Artifacts are a favorite SPI exam topic because they test whether you understand the underlying physics, not just whether you can recognize a pattern on a screen. Every artifact below has a clear physical cause — and the exam usually asks about the cause, not just the name. Here are the seven that come up most often.
1. Reverberation
Reverberation occurs when sound bounces back and forth between two strong, parallel reflectors — often the transducer face and a superficial reflective structure — before finally returning to the transducer. Each "bounce" creates an additional echo, displayed at increasing depth, producing a series of equally spaced parallel lines that get fainter with depth.
The exam may also test "ring-down" artifact as a related concept — a continuous reverberation often associated with gas bubbles, appearing as a solid streak rather than discrete lines.
2. Comet-tail artifact
A specific, dense form of reverberation, comet-tail artifact appears as a bright, tapering trail of echoes extending from a strongly reflective small object — classically associated with metallic objects (like a surgical clip or IUD) or small gas bubbles. The "tail" results from very rapid, closely spaced reverberation between the object's surfaces.
3. Posterior acoustic shadowing
Shadowing appears as a dark band extending posterior to (behind) a structure that strongly attenuates or reflects sound — most commonly calcifications, bone, or dense fibrous tissue. Because so little sound energy makes it past the structure, very little returns from the tissue behind it, so that region appears dark on the image.
Shadowing is a useful diagnostic clue, not just an artifact to correct — its presence or absence helps differentiate, for example, a calcified gallstone (which shadows) from a polyp (which typically does not).
4. Posterior acoustic enhancement
The opposite of shadowing — enhancement appears as a bright band posterior to a structure that attenuates sound very little, classically a simple fluid-filled structure like a cyst or the bladder. Because the fluid absorbs less sound than the surrounding tissue would, more sound energy reaches the tissue behind it, and that tissue appears artificially bright by comparison to the surrounding area at the same depth.
5. Mirror image artifact
Mirror image artifact occurs when sound reflects off a strong curved or flat reflector (commonly the diaphragm) and continues on to image a real structure, then returns along the same reflected path. The system assumes sound traveled in a straight line, so it displays a duplicate of the real structure on the opposite side of the strong reflector — at a depth corresponding to the total path length traveled.
Classic example: a liver lesion appearing to be duplicated above the diaphragm, inside what should be the lung field — the "duplicate" is the mirror image, not a second real structure.
6. Refraction artifact
Refraction occurs when sound crosses a boundary between two tissues with different propagation speeds at an oblique angle, causing the beam to bend. Because the system assumes the beam travels in a straight line, a structure can appear to be displaced from its true position — sometimes appearing duplicated or split.
A commonly cited example is at the edge of the rectus abdominis muscles, where refraction can cause a structure (like a gestational sac) to appear duplicated, side by side.
7. Side lobe and beam width artifacts
Real ultrasound beams aren't perfectly narrow — they have a main beam plus weaker side lobes (or, in array transducers, grating lobes). Energy from these secondary beams can return from structures outside the main beam's path, and the system displays that energy as if it came from the main beam — producing artifactual echoes inside structures that should be anechoic, such as artifactual "debris" appearing within a simple cyst.
Beam width artifact is the related concept where the main beam itself is wider than displayed, causing structures at the edge of the beam to be smeared across a wider area than their true size — particularly noticeable at the focal zone boundaries.
How the exam tests this: rather than just asking "what is this artifact called," questions often describe a scanning scenario and ask you to identify the artifact from the physical setup — e.g., "a strongly reflective curved structure near the diaphragm, with a duplicated structure appearing above it" (mirror image), or ask what setting change would reduce a described artifact. Recognizing the underlying cause is what lets you answer both question types from the same knowledge.
A quick reference table
| Artifact | Visual appearance | Underlying cause |
|---|---|---|
| Reverberation | Equally spaced parallel lines, fading with depth | Sound bouncing between two strong parallel reflectors |
| Comet-tail | Bright tapering trail behind a small reflector | Rapid reverberation in metal or gas |
| Shadowing | Dark band behind a structure | Strong attenuation/reflection by that structure |
| Enhancement | Bright band behind a structure | Minimal attenuation (fluid-filled structure) |
| Mirror image | Duplicate structure across a strong reflector | Reflected path mistaken for a straight path |
| Refraction | Structure displaced or duplicated | Beam bending at a tissue boundary |
| Side lobe / beam width | Artifactual echoes inside anechoic structures | Energy from secondary beam paths |