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b
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2 Probes andImaging Modes
Fig. 2.5 (a) Echoes
(red) from a stationary
object return at the same
frequency as the
transmitted ultrasound
(blue). (b) Echoes from
an object moving away
from the transducer have
decreased frequency. (c)
Echoes from an object
moving toward the
transducer have
increased frequency
33
Fig. 2.6 Pulsed-wave
Doppler of the carotid
artery. The angle
correction line (arrow) is
adjusted to be parallel to
blood ow to improve
the accuracy of velocity
calculation.
Measurements are
inaccurate at angles
greater than 60°
In addition to constraints from the angle of
insonation, there are limits to the maximum
Doppler shift that can be measured. A waveform
must be sampled at least twice its frequency to be
accurately represented, according to the NyquistShannon sampling theorem [6] (Fig. 2.7a—
Nyquist limit). The sampling rate is the pulse
repetition frequency (PRF), how many pulses of
ultrasound are sent out per second. The maximum
magnitude Doppler shift that can be accurately
measured is PRF/2, known as the Nyquist limit.
Velocities that produce a Doppler shift exceeding
the Nyquist limit will be misinterpreted in a phenomenon known as aliasing and may appear as
ow in the opposite direction (Fig.2.7b—alias-
ing). This is analogous to the spinning wheels of a
34
PRF =
2*Doppler shif
PRF <
2*Doppler shif
(alias)
a b
t
t
C. Allison
sample points
Fig. 2.7 (a) A waveform sampled at two different pulse
rate frequencies (PRF). Above, the PRF is at the Nyquist
limit, so the red dots at sampling points accurately recreate the original waveform. Shown below, at too low a PRF
the red dots at too infrequent intervals create the appear-
apparent lower frequency
car on a lm appearing to rotate in reverse, which
can happen when the sample rate of the camera is
slower than twice the wheel rotation frequency
(i.e., the camera’s Nyquist limit).
Values derived from PRF include the pulse
repetition period (PRP), which is the time
between the beginning of the reach pulse:
PRP=1/PRF.Pulse duration is the length of time
of each pulse. The Duty factor is the fraction of
time that the transducer is generating sound,
which is pulse duration/PRP.
In various Doppler modes, Doppler gain will
adjust the sensitivity of the machine to ow. This
should be adjusted until the artifact is not visible.
Machine presets are helpful for starting PRF,
Doppler gain, etc. that are likely to work well for
an application. For example, a cardiac preset will
have a high PRF for the high ow velocities
encountered in the heart.
Color Doppler
Color Doppler superimposes velocities calculated by Doppler shift in the form of color onto
2D images from B mode, also termed duplex
ance of a lower frequency waveform (an alias, green
dashed line) than the original. (b) Color Doppler ultrasound showing aliasing resulting in the appearance of
reversal of ow direction at the sites of higher velocity
blood ow in the center of a blood vessel
scanning. Usually, velocities in a range consistent with blood ow are of interest, so lters are
set so that low velocities that are likely artifactual
or from probe movement are not displayed.
Typical lters exclude frequencies below
50–100MHz [5]. These may need to be adjusted
if trying to detect low-velocity ow.
The color display provides information on
velocity and direction. While the default convention is to represent the movement away from the
probe as blue and toward the probe as red (leading to the mnemonic BART: blue-away, redtoward); this is sometimes reversed, and the user
should refer to the legend displayed on the
machine (Fig.2.8—color Doppler).
The user can adjust the area of the 2D image
being sampled for velocities, i.e., the color box. A
larger color box requires more computing power,
often resulting in a lower frame rate. Linear
transducers often have available an angulated
color box that steers the angle of insonation in
addition to the angle of the probe contacting the
skin, facilitating an angle of less than 60°.
Turbulent ow appears as a mix of colors representing different ow directions, but care
should be taken that this has not been caused by
2 Probes andImaging Modes
Fig. 2.8 Color Doppler
ultrasound of a kidney,
showing blood ow of
intrarenal vasculature
35
aliasing from velocities causing Doppler shifts
exceeding the Nyquist limit for the set PRF.
Spectral Doppler
Spectral Doppler imaging represents velocities
as a spectrogram with time on the x-axis and
velocities (calculated from Doppler shift) on the
y-axis. This information is taken along a narrow
line from the transducer surface. Note that multiple velocities are detected and displayed for
one time point, representing the variation in ow
in, for example, different locations within a single blood vessel. The brightness corresponds,
qualitatively, to the number of blood cells moving at that velocity. The displayed brightness is
adjusted with Doppler gain. The location of the
baseline on the screen and the velocity scale for
the y-axis are adjusted for optimal appearance
(Fig.2.9—spectrogram).
Two main forms of spectral Doppler imaging
are pulsed-wave Doppler (PWD) and continuouswave Doppler (CWD). In PWD, the transducer
intermittently emits pulses of ultrasound, then
“listens” between (Fig. 2.10—PW vs. CW
Doppler).The user species the depth from which
to sample velocities in the form of a range gate.
This allows the measurement of velocities in a
specic area, for example from only the left ventricular outow tract, despite other blood ow
occurring along the same line of measurement.
Because ultrasounds are sent in pulses, aliasing
will occur when velocities exceed the Nyquist
limit for the set PRF.In contrast, CWD has separate transmitters continuously emitting and “listening” for ultrasound. This eliminates aliasing at
velocities encountered clinically but does not
allow the assessment of depth along the line of
measurement [6]. CWD is used for measuring the
highest velocities encountered clinically with
accuracy, such as those of a stenotic heart valve.
36
Fig. 2.9 Pulsed wave
ultrasound of the carotid
artery demonstrating
velocities within the
gated area over multiple
cardiac cycles
C. Allison
Fig. 2.10 Comparison of pulsed wave (left) and continuous wave Doppler (right). Note the aliasing present in pulsed
wave Doppler. Continuous wave Doppler has no aliasing and can measure high velocities
Tissue Doppler
Pro-tips
Tissue Doppler is a subtype of spectral Doppler
imaging that is optimized for moving soft tissue, usually myocardium, instead of blood
cells. Filters for slower velocity, but higher
amplitude movement allow Doppler shifts
from moving tissue to be isolated from those of
blood [7].
In spectral Doppler the brightness corre-
sponds, qualitatively, to the number of
blood cells moving at that velocity. The dis-
played brightness is adjusted with Doppler
gain. Two main forms of spectral Doppler
imaging are pulsed-wave Doppler (PWD)
and continuous-wave Doppler (CWD).
2 Probes andImaging Modes
Evolving Evidence
Cardiac presets often 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.
Summary Points
• For supercial structures, select a highfrequency, linear probe
• For visualizing deep structures, choose a lowfrequency curvilinear (abdominal)probe or a
phased-array (cardiac) probe
Questions
1. Which of the following is most appropriate
for ultrasound guidance of radial arterial line
placement?
A. 3.5MHz, curvilinear probe
B. 13MHz, linear probe
C. 5MHz, phased array probe
D. 8MHz, linear probe
Answer: (B). Visualization of a small,
supercial structure like the radial artery is
optimized with a higher resolution linear
probe
2. Which of the following is true of harmonic
imaging?
A. Interpretation echoes at higher frequen-
cies generated at multiples of the fundamental frequency can generally provide
higher-resolution images.
B. It provides better axial resolution for very
deep structures.
C. Limited evidence suggests it improves
visualization of sonographic B lines.
D. It facilitates wide-angle imaging from a
smaller footprint, for example, through rib
spaces.
Answer: (A). The harmonic frequencies
generally improve resolution and image quality, but visualization of deep structures is limited, and it can reduce the generation of useful
artifacts used in lung ultrasound. Harmonic
imaging can reduce the axial resolution.
37
Harmonic imaging will have minimal to no
effect on the angle of the image.
3. On Color Doppler of the abdominal aorta,
blood ow velocities away from the probe are
seen as blue color around the periphery of the
vessel lumen, but velocities in the center of
the lumen appear red corresponding to ow
toward the probe. Which of the following
explains this artifact?
A. The Doppler gain is set too high
B. Edge artifact causing acoustic shadowing
at the edges of the vessel
C. The pulse repetition frequency is less than
the Nyquist limit required to measure the
velocities of the blood in the center of the
lumen
D. Aortic calcications causing reverbera-
tion artifact, leading to misinterpretation
of velocity direction
Answer: (C). This is an example of aliasing
occurring at the higher velocities at the center
of the artery lumen. Likely the pulse repetition frequency was inappropriately decreased
to less than what was needed for this
application.
References
1. Bushberg JT.The essential physics of medical imaging. 3rd ed. Wolters Kluwer Health/Lippincott
Williams & Wilkins; 2012.
2. Desser TS, Jeffrey RB.Tissue harmonic imaging techniques: physical principles and clinical applications.
Semin Ultrasound CT MR. 2001;22(1):1–10. https://
doi.org/10.1016/s0887- 2171(01)90014- 9.
3. Hedrick WR, Metzger L. Tissue harmonic imaging:
a review. J Diagn Med Sonogr. 2005;21(3):183–9.
https://doi.org/10.1177/8756479305276477.
4. Matthias I, Panebianco NL, Maltenfort MG, Dean
AJ, Baston C. Effect of machine settings on ultrasound assessment of B-lines. J Ultrasound Med.
2020;40(10):2039–46. https://doi.org/10.1002/
jum.15581.
5. Rumack CM, Wilson SR, Charboneau JW, Levine
D.Diagnostic ultrasound. 4th ed. Mosby; 2011.
6. Rubin JM. Spectral Doppler US. Radiographics.
1994;14(1):139–50. https://doi.org/10.1148/
radiographics.14.1.8128046.
7. Ho CY, Solomon SD. A clinician’s guide
to tissue Doppler imaging. Circulation.
2006;113(10):e396–8. https://doi.org/10.1161/
CIRCULATIONAHA.105.579268.
Ultrasound Artifacts
TaroMinami
3
Learning Objectives
1. Dene a radiographic image artifact
2. Describe common image artifacts
3. Examine how ultrasound image artifacts arise
An ultrasound machine creates images based on
assumptions from the physical properties of the
ultrasound wave. For example, the sound travels
in the human body, hits the target, and returns to
the probe. If we know the time it takes to return to
the probe, we can calculate the distance between
the probe and the target as the sound travels at a
xed speed of 1540 m/s in the human body.
Based on the intensity of the ultrasound wave
returning to the probe and the time it takes to
travel, a machine can draw a dot on the screen
with a color ranging from white to black.
An ultrasound machine assumes many things
to create images. It assumes that the ultrasound
beam moves at a xed speed in a human body at
1540 m/s. It also assumes that the beam goes
straight, hits the target, and returns once. In the
Supplementary Information The online version contains supplementary material available at https://doi.
org/10.1007/978- 3- 031- 80038- 2_3.
T. Minami (*)
Division of Pulmonary, Critical Care, and Sleep
Medicine, Department of Medicine, The Warren
Alpert Medical School of Brown University,
Providence, RI, USA
ideal world, ultrasound follows these
assumptions.
However, realities often differ from our
assumptions. Sound travels much faster in the
bone and much slower in the air. The ultrasound
beam may not propagate as expected, sometimes
going back and forth between the probe and the
target multiple times.
Artifacts arise from such deviations. For
example, if the sound travels twice between the
probe and the object, the ultrasound machine
would assume it came back from twice as deep a
position. It may also assume that the ultrasound
came back from the shallower location if it travels much faster and thus takes less time.
Understanding ultrasound artifacts stems from
understanding ultrasound properties and assumptions the ultrasound machines make.
Why It Is Important toUnderstand
Ultrasound Artifacts
Ultrasound artifacts happen often, and we
encounter them daily: Artifacts may confuse us
daily if we do not recognize them properly. They
may give clinicians wrong impressions of the
structure they are observing, thus leading them to
the wrong diagnosis and, ultimately, the wrong
treatment. On the other hand, recognizing specic image artifacts can help us make a particular
© 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_3
39
40
T. Minami
diagnosis. For example, we make diagnoses
based on artifact patterns in lung ultrasound.
Since we cannot observe the lung structures
directly, as we do for heart or vascular structures,
we assume lung pathophysiology through artifact
patterns, owing much to the presence of air that
attenuates and dissipates ultrasound signals. We
utilize reverberation artifacts (A-lines or
A-prole) to assume the presence of air and ringdown artifacts (B-lines) to assume the presence
of water. Understanding ultrasound artifacts is
thus vital as we must (1) avoid confusion by the
false structures or functions created by artifacts
that do not exist and (2) deepen our understanding of the pathophysiologies artifacts imply.
Types ofArtifacts
There are many image artifacts under different
mechanisms which can confuse learners. To simplify image artifacts, we can classify them into three
major categories: artifacts associated with sound
reections, artifacts associated with ultrasound
attenuation, and artifacts associated with ultrasound
beam characteristics [1]. For additional specic
examples of cardiac artifacts, refer to Chap. 10.
1. Reections: reverberation, ring-down, comet
tail, mirror
2. Attenuation: shadow artifact, enhancement
artifact
3. Beam characteristics: side-lobe artifacts
Pro-tip
Be on the lookout for artifacts when dealing with strong reectors (pericarditis,
mechanical valve, etc.).
Artifacts Associated withReections
An ultrasound machine creates an image based
on the assumption that ultrasound travels once
straight to the target before returning to the probe
at a constant speed through the human body, i.e.,
1540m/s. The ultrasound machine translates the
time the ultrasound takes to return and the
strength of the sound coming back into an image
[2]. The more time it takes, the location at which
it reects should be located deeper (thus, the dot
is drawn at the lower part of the screen), and the
stronger the signal coming back, the whiter it
appears on the screen. Conversely, if the coming
back signal is weak, it will appear darker on the
screen. In other words, the traveling time determines the location and the attenuation determines
the location. That is why water appears black,
bones, air, lungs, and membranes appear white,
and the liver and spleen, soft tissues, appear gray.
Pro-tip
Different authors may refer to the same or
similar artifacts by different names: ringdown vs. reverberation artifact.
Reverberation Artifacts
The amount of ultrasound reected off a tissue
surface depends on the difference in acoustic
impedance of the two tissues. If the difference is
small, the reection is small; if the difference is
large, the reection is large. A typical example
of multiple reections is the A-line caused by
reections from the pleural surface of the lungs.
First, ultrasound waves emitted from the
ultrasound probe are reected by the pleura,
then some of the ultrasound waves are reected
back to the skin/probe surface, then again to
the pleura, and so on. Since the ultrasound
machine cannot tell the difference between the
sound traveling once, twice, or sometimes
three times, it thinks that the sound traveling
twice or three times is coming back from a
deeper location.
A famous example would include A-lines seen
in lung ultrasounds (Image 3.1, Lung ultrasound
with regularly spaced horizontal A-lines indicating aerated lung or pneumothorax).
Pro-tip
Doppler, including color ow, is suscepti-
ble to artifacts as well.
3 Ultrasound Artifacts
41
Image 3.1 Examples of reverberation artifact: A-lines.
Please note that A-lines are artifacts created from the
pleura and hence note that the distance of the surface-
Comet-Tail Artifact
There is a lot of confusion about this artifact.
First, the comet-tail artifact—though it looks
similar—is not the ring-down artifact as its
mechanism is quite different. Comet-tail artifact
is a variation of a reverberation artifact and, thus,
not the same as a ring-down artifact.
Ring-Down Artifact
Ultrasonic energy transmitted from the probe
causes resonant vibrations within the uid surrounded by air. This vibration produces a continuous sound wave toward the probe, which the
ultrasound device interprets as a backward-facing
linear structure, which looks like a white line
extending down from the air-enclosed structure.
This is the B-line, a common nding on lung echo.
This is a common B-line nding on a lung echo
because water comes from an air-lled space.
A famous example of the ring-down artifact is
seen in lung ultrasound as B-lines (Image 3.2,
Lung ultrasound with vertical B-lines indicating
extravascular lung water).
Mirror Image Artifacts
A typical example of a mirror image artifact is
that of the liver, which is in front of the diaphragm on the image, and appears as if it is
pleura and pleura-A-line is same. Also note although it
may seem subtle, the pleura is the most echogenic (most
white) line, followed by A1, then by A2 (less white)
reected in a mirror behind the diaphragm. This
is caused by the strong reection of ultrasound
waves from the diaphragm, which causes the
ultrasound waves emitted from the probe to travel
in the following order: diaphragm, liver, diaphragm, and probe. In addition to the diaphragm,
similar images across a strongly reecting surface, such as the pericardium or bladder wall,
may be mirror images. Color ow and Spectral
Doppler can also be susceptible to mirror
artifacts.
Artifacts Associated withAttenuation
The further ultrasound travels, the more its
energy lessens. This is called attenuation. The
higher the ultrasound frequency, the more the
energy attenuates over the same distance. Some
tissues (e.g., water (blood)) retain more energy
than others (e.g., bones and lungs). The attenuation unit is Db/cm, and the higher the number, the
more attenuation the ultrasound has over the
same distance.
High attenuation (bone, lungs, and air) means
more ultrasound is reected at the site, while low
attenuation (water, blood) means little ultrasound
is reected at the site. Ultrasound waves pass
42
Image 3.2 Lung
ultrasound with vertical
B-lines. Examples of
ring-down artifact.
B-lines can be observed
which arise from the
pleura surface
T. Minami
through water without much attenuation (or, conversely, with almost no reection). In contrast, in
bone and air, ultrasound waves are so attenuated
(mostly reected) that they cannot pass beyond a
certain distance. Soft tissues, such as the liver,
transmit a portion of the ultrasound and reect
the rest [3].
If the ultrasound is not reected and passes
through the area unimpeded (e.g., uid), the
ultrasound signal that is reected back is less and
appears black. On the other hand, if ultrasound
waves are only reected off the surface of a material (e.g., bone surface and membranes) more
ultrasound waves will be returned to the probe,
and the brightness on the screen will be higher. In
the case of parenchymal organs such as the liver
and kidneys, some ultrasound waves pass
through, and some are reected, which is why
they appear gray on the screen [4].
Shadowing Artifact
When ultrasound passes through a target with
high attenuation (e.g., bone and air), the ultrasound is reected from the surface, making it difcult for an ultrasound to reach the area behind
the target, resulting in a black shadow beneath
the target on the image. In this case, the black
shadow does not mean that water is present, but
rather it means that there is no signal coming
back beyond the target (Image 3.3).
Enhancement Artifact
Conversely, when ultrasound travels through a
target with low attenuation (e.g., water), it maintains relatively high energy compared to other
locations. In such cases, the signal reected back
will be higher than in other areas because of the
higher energy. This is called Enhancement
Artifact (Image 3.4).
Artifacts Associated withUltrasound
Beam Properties
The type of assumption discussed here is that
there is only one ultrasound beam: However,
there are other beams, such as side lobes and
grating lobes. These beams could, from time to
time, generate ultrasound reection strong
enough to be detected by the probe and create
artifacts.
Side-Lobe Artifacts
Side lobe beams are often generated in linear
array transducers; they could generate a detectable reection of ultrasound beam by an object
3 Ultrasound Artifacts
Image 3.3 Example of
the shadow artifact. You
can observe shadow
artifact behind the gall
stone, which has a
highly echogenic surface
Image 3.4 Enhancement artifact. Note the
high echogenic zone
behind the gall bladder,
which itself is anechoic
43
with strong reector properties but does not exist
in the path of the main beam. Since the ultrasound machine assumes there only exists one
beam, thus it will display the reector in the position it should not exist.
Pro-tip
Image from multiple windows and views to
better recognize artifacts.