Ultrasound Physics & Instrumentation
4th Edition
Volume II
Companion Presentation
Frank R. Miele
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Volume II Outline
 Chapter 7: Doppler
 Chapter 8: Artifacts
 Level 2
 Chapter 9: Bioeffects
 Chapter 10: Contrast and Harmonics
 Chapter 11: Quality Assurance
 Chapter 12: Fluid Dynamics
 Chapter 13: Hemodynamics
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Chapter 8: Artifacts
Artifacts are any perturbation of a signal which distorts a display from
“truth”.
Understanding the principles and mechanisms of artifacts is one of the most
important aspects of learning to perform and interpret ultrasound.
 There are times when artifacts are extremely useful for making a correct
diagnosis. The utility of these artifacts exists only when there is an
understanding of the physics which results in these artifacts.
 There are other times when artifacts obscure the information necessary.
In these cases, minimizing the artifacts is critical and can only be
achieved by a thorough understanding of the physical mechanisms
which produce these artifacts.
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Detail Resolution
Artifacts associated with limited detail resolution include an inability to
correctly visualize dimensions or even the presence of structures
laterally, axially, and/or elevationally.
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Lateral and Axial Resolution
Pin Group 1 demonstrates the limit in axial
resolution. Notice that the red arrow indicates
the closest axially spaced pins still easily
distinguishable.
Pin Group 2 demonstrates the limit in lateral
resolution. The green arrow indicates the
closest laterally spaced pins still
distinguishable.
Pin 3, indicated by the yellow arrow shows
how much lateral distortion there is at greater
depths, where the beam is diverging. Pin 3
has the same physical dimension as all of the
pins located in the line of pins directly above.
Fig. 1: (Pg 595)
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Elevation Resolution
Elevation is usually the “forgotten” dimension, since (unless using 3-D imaging)
there is no way of directly visualizing the elevation plane which corresponds to
the image slice thickness.
“Noise” from “slice thickness”
“Noise” eliminated by harmonics
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Fig. 2 & 3: (Pg 596)
Locational Artifacts
As the name suggests, “locational” artifacts result in structures appearing in an
incorrect positions within the image. There are many sources of locational
artifacts such as:
•
Refraction
•
Reverberation
• Comet tail
• Ring Down
•
Multipath
•
Grating lobes (and side lobes)
•
Speed Error
•
Range Ambiguity
•
Mirror Image
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Refraction Artifact
Refraction was discussed in detail in Chapter 3. Refraction artifact
results in a lateral displacement of the structure within the image.
Refracting Interface
Strong Reflector (real)
Displayed Image (Artifact)
Fig. 4: (Pg 597)
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Refraction Artifact
Fig. 5: (Pg 597)
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Refraction Artifact (Animation)
(Pg 598)
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Reverberation
Reverberation results in spurious structures caused by sound which reverberates,
or rings, between two or more surfaces.
 Reverberation is more likely when there is a large acoustic impedance
mismatch and relatively specular reflection
 Reverberation is highly angularly dependent
 Reverberation is usually worst when the sound is perpendicular to the
specular reflecting interface.
 Reverberation causes all structures and tissue between the reverberating
structures to be replicated as well as the reverberating structure itself.
 Reverberation is very common and often not identified by the person
scanning or by the person interpreting the scan.
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Reverberation Artifact (simple case)
Fig. 7: (Pg 599)
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Reverberation Artifact (simple case)
As displayed in the last slide, the simple mechanism for this reverberation
artifact is reverberation with the transducer surface.
Fig. 7: (Pg 599)
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Reverberation Artifact (Animation)
Reverberation from a 20 gauge needle during a biopsy of a superficial mass.
(Pg 599)
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Reverberation: Simple Continued
In this case, the reverberation occurs between the surface of the transducer
and the anterior surface of a structure.
123
Real
1 cm
2 cm
3 cm
Artifactual
Artifactual
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Reverberation: Increasing Complexity
In this case, there are multiple reverberation paths.
1 2
3
True structures
1 cm
1
2 cm
2
3
3 cm
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Don’t believe
everything you
see!
Reverberation
In this image of a silicon breast implant, there are many examples of
reverberation artifact.
3 2 1
1 2 3
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Reverberation (multiple specular reflectors)
Subclavian artery with multiple reverberation paths
Strong Reflecting
Surfaces
Fig. 8: (Pg 600)
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Reverberation with Color
Reverberation causing false color vessel – often referred to as a “Mirror” artifact.
Fig. 9: (Pg 601)
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Reverberation (Animation)
Note that the same reverberation path causes color and spectral artifacts as well
as the 2-D artifact.
(Pg 601 B)
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Reverberation
There are times when reverberation can appear very similar to a thrombus.
Fig. 10: (Pg 602)
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Reverberation
Artifactual pedunculated mass caused by reverberation artifact.
Fig. 11: (Pg 602)
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Reverberation (Ring Down)
The sound is ringing between the boundary of the air sacs within the biliary tree
causing a reverberation artifact often called “Ring Down”.
Fig. 12: (Pg 603)
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Ringdown
This image from the
animation CD is a
transverse scan of a liver in
a patient with a stent in the
common bile duct. The
stent has allowed air into
the biliary system causing
pneumobilia. The air is the
cause of the obvious ring
down artifact.
(Pg 603 A)
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Reverberation: Comet Tail (Animation)
Comet tail (reverberation within a metallic structure or high impedance
medium) in a prosthetic valve (St Jude).
(Pg 604 A)
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Reverberation: Comet Tail
Comet tail from a calcification in the prostate gland (from Animation CD).
(Pg 604 B)
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Twinkle Artifact: Comet Tail
“Twinkle” artifact is refers to the resulting color artifact equivalent to comet tail
reverberation. “Twinkle” in this case is caused by a small stone in the distal
ureter at the ureterovesical junction (from Animation CD).
(Pg 604 C)
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Twinkle Artifact: Comet Tail
In this case, the color “comet tail” is from the metal of a prosthetic aortic valve
(from Animation CD).
(Pg 604 D)
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Multi-path Artifact
Multi-path artifact results in a structure appearing deeper than reality because
of the elongated path length.
Fig. 14: (Pg 604)
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Grating Lobes
Grating lobes (for multi-elements and side lobes for single elements) result in a
lateral displacement of structures within an image. As the name suggest, there
is energy in regions other than the main beam which cause reflections.
(Grating lobes are weaker lobes of energy in
direction other than main beam direction)
Spurious second aortic valve root
Beam Directivity
Fig. 15: (Pg 605)
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Speed Error
The system assumes a propagation speed of 1540 m/sec. When the propagation
speed is different than 1540 m/sec, structures are drawn at incorrect depths.
Fig. 16: (Pg 606)
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Speed Error Artifact Example
This speed error case results from a needle entering into a cystic structure. The
needle appears bent giving rise to the name “broken needle” or “bayonet” sign.
Fig. 17: (Pg 606)
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Range Ambiguity Artifact
As mentioned in Chapter 7, all pulsed
wave modalities, including 2-D imaging,
suffer from range ambiguity artifact. Recall
that range ambiguity artifact is the result of
reflected data from the previous acoustic
transmit adding to the reflection of the
current acoustic line. Notice in the figure
that the dotted line represents the
unwanted data returning from the previous
transmit (in the depth region of 1 cm to 2
cm) that superimposes into the current line
of data (in the depth region of 0 cm to 1
cm).
Fig. 18: (Pg 607)
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Range Ambiguity
Every acoustic line of the image is “corrupted” by data from the previous transmit.
0
x
2x
Notice how the image at depth “0” is affected by the reflection from depth “x” of
the previous line. Similarly, the image just slightly deeper than “0” is affected by
the reflections from just slightly deeper than the depth of “x” from the previous
line. This process continues until the depth of “x” is affected by the reflection from
the previous line reflection from “2x”.
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Mirroring
Mirroring results in an artificial structure symmetric to the actual structure across
the “mirroring structure”.
1
2
Real Structure
drawn correctly by
beam 2
Strong
Reflector
“Mirrored” Structure
caused by reflection of
beam 1
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Mirror Artifact
This mirror image of a calcification in the liver is produced by reflection from the
diaphragm.
Fig. 19: (Pg 607)
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Mirror Artifact
This image shows a large mirror artifact of a multi-nodular goiter reflected across
the trachea.
Fig. 20: (Pg 608)
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Shadowing Artifact
Shadowing is caused by excessive reflection, absorption, or refraction. This
case is shadowing produced by excessive reflection from a gallstone.
Fig. 21: (Pg 608)
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Shadowing Artifact Example
This image demonstrates shadowing caused by excessive reflection as well as
the obvious “ring down” artifact caused by pneumobilia.
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Shadowing Artifact Example
In this image there are many artifacts, including shadowing caused by
calcifications within a heterogeneous plaque.
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“Edge” (refractive) Shadowing
Edge shadowing is caused by excessive refraction and commonly occurs from
the edges vessels, cystic structures, and bones.
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Refractive (edge) Shadowing
The shadowing evident in this fetal skull image is caused by refraction, not
reflection from the bone. Notice how the shadow only exists in regions where
the incident angle is far away from 0 degrees (recall Snell’s law).
Fig. 22: (Pg 609)
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Calculating the Critical Angle
ci sin  critical 
1540 sin  critical 



  critical   25
ct
sin t 
4080
sin  90 
Fig. 23: (Pg 609)
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The calculation shows
that total internal
reflection occurs at an
angle of approximately
35 degrees. Recall
that the transmitted
angle equals 90
degrees for the critical
incident angle.
Shadowing: Case 1 (from Animation CD)
Shadowing caused by reflection and absorption from the spiny process above a
normal vertebral artery.
(Pg 609 A)
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Shadowing: Case 2 (from Animation CD)
The same vertebral artery
as last slide. Notice how
the angle of the shadow is
different for the 2D image
than for the color image
(since the color is steered
separately from the 2D).
(Pg 609 B)
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Shadowing: Case 3 (from Animation CD)
(Pg 609 C)
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Shadowing: Case 4 (from Animation CD)
Image of a right kidney with shadowing from 2 kidney stones.
(Pg 609 D)
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Shadowing: Case 5 (from Animation CD)
Image of a breast with a silicon implant. The combination of a shadow and
reverberation produce what is sometimes referred to as a “dirty shadow”.
(Pg 609 E)
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Enhancement
Enhancement from an anechoic cyst in the liver.
Fig. 25: (Pg 610)
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Enhancement Artifact
Enhancement is the reciprocal of shadowing. Enhancement is most common
deeper than a fluid filled structure. In this case, enhancement is from the
blood pool in the femoral artery.
Fig. 24: (Pg 610)
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Enhancement: Case 1 (from Animation CD)
Image of a fatty breast cyst creating enhancement.
(Pg 610 A)
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Enhancement: Case 2 (from Animation CD)
Enhancement from the blood pool within a femoral vein.
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(Pg 610 B)
Doppler Artifacts
There are many Doppler artifacts to consider such as:
 Aliasing
 Range Ambiguity
 Spectral Mirroring
 Spectral Broadening
 Blossoming
 Circuit Saturation
 Refraction
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Aliasing
Aliasing results when the Nyquist criterion is violated. When the Doppler
frequency shift is greater than one half of the PRF, the Doppler signal wraps
around either the spectrum (spectral Doppler) or the color scale (Color Doppler).
Fig. 26: (Pg 611)
Note that in this case, even though there is a true alias, since the peak velocity falls within
the spectral window, it is still possible to determine the true peak velocity.
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Aliasing
Note that in this case, the aliasing is a true alias in that it is not possible to
determine the actual peak velocity.
Fig. 27: (Pg 611)
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Aliasing: (from Animation CD)
A: Aliased Flow
B: Shift Baseline (still aliased)
C: Increase Scales (no aliasing)
(Pg 611 A)
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Aliasing (Animation)
(Pg 611 B)
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Aliasing Animation
(Pg 611 C)
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Color Aliasing
As discussed in Chapter 7, aliasing is more likely to occur within the center of a
vessel, whenever there is a bend, branch, narrowing, etc., when the angle is
closer to 0 or 180 degrees, when the scales are low, and the transmit frequency
is high. How many reasons can you find for why there is aliasing in this image?
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Fig. 28: (Pg 612)
Color Aliasing Case 1: (from Animation CD)
(Pg 612 A)
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Color Aliasing Case 2: (from Animation CD)
(Pg 612 B)
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Color Aliasing Case 3: (from Animation CD)
(Pg 612 C)
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Color Aliasing Case 4: (from Animation CD)
(Pg 612 D)
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Range Ambiguity
In Chapter 7, the concept of range ambiguity was discussed in detail. Recall that
range ambiguity is the potential to detect signals from deeper depths as the echo
from previous transmit events overlaps the echoes from the current transmit
event.
Not only do all pulsed wave modalities exhibit range ambiguity, its mechanism is
so predictable, that the spectral Doppler modality of HPRF is created by
intentionally manipulating the range ambiguity artifact.
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Spectral Mirroring
Spectral mirroring results in an artificial Doppler signal to be displayed
in the opposite direction of the true flow. The following conditions
exacerbate spectral mirroring:
 excessive transmit
 excessive receive gain
 superficial Doppler with high frequency transducer
 Insonification angle close to 90 degrees
 poor electronic design (poor separation between I and Q channels)
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Spectral Mirroring: Case 1
An obvious case of spectral mirroring. The flow below the baseline is
purely artifactual.
Fig. 29: (Pg 612)
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Spectral Mirroring: Case 2
(from Animation CD)
As the gain is reduced, the apparent spectral mirroring decreases.
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(Pg 613 A)
Spectral Mirroring: Case 3
(from Animation CD)
Sometimes it is difficult to tell what is real and what is artifact. Look for
symmetry of the highest amplitude signals (brightest) about the baseline.
(Pg 613 B)
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Mirroring or Reverse Flow?
Is the reverse flow real or a spectral mirror?
Answer: Real flow at bifurcation in transcranial Doppler
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Mirroring or Reverse Flow?
(from Animation CD)
Is the reverse flow real or a spectral mirror?
(Pg 613 C)
Answer: Mirroring
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Spectral Mirroring Caused By Angle
When the insonification angle is close to 90 degrees, elements on one side of
the steer line will see flow towards the transducer, whereas elements on the
other side of the steer line will see flow away – resulting in a spectral mirror.
< 90° “Forward Flow”
> 90° “Reverse Flow”
Fig. 30: (Pg 613)
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Spectral Spread (Broadening)
Spectral spread causes overestimation of the peak velocity as well as the
diminishment of a “spectral window”.
True Spectrum
70°
80° (cosine
correction too large)
60° (cosine
correction too small)
Peak too high
Window Diminished
Fig. 31: (Pg 613)
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Fig. 32: (Pg 614)
Minimizing Spectral Spread
Spectral spread is exacerbated by:
 large array transducers (linear arrays)
 superficial gate location
 large insonification angles (especially as the angle get larger
than 60º)
 excessive gain
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Spectral Broadening
As just mentioned, spectral Broadening is exacerbated by larger insonification
angles. In this case, compare the spectrum at 55 ° with the spectrum at 70°.
55 °
70 °
Fig. 33a & 33b: (Pg 614)
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Blossoming Artifact
Blossoming results when the
signal is overgained.
Blossoming results in a
higher than true peak velocity
as well as the potential loss
or decrease of the spectral
window (when one exists) in
PW Doppler.
Appropriate Gain
Blossoming Artifact
Fig. 34 & 35: (Pg 615)
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Blossoming Artifact (from Animation CD)
(Pg 615)
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Wall Filter Saturation
As discussed in Chapter 7, when the dynamic range is not adequately reduced
by the wall filters, circuit saturation occurs as evidenced in this spectrum.
Fig. 36: (Pg 616)
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Refraction and Total Internal Reflection
Refraction was discussed in detail in Chapter 3. Total internal reflection can
result in a loss in spectral signal as indicated in the diagram below.
XDCR
θi= θ critical
Total Internal Reflection
Fig. 37: (Pg 616)
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