Ultrasound Physics & Instrumentation
4th Edition
Volume I
Companion Presentation
Frank R. Miele
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License Agreement
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Volume I Outline
 Chapter 1: Mathematics
 Chapter 2: Waves
 Chapter 3: Attenuation
 Chapter 4: Pulsed Wave
 Chapter 5: Transducers
 Chapter 6: System Operation
 Level 1
 Level 2
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Chapter 6: System Operation - Level 1
System operation deals with both the processing of the returning
echoes and the system controls.
Level 1 focuses on:
 general signal processing of the received radio frequency (RF) echoes
 the basic controls of receiver gain
 time gain compensation
 the concept of signal to noise ratio (SNR).
Level 2 focuses on overall system design, more in depth discussion about
compensation, scan conversion, compression, measurements, and display.
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System Major Subsystems
There are many functions that an ultrasound system performs. The
system is commonly subdivided into two major sub-systems:
 The Front End (often referred to as the receiver)
 The Back End (often referred to as the scan converter
In reality, there are many ways in which the system functions could be
partitioned including separating out the display monitor and the data storage
systems.
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Basic Processes of Real-Time Imaging
There are six core functions that an ultrasound system must perform:
 Transmit beams (Front end)
 Receive beams (Front end)
 Process the returned data (Front end and Back end)
 Perform measurements in the processed data (Back end)
 Display the processed data (Back end)
 Store the processed data (Back end)
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Basic System Functions
Fig. 6: (Pg 308)
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Transmitter (Pulser)
In Chapter 4 we learned about pulsed wave. In Chapter 5 we learned about
steering and focusing by phasing, and sequencing. The transmit beamformer
is responsible for creating all of the timing, phase delays, and transmit signals
which create each individual beam and, over time, a scan.
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The “Receiver”
The receiver of the front end of the ultrasound system performs many
functions such as:
 Amplification
 Compensation
 Compression
 Demodulation
 Rejection
(The above functions are the five functions that are generally included on the
credentialing exams. In reality there are many major functions not included
in this list.)
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The Receiver
In reality there are many more functions performed by the receivers
such as:
 A/D conversion
 beamforming (phasing and adding together each channel signal)
 frequency filtering
 parallel processing
 and many more
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The Concept of Signal to Noise Ratio
One of the most important concepts to learn is the concept of signal to
noise ratio (SNR).
The signal to noise ratio is a measure of how strong a signal is relative
to the background noise, or: the ratio of the signal amplitude to the
noise amplitude.
Note that a large signal does not guarantee a high quality image, since
even a large signal could be masked by noise. In essence poor SNR
can result from a low amplitude signal, a high noise level, or both.
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The Concept of Apparent SNR
A distinction should be made between true SNR and apparent SNR.
If the system settings are set inappropriately, the SNR may appear to
be poor, even when the true SNR is relatively good.
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Signal to Noise Ratio
Notice that in two of the three cases, the SNR is poor. In the middle picture,
the SNR is low because of high amplitude noise. In the picture on the right,
the SNR is poor because the signal has a low amplitude.
Fig. 1: (Pg 304)
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Apparent SNR
Although the SNR is
the same in both of
these cases, the first
case results in poor
apparent SNR,
whereas the second
case appears as
good SNR.
Fig. 2: (Pg 305)
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Varying Receive Gain & Apparent SNR
Both of the images were produced using the same transmit power but different
receive gain. The lower gain of Figure 3a results in poor apparent SNR,
whereas the higher receive gain of 3b appears as good SNR.
Fig. 3a: Signal Appears Weak
Fig. 3b: Signal Appears Strong
Fig. 3: (Pg 305)
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Varying Transmit Power & SNR
Figure 4a has poor SNR because of low transmit power. Figure 4b shows good
SNR as a result of higher transmit power. Both images used the same receive
gain.
Fig. 4a: Poor SNR (Weak Signal)
Fig. 4b: Good SNR
Fig. 4: (Pg 306)
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Too Much Transmit vs. Too Much Gain
Notice that Figure 5a appears bright but has good SNR as a result of high
transmit power. Figure 5b, appears too bright but has poor SNR as a result of
lower transmit and excessive receive gain.
Fig. 5a: Transmit too High but Good
Fig. 5b: Receive Gain too High,
SNR
SNR Good but Apparent SNR Worse
Fig. 5: (Pg 305)
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SNR (from Animation CD)
307 A: Poor SNR
307 D: Poor Apparent SNR
307 B: Good SNR
307 E: Good SNR
(Pg 307)
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307 C: Poor Apparent SNR
307 F: Poor Apparent SNR
Improving SNR
There are many ways of improving the signal strength.
 Increase transmit power
 Use a lower frequency transducer (for deeper imaging depths)
 Use a different imaging plane
 Maneuvers to remove attenuators such as lung and gas
 Move transmit focus deeper
 Use a larger aperture transducer (allows for deeper focus)
 Use semi-invasive techniques (“endo-probes” and transesophageal)
Note that increasing the receiver gain does not improve the SNR – it increases
both the signal and the noise proportionally.
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Raw RF Signal
The returning RF signals are changed (modulated) by the mechanical interaction
of the wave with the body. Larger acoustic impedance mismatches result in
higher amplitude signals. Signals from deeper depths (later in time) are also
attenuated more than reflections from shallower depths (earlier in time).
Fig. 7: (Pg 311)
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Amplified Raw RF Signal
Amplification is the process of multiplying the received signal to make the signal
larger. Amplification results in an increase of the amplitude for signals from all
depths uniformly.
Fig. 8: (Pg 311)
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Amplification Visualized
Amplitude
Amplitude
The image on the left provides a reference for the amplified image on
the right.
Time
Time
Raw RF Signal
Amplified Raw RF Signal
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Varying Receive Gain (Amplification)
9a: Severely Undergained
9d: Appropriately Gained
Fig. 9: (Pg 312)
9b: Badly Undergained
9e: Optimally Gained
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9c: Undergained
9f: Slightly Overgained
Amplification (Animation)
(Pg 312)
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Compensated RF Signal
More amplification is required for signals from deeper depths to compensate for
the increased attenuation. Notice in the image below that the signals later in
time are amplified more than the signals earlier in time (compare with image of
Figure 8).
Fig. 10: (Pg 313)
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Compensation Visualized
Amplitude
Amplitude
The image on the left provides a reference for the compensated image
on the right.
Time
Compensated RF Signal
Amplified Raw RF Signal
Compensation is performed by the system TGCs.
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Time
Receive Gain and TGC
Fig. 11: (Pg 314)
Fig. 12: (Pg 314)
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Incorrect TGC Settings
Inappropriate TGC settings can result in regions appearing too light or
too dark as seen in the two images below.
Fig. 13a: (Mid-range TGCs Too Low)
Fig. 13b: (Mid-range TGCs Too High)
Fig. 13: (Pg 314)
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Simple Compression Map
Compression is a method to reduce the dynamic range by mapping a larger
range of signals into a smaller range of signals. The following simplistic map
shows a 5 to 1 reduction in signal dynamic range.
Fig. 14: (Pg 315)
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Log Compressed RF Signal
Compression is needed since the dynamic range of the returning echoes
is much greater than the dynamic range visible to the human eye.
Through compression, the ratio of the maximum to the minimum signal is
significantly reduced.
Fig. 15: (Pg 316)
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Compression Visualized
Amplitude
Amplitude
The image on the left provides a reference for the amplified image on the right.
Time
Log Compressed RF Signal
Compensated RF Signal
Time
Compression in the receiver is not under user control. However there is more signal
compression which takes place in the back end of the system that is under user control.
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Various Compression Maps
16a:Most Contrast
16d
(Pg 317)
16b
16c
16e
16f: Least Contrast
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Compression (Animation)
(Pg 317)
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Rectification of RF Signal
The process of signal detection (demodulation) is actually comprised of two,
more fundamental steps: rectification and envelope detection. The following
image demonstrates the process of rectification, converting the signal from being
bi-polar to uni-polar.
Fig. 17: (Pg 318)
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Envelope Detection of RF Signal
The second stage of signal detection is envelope detection. The output of this
stage is basically the early form of ultrasound referred to as amplitude mode (Amode). Notice how the height of the amplitude corresponds to the amplitude of
the signal.
Fig. 18: (Pg 319)
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Demodulated Signal (A-mode)
In A-mode, the horizontal axis corresponds to time (which is related to depth)
and the vertical axis corresponds to the signal strength (amplitude).
Fig. 19: (Pg 319)
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Demodulation Visualized
Amplitude
Amplitude
The image on the left provides a reference for the demodulated image
on the right.
Log Compressed RF Signal
Time
Demodulated Signal (A-Mode)
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Time
Demodulated Signal after Rejection
The premise of rejection is that signals below a “threshold” are eliminated as too
weak to be of value. The reality is that “rejection” is more a natural limit that
results from noise in the image. Signals below the noise floor are masked by the
noise in the image.
Fig. 20: (Pg 320)
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Rejection Visualized
Amplitude
Amplitude
The image on the left provides a reference for the application of
rejection to the image on the right.
Demodulated Signal (A-Mode)
Time
Demodulated Signal After Rejection
Time
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A-mode from a Radial Artery
Fig. 21: (Pg 321)
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