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ARTIFACTS IN ULTRASONOGRAPHY

ARTIFACTS IN ULTRASONOGRAPHY

I was a participant at the ultrasound course, where Pete Mantis said to us, that he is not going to let us do the practicals, as long as we don’t recognize the artifacts on the pictures, that he was showing during the lectures. Being familiar with the most common artifacts, which are to some degree encountered during every ultrasound examination, is very important to avoid interpretation errors, because they may obscure normal anatomy or disease, or may be misinterpreted as pathology. On the other hand, some artifacts may help us make the diagnosis.

Artifacts are any alterations in the image which do not represent an actual image of the examined area. They may be produced by technical imaging errors or result from the complex interaction of the ultrasound with biological tissues.

REVERBERATION

Reverberation artifacts appear as a series of equally spaced lines. They are produced by an ultrasound beam repeatedly bouncing back and forth between two highly reflective interfaces or between the transducer and a strong reflector. They can obscure deeper structures but can also be useful when detected in unexpected locations.
Repetition artifacts are produced each time the sound wave pulse returns to the transducer after it hit a reflective surface (such as gas, bone or metal, particularly if this interface is near the transducer). This echo is partly captured by the probe, producing a hyperechoic line. The surface of the probe will reflect the high-intensity echo and send it back and forth, resulting in several equidistant hyperechoic lines, each one deeper. The number of reverberation images depends on the penetrating power of the beam and the sensitivity of the probe.
Comet tail artifacts can be seen with gas bubbles in the intestinal loop, which form thin layers, separated by liquid; the waves rebound between the layers, resulting in many echoes that return to the probe at irregular intervals, making a trail of closely spaced, discrete, very bright, small echoes, resembling a comet’s tail. This artifact can also be caused by metallic pellets, surgical clips or a biopsy needle.

MIRROR IMAGE

A strongly reflective, smooth, curved interface (= mirror) may reflect the sound distally instead of returning it to the transducer. Objects within the direction of the beam reflect the sound beam back to the mirror and from there back to the transducer. The path of the reflected echoes is longer and because the ultrasound machine does not predict such refraction of the beam (it assumes that pulses and echoes travel in a straight line), it places mirror image at a deeper location along the beam axis. This can lead to misinterpretation of the location of an organ or structure.
An example of a “mirror” is the diaphragm, which is highly reflective because of the air-filled lung behind it. On the ultrasound images, the liver and gallbladder can look like being cranial to the diaphragm in the thoracic cavity. It is important to recognize this artifact to avoid misdiagnosing the diaphragmatic rupture or lung consolidation. This artifact will not occur in the presence of pleural effusion.

ACOUSTIC SHADOWING

Shadowing is created by nearly complete absorption or reflection of the sound beam at the structure of high attenuation. If the sound is reflected (in case of soft tissue-gas interface), the area below the structure looks inhomogeneous (dirty shadowing) because of multiple reflections or reverberations. If a significant portion of the ultrasound beam is absorbed and reverberations are absent (with soft tissue-bone or calculi interface), this results in poorly echoic or anechoic (clean) shadowing. Shadowing can obscure deeper structures but is also useful to identify calculi within the urinary tract.

Edge shadowing can appear as acoustic shadowing zones distal to the lateral margins of fluid-filled curved structures (e.g. gallbladder, bladder, cyst, kidney, adrenal glands). The sound waves penetrating the edge of a structure may be refracted, producing a linear or triangular anechoic zone below the lateral edges of the structure.

ACOUSTIC ENHANCEMENT

Acoustic enhancement is a localized increase of echo amplitude distal to a structure of low attenuation, seen as an area of increased brightness. When sound waves pass through a poorly attenuating structure that allows them easy passage, there is less tissue reflection and an area of artifactual increased echogenicity is produced right under the structure because more sound waves are present in this area compared with tissues at the same depth around. It is typically seen with fluid-filled structures in a soft tissue background (e.g. gallbladder, liver cyst). Acoustic enhancement can help differentiate fluid-filled structures from solid, hypoechoic masses.

SLICE THICKNESS/BEAM WIDTH

The ultrasound beam is not equally wide everywhere. When exiting the probe, its width is similar to the probe, then it becomes narrowest at the focal zone and widens again deeper. When a wider part of the beam includes part of a cystic structure and its surrounding tissues, the echoes from the tissue are mistakenly displayed within the cystic structure (bladder, gallbladder), imitating the presence of sediment (pseudo-sludge). The echoes disappear if the entire width of the beam is inside the cystic structure, consequently placing a focal zone wisely reduces this artifact.

LITERATURE

Barr F., Gaschen L.: BSAVA Manual of Canine and Feline Ultrasonography. BSAVA, 2011

Penninck D., d’Anjou M.: Atlas of Small Animal Ultrasonography, 2nd Ed. Wiley Blackwell, 2015

Mattoon J. S., Nyland T. G.: Small Animal Diagnostic Ultrasound, 3rd Ed. Saunders Elsevier, 2015