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1 Physics ofUltrasound
13
Bioeects ofUltrasound
Ultrasound is considered a relatively safe noninvasive diagnostic tool. However, that does not mean
it comes without potential harm. There are bioeffects that result from the interaction of ultrasound
energy with tissue. These can be classied as
Thermal effects and Mechanical effects.
Beginning in 1993, an output display standard that
reports the bioeffects of ultrasound on tissue was
required by the FDA.The standard display of the
bioeffects of ultrasound was necessary to allow
machines to operate at higher energy outputs, and
to have greater diagnostic capabilities than previously possible. The display of the indices of thermal and mechanical effects of ultrasound is now
standard, and they serve as a real-time alert to the
operator so that they can implement the As Low As
Reasonably Achievable (ALARA) principle and
use this diagnostic tool safely.
Thermal Eects
extreme temperature increases, free radical generation, and even tissue liquefaction. The
mechanical effect of ultrasound on tissue is quantied and reported as the “Mechanical Index,”
which quanties the likelihood of ultrasound
exposure producing cavitation. The FDA requires
the mechanical index to remain below 1.9 for
safe diagnostic ultrasound use.
The mechanical index (MI) = Peak rarefactional
pressure/sqrt of
frequency
The mechanical effect of ultrasound has signicant diagnostic benets when using contrast
media designed for ultrasound. There is also a
potential therapeutic benet of cavitation in
intraocular drug delivery which is likely related
to the transient, reversible cavitation effects of
ultrasound on the cornea (evolving evidence).
Imaging Modes andProbes
As ultrasound propagates through tissue, it is
converted from sound to other forms of energy
through the process of attenuation. Most of this
conversion is into heat energy, resulting in a thermal effect which can be harmful to sensitive tissues (e.g., fetal tissue). The heat-generating
capacity of ultrasound is quantied and reported
as the “Thermal Index.”
The thermal index (TI) = power generated by
the transducer/power required to raise tissue temperature by 1°C.
Mechanical Eects
The mechanical effect of ultrasound refers to
damage caused by the oscillation of tissue as a
result of a sound wave passing through it. The
most common mechanical effect on tissue is
referred to as “cavitation.” Cavitation is caused
by the oscillation of small gas bubbles within the
ultrasound eld. In certain circumstances, these
bubbles may grow in size or collapse, generating
high energies that affect adjacent tissue, causing
Ultrasound imaging can be used in multiple ways
to create different imaging modes. Each mode
comes with unique characteristics that are well
exploited in different clinical situations.
Two-Dimensional andThreeDimensional Imaging
As mentioned in the prior section, diagnostic
ultrasound imaging uses short pulses of sound to
create representations of the anatomic structures.
These pulses of sound interact with the tissue and
are reected back to the ultrasound system along
individual scan lines. These scan lines “sweep”
through a sector of tissue, and the reections
received by the system are analyzed and displayed using a gray scale. This scale is based on
the intensity of the ultrasound beams returning to
the system, ranging from black (no ultrasound
signal) to white (maximum signal intensity). The
location (or depth) of a reecting structure on the
image is determined by the transit time to and
from the reector.
14
Z. S. Shaman and F. S. Qadir
Scan lines are then repeated and assembled
into a frame, creating a two-dimensional (2-D)
image. The process is then repeated to create
more frames, making the image come to life.
Two-dimensional imaging is the most used and
easily recognizable mode of ultrasound imaging.
Other than being the most used imaging technique, 2-D imaging allows for a wide view of the
underlying structures in addition to signicant
details of the examination area. This balance of
breadth makes 2-D imaging useful for most clinical applications.
Three-dimensional (3-D) imaging is similar
to 2-D imaging in concept, with the addition of
multiple scan slices to create a three-dimensional image (Fig.1.10). In 3-D imaging, multiple swaths of tissue are imaged along the
perpendicular axis of the 2-D scan (the z-axis)
and are reconstructed to be displayed on the
screen using 3-D rendering. This can be
achieved by using arrays of sound-generating
elements or by mechanically moving regular
linear arrays of elements. This imaging modality is most commonly used in cardiac and
obstetric imaging.
M-Mode Imaging
In motion mode (M-Mode) imaging, the information from a single scan line is plotted against
time to create an image. Structures along the scan
line are displayed vertically according to their
depth, while the horizontal axis on the screen
represents time. Given that the signal is sent and
received along a single scan line, multiple signals
can be sent at a high rate, giving M-Mode imaging an excellent temporal, as well as axial
resolution.
For optimal use of M-mode, it is important to
start the examination in 2-D mode, and then to
align the M-Mode scan indicator along the area
of interest before activating M-Mode imaging.
While M-Mode imaging allows for higher
resolution, it does sacrice the breadth of the
examination. When the details of movement
along the axis of the beam are of signicant interest, M-Mode imaging becomes very useful.
Therefore, M-Mode is often used in evaluating
the movement of heart valves and cardiac walls,
in addition to applications related to vascular,
diaphragmatic, and pleural changes relative to
Fig. 1.10 Schematic of
the waya threedimensional image is
created by adding
multiple layers of the
x-axis and y-axis
two-dimensional scans
along the z-axis
ab
()
1 Physics ofUltrasound
15
Fig. 1.11 Schematic showing the interaction of ultrasound with a moving reector creating a positive and a
negative Doppler shift. Panel A represents a positive shift
respirations. For example, M-Mode imaging is
used to assess cardiac parameters such as tricuspid annular plane systolic excursion (TAPSE),
assessing changes in large vessel caliber (respiratory variability of the IVC), and visceral pleural
sliding against the parietal pleura during assessment for pneumothorax.
The Doppler Principle
So far, we have presented sound wave echoes
bouncing off stationary reectors. However,
sometimes sound waves reect after interacting
with a moving reector. Here, the reector causes
an additional effect on the wave, changing its fre-
(depicted as a red arrow) and panel B represents a negative shift (depicted as a blue arrow)
quency. This change in frequency is called the
Doppler shift (Fig. 1.11). The term “Doppler
effect” was coined by the Austrian mathematician and physicist Christian Doppler who
described how light wave frequency depends on
the relative speed of binary stars. Similarly, in
ultrasound, the Doppler effect is dened as the
change in frequency of a sound wave transmitted
from a source that is in motion relative to the
receiver.
By comparing the frequency of returned ultrasound waves relative to transmitted ones, the
Doppler effect can be used to detect the motion of
tissue and uid being imaged and to measure
their velocity. The velocity of the moving reector can be calculated using this equation:
reflector speed source frequencycosinethe a
Doppler shift
×× ×2
=
sound propagation speed
Therefore, the change in the frequency of the
beam is proportional to the magnitude of the relative velocity of the source (reector) and the
receiver to each other. Also, if the direction of
motion of the reecting structure is at an angle
(θ) to the ultrasound beam (angle of insonation),
Note that the Doppler shift is bidirectional.
The shift is considered positive (resulting in a
higher frequency) if the reector and the receiver
are moving toward each other, or negative (resulting in a lower frequency) if the reector and the
receiver are moving away from each other.
nngleofincidence
the Doppler shift will be a fraction (cosine θ) of
the expected shift(Table1.7). Hence, if the direction of motion is perpendicular to the incident
Doppler Imaging
beam (at 90 degrees), then the relative velocity of
0), leading to no Doppler shift, and no motion
will be detected by the ultrasound system
(Fig.1.12).
the reector to the probe will be nil (cosine 90° =
Ultrasound systems use the Doppler principle to
measure the velocity of reectors in motion.
This is done by continuous measurement (called
Continuous Wave Doppler) or intermittent
θ
16
Z. S. Shaman and F. S. Qadir
Table 1.7
Insonation angle (θ) Cosine θ
0° 1
15° 0.97
30° 0.87
45° 0.71
60° 0.5
75° 0.26
90° 0
Fig. 1.12 Schematic showing the interaction of ultrasound with a moving reector creating no Doppler shift.
Panel C shows that when the sound beam interacts with a
moving reector at a right angle (90 degrees) there will be
no frequency shift between the incident beam and the
returned beam
The values of the cosine of select angles
measurement of the frequency of the reected
waves. When intermittent measurement is performed, the results are presented in a quantitative (called Pulsed Wave Doppler) or in a
semiquantitative way (called Color Flow
Doppler).
There are, of course, advantages and disad-
vantages to each of the three modes (Table1.8).
Continuous Wave (CW) Doppler
In this mode, the ultrasound uses two crystals,
one to continuously emit ultrasound signals,
and one to continuously receive signals. This
allows an accurate detection of Doppler shifts
and therefore speed but lacks localization of
the reectors. CW Doppler imaging accurately
measures high velocities such as regurgitant
jets across heart valves; however, the location
of movement along the length of the beam is
unknowable. This depth uncertainty is called
range ambiguity. Therefore, in CW Doppler,
velocity is displayed most accurately while the
location of the moving structures is now
known.
Pulsed Wave (PW) Doppler
In this mode, one crystal alternates between emitting ultrasound pulses and receiving echoes.
Ultrasound reections from a specic area of
interrogation, called “sample volume” or “gate,”
are analyzed. This localization of the area of
interrogation is determined by the operator and is
called “range gating” because the depth (or
range) of frequencies that are analyzed is limited
to a specic area (or gate). Range gating eliminates range ambiguity; however, high velocities
may not be accurately detected, because of an
artifact called aliasing, which is described below.
Therefore, in PW Doppler there is information
about both the velocity and the location of the
movement, but both are somewhat limited in
accuracy.
When using pulsed wave and continuous
Doppler, the ultrasound system analyzes the frequency shift of the incoming sound waves, yielding a graphical representation of ow velocity over
time, and is called spectral analysis. Spectral
Doppler analysis allows measurements to be made
of the ow velocity and the relationship of velocity
to time. Spectral analysis of ow is often used in
vascular and in cardiac ultrasound imaging.
Color Flow (CF) Doppler
This is a form of pulsed wave Doppler, in which
the frequencies of the pulses of reected sound,
from multiple sample volumes, are analyzed.
The ow at each sampling point is displayed as
BART Blue AwayRed Toward=− −,
1 Physics ofUltrasound
17
Table 1.8
modes of ultrasound
Doppler imaging mode Velocity accuracy Location of measurement
Continuous wave (CW) Accurate, no aliasing at any speed Along a single scan line but at all
Pulsed wave (PW) Limited, may be affected by aliasing Only at the sample volume of the
Color ow (CF) Semiquantitative, may be affected
The benets of each mode are presented as relative “+” signs
a color map of speed and direction. The color
map is presented on the screen overlaying a conventional 2-D image. Color Doppler gives infor-
Comparing the accuracy of velocity measurement and the location of measurement of different Doppler
points of the line
+++ +
single scan line
++ ++
by aliasing
+ +++
At multiple sample volumes along
multiple scan lines
myocardial motion speed can be detected. Tissue
Doppler imaging is commonly used as PW and
Color modalities.
mation about velocity in a semiquantitative way
only. It is by convention that the color used to
represent positive frequency shift is red and the
color used to represent negative frequency shift
is blue. Yellow hue color transitions from red
and blue are used in a semiquantitative way to
represent higher frequency shifts in both directions. The acronym “BART” is one way to
remember that in CF Doppler “Blue” means the
movement is “Away” from the transducer, and
“Red” means the movement is “Toward” the
transducer. Yellow color is used to represent
Pulsed Wave TDI
This is similar to a traditional pulse wave doppler. The ultrasound system uses lters specically designed to pick up low-velocity tissue
structures and is therefore able to provide a precise measurement of tissue velocity within the
designated sample volume. This modality is primarily used in the assessment of cardiac left ventricular diastolic dysfunction, where the sample
volumes are set at the insertion points of the
mitral valve annulus [7].
higher positive, as well as, negative frequency
shifts. While CF Doppler allows an accurate
localization of ow, velocity determination is
analyzed in a semiquantitative way only.
Therefore, speed accuracy is sacriced for
information about location.
Color TDI
The ultrasound system superimposes color-coded
tissue velocity images onto the 2-D image in real
time. The system then displays the mean velocity
of each pixel against time. This is useful in comparative assessment of regional velocity as well
as direction but compromises on quantitative
assessment. This is helpful, for example, in
assessing regional wall motion abnormalities of
Tissue Doppler Imaging (TDI)
the left ventricle.
Color ow, pulse wave, and continuous wave
Doppler imaging are usually used to measure
blood ow velocity [6]. Cardiac structures (the
myocardium in particular) move at a much slower
velocity when compared to blood ow, therefore,
special Doppler settings are needed to detect relatively slower tissue motion. When the transducer is set to detect small shifts in frequency,
Tissue Harmonics Imaging (THI)
When ultrasound energy interacts with the tissue, it returns to the transducer echoes of the
original frequency as well as echoes of multiples
of the original frequency (called harmonic frequencies). This occurs because of nonlinear
18
Z. S. Shaman and F. S. Qadir
propagation of ultrasound waves throughtissue.
These returning harmonic frequencies can be
detected by the ultrasound system. Harmonic
frequencies are then separated from the main
beam frequency and are amplied to generate an
image{Anvari, 2015 #12;Anvari, 2015
#12;Zderic, 2004 #23}. This mode of imaging
offers several advantages [8]:
– Improved signal-to-noise ratio which results
in better denition of subtle parenchymal
details
– Improved lateral resolution becausethe effects
of harmonic waves are most prominantat the
center of the beam therefore resulting in a narrower imaging plane and improving lateral
resolution
– Minimizing reverberation, side lobe, and grat-
ing lobe artifacts
– Minimize near-eld noise because harmonic
waves are not produced by supercial
structures
– Improved imaging of deeper tissue because
the effects ofharmonic waves are most prominant atdeeper structures
Because of the benets above, harmonic
imaging is standard in cardiac ultrasound
examination
greater penetration of tissue, and hence allows
for an accurate assessment of deeper structures in
the abdomen and pelvis.
Linear Array Transducers
Also known simply as linear transducers, these
use crystals arranged along a straight line on the
surface. These transducers generate highfrequency parallel beams, allowing for a greater
and uniform near-eld resolution, although at the
expense of penetration. A rectangular 2-D image
is formed by this probe. Linear probes are best
utilized to assess vascular structures during
ultrasound- guided vascular access [9].
Phased Array Transducers
Also known as sector array transducers, these
are designed specically for cardiac imaging.
The crystals used are smaller and are arranged
in a more compact arrangement, either horizontal or circular. The transducer utilizes variable
timing of crystal activation, resulting in a fanshaped image generation. The shape of the
ultrasound beam, as well as the narrow footprint
of the transducer, makes it ideal for imaging
through narrow windows, such as the intercostal
spaces, in cardiac imaging. These transducers
allow imagingat increased depth, although that
compromises signicantly on near-eld resolution [7].
Probe Selection
There are many transducer types that are generally available in ultrasound systems. Each type is
specically designed for optimal imaging of a
particular body region. The most commonly used
transducers in the assessment of critically ill
patients are three.
Curved Linear Array Transducers
Also known as convex transducers, these are
designed for abdominal and pelvic ultrasonography. Sound emitting elements are arranged side
by side along a curved surface, allowing for a
cone-shaped, wide eld of view, which gets
wider at an increased depth. It also allows for
good far-eld resolution. The beam generated is
generally of low frequency, which allows for
Ultrasound Artifacts (See Chap. 3)
Ultrasound artifacts are images that do not accurately represent the structures underneath the
transducer. Artifacts occur when the assumptions
behind image generation deviate from the reality
of space/time, attenuation, and/or frequency
(Doppler) shifts.
Artifacts are frequently encountered in diagnostic ultrasound, and while some are unwanted,
others may provide valuable information related
to the structural features of the underlying tissue.
Therefore, it is important to recognize and to
understand common artifacts. This will help
avoid image misinterpretation.
Once an artifact is detected, it is not too difcult to deduce how the artifact occurred. The
1 Physics ofUltrasound
Fig. 1.13 A subcostal
ultrasound view of the
heart. The left atrium
and the mitral valve
(solid oval) are mirrored
and appear deep to the
pericardial line (dotted
oval). This occurs
because the ultrasound
beam reects between
the pericardium and
other structures in the
heart
19
challenge, however, is to decide if an artifact
exists to begin with. The general rule in twodimensional imaging is to obtain multiple views
of every area of interest, as most artifacts will not
withstand the test of multiple views.
“One view is no view”; always obtain at least
two (preferably perpendicular) views.
Space/Time Artifacts
Refraction
When the ultrasound beam encounters an interface at an angle, the difference in propagation
velocities between the two media can cause
refraction of the ultrasound beam. If the refracted
beam reects off a structure to the ultrasound
system, this will result in that structure being
assigned an incorrect location. This occurs
because the ultrasound system assumes beams
travel in straight lines. To resolve this artifact,
the transducer can be adjusted to make the angle
of the ultrasound beam perpendicular to the
interface.
Mirror Image
Similar to refraction, when an ultrasound beam
interacts with a structure on the way to a strong
reector, the primary beam may reect back and
forth between the structure and the strong reec-
tor. This results in a second image of the structure
being assigned a deeper location than the originalreections (Fig. 1.13) because the ultrasound
system assumes that the beam only reected once
and that the later echoes were generated from a
deeper location. Examples of reective surfaces
that can produce a mirror image artifact are the
pericardium and the diaphragm. The former is
responsible for the appearance of cardiac mirror
artifacts and the latter for the appearance of false
pleural effusions in patients with ascites. Mirror
imaging is type ofreverberation artifact.
Reverberation
When the ultrasound beam encounters two strong
reectors that are usually (but not necessarily)parallel to one another, the beam may bounce
continuously between the two reectors. Here,
the ultrasound system interprets the incoming
signals as returning from deeper and deeper
structures, due to the longer time it takes for the
echoes to return to the transducer (Fig. 1.14).
Examples of such strong reectors include pacemaker leads in cardiac ultrasound, pleural surfaces, and procedural needles. This artifact is
usually helpful in identifying strong reectors.
Ring-down andComet Tail
A type of resonance artifact that occurs when an
ultrasound wave causes cholesterol crystals or
20
Z. S. Shaman and F. S. Qadir
Secondary Lobes andBeam Width
Whereas most of the ultrasound beam propagates
along the central (main) beam, some lowintensity waves may be directed toward the sides
of the central beam. Side lobes appear lateral to
the central beam and the grating lobes usuallyappear lateral to the side lobes (Fig. 1.16).
The energy from these lobes is usually inconsequential, but if there is a strong reector present
lateral to the central beam, reections off these
side beams would be displayed in the center of
the image as if originating from the central beam.
Side lobe artifacts create reections of ghost
images that can be reduced by obtaining images
in planes perpendicular to each other. A similar
spatial artifact may occur that relates to the width
of the beam and is called the Beam Width artifact. In this case, as the beam widens in the far
eld, an echo returning from the edge of the
wider beam is assumed to return from the center
of the beam causing lateral structures to be superimposed on the central image.
Fig. 1.14 Ultrasound view of the anterior chest showing
the pleural line (top dotted red arrow) and reverberations
of the pleural line (subsequent dotted red arrows). As the
incident beam reects back and forth between the pleural
line and the transducer (solid blue and red arrows), the
processor assumes the later signals to return from deeper
and deeper structures
the uid trapped between gas bubbles to resonate. The vibration of such crystals or bubbles
will send a continuous signal back to the transducer. The ultrasound system will interpret this
continuous signal of sound waves as a series of
ladder-like bands in a straight line (Fig. 1.15).
This artifact has important practical implications
and is useful as a diagnostic tool in lung ultrasound where it is the mechanism by whichvertical B-lines are created. Ringdown artifacts are
similar in appearance to Comet Tail artifacts;
however, the mechanism of Comet Tail artifacts
is actually a reverberation between reectors that
are very close to each other. The returned echoes
are presented on the screen as very short horizontal lines that stack up to create a bright vertical
line [10].
Bayonet
This is a type of speed assumption error artifact.
The ultrasound system calculates the depth of a
structure by multiplying echo return time by the
speed of ultrasound. Whereas sound velocity
through soft tissue is assumed to be constant,
there may be slight differences in adjacent tissues. The bayonet artifact is the appearance of
bending of a straight structure like a needle being
used for the vascular access procedure. When the
needle traverses different tissues, the ultrasound
beam may move at different speeds causing the
needle to appear “bent” in shape (Fig.1.17).
Edge
The edge of a structure may cause the ultrasound
beam to refract and may not return to the transducer. This specic form of refraction causes areas
immediately deep to the sides of vessels to appear
dark and devoid of signal (Fig.1.18). Not to be
confused with shadowing (see below), an edge artifact occurs at boundaries that are nearly parallel to
the beam direction while shadowing occurs at
boundaries that are nearly perpendicular to the signal direction by attenuating the signal energy [11].
1 Physics ofUltrasound
Fig. 1.15 Ultrasound
view of the pleural
surface showing
ringdown generated by
air/uid boundaries in a
secondary lobular septal
insertion point. The inset
(dotted red box) is
expanded to show the
effect of the continuous
signal that is received by
the transducer from the
area of resonance
21
Fig. 1.16 A foreshortened parasternal long-axis ultrasound view of the heart. On the left side, a line (red
arrows) is seen inside a short axis view of the descending
aorta (dotted red circle). The explanation is shown on the
right side image. Side lobe beams (dotted blue oval) are
Attenuation Artifacts
Shadowing
This represents a signal void behind structures with high attenuation coefcients. This
commonly occurs behind solid structures, such as
bones and calcied masses (Fig. 1.19). Other
reected by the pleura (dotted blue arrows) and return to
the transducer. The echoes are misinterpreted as generated
by the main beam (solid oval) and are therefore presented
closer to the center of the image (solid arrows)
examples include implants and prosthetic valves
in the heart. Acoustic shadowing may be useful in
identifying structures, such as gallstones, or to
identify a needle in soft tissue during ultrasoundguided vascular access. Acoustic shadowing may
also make it hard to identify structures directly
behind the attenuating structure.
22
Z. S. Shaman and F. S. Qadir
Fig. 1.17 Ultrasound image of a needle in soft tissue traversing two soft tissue media of different properties causing the speed of sound to be slightly different. This causes
echoes from the needle to return slightly faster on one side
Fig. 1.18 An ultrasound view of a short axis of the
carotid artery. The incident beam changes direction at the
lateral borders of the vessel (dotted blue arrows) causing a
signal void deep to the area. A similar pattern is indicated
by the red arrow on the other side of the vessel
and for the needle to appear broken or bent in shape (red
arrow). Image adapted from Radiology key. In: Radiology
Key. http://www.radiologykey.com/. Accessed 7 Mar
2022
Enhancement
This represents unexpectedly strong signals
returning from areas behind structures that are
exceptional transmitters of sound waves, such as
uid. Enhancement can be seen with echoes deep
to uid-lled structures such as cysts, blood vessels, the urinary bladder, and the gallbladder
(Fig.1.20). Since ultrasound waves undergo little attenuation through uid, higher amplitude
ultrasound waves will be reected from tissues
deep into these structures. This will result in the
appearance of higher echogenicity of deep
tissues.
Doppler Artifacts
Aliasing
In Doppler modes that utilize intermittent pulses
of signal (that is, in modes other than continuous
wave Doppler), an artifact may arise that makes
the speed and direction of the reector ambiguous to the receiving transducer. This is called
Aliasing (Fig.1.21).
Aliasing occurs when the sampling rate from
the specic gate is too slow as compared to the
frequency shift. The magnitude of the frequency