Curriculum · Ultrasound Artifacts
Propagation Artifacts & the Assumptions Framework
The assumptions the beamformer makes, then the propagation artifacts they create: reverberation, comet-tail vs ring-down, mirror image, refraction/edge shadowing, speed error, range ambiguity, and side vs grating lobes.
~40 min · level: intermediate · SPIPOCUS draft — pending clinical review
Learning objectives
- State the assumptions the scanner makes and map each artifact to the violated assumption.
- Differentiate reverberation, comet-tail, and ring-down by mechanism and appearance.
- Explain mirror-image, refraction/edge, speed-error, and range-ambiguity artifacts and how to eliminate them.
- Distinguish side lobes from grating lobes.
Every artifact is the visible consequence of violating a fixed assumption the beamformer is hard-wired to make when it converts echo timing and amplitude into a pixel. The scanner computes depth as with fixed at 1540 m/s, and assumes:
| Assumption | Violated by |
|---|---|
Constant 1540 m/s speed | Speed-error / bayonet |
Straight-line travel | Refraction/edge, mirror image |
One reflection, from the latest pulse | Reverberation, comet-tail, ring-down, range ambiguity |
Echoes arise on the central beam axis | Side lobes, grating lobes, beam-width |
Uniform attenuation | Shadowing, enhancement |
Reverberation occurs when the pulse bounces between two strong parallel reflectors (or a reflector and the transducer face). Each extra round trip is read as a deeper echo, producing equally spaced parallel lines that fade with depth. The pleura's reverberation produces lung A-lines — a sign of normal aerated lung.
Mirror image: a strong specular reflector (the diaphragm) acts as an acoustic mirror; sound takes a longer multipath route, so a duplicate of a real structure is placed deep to the mirror (e.g., liver "above" the diaphragm). It is angle-dependent — changing the angle makes it move or vanish. Its presence at the diaphragm implies aerated lung above (no effusion).
Refraction / edge shadowing: at the curved edge of a rounded fluid structure, the beam bends (Snell) and, at/beyond the critical angle, is reflected away — leaving thin hypoechoic shadow lines off the lateral edges of cysts, follicles, and the fetal skull. It shifts with angle (unlike a true calcific shadow).
Speed-error (propagation-speed) artifact: when sound crosses a medium with (e.g., fat ~1450 m/s, slow), echoes return late and are placed too deep; a once-straight interface is broken into a step ("bayonet"). A straight needle traversing fat and muscle appears bent. This confirms a fat-containing lesion (lipoma).
Range ambiguity: if PRF is high, a new pulse fires before a deep echo returns, so that echo is timed against the new pulse and placed too shallow. Lower the PRF (increase depth) to resolve it.
| Side lobes | Grating lobes | |
|---|---|---|
Origin | Radial vibration of a single element | Periodic element spacing (arrays only) |
Energy | Weak (~1%) | Stronger (near main-lobe) |
Reduced by | Apodization, harmonics | Element pitch ≤ λ/2, subdicing |
Effect | Pseudosludge in cysts/bladder | Bright off-axis duplicate of a reflector |
Key takeaways
- Every propagation artifact is the visible result of violating an assumption the beamformer hard-wires into D = c*t/2 with c fixed at 1540 m/s (constant speed, straight-line travel, one reflection from the latest pulse, echoes on the central beam axis, uniform attenuation).
- Comet-tail is reverberation between very closely spaced solid reflectors that tapers and fades with depth, while ring-down is gas-trapped fluid resonance producing a non-tapering stripe to the bottom of the image (comet-tail = solid, ring-down = gas).
- Speed-error (bayonet) artifact arises when sound crosses tissue with c not equal to 1540 (fat ~1450 m/s, slow), returning echoes late and placing them too deep so a straight needle or interface appears bent/stepped, confirming a fat-containing lesion.
- Mirror-image and refraction/edge-shadowing artifacts are both angle-dependent and shift or vanish when the insonation angle changes, distinguishing them from true findings like a real calcific shadow.
- Side lobes come from radial vibration of a single element (weak ~1%, reduced by apodization and harmonics), whereas grating lobes are array-specific from periodic element spacing (stronger, reduced by element pitch <= lambda/2 and subdicing).
Check your understanding
Registry-style items with worked rationales.
1A non-tapering echogenic stripe extends to the bottom of the screen deep to a collection of bowel gas. This is most consistent with:analysis
2A straight biopsy needle appears bent ('bayonet') as it passes from fat into muscle. The cause is:analysis
3Grating lobe artifact differs from side lobe artifact in that grating lobes:recall
Go deeper — trusted free resources
Hand-picked, verified links to authoritative open resources. Opens in a new tab.
A consolidated reference article cataloguing the major ultrasound artifacts (reverberation, comet tail, ring down, mirror image, refraction, speed/propagation errors) with mechanisms and example images.
A free, peer-reviewed chapter that frames artifacts as violations of core imaging assumptions (straight-line travel, constant 1540 m/s speed, single direct reflection), making it ideal grounding for the assumptions framework.
Focused article on how beams bouncing between two strong reflectors create equally spaced false echoes, with comet tail and ring down discussed as special variants.
Explains how a strong reflector such as the diaphragm produces a duplicated structure deep to it, plus scanning maneuvers to recognize and avoid the artifact.
References
- Feldman MK, Katyal S, Blackwood MS. US Artifacts. RadioGraphics. 2009;29(4):1179–1189.
- Kremkau FW. Sonography Principles and Instruments. 9th ed. Elsevier; 2016.
- Edelman SK. Understanding Ultrasound Physics. 4th ed. ESP Inc.; 2012.
- Avruch L, Cooperberg PL. The ring-down artifact. J Ultrasound Med. 1985;4(1):21–28.