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Nyquist limitPulseRepetitionFrequency= /2
1 Physics ofUltrasound
Fig. 1.19 Ultrasound
image of the thorax,
with multiple rib
shadows creating areas
of signal void. The
central rib and shadow
are outlined by a blue
line
Fig. 1.20 Ultrasound
view of the liver and gall
bladder, showing the
area deep to the gall
bladder (dotted red oval)
with a brighter signal
than the areas to the side
of it. The blue lines
represent the path of the
ultrasound beam
23
shift at which aliasing takes place is called the
Nyquist Limit. The Nyquist limit is equal to half
the pulse repetition frequency(PRF).
An example of aliasing is described here: If an
observer of a fast-turning clock hand is unable to
monitor the clock continuously but can take
snapshots of the face of the clock at certain intervals (limited sampling rate), she may not be able
to tell the direction of movement (clockwise or
counter-clockwise) if the sampling rate is too
slow. For the observer to be able to determine the
direction of movement, she needs to take snap-
24
Z. S. Shaman and F. S. Qadir
Fig. 1.21 A schematic of sampling rate and aliasing. In
the upper panel, the sampling rate (clock face snapshots)
occurs at more than twice the event rate (clock revolutions) which allows the observer to determine the rotation
Fig. 1.22 Aliasing in CF Doppler (left panel) and in PW
Doppler (right panel). In the left panel, the fast central
ow of the left ventricular outow in an apical view of the
heart is designated a color that is the opposite in direction
to the lower ow around it. In the right panel, the spectral
shots at more than twice the frequency of a full
revolution (see image below).
In ultrasound, aliasing shows as a reversal of
(or the inability to determine) the direction of
ow. In PW Doppler, the ow signal may appear
below the baseline when blood ows toward the
transducer at high speeds, such as in mitral valve
stenosis. Aliasing may also cause CF Doppler to
show a mix of colors in an arterial ow signal
when the pulse repetition frequency is set to
detect a slower venous ow (Fig.1.22).
rate and direction accurately. In the lower panel, because
the sampling rate is less than twice the event rate, the
observer is unable to determine the direction of clock rotation accurately
ow of the same area is assigned a positive direction while
it is generated by blood ow that is moving away from the
transducer. In both cases, the fast ow velocity causes frequency phase shifts that exceed the Nyquist limit
Because aliasing depends on the PRF,
reducing aliasing is contingent on the ability
of the operator to modify the PRF, and
includes:
– Increasing the PRF, which is mostly possible,
with certain limits
– Setting the sample volume at a shallower loca-
tion, which increases the PRF
– Selecting a lower frequency transducer, which
reduces the Doppler shift
1 Physics ofUltrasound
25
– Shifting the spectral analysis view baseline,
which applies a mathematical correction
– Using CW Doppler instead of PW or CF
Doppler because CW Doppler suffers no
aliasing
Questions
Please choose the best single answer
1. Traveling through soft tissue, which beam
will lose most energy echoing back to the
transducer from a 5cm reector?
A. The beam that takes 65 μs for the round
trip
B. The beam with the shortest wavelength
C. The beam with the highest amplitude
D. The beam with the longest pulse duration
E. The beam that has the longest period
The correct answer is B: The beam with
the shortest wavelength.
Explanation: All beams are assumed to
have the same speed of 1540 m/s in soft tissue. It is a good rule to remember that it takes
an ultrasound beam 13 μs to make a round trip
to and from a 1cm deep reector. Amplitude
and pulse duration do not affect attenuation.
Since attenuation = coefcient x distance x
frequency, and since the medium and the distance are the same, the only factor affecting
attenuation in this scenario is the frequency.
Because frequency and wavelength are
inversely related, the shortest wavelength
beam will have the highest frequency and will
suffer the most attenuation (option B is correct). The period is directly related to the
wavelength, therefore long wave periods will
result in lower frequency and lower
attenuation.
2. Which of the following occurs as a result of
decreasing the depth on the ultrasound image
from 20cm to 10cm?
A. The pulse period increases
B. The pulse duration decreases
C. The beam frequency increases
D. The duty factor increases
E. The spatial pulse length decreases
The correct answer is D: The duty factor
increases.
Explanation: Decreasing the depth
decreases the pulse period, which means that
the pulse repetition frequency increases. Pulse
duration (and spatial pulse length) are inherent characteristics of the transducer. Since the
duty factor = pulse duration/pulse period, the
duty factor increases as the depth of the scan
is decreased (option D is correct). The beam
frequency is not affected by the depth of the
image.
3. Axial resolution in the far (deep) eld can be
improved by which of the following?
A. Increasing the far-eld time gain
compensation
B. Moving the electronic focal zone farther
(deeper)
C. Using higher wavelength options to scan
D. Increasing the frame rate by narrowing the
sector
E. Switching to a higher frequency
transducer
The correct answer is E: Switching to a
higher frequency transducer.
Explanation: While higher gain will make
distant structures appear brighter on the
screen, post-processing manipulations do not
affect resolution. The focal zone affects lateral resolution and the frame rate affects temporal resolution, but neither will affect axial
resolution. Wave frequencies will affect the
spatial pulse length, and axial resolution =
SPL/2, so higher frequency transducers will
have a better axial resolution (option E is correct). Longer waves mean lower frequency
which is the opposite of what you need to
improve axial resolution, although you will
have better penetration into the tissue.
4. While examining the heart in an apical
5-chamber view using a phased array transducer, the structural details of the aortic valve
(which is a deep structure) can be improved
by which of the following maneuvers?
A. Turning on M-Mode imaging
B. Using tissue Doppler imaging
C. Increasing the near-eld time gain
compensation
D. Changing to a curvilinear transducer
E. Turning on tissue harmonics imaging
26
Z. S. Shaman and F. S. Qadir
The correct answer is E: Turning on tissue
harmonics imaging.
Explanation: It is true that M-mode imaging offers great axial and temporal resolution;
however, M-Mode imaging shows information from a single line of sight against time,
and this information is not useful when examining the structure of a valve. Again, tissue
Doppler imaging (in color tissue Doppler or
in pulsed wave tissue Doppler) will show
velocities but not structural details of the area
of interest. Increasing the time gain compensation is post processing and will make the
image brighter on the screen but does not add
to the details. However, in this case, since the
valve location is deep, changing the near eld
will not affect the area of interest. Because
curvilinear transducers have more soundemitting elements, they do offer better lateral
resolution than phased array transducers, but
in the case of an apical view of the heart, the
large footprint of the curvilinear transducer
will result in signicant rib shadowing and
image degradation. Tissue harmonics imaging
is the standard in cardiac ultrasonography
because it improves the signal-to-noise ratio
particularly from deep structures, improves
lateral resolution, and minimizes unwanted
artifacts.
5. Using a 10MHz linear transducer to examine
the carotid artery, the measured speed of
blood ow will be half the actual speed when
the angle of insonation is:
A. 15 degrees
B. 30 degrees
C. 45 degrees
D. 60 degrees
E. 75 degrees
The correct answer D: 60 degrees.
Explanation: Doppler shift = 2×reector
speed×source frequency×cosine the angle
of incidence (θ)/sound propagation speed.
Therefore, when estimating the speed of
blood ow using any particular ultrasound
beam frequency, and since the speed of sound
is assumed to be constant in soft tissue, the
Doppler shift in the beam reection will
depend on the cosine of the angle of
insonation. The table below show the cosine
values of representative angles. Note that
being “off” by 15 degrees results in an underestimation of velocity by 3% only (Table1.7).
6. While examining the blood ow in the carotid
artery using color ow Doppler, severe aliasing appears with mosaic color patterns that
limit the ability to interpret the ndings. How
can aliasing be reduced?
A. Increase transducer frequency
B. Increase pulse repetition frequency
C. Increase Doppler gain compensation
D. Increase the depth of the interrogation
eld
E. Increase the width of the interrogation
eld
The correct answer is B: Increase pulse
repetition frequency.
Explanation: Color Doppler aliasing can
be eliminated by elevating the Nyquist limit.
This can be achieved by decreasing the transducer frequency therefore lowering the frequency shifts (option A is incorrect), or by
increasing the pulse repetition frequency
(option B is correct). Changing the Doppler
gain will only change the color brightness on
the screen but will not change the color itself
(option C is incorrect). Increasing the size of
the interrogation eld will decrease the temporal resolution of the color ow Doppler but
will not affect the Nyquist limit (options D
and E are incorrect). Of note, decreasing the
depth of the area of interest would allow for
higher PRF and therefore decrease aliasing
(the option not presented).
7. An operator notices a linear hyperechoic density in the lumen of the descending aorta in
the short axis on a parasternal view of the
heart. How can she minimize the possibility
of an artifact causing the appearance of a dissection ap?
A. Let the area of interest fall deeper thanthe
focal zone
B. Widen the imaging sector
C. Turn off harmonics imaging
D. Examine the area in a different plane
E. Increase the Time Gain Compensation
1 Physics ofUltrasound
27
The correct answer is D: Examine the area
in a different plane.
Explanation: Side lobe and beam width
artifacts are possible reasons for a lateral
reector to appear as if it is generated by the
central beam. Decreasing scanning depth to
the focal zone will decrease the beam width
artifact (option A is incorrect). Narrowing the
sector width will improve lateral resolution by
increasing the number of scan lines in the area
of interest but does not affect the beam width
or side lobe artifacts (option B is incorrect).
The nonlinear relationship between the harmonic generation and the original ultrasound
wave can improve visualization and decrease
the amount of side-lobe clutter (option C is
incorrect). Viewing the structure of interest
through multiple acoustic windows at different
angles is unlikely to reproduce the same artifact in the same area (option D is correct).
Increasing the gain is the opposite of what one
should do because that will increase the brightness of the artifact (option E is incorrect).
8. When performing ultrasound-guided vascular
access, when is the best time to image the target vessel, and make necessary adjustments to
obtain an ideal image?
A. During procedure planning, prior to tak-
ing sterile precautions
B. After sterile precautions, prior to starting
a procedure
C. While performing the procedure, in real
time
D. Image adjustments are best done after the
procedure
E. None of the above. Machine default set-
tings are optimal
The correct answer is A: During procedure
planning, prior to taking sterile precautions.
When performing ultrasound-guided procedures, such as deep vascular access, procedural planning and setup are critical to the
success of the procedure. Obtaining an ultrasound image of the target vessel is best done
prior to taking sterile precautions and
equipment set up (option A is correct). The
proceduralist should use this time to identify
the correct probe, obtain the image, and per-
form adjustments to the image parameters
(e.g., depth and gain). These adjustments are
usually needed to ensure a large, and clear
eld of view to visualize the blood vessel and
the soft tissue around it (options D and E are
incorrect). The proceduralist can also identify
the sterile eld and mark the site (if indicated). Importantly, the proceduralist can also
use this time to assess the target vessel and
surrounding tissue in detail and look for
potential barriers (e.g., strictures and blood
clots), prior to equipment set up. This task
may be more difcult once sterile precautions
have been taken, as the ultrasound machine
may not always be within reach, and the
machine controls will not be part of the sterile
eld (option B is incorrect). It is not recommended to perform image adjustments while
performing the procedure, due to the risk of
losing view of the needle and increasing risk
of complications (option C is incorrect).
9. While attempting to obtain an ultrasound
image of a patient’s gall bladder, the sonographer encounters an air artifact. Which of the
following maneuvers can help the sonographer obtain the desired image?
A. Apply gentle pressure with the probe on
the abdomen
B. Slide the probe over the abdomen until an
image is obtained
C. Attempt to obtain the image from a differ-
ent anatomical landmark
D. Apply more ultrasound gel between the
transducer and the skin
E. All of the above
The correct answer is E: All of the above.
Explanation: During abdominal ultrasound, air artifacts are frequently encountered
due to bowel gas. As discussed above, ultrasound waves are disrupted by air, and this is
detrimental during abdominal imaging. All of
the above-mentioned techniques can be helpful in attaining the desired image (Option E is
correct). Applying gentle pressure on the
abdomen can displace air-containing bowel
loops between the probe and the target organ.
Fine movements such as sliding the probe can
change the angle of view and avoid the air
28
Z. S. Shaman and F. S. Qadir
containing bowel loops, as well as imaging
from a different anatomic landmark (laterally,
for example). Applying more ultrasound gel is
always a good idea when unable to obtain a
good image, as a lack of gel will prevent the
ultrasound waves from transmitting from the
probe into the soft tissue, resulting in poor
image quality and artifacts.
References
1. Lau VI, Jaidka A, Wiskar K, et al. Better
with ultrasound: transcranial Doppler. Chest.
2020;157:142–50.
2. NEMA. Standard for real-time display of thermal
and mechanical acoustic output indices on diagnostic
ultrasound equipment. Rockville, MD: The Institute;
1992.
3. Thomas JD, Rubin DN. Tissue harmonic imaging: why does it work? J Am Soc Echocardiogr.
1998;11:803–8.
4. Huang Q, Zeng Z. A review on real-time 3D ultrasound imaging technology. Biomed Res Int.
2017;2017:6027029.
5. Otto C. Textbook of clinical echocardiography.
Philadelphia, PA: Elsevier, Inc.; 2018.
6. Ellens N, Hynynen K.Frequency considerations for
deep ablation with high-intensity focused ultrasound:
a simulation study. Med Phys. 2015;42:4896–10.
7. Klein AL.Clinical echocardiography review a selfassessment tool. Philadelphia, PA: Wolters Kluwer/
Lippincott Williams & Wilkins Health; 2011.
8. Allison C, Cellum B, Karpinecz B, Nasrallah F,
Zderic V. Ultrasound-enhanced transcorneal drug
delivery for treatment of fungal keratitis. Cornea.
2022;41:894–900.
9. Anvari A, Forsberg F, Samir AE. A primer on the
physical principles of tissue harmonic imaging.
Radiographics. 2015;35:1955–64.
10. Radiopaedia. http://www.radiopaedia.org/. Accessed
7 Mar 2022. 2022.
11. Edelman S. Ultrasound physics and instrumentation. Woodlands, TX: Education for the Sonographic
Professional; 1999.
Probes andImaging Modes
First Harmonic
Amplitude
1.3 MHz 2.6 MHz
3.9 MHz
ChrisAllison
2
Learning Objectives
1. Describe the advantages and disadvantages of
each type of probe by frequency
2. Describe the potential pitfalls of harmonic
imaging
3. Identify strategies for obtaining optimal gain
4. Know the basic ultrasound imaging modes
and the physical principles behind them
5. Understand how Doppler images are obtained
Probe Selection
There is a plethora of ultrasound probes available
from various manufacturers. Probes generate ultrasound waves from an array of piezoelectric crystals
at the surface, i.e., “footprint,” of the probe.
When selecting a probe, one should ask two
simple questions: Is the structure you are visualizing supercial (for simplicity, less than about 5cm)
or deep? Is the sonographic window small, i.e.,
between ribs? If supercial, select a high- frequency,
linear probe. High-frequency probes are capable of
at least 10MHz (note probes typically have their
wavelength capabilities labeled on them).
1
At higher frequencies, and thus shorter wavelengths, the axial resolution is improved.
However, the higher frequency ultrasound beams
attenuate more per depth, preventing visualization of deep structures. Typical applications using
a high-frequency linear probe include vascular
access, lung pleura (e.g., pneumothorax), diaphragm thickness, DVT assessment, nerve
blocks, ocular ultrasound, and some musculoskeletal/soft tissue studies. Within high- frequency
linear probes, a higher frequency (e.g., 15MHz)
would be most useful for very small, supercial
structures such as the radial artery; whereas a
lower frequency (e.g., 8–10MHz) would enable
better visualization of the femoral and popliteal
veins in a larger patient (Fig. 2.1).
For visualizing deep structures, users choose
between a low-frequency curvilinear probe and a
phased-array probe (commonly called a “cardiac”
Second Harmonic
Third Harmonic
1
With the exception of some portable ultrasound devices
that use an alternative to piezoelectric crystals.
C. Allison (*)
Umass Chan Medical School—Baystate,
Springeld, MA, USA
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2025
M. J. Lanspa, A. T. Levinson (eds.), Echocardiography and Ultrasonography in the ICU,
Respiratory Medicine, https://doi.org/10.1007/978-3-031-80038-2_2
Fig. 2.1 A high-frequency linear probe (a) low- frequency
curvilinear probe (b), and a phased-array probe (c)
29
30
C. Allison
probe) . Phased-array probes differ from curvilinear and linear probes in that the piezoelectric
crystals sequentially generate ultrasound waves
with tiny delays along the probe, creating inference waves that steer the beam left and right; creating a larger eld of view from a smaller
footprint on the screen [1]. This allows for a
transducer footprint that ts between ribs, making phased-array probes preferred for cardiac and
other intrathoracic assessment. The curved
arrangement of crystals on curvilinear probes
allows for a broader eld of view. Pragmatically,
for the level of sophistication required for
physician- performed point-of-care-ultrasound, a
phased array probe can accomplish most studies
that would often be performed with a curvilinear
probe (FAST exam, gall bladder assessment).
An additional practical consideration is that
modern ultrasound machines have presets available based on the transducer that optimizes the
frequency and other settings for the selected
study. Cardiac presets will typically only be
available for phased-array probes.
Harmonic Imaging
Ultrasound transducers generate ultrasound
waves at a set frequency termed the fundamental
frequency. Unequal speeds of compression and
rarefaction during ultrasound transmission
through tissue result in the generation of additional higher frequency sound waves at multiples
of the fundamental frequency, known as harmonics. Because of the distortion of the fundamental
frequency, analyzing the harmonic frequencies
returning to the probe can actually result in a
higher resolution images with less noise [2, 3].
This is particularly useful when scanning through
adipose tissue. The disadvantage is that the harmonics are not generated very close to the transducer, making this less useful for supercial
imaging; also, the higher frequency harmonics
attenuate more quickly with depth, limiting their
use for imaging deep structures. Harmonic imaging mitigates reverberation artifacts, which generally improves image quality; however,
assessment for extravascular lung water through
sonographic B lines relies on artifact generation.
Limited clinical data suggest B line visualization
is improved with harmonic imaging [4].
Presets may use harmonic imaging. Some
modern machines will include a “lung” preset
with harmonics off to improve visualization of B
lines. Users will note diminished image quality if
such presets are used for other applications.
Imaging Modes
The default mode for ultrasound mode is known
as “B” mode AKA “brightness” mode AKA 2D
mode. (The naming has a historical basis in distinguishing this mode from “A” mode, which displays a graph of echo intensity at various depths)
(Fig.2.2). In the B mode, the ultrasound transducer footprint is displayed at the top of the
screen, creating an “x-axis” of the position along
the probe surface. The “y-axis” is time, but with
the estimation of the speed with which ultrasound
waves travel to and back from the tissue that
reects them, the y-axis is interpreted as depth.
The intensity of the ultrasound waves that return
at different time points (thus depths) is measured
by the machine and then displayed in grayscale,
with whiter being more intense and blacker less
intense.
This leads us to the rst two basic settings we
modify to optimize images: depth and gain.
Changing depth adjusts what amount of “ultrasound listening time” (thus depth) is displayed on
the screen. It is a good practice to start with more
depth than needed displayed on the screen to
ensure important information/pathology is not
missed in the far eld, then reduce the depth such
the entire structure of interest is visible without
wasted space deep to it. Gain changes what grayscale values for displayed pixels correspond to
absolute values of echo intensities received by
the probe; practically making the image brighter
or darker. Gain should be adjusted to optimize
contrast between tissue densities; too little or too
much gain reduces this (Fig.2.3—too much and
too little gain). A good rule of thumb is to have
the gain lowered just until a structure known to
be anechoic (thus black) such as the lumen of a
2 Probes andImaging Modes
Fig. 2.2 In B mode, the
transducer footprint is
displayed. Intensities of
echoes returning are
displayed in grayscale
31
ab c
Fig. 2.3 Gain is optimized in B mode so images are not too dark (a) or too bright (b), maximizing contrast (c)
blood vessel becomes black without artifactual
echoes within it. Time-gain compensation adjusts
the gain at different depths, allowing a consistent
image despite the attenuation of ultrasound intensity that has returned from the far eld.
M mode can be used to visualize and measure
moving structures. In this mode, pixels along a
line originating from a single point on the probe
surface are propagated over time (Fig.2.4—M
mode). The x-axis is time (in seconds, much
longer than the time it takes ultrasound to travel
to and from visualized tissue). The y-axis is
depth. Care must be taken to not move the transducer during M mode to ensure that changes in
the propagated line only represent movement of
the visualized structure itself. M mode is used to
measure the distances heart structures move during a cardiac cycle, fetal heart rates, and has been
used for assessment of pleural sliding. M mode
was particularly important historically when
32
Doppl
v
c
.
c
s.
Fig. 2.4 M mode shows
respiration variation in
the inferior vena cava.
The single line selected
by the user is propagated
over time, allowing
visualization and
measurement of moving
structures. Here, a
complete collapse of the
IVC is seen
C. Allison
exporting videos (for example, to VHS) was
cumbersome, as it allows movement to be displayed in a single still image. M mode remains
useful for measuring the movement of cardiac
structures, such as tricuspid annulus plane systolic excursion (TAPSE).
Doppler Imaging (See Chap. 7
forEchocardiographic Applications)
We are able to measure velocities of blood ow
and moving tissue using ultrasound by taking
advantage of the Doppler effect. If an object is
stationary, echoes will return to the transducer
with the same frequency. However echoes returning from a moving object after a change in frequency, known as the Doppler shift = f
where fr is the frequency of reected ultrasound
waves and fT is the frequency of transmitted
sound waves. Echoes returning from an object
moving toward the transducer have increased frequency, and those reected from an object moving away from the transducer have decreased
frequency (Fig. 2.5—showing Doppler shift,
waves closer together).
− fT,
r
The Doppler shift is predictable based on the
speed of sound in tissue (c) and object velocities
(v):
er shiftT=∗ ∗2 f
However, the Doppler shift also depends on the
angle at which the ultrasound waves strike the
moving object, known as the angle of insonation
(θ). Doppler shift is directly proportional to the
cosine of θ:
angle-corrected
Doppler shift
=∗ ∗∗2 f
v
co
T
θ
Recall that cos 0° = 1, so the Doppler shift is
unchanged if scanning a moving object “head on” but this is usually impractical. cos90° =0,
which corresponds to the intuitive fact that you
cannot detect a Doppler shift if scanning perpendicularly to the moving object. However, if the
angle of insonation exceeds 60°, these corrections become inaccurate to calculate velocities
[5]. Users can manipulate angle correction on
spectral Doppler studies [5] (Fig.2.6—angle correction in spectral Doppler).