation
 Function of the receiver is to obtain the signal from the
 Johnson noise and increased receiver amplifier noise due to
transducer
raised source impedance
 To extract from it the best possible representation of an
echo pattern
 Receiver bandwidth is about twice the effective transducer
bandwidth
 Transmitter-Receiver Matching
 A common source-receiver of ultrasound - sensitive input
stage of the receiving amplifier must be protected from the
high voltage transmission pulse

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Basic Pulse-echo Apparatus Basic Pulse-echo Apparatus
 Improved circuit to provide more effective transmitter/  Wide Band Amplifier
receiver switching  Echo-signals received at the receiving transducer are in
the form of modulated carrier frequency and may be as
small as a few microvolts
 Sufficient amplification before being fed to a detector
circuit for extracting modulating signals which carry the
useful information
 Desirable gain of wide band amplifier is of the order of 80-
100 dB
 Amplifier does not operate in the non-linear regions with
large input signals
 Low noise level to receive echoes from deep targets

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Basic Pulse-echo Apparatus Basic Pulse-echo Apparatus
 Wide Band Amplifier  Swept Gain Control
 A dual gate MOSFET which is very suitable for high  Reduces the amplification for the first few centimetres of
frequency signals and provides a high input impedance to body tissue and progressively increases it to a maximum
the signals from the transducer for the weaker echoes from the distal zone
 Log amplifier is usually utilized  Detector
 One can see small relative differences in both low amplitude and  After the logarithmic amplification, the echo signals are
high amplitude echoes in the same image rectified in the detector circuit
 Swept Gain Control  Conventional diode-capacitor type with an inductive filter to
 Stronger echoes are received from the more proximal have additional filtering of the carrier frequency
zones under examination than from the deeper structures  Demodulator circuit - synchronous demodulation intended
 Receiving amplifier can only accept a limited range of for FM sound demodulation in television receivers
input signals without overloading and distortion

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Basic Pulse-echo Apparatus Basic Pulse-echo Apparatus
 Video Amplifier  Time Delay Unit
 Signal requires further amplification after its demodulation  SCR is fired
in the detector circuit before it can be given to the Y-plates  Start of the trace can be delayed by the time delay unit so
of the CRT that the trace can be expanded to obtain better display
 Output of the detector circuit is typically around 1 V
 Time Base
 But for display on the CRT, the signal must be amplified to
 Time base speed is adjusted
about 100 to 150 V
 So that echoes from the deepest structures of interest will appear on
 Amplifier must have a good transient response with the screen before the beam has completely traversed it
minimum possible overshoot  Speed of ultrasound in soft tissue to be about 1,500 m/s, a
 Video amplifier is the RC coupled type time of 13.3 µs must be allowed
 Horizontal sweep generator is controlled by the PRF
generator

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Basic Pulse-echo Apparatus

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Time Marker
Produces pulses that are a known time apart
Therefore correspond to a known distance apart
in human tissues
Marker pulses are given to the video amplifier and
then to the Y plates for display along with the
echoes
Display
CRT is not only a fast-acting device but also gives
a clear presentation of the received echo signals
‘Reject’ and ‘Damping’ controls
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