Propagation Artifacts

Why Artifacts Exist: The Machine's Built-In Assumptions

A B-mode image is not a photograph of tissue — it is a picture reconstructed from pulse-echo timing under a short list of assumptions. Whenever real tissue breaks one of these assumptions, the machine still follows its rules faithfully, and the result is an artifact: an echo displayed with the wrong brightness, position, or shape relative to the true anatomy. Four assumptions explain nearly every propagation artifact on the SPI exam:

1. Straight-line travel — sound travels only along the central axis of the beam, out and back, with no bending.
2. Constant speed — sound travels at the assumed 1540 m/s in every tissue the pulse crosses, so depth = (speed × time)/2.
3. Single reflection — a pulse produces exactly one echo per interface, then returns directly to the transducer with no further bouncing.
4. One pulse in flight — the system never transmits pulse n + 1 until every echo from pulse n has been received, so any echo received is assumed to belong to the most recently transmitted pulse.

Reverberation

Reverberation occurs when sound bounces repeatedly between two strong, closely spaced, roughly parallel reflective interfaces (often the transducer face and a superficial strong reflector, or two adjacent tissue interfaces) before finally returning to the transducer. Each round trip produces one more delayed echo, so the display shows equally spaced parallel bands of progressively decreasing brightness deep to the true reflector — a "picket fence" appearance. This violates the single-reflection assumption: the sound bounced back and forth multiple times, and each extra trip is misread as a separate, deeper interface.

Comet-Tail Artifact

Comet tail is a special case of reverberation produced by very small, very closely spaced strong reflectors — metallic foreign bodies, surgical clips, or cholesterol crystals (as in adenomyomatosis). Because the reflectors are closer together than the system's axial resolution, the individual reverberation echoes merge into a solid, tapering, triangular tail rather than discrete bands. It violates the same single-reflection assumption as reverberation, differing only in reflector spacing.

Ring-Down Artifact

Ring-down looks similar to comet tail but has a different mechanism: it arises from a cluster of trapped gas bubbles (biliary or portal venous gas, gas within bowel) whose fluid resonates at the transducer's frequency and continuously re-radiates sound back to the transducer for as long as the pulse's energy persists. The result is a solid or streaky band of echoes trailing from the gas collection. It still violates the single-reflection assumption, but through resonant re-emission rather than simple bouncing.

Mirror-Image Artifact

A mirror-image artifact appears when a real structure lies near a strong, smooth, curved reflector — classically the diaphragm. Sound reflects off the real structure, strikes the strong reflector, bounces back to the structure, and only then returns to the transducer. The machine assumes straight-line travel (assumption 1) with no intervening bounce, so it plots a duplicate of the real structure at an equal distance on the far side of the strong reflector — for example, a liver lesion appearing to sit above the diaphragm within the lung field.

Speed-Displacement (Propagation-Speed) Artifact

The system always converts echo return time to depth using the assumed 1540 m/s. When the true path includes a tissue with a different propagation speed — fat conducts sound more slowly, roughly 1450 m/s — the round-trip time no longer matches what 1540 m/s predicts. Because fat is slower, the echo returns later than expected, and the machine, still using 1540 m/s, computes too great a distance: the structure is displayed deeper than its true position, and a smoothly curved border like the diaphragm can show an artifactual step where the beam crosses a fat pad. This violates the constant-speed assumption.

Range Ambiguity

Range ambiguity violates the one-pulse-in-flight assumption. If the pulse repetition frequency is set very high (as in high-PRF Doppler techniques used to sample high velocities at depth), the pulse repetition period may become shorter than the round-trip time to a deep reflector. A second pulse is transmitted before the echo from the first pulse's deepest reflector has returned. That late echo is then received during the second pulse's listening window and misassigned as if it came from that second, more recent pulse — placing a deep structure erroneously close to the transducer.

Quick discriminator: evenly spaced repeating echoes point to reverberation or comet tail; an artifact trailing continuously from a gas pocket points to ring-down; a normal-appearing duplicate beyond a strong curved reflector points to mirror image. On the exam, the fix for reverberation-family artifacts is usually to change transducer angle or patient position so the beam no longer strikes the reflector pair perpendicularly — not to adjust gain or TGC, which cannot remove a geometry-driven artifact.