10.2 Grayscale Artifacts

Why Artifacts Matter

An artifact is any echo on the display that does not correspond to a real reflector at the displayed location — or a real structure that is missing, misplaced, or mis-shaped. Artifacts arise because the machine makes fixed assumptions: sound travels in a straight line, at a constant 1540 m/s in soft tissue, reflects only once along each path, and returns only from the main beam axis. Whenever tissue violates one of these assumptions, an artifact appears. The ARRT outline explicitly lists gray scale artifacts — reverberation, mirror image, shadowing, posterior enhancement, and comet tail — so expect to name each one, state its cause, and choose how to reduce it or exploit it.

Reverberation

Reverberation occurs when the pulse bounces repeatedly between two strong reflectors — often the transducer face and a near-field interface, or two parallel fascial planes. The system times each round trip and assumes every returning echo made a single trip, so it plots duplicate reflectors at equally spaced multiples of the true depth, fading in amplitude with depth. Classic sites are the anterior urinary bladder wall and the superficial abdominal wall. Reduce reverberation by changing the scan angle, moving the transducer, lowering the gain, or using tissue harmonic imaging.

Comet Tail and Ring-Down

Comet tail is a short, tapering train of closely spaced reverberations deep to a small, highly reflective object — cholesterol crystals in gallbladder adenomyomatosis, surgical clips, or an intrauterine device. Ring-down is its cousin: a continuous solid streak generated when the pulse excites resonance in a cluster of gas bubbles or a fluid-gas interface, as with pneumobilia or gas in an abscess. Both are useful signs rather than nuisances — a comet tail rising from a gallbladder wall fold essentially confirms adenomyomatosis, and ring-down flags gas where none should be.

Mirror Image

A mirror-image artifact duplicates a structure across a strong specular reflector, most classically the diaphragm. Sound reflects off the diaphragm, strikes a hepatic lesion or vessel, returns to the diaphragm, and only then travels back to the transducer; the longer round trip places a false copy deeper than the diaphragm, so a liver lesion appears to sit inside the lung. The true structure and its phantom are equidistant from the reflecting surface. Changing the scan angle or intercostal space destroys the geometry and the artifact vanishes — which is also how you confirm it.

Posterior Shadowing vs. Posterior Enhancement

Posterior acoustic shadowing is an anechoic band deep to a strongly attenuating structure — calcification, stone, bone, or gas — that reflects and absorbs the beam so little energy remains to image the tissue behind it. Clean shadowing (sharp and echo-free) typifies calcified gallstones and bone; dirty shadowing contains reverberation and ring-down and indicates gas. Shadowing is exploited diagnostically: the wall-echo-shadow (WES) sign — gallbladder wall line, bile echo, then clean shadow — identifies a stone-packed gallbladder even when individual stones are not resolved.

Posterior acoustic enhancement (through transmission) is the opposite: fluid attenuates far less than soft tissue, so the TGC curve that corrected for soft tissue over-amplifies the echoes deep to a cyst, bladder, or gallbladder, making that region abnormally bright. Enhancement behind a round anechoic structure is strong evidence the structure is simple fluid.

Edge (Refraction) Shadowing

At the curved margin of a rounded structure — a cyst wall, the gallbladder neck, the fetal skull — the beam strikes at an oblique angle of incidence and refracts, bending away from the region directly deep to the edge and leaving a thin anechoic stripe. This is a physics artifact, not a vessel, tear, or wall defect; it shifts or disappears when you change the approach angle or center the structure in the beam.

Side Lobes and Grating Lobes

Transducers emit weak off-axis beams — side lobes in single-element designs and grating lobes in array transducers. A strong reflector such as bone, gas, or the diaphragm caught in an off-axis lobe returns an echo that the machine plots along the main beam axis, layering false low-level echoes into anechoic spaces: pseudo-sludge in the gallbladder or phantom debris in a simple cyst or bladder. The giveaway is that artifactual sludge does not layer to the dependent wall or shift with repositioning. Reduce it with tissue harmonics, lower gain, and a changed angle of approach.

Section Thickness, Speed Error, and Multipath

The beam has real thickness in the elevational plane, so section-thickness (partial-volume) artifact averages echoes from just outside the intended slice into it — another source of pseudo-debris in small cystic structures. Speed-error artifact appears when sound crosses tissue slower or faster than the assumed 1540 m/s (fat propagates near 1450 m/s): the machine miscalculates range, displacing or breaking structures deep to the slow medium — a refractile step-off in the diaphragm line is the classic look. Multipath artifact is related misregistration: echoes that ricochet off intermediate reflectors take a longer route home and are plotted too deep.

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