Artifacts and Pitfalls

Published on 06/02/2015 by admin

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Last modified 22/04/2025

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Artifacts and Pitfalls

Christopher J. Gallagher and Gian Paparcuri

When you are in the OR, you are forever asking yourself, “Is it real, or is it Memorex?”. We get seduced by the great images, and you have to shake yourself and say, “This is not a REAL LIVE picture, this is an ULTRASOUND CREATION OF A PICTURE, and ultrasound can fool you”.

Let’s grind through these monsters. Pay close, close, close attention to the aortic dissection artifacts; that is what will scare the hell out of you in the middle of the night.

Useful for any and all artifacts is the mantra, “Change the viewing angle.”. That may allow you to see around a calcified or prosthetic valve. Plus, if you see the same thing from a bunch of different angles, guess what? It’s really there! (Maybe. We live in an uncertain world.)

Artifacts change their appearance and appear or disappear depending on the view.

Real structures will remain constant and can be seen in multiple views.

(See also CFD artifacts, pitfalls, and masses.)

Acoustic imaging assumptions (to assign the location and intensity of received echoes):

image Reflections arise only from structures along the beam’s main axis. Echoes detected originated from within the main ultrasound beam. Problem: side lobes, grating lobes.

image Sound travels directly to a reflector and back (straight line). Problem: refraction.

image Echo returns to transducer after a single reflection. Problem: mirror images, reverberations.

image Sound (sound beam and its echo) travels in a straight line.

image Sound travels exactly at 1540 m/sec (constant, average speed → time is distance). Problem: when the propagation speed errors occurs, reflectors are placed in improper positions or at incorrect depth (time is not equal to distance).

image The depth of an object is directly related to the amount of time for an ultrasound pulse to return to the transducer as an echo (round trip). The length of time for a single round trip of an echo is related only to the distance traveled by the echo.

image Reflection’s intensity (strength) is related to tissue characteristics. Problem: attenuation.

image Imaging plane is extremely thin. Problem: ultrasound beam has some thickness in the perpendicular beam plane, reflections from structures above or below a target reflector may be placed with the target reflector on the display screen.

image Acoustic energy in an ultrasound field is uniformly attenuated.

Artifacts

Artifacts Associated with US Beam Characteristics

US beam exits transducer as a complex 3-D bowtie shape (below figure) with additional off-axis low-energy beams (side lobes and grating lobes). Strong reflector (highly reflective object) located outside the main US beam (peripheral field) may generate detectable echoes that will be displayed as having originated from within the main beam. Misplaced echoes overlapping anechoic structure. Adjust focal zone to the level of interest and place transducer at the center of the object of interest.

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Beam width: distal beam (widened beam) widens beyond the actual transducer width (below figures).

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Side lobe: multiple beams of low-amplitude ultrasound energy projecting radially (off axis) from the main beam axis (from the edges of the transducer elements).

Multiple beams encountering same object duplicate the structure (false targets), generating artifactual echoes of the true reflector, at its correct depth but laterally from the true anatomy (the system assumes they originated from the main beam).

Weaker than the main beam, less acoustic energy, weaker returning echoes, many are not actually seen (overshadowed by true echoes). Evident when they do not conflict with real echoes or originate from strong reflectors.

If the beam is oscillating rapidly, the multiple artifactual echos produced by the side lobes are displayed as a curved line at the same level as the true object.

Options: reduce power (below figures).

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Edge shadow: shadowing as a result of refraction at the edge of a circular structure (dropout phenomena). US beam intercepting tangentially (poorly imaged around 10 and 2 o’clock).

Artifacts Associated with Multiple Echoes

In the presence of two parallel highly reflective surfaces, echoes may be repeatedly reflected back and forth before returning to the transducer for detection, displaying multiple echoes. The echo returning to the transducer after a single reflection will be properly displayed (proper location). Sequential echoes will take longer to return and the processor will erroneously place the delayed echoes at an increased distance from the transducer (multiple equidistantly spaced linear reflections).

Reverberation artifact: echo signal returning to the transducer is reflected back to the tissues by the surface of the transducer or a dense object (two strong reflectors lying in the line, along the main axis of the ultrasound beam) creating multiple unreal superimposed images, equally spaced, ladder-like appearance at ever increasing depth. Pulse bounces back and forth between the two reflectors, sending multiple echoes back to the transducer. The first two echoes arise from true anatomic structures in their correct position. Example: PAC, calcified aorta (strong reflectors), makes sound wave ricochets (rebounding) back and forth between transducer and structure. Readjust the angle; reducing gain or the contrast might not eliminate ‘em.

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Linear reverberation: ascending aorta, mimics dissections.

Comet tail artifact (form of reverberation with a triangular, tapered shape): reflective interfaces, sequential echoes and displayed images are so closely spaced that individual signals are not perceived. The last echo may have decreased amplitude secondary to attenuation (decreased displayed amplitude).

Ring down artifacts (supposed comet tail variant): main US beam encountering a ring of bubbles with fluid trapped centrally. Resonant vibrations from the pocket of fluid between air bubbles creates a continuous sound wave transmitted back to the transducer. Displayed as a bright reflector with echogenic line (or series of parallel bands) extending posteriorly (below figures).

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Mirror image artifact (type of reverberation): duplicate of real structure. Primary beam encounters a highly reflective interface (mirror-like specular reflector), the mirror redirects the ultrasound beam towards the second reflector (the structure), the reflected echoes then encounter the “back side” of the second reflector (the structure) and are reflected back toward the reflective interface before being reflected back finally to the transducer. The transducer “thinks” the sound is coming from the original direction in which it was sent, but it takes longer to return, so it “sees” the object farther, deeper. Duplicated echogenic lesion is misplaced distant—deeper—farther away and equidistant from the deeper strongly reflective interface. Common at the level of the pericardium or diaphragm, with the pleural–air interface acting as the strong reflector (below figures). Options: change the angle (different orientation).

image

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Artifacts Associated with Velocity Errors

The speed of sound within different materials (air 330, fat 1450, soft tissue 1540, bone 4080 meters per second) depends on their density and elastic properties. US image processing assumes a constant speed of sound of 1540 m/sec. When sound travels through material with a velocity significantly slower than the assumed 1540 m/sec, the returning echo will take longer to return to the transducer and will be displayed deeper on the image.

Speed displacement artifact: the ultrasound system assumes a travel velocity of 1540 m/s (since time is distance, if ultrasound beam travels faster than assumed, faster speed equals earlier return, echoes will be falsely displayed closer than they really are).

Refraction: change in direction (bending away from straight line path) of sound wave traveling from one medium to another (≠propagation speed). Occurs only when there is an oblique incidence—non-perpendicular (Snells’s–Descartes’ law).

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θ1 (incidence angle) > θ2 (transmission angle) because n1 > n2

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θ1 (incidence angle) < θ2 (transmission angle) because V1 < V2

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Refraction artifact: based on the assumption that beam travels in a straight line, returning echoes from refraction misplace their true location (below figures).

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Artifacts Associated with Attenuation Errors

Attenuation: absorption and scatter (greater distance traveled, more attenuation). Compensation amplification (TGC) of echoes that take longer to return to the transducer, that return later (>earlier returning echoes) make image appear more uniform in he deep field.

Shadowing: when the US beam encounters a focal material that attenuates the sound to a greater extent [or a lesser extent] than the surrounding tissues (high acoustic impedance), the strength of the beam distal to this structure will be weaker—shadowing—little US energy remains to image deeper tissues [or stronger—increased through transmission].

Thus when US beam encounters a strongly attenuating or highly reflective structure (structure with high attenuation), the amplitude of the beam distal to this structure is diminished (ultrasound beam is weakened), the echo returning from structures beyond (deeper) the highly attenuating structure will also be diminished, weakened (dark or hypoechoic band), resulting in poor resolution within the shadow.

Calcified tissue and metallic valves eat up all the ultrasound waves (looking white) and do not allow ultrasound to go further, throwing a distal shadow.

image

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Near-field clutter: noise near transducer arising from high-amplitude oscillations of piezoelectric elements. Problem: identify echogenic structures in the near field.

Electronic interface: linear “dot” artifact affecting the whole image (electrocautery).

Incorrect gain: create and obscure information, reduces lateral resolution. Erroneous appearance of spontaneous echo contrast (SEC), no “swirling” with grainy appearance.

Range ambiguity: At high pulse repetition frequency (don’t be afraid to go back and look at the physics in Chapter 5 again), the ultrasound can get fooled into thinking an object is in the wrong location. A second pulse is sent out before the first one comes back, so the machine can’t tell where the signal came from.

The end result of this chicanery? A Swan might appear in the left ventricle (oops!), or an aortic valve might appear in the middle of a chamber (how can the patient be so stable with his aortic valve sitting in the middle of the left ventricle?). Change the depth of the image (that will alter the pulse repetition frequency) and that should make the “mystery object” disappear.

Doppler Artifacts and Pitfalls

The main Doppler headache, and a recurring point in the meeting and tapes, is aliasing. You will hear a million times (and no doubt be asked) to differentiate between continuous-wave Doppler (range ambiguity but no problem with aliasing) and pulsed-wave Doppler (range certainty but problems with aliasing).

As before, don’t be afraid to go through the physics in Chapter 5 again. You finished physics a looooooooooooooooooooooooong time ago, and this stuff can be a little tricky.

Aliasing: you can’t measure a maximum velocity.

Wrong angle: remember the cosine thing from long ago in a galaxy far, far away? To get an accurate measure, you have to be looking “straight up the pipe”.

If the angle is more than 20 degrees off kilter, your Doppler will be inaccurate.

Beam width: this is the “Doppler equivalent” of the problem with side lobes. You are trying to see, for example, the left ventricular inflow from the left atrium, but your beam width is too wide and you also see an aortic regurgitant jet at the same time, screwing up the signal.

image

Another problem with beam width is “the third dimension”. The beams are not perfect 2-D structures. There is a thickness to them, so you may get an abnormal signal from something “out of the plane” of the image, but still “pick-upable” from the beam.

Mirror image artifact: A symmetric but weaker signal appears in the opposite direction of what you measure.

When this happens, reduce the gain.

Ghosting: remember the old chestnut about seeing the “green flash” of the sun just as it is setting? No doubt many a retina has gotten fried to a crisp looking for that. Ghosting is sort of like that.

With color Doppler, if the patient has a strong reflector, like a metallic valve, you can get a brief flash of blue or red that doesn’t correspond to any flow pattern. It’s like, you know man, just like this big flash, so don’t get all bent out of shape, man.

Structures Mimicking Pathology

This is total Visual City, USA, and a lot of it you’ve heard of before.

All kinds of normal things and embryonic leftovers are floating around the heart, gumming up the works and throwing you for a loop. The list is quasi long, but each individual one just takes a little memory work and pattern recognition.

Moderator band: A big muscle band in the apical third of the right ventricle (never in the left). The moderator band has part of the conduction system in it.

image

Pleural effusion: well, this isn’t exactly mimicking pathology, hell, it IS pathology. But anyway, you see this lateral and posterior to the heart. Since the heart is on the left side (usually), you usually only see left pleural effusions.

Nodulus arantii: kick ass name, huh?

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Little knobby fibrous thingies at the center of the free edge of each cusp of the aortic valve. Dig the Latin name.

Lambl’s excrescences: better name, even, than nodulus arantii. I hope I never get excrescences anywhere.

Filamentous thingamabobs attached (usually) to the aortic side of the aortic valve leaflets.

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Coumadin ridge: atrial tissue dividing the atrial appendage from the left upper pulmonary vein. Can look like a clot, but don’t fall for it.

I already drew this once, don’t get greedy.

Pectinate muscles: parallel ridges along endocardial surfaces of the left and right atria, as well as both appendages.

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Crista terminalis: little valve-like thingie at the junction of the superior vena cava and the right atrium (so you’ll see it to the right side of a bi-caval view).

Eustachian valve: same kind of deal over on the other side, where the inferior vena cava meets the right atrium.

Both of these were drawn a while ago. Re-read the book, if you missed it.

Thebesian valve: same kind of deal, but now at the entrance to the coronary sinus. This can make it hard to place the retrograde cannula, since it is, in effect, a valve.

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Chiari network: kind of like a Lambl’s excrescence in the atrium.

Wall motion abnormality: this isn’t really an anatomic pitfall, but it’s worth noting here, as Dr. Grichnik did in her lecture (and from which I hope she doesn’t mind I stole everything!).

Epicardial pacing can make the septal wall appear hypokinetic or dyskinetic. The normal sequence of depolarization doesn’t occur, so you could be fooled into thinking there was a coronary occlusion leading to this regional wall motion abnormality.

Answers

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