Medical Imaging

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Medical Imaging
By Prof. M.M. Mohamed
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ULTRA
DIAGNOSTIC ULTRASOUND 2 - 10 MHz
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Frequency Range
Name
lt 20 Hz
Infrasound
20 Hz - 20 kHz
Audible Sound
gt 20 kHz
Ultrasound
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BUT...
Because of High Impedance Mismatch
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Historical Overview

• 1880... Piezoelectric effect

• 1887 Waves which are used in ultrasonic devices

• 1917 Transmit sound waves under the see water

• 1920 Study the interaction between ultrasound
  and living systems.

• 1940 - 1950 Slow evolutionary period.

• 1960s Increasing number of physicians accept
  ultrasound in the clinic.

• 1970s - until now Widespread use of
• ultrasound, as well as the development of new
  techniques.

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Ultrasound Applications

• Ultrasound is used in a variety of applications
  and devices.
• We see it used in everything from toothbrushes
  and TV remotes to that Ultrasound equipment that
  we most commonly see in the Medical environment.
• Of the this equipment the most commonly confused
  are Therapeutic and Medical Imaging. That is to
  say Ultrasound equipment used to stimulate tissue
  repair and that equipment that is used for
  Diagnostic Imaging have more differences than
  similarities.

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• One hertz simply means "one per second.
• Frequency is the measurement of the number of
  times that a repeated event occurs per unit time.
  It is also defined as the rate of change of phase
  of a sinusoidal waveform.
• Phase describes the current state of something
  that changes cyclically (oscillates).

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Acoustic Impedance, Z
As stated earlier, when an ultrasound wave meets
a boundary between two different materials some
of it is refracted and some is reflected. The
reflected wave is detected by the ultrasound
scanner and forms the image. The proportion of
the incident wave that is reflected depends on
the change in the acoustic impedance,
Z. Acoustic Impedance, Z of a medium is defined
as Z ?c Where ? the
density of the material, kgm-3 c
speed of sound in that material, ms-1
TASK What are the units of Z? See page 201/203
of textbook for typical values
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Characterization of Acoustic Wave
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Acoustic energy and intensity
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Acoustic Properties of Common Material
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Acoustic properties of tissues
Tissue / Ultrasound waves Interaction

• Absorption.

• Scattering.

• Attenuation.

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Reflection and Refraction Geometric
Characteristics
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1) Absorption Absorption mechanisms converts
the energy of an acoustic wave to heat as the
wave propagates through a medium. A plane
ultrasonic wave in an absorbing medium will lose
intensity as
2) Scattering The scattering of a wave on an
obstacle is a very complicated process, where it
depends on its cross-section.
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3) Attenuation
The term attenuation refers to loss in energy
from the ultrasonic beam passing through a length
of tissue.
db/cm for f gt 0
Where, f is the frequency, a(f) is the
frequency dependent attenuation coefficient, b
is the attenuation coefficient slope with
frequency, and n is the non linearity
frequency attenuation parameter.
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Ultrasound System

• Ultrasound systems must contain some form of the
  five system blocks.
• Display - The system will have some way of
  displaying the data it acquires.
• User Interface - It must have a user interface,
  this may be mechanical or voice activated.
• Transducer Your ultrasound system will have a
  transducer to convert electrical impulses to
  sound and back.
• Image Processing - The ultrasound machine will
  have some sort of image processing. This may be
  analog or digital.
• Power Supply - Finally it will have a power
  supply, again analog or digital.
• Peripherals ( may include cameras, or printers).

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Transducer

• Linear Array
• Sector Phased Array
• Vector Phased Array
• Linear Phased Array
• Curved Phased Array
• Mechanical Endo Cavity
• Phased Endo Cavity
• Mechanical, rotating wheel
• Mechanical, wobblers
• Mechanical, acoustic mirror

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Ultrasonic transducer

• In the case of ultrasound two transducer function
  are recognized
• conversion of ac electric oscillation into
  acoustic vibration, and
• Conversion of acoustic vibrations into ac
  oscillations of the same frequency.
• These two functions are the transmitter and
  receiver transducers.

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Piezoelectric materials

• Natural quartz,
• Barium titanate,
• Rochelle salts,and
• Lead zirconate titanate

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Piezoelectric effect
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Piezoelectric element (a) at rest, (b) defect
left, (c) defect right
(a)
(b)
(c)
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Intensity reflection coefficient, ?
At a boundary between mediums, the ratio of the
intensity reflected, Ir to the intensity
incident, I0 is known as the intensity reflection
coefficient, ?.
? Ir
I0 The
intensity of both the reflected and incident
ultrasound waves depend on the acoustic
impedance, Z of the two mediums. Therefore the
fraction of the wave intensity reflected can be
calculated for an ultrasound wave travelling from
medium 1, (acoustic impedance Z1) to medium 2
(acoustic impedance Z2). ? Ir Z2 - Z1

I0 Z2 Z1
If 2 mediums have a large difference in
impedance, then most of the wave is reflected.
If they have a similar impedance then none is
reflected.
2
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Impedance Matching / Gel
When ultrasound passes through two very different
materials the majority of it is reflected. This
happens between air and the body, meaning that
most ultrasound waves never enter the body. To
prevent this large difference in impedance a
coupling medium (gel) is used between the air and
the skin. The need to match up similar impedances
to ensure the waves pass through the body is
known as impedance matching.
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A-Scan

• A-Scan (Amplitude scan)
• Gives no photo image
• Pulses of ultrasound sent into the body,
  reflected ultrasound is detected and appear as
  vertical spikes on a CRO screen.
• The horizontal positions of the spikes indicate
  the time it took for the wave to be reflected.
• Commonly used to measure size of foetal head.

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B-Scan

• B-Scan (Brightness scan)
• An array of transducers are used and the
  ultrasound beam is spread out across the body.
• Returning waves are detected and appear as spots
  of varying brightness.
• These spots of brightness are used to build up a
  picture.

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The Doppler Effect
The apparent frequency of a wave increases when
the source of a wave is moving towards you.
http//paws.kettering.edu/drussell/Demos/doppler/
carhorn.wav
The Doppler effect can be used to measure blood
flow in adults, children and developing
babies. Both the time for the reflected
ultrasound wave and the new frequency of the
reflected wave are measured. This enables the
speed of blood flow to be calculated. The
greater the difference between the original
frequency and the reflected frequency, the
greater the speed. Computers then display this
info as moving images by updating data several
times per second.
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• Linear Array probes have a distinctive format.
• Sector phased array have a characteristic point
  to the image.
• Vector phased array images are similar but have
  flattened tops.
• Linear phased arrays have a rectangular image.
• Curved arrays are arched at the top.
• There are many other configuration, some very
  exotic

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• This is a simplistic view of a transducer but it
  contains all the basic elements.
• The cable provide the electrical connection.
• The strain relief supports the very fine coaxial
  cables in the cable.
• The case protects the internal crystal
  connections.
• The damping material isolates the crystal element
  from mechanical noise and provides mechanical
  support.
• The piezoelectric element convert electrical
  impulses to mechanical motion and back.
• The filler or lens provides mechanical isolation
  for the crystal element, impedance matching and
  its shape provides focus.

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Array construction

• Array construction contains the same basic parts.
• The main difference is in the Piezoelectric
  Material. Instead of a single crystal the array
  is sliced transversely to create a large number
  of small elements.
• Arrays of linear or phased construction are
  similar but differ when it comes to system
  construction.
• The filler material does more than protect the
  array, it is a specifically designed acoustic
  lens.

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Ultrasound Modes

• A Mode presents reflected ultrasound energy on a
  single line display. The strength of the
  reflected energy at nay particular depth is
  visualized as the amplitude of the waveform.
• B Mode converts A Mode information into a two
  dimensional grayscale display.
• C Mode is a color representation of blood flow
  velocity and direction.
• D Mode is a spectral representation of blood flow
  velocity and direction.
• P Mode is used to visualize very low blood flows
  in color. Unlike C Mode, this mode does not show
  the operator flow direction.
• Triplex is the simultaneous operation of B Mode,
  C Mode and D Mode.
• M Mode is a scrolling display allowing the
  operator to view and record organ motion.

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Axial Resolution

• Another concern is Resolution.
• Axial resolution is corresponds directly to the
  wave length characteristics of the Ultrasound
  wave. As frequency increases wave length shortens
  allowing for greater resolution. What we loose is
  penetration. Again as frequency increases
  penetration decreases. Higher frequencies also
  provide finer tissue grain or smoothness. A less
  grainy look.

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Lateral resolution

• In simple ultrasound systems Lateral resolution
  is attributed to physical focus characteristics
  of the crystal element. The concaved shape of the
  element provides focus to the beam and the width
  of the beam at any particular point effects the
  ability of the ultrasound system to resolve small
  objects that are side by side.

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Transverse resolution

• Transverse resolution is unique to the phase
  array probe. It is the ability of the probe to
  resolve objects side by side, as in lateral
  resolution, but in this case it is measured
  transverse to what would be considered the normal
  imaging plane. Again this is assuming simplest
  probe construction.

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Contrast Resolution

• The ability of the system to resolve adjacent
  bright reflectors is called contrast resolution.
  This is in small part due to the cumulative
  effects of axial and lateral resolution. The
  systems scan converter plays a large role is this
  characteristic.

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Diagnostic Ultrasonography
Displaying Monitor
Transducer / Probe
Keyboard
Probe Connector
Printer (B/W Color)
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TRANSDUCERS

• Device that can change one form of energy into
  another.

Piezoelectric property
Piezoelectric material

• The necessary element for generating acoustic
  waves.

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Piezoelectric effect
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Transducer Design
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Echoes from Two Interfaces
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Echoes from Internal Organ
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DISPLAY TECHNIQUES
A- mode
M- mode
B- mode
Doppler
Pulsed
Continuos
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1) A- mode
Block diagram of an A-scan instrument. A pulser
circuit triggers the transducer, and the
saw-tooth generator. The T/R switch isolates the
receiver amplifier during transmission.
Amplitudes of the received echo signals are
presented on the vertical axis of the CRT.
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2) B-mode
Brightness modulation
CRT
Vert.
Time variable gain
Amp
Horiz.
Pulser circuit
Beam steering control unit
T/R switch
Saw tooth voltage sweep
A pulser circuit triggers the transducer, and the
saw tooth generator. The T/R switch isolates the
receiver amplifier during transmission. For each
scanning line, the amplitudes of the received
echo signals are modulated to brightness.
Steering unit is controlling the synchronization
process.
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3) M- mode
Slow voltage ramp
CRT
B
Vert.
Time variable gain
Amp
Horiz.
Pulser circuit
Sawtooth voltage sweep
Trigger
T/R switch
Body
B
A
Fixed organ
Transducer
Moving Organ
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TRANSDUCER MODES
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TRANSDUCERS TYPES
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TRANSDUCERS TYPES
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Linear Probe Image
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Sector (Phased array) Probe
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Convex Probe Image
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Real Time 3D
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Fetal Spine
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Reconstructional 3-D
Obstetrics
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Ultrasound Machines
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Ultrasound Machines

• Function
• Diagnostic ultrasound machines are used to give
  images of structures within the body. This
  chapter does not deal with other kinds of machine
  (e.g. therapeutic and lithotripsy). The
  diagnostic machine probes, which produce the
  ultrasound, come in a variety of sizes and
  styles, each type being produced for a particular
  special use. Some require a large trolley for all
  the parts of the unit, while the smallest come in
  a small box with only a audio loudspeaker as
  output. They may be found in cardiology,
  maternity, outpatients and radiology departments
  and will often have a printer attached for
  recording images. Unlike X-rays, ultrasound poses
  no danger to the human body.
• How it works
• The ultrasound probe contains a crystal that
  sends out bursts of high frequency vibrations
  that pass through gel and on through the body.
  Soft tissue and bone reflect echoes back to the
  probe, while pockets of liquid pass the
  ultrasound straight through. The echoes are
  picked up and arranged into an image displayed on
  a screen. The machine offers a number of
  processing options for the signal and image and
  also allows the user to measure physical features
  displayed on the screen. This requires the
  machine to incorporate a computer.

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How TO CHOOSE TRANSDUCER
1) FREQUENCY
2) SCANNING ANGLE
3) RADIUS or LENGTH
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WHAT WE HAVE TO KNOW BEFORE PURCHASING A G.P.U.S
WHAT TYPES OF EXAMINATION ARE TO BE CARRIED OUT ?
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1. TRANSDUCER Curvilinear or combination of
linear and sector.
2. FREQUENCY Standard transducer should have
central frequency of 3.5 MHz.
3. ANGLE for Sector probe should be 40 or more,
linear array should be 5 - 8 cm
long.
4. FRAM RATE 15 - 30 Hz for linear array,
5 - 10 Hz for sector array.
5. FRAM FREEZE DENSITY at least 5125124 bits
to provide 16 gray levels
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6. ELECTRONIC CALIPERS one pair at least, with
Quantitative readout.
7. ADD DATA IS POSSIBLE patient identification,
hospital name, date of examination etc.
8. HARD COPY should be possible.
9. MONITOR at least 13 cm 10 cm (preferably
larger)
10. STABILIZING should be able to stabilize
voltage variation of /- 10.
11. Biometric tables (it may not be universal
and should be adjusted for local standards.
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WHAT WE HAVE TO CHECK WHEN WE RECEIVE THE SCANER
SERVICE MANUAL
USERS MANUAL
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Checking every instruction in the manual may
takes time. But if you do so, you will save time,
money, and frustration.
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CHECK LIST
1. Voltage setting should be compatible with the
electrical supply.
2. Interference on the screen/ whit sparks.
3. Transducer and cables test.
4. Check the cursor / measuring length,
5. Check the accessibility of the biometrics or
measurement tables.
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IMAGE ARTIFACTS
Any missing or distorted image that does not
match the real image of the part being examined
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ARTIFACT'S CAUSES

• Acoustic characteristics of the tissues.
• Scanners settings.
• Lack of users experience.
• Defected part within the scanner.

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To confirm any suspected abnormality, Use
multiple projections at different angles

• COMMON ARTIFACTS

• Cysts artifact (strong back-wall effect).
• Abdominal wall artifact.
• Gas artifact.
• Reverberation artifact.
• Incomplete imaging artifact.
• Gain artifact.
• Shadows artifact.

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DAILY CHECKS "visual"

• Visually inspects all transducers.
• Cable, cracked surface, punctured,
  discolored casing

• Visually inspect the power cords.

• Verify that the trackball and DGC controls
  appears clean and free from gel or other
  contaminants.

Once the system is powered on
Verify that the monitor displays CORRECTLY the
connected transducers identification, current
date, time.
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DAILY ADJUSTMENT

• FOCUS.
• DEPTH GAIN COMPENSATION.
• OVERALL GAIN.
• ZOOM.
• MONITOR (B/C).

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CLEANING AND DISINFECTING