Transcript Ultrasound

Medical Imaging
By
Prof. M.M. Mohamed
WHAT IS ULTRA SOUND?
Audible
Sound
Up to 20 kHz
Ultrasound
2-10 MHz
DIAGNOSTIC ULTRASOUND
2 - 10 MHz
Frequency Range
Name
< 20 Hz
Infrasound
20 Hz - 20 kHz Audible Sound
> 20 kHz
Ultrasound
X-RAYs
NUCLEAR
MRI
ULTRASOUND
NONINVASIVE
NON-TRAUMATIC
LESS EXPENSIVE
BUT……………...
• Cannot penetrate mature
adult bone (brain).
• Cannot penetrate any
substantial gas-layer.
Because of High Impedance Mismatch
• 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.
• 1960’s… Increasing number of physicians
accept ultrasound in the clinic.
• 1970’s - until now… Widespread use of
ultrasound, as well as the development of new techniques.
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.
• 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).
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
Characterization of Acoustic
Wave
Acoustic energy and intensity
Acoustic Properties of Common
Material
Acoustic properties of tissues
Tissue / Ultrasound waves Interaction
• Absorption.
• Scattering.
• Attenuation.
Reflection and Refraction:
Geometric
Characteristics
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
I ( x)  I 0 e
2x
2) Scattering
The scattering of a wave on an obstacle is a very complicated
process, where it depends on its cross-section.
The term attenuation refers to loss in energy from
the ultrasonic beam passing through a length of
tissue.
 ( f )  f
n
db/cm
for f > 0
Where,
f
a(f)
b
n
is the frequency,
is the frequency dependent attenuation coefficient,
is the attenuation coefficient slope with frequency, and
is the non linearity frequency attenuation parameter.
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).
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
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.
Piezoelectric materials
•
•
•
•
Natural quartz,
Barium titanate,
Rochelle salts,and
Lead zirconate titanate
(A)
-
(B)
(C)
+ + +
- - - - - - - - - - - - - -
+
+
-
+
+ + + + + + + + +
+ + + + + +
+
+
+
-
- - - - - - - - - - - - - -
Piezoelectric element (a) at rest, (b)
defect left, (c) defect right
(a)
(b)
(c)
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).
 = I r = Z2 - Z1
I0
Z2 + Z 1
2
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.
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.
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.
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.
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.
• 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
• 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 it’s shape
provides focus.
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.
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.
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.
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.
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.
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.
Diagnostic Ultrasonography
Displaying Monitor
Transducer / Probe
Keyboard
Probe Connector
Printer (B/W & Color)
• Device that can change one form of energy into another.
Electrical excitation
into
motion and pressure.
• The necessary element for generating acoustic waves.
(A)
-
+
+
+
(B)
(C)
+ + +
- - - - - - - - - - - - - -
+
+
-
+
+ + + + + + + + +
+ + + + + +
+
+
+
-
- - - - - - - - - - - - - -
Transducer Design
Echoes from Two Interfaces
Echoes from Internal Organ
A- mode
M- mode
B- mode
Doppler
Pulsed
Continuos
CRT
V
Time variable gain
Amp
H.
Pulser
T/R
Saw tooth voltage
sweep
Trigger
switch
Body
Organ
Transducer
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.
Brightness
modulation
CRT
Time variable gain
Vert.
Amp
Horiz.
Pulser circuit
T/R
switch
Beam steering control
unit
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.
Slow voltage ramp
CRT
B
Time variable gain
Vert.
Amp
Horiz.
Pulser circuit
T/R
Sawtooth voltage
sweep
Trigge
r
switch
Body
B
A
Transducer
Fixed
organ
Moving
Organ
TRANSMITTER
RECEIVER
PULSED
CONTINUES
Linear Probe Image
Sector (Phased array) Probe
Convex Probe Image
Real Time 3D
Fetal Spine
Reconstructional 3-D
Obstetrics
Ultrasound Machines
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.
WHAT TYPES OF EXAMINATION ARE TO BE
CARRIED OUT ?
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 512*512*4 bits to
provide 16 gray levels
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.
WHAT WE HAVE TO CHECK
WHEN WE RECEIVE THE SCANER
USER’S MANUAL
SERVICE MANUAL
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.
Any missing or distorted image that does not
match the real image of the part being examined
• Acoustic characteristics of the tissues.
• Scanner’s settings.
• Lack of user’s experience.
• Defected part within the scanner.
• COMMON ARTIFACTS:
• Cyst’s artifact (strong back-wall effect).
• Abdominal wall artifact.
• Gas artifact.
• Reverberation artifact.
• Incomplete imaging artifact.
• Gain artifact.
• Shadows artifact.
• 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 transducer’s identification, current date, time.
• FOCUS.
• DEPTH GAIN COMPENSATION.
• OVERALL GAIN.
• ZOOM.
• MONITOR (B/C).
TRANSDUCERS
MAIN UNIT
Linear, convex, …..
Surface of the system.
Endocavity, interoperative, …
DGC slides control.
Trackball.
Unit filters.