Pegasus Lectures, Inc. COPYRIGHT 2006 Volume II Companion Presentation Frank R. Miele Pegasus Lectures, Inc. Ultrasound Physics & Instrumentation 4 th

Pegasus Lectures, Inc.COPYRIGHT 2006

Volume II

Companion Presentation

Frank R. MielePegasus Lectures, Inc.

Ultrasound Physics & Instrumentation4th Edition


License Agreement

This presentation is the sole property of Pegasus Lectures, Inc.

No part of this presentation may be copied or used for any purpose other than as part of the partnership program as described in the license agreement.

Materials within this presentation may not be used in any part or form outside of the partnership program. Failure to follow the license agreement is a violation of

Federal Copyright Law.

All Copyright Laws Apply.


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


Chapter 8: ArtifactsArtifacts 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.


Detail ResolutionArtifacts associated with limited detail resolution include an inability to correctly visualize dimensions or even the presence of structures laterally, axially, and/or elevationally.


Lateral and Axial Resolution

Fig. 1: (Pg 595)

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.


“Noise” eliminated by harmonics

Elevation ResolutionElevation 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.

Fig. 2 & 3: (Pg 596)

“Noise” from “slice thickness”


Locational ArtifactsAs 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


Fig. 4: (Pg 597)

Refraction ArtifactRefraction 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. 5: (Pg 597)

Refraction Artifact


(Pg 598)

Refraction Artifact (Animation)


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.

Reverberation


Fig. 7: (Pg 599)

Reverberation Artifact (simple case)


Fig. 7: (Pg 599)

Reverberation Artifact (simple case)As displayed in the last slide, the simple mechanism for this reverberation artifact is reverberation with the transducer surface.


(Pg 599)

Reverberation Artifact (Animation)Reverberation from a 20 gauge needle during a biopsy of a superficial mass.


In this case, the reverberation occurs between the surface of the transducer and the anterior surface of a structure.

Reverberation: Simple Continued

1 2 3

1 cm

2 cm

3 cm

Artifactual

Artifactual

Real


Reverberation: Increasing ComplexityIn this case, there are multiple reverberation paths.

Don’t believe everything you see!

1 cm

2 cm

3 cm

1

2

3

True structures1 2 3


ReverberationIn this image of a silicon breast implant, there are many examples of reverberation artifact. 1 2 3123


Fig. 8: (Pg 600)

Reverberation (multiple specular reflectors)

Subclavian artery with multiple reverberation paths

Strong Reflecting Surfaces


Fig. 9: (Pg 601)

Reverberation with ColorReverberation causing false color vessel – often referred to as a “Mirror” artifact.


(Pg 601 B)

Reverberation (Animation)Note that the same reverberation path causes color and spectral artifacts as well as the 2-D artifact.


Fig. 10: (Pg 602)

ReverberationThere are times when reverberation can appear very similar to a thrombus.


Fig. 11: (Pg 602)

ReverberationArtifactual pedunculated mass caused by reverberation artifact.


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)


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)


(Pg 604 A)

Reverberation: Comet Tail (Animation)

Comet tail (reverberation within a metallic structure or high impedance medium) in a prosthetic valve (St Jude).


(Pg 604 B)

Reverberation: Comet TailComet tail from a calcification in the prostate gland (from Animation CD).


(Pg 604 C)

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 D)

Twinkle Artifact: Comet TailIn this case, the color “comet tail” is from the metal of a prosthetic aortic valve (from Animation CD).


Fig. 14: (Pg 604)

Multi-path ArtifactMulti-path artifact results in a structure appearing deeper than reality because of the elongated path length.


Fig. 15: (Pg 605)

Grating LobesGrating 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. 16: (Pg 606)

Speed ErrorThe 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. 17: (Pg 606)

Speed Error Artifact ExampleThis 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. 18: (Pg 607)

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).


Range AmbiguityEvery acoustic line of the image is “corrupted” by data from the previous transmit.

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”.

2x

x

0


MirroringMirroring results in an artificial structure symmetric to the actual structure across the “mirroring structure”.

Strong Reflector

“Mirrored” Structurecaused by reflection of beam 1

Real Structuredrawn correctly by beam 2

12


Fig. 19: (Pg 607)

Mirror ArtifactThis mirror image of a calcification in the liver is produced by reflection from the diaphragm.


Fig. 20: (Pg 608)

Mirror ArtifactThis image shows a large mirror artifact of a multi-nodular goiter reflected across the trachea.


Fig. 21: (Pg 608)

Shadowing ArtifactShadowing is caused by excessive reflection, absorption, or refraction. This case is shadowing produced by excessive reflection from a gallstone.


Shadowing Artifact ExampleThis image demonstrates shadowing caused by excessive reflection as well as the obvious “ring down” artifact caused by pneumobilia.


Shadowing Artifact ExampleIn this image there are many artifacts, including shadowing caused by calcifications within a heterogeneous plaque.


“Edge” (refractive) ShadowingEdge shadowing is caused by excessive refraction and commonly occurs from the edges vessels, cystic structures, and bones.


Fig. 22: (Pg 609)

Refractive (edge) ShadowingThe 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).


Calculating the Critical Angle

sin sin1540

25sin 4080 sin 90

critical criticalicritical

t t

c

c

Fig. 23: (Pg 609)

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.


(Pg 609 A)

Shadowing: Case 1 (from Animation CD)Shadowing caused by reflection and absorption from the spiny process above a normal vertebral artery.


(Pg 609 B)

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 C)

Shadowing: Case 3 (from Animation CD)


(Pg 609 D)

Shadowing: Case 4 (from Animation CD)Image of a right kidney with shadowing from 2 kidney stones.


(Pg 609 E)

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”.


Fig. 25: (Pg 610)

EnhancementEnhancement from an anechoic cyst in the liver.


Fig. 24: (Pg 610)

Enhancement ArtifactEnhancement 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.


(Pg 610 A)

Enhancement: Case 1 (from Animation CD)

Image of a fatty breast cyst creating enhancement.


Enhancement: Case 2 (from Animation CD)

Enhancement from the blood pool within a femoral vein.

(Pg 610 B)


Doppler ArtifactsThere are many Doppler artifacts to consider such as:

Aliasing

Range Ambiguity

Spectral Mirroring

Spectral Broadening

Blossoming

Circuit Saturation

Refraction


AliasingAliasing 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).

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.

Fig. 26: (Pg 611)


Fig. 27: (Pg 611)

AliasingNote that in this case, the aliasing is a true alias in that it is not possible to determine the actual peak velocity.


(Pg 611 A)

Aliasing: (from Animation CD)

A: Aliased Flow

B: Shift Baseline (still aliased)

C: Increase Scales (no aliasing)


(Pg 611 B)

Aliasing (Animation)


(Pg 611 C)

Aliasing Animation


Fig. 28: (Pg 612)

Color AliasingAs 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?


(Pg 612 A)

Color Aliasing Case 1: (from Animation CD)


(Pg 612 B)



(Pg 612 C)



(Pg 612 D)



Range AmbiguityIn 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.


Spectral MirroringSpectral 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)


Spectral Mirroring: Case 1An obvious case of spectral mirroring. The flow below the baseline is purely artifactual.

Fig. 29: (Pg 612)


Spectral Mirroring: Case 2 (from Animation CD)

As the gain is reduced, the apparent spectral mirroring decreases.

(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)


Mirroring or Reverse Flow?Is the reverse flow real or a spectral mirror?

Answer: Real flow at bifurcation in transcranial Doppler


Mirroring or Reverse Flow? (from Animation CD)

Is the reverse flow real or a spectral mirror?

Answer: Mirroring

(Pg 613 C)


Spectral Mirroring Caused By AngleWhen 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.

Fig. 30: (Pg 613)

< 90° “Forward Flow”> 90° “Reverse Flow”


Fig. 32: (Pg 614)

Spectral Spread (Broadening)Spectral spread causes overestimation of the peak velocity as well as the diminishment of a “spectral window”.

Fig. 31: (Pg 613)

80° (cosine correction too large)

60° (cosine correction too small)

70°

True Spectrum

Peak too high

Window Diminished


Minimizing Spectral SpreadSpectral 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


Fig. 33a & 33b: (Pg 614)

Spectral BroadeningAs 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. 34 & 35: (Pg 615)

Blossoming ArtifactBlossoming 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


(Pg 615)

Blossoming Artifact (from Animation CD)


Fig. 36: (Pg 616)

Wall Filter SaturationAs 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. 37: (Pg 616)

Refraction and Total Internal ReflectionRefraction was discussed in detail in Chapter 3. Total internal reflection can result in a loss in spectral signal as indicated in the diagram below.

Total Internal Reflection

θi= θ critical

XDCR


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Pegasus Lectures, Inc. COPYRIGHT 2006 Volume II Companion Presentation Frank R. Miele Pegasus Lectures, Inc. Ultrasound Physics & Instrumentation 4 th