Ultrasound Artefacts: What They Are and What They Tell You
An artefact is anything on the screen that does not match the anatomy. Some hide disease, some imitate it, and some — the shadow behind a stone, B-lines in the lung — are the diagnosis. Why the machine makes them, how to recognise each one, and how presets, harmonics and compounding change what you see.
Why artefacts happen
The machine builds the image on a few assumptions: sound travels in a straight line at 1,540 m/s through all tissue, each echo comes from a single reflection, and sound weakens at a steady rate with depth. When an assumption fails, the image shows something that is not there, puts something in the wrong place, or hides it.
| Assumption | When it fails | Artefacts it causes |
|---|---|---|
| Sound travels in a straight, narrow beam | Sound bends at tissue boundaries, and weaker side beams leave the probe | Refraction duplication, edge shadowing, side lobes |
| Each echo comes from one reflection | Sound bounces between strong reflectors before returning | Reverberation, comet tail, ring-down, mirror image |
| Sound weakens at the same rate everywhere | Some tissues absorb much more or much less than average | Acoustic shadowing, posterior enhancement |
| Speed is 1,540 m/s everywhere | Sound is slower in fat and faster in some other tissues | Speed displacement: structures drawn slightly too deep or too shallow |
| The beam has no thickness | The beam has a real slice thickness | Partial volume: false echoes inside cysts and the bladder |
Shadowing and enhancement
Acoustic shadowing
A dark band behind something that reflects or absorbs most of the sound — gallstones, kidney stones, bone, calcification. A bright focus with a clean, dark shadow in the gallbladder supports a stone.
Dirty shadowing
Gas reflects nearly all sound, giving a grey, streaky shadow with reverberation in it. It is typical of bowel gas and helps tell gas from a stone.
Posterior acoustic enhancement
A brighter area behind fluid — cysts, the gallbladder, a full bladder — because fluid absorbs less sound than the machine expects. It helps separate a simple cyst from a solid lump, though some solid lesions also enhance.
Edge shadowing
Thin dark lines falling from the curved edges of round structures such as cysts, the fetal head or the testis, caused by sound bending at the edge. It is not a stone.
Reverberation, comet tail and ring-down
Reverberation
Evenly spaced bright lines repeating deeper and deeper, when sound bounces between the probe and a strong reflector. A-lines in the lung are reverberations of the pleural line; the same effect puts haze in the front of the bladder.
A-lines and B-lines in lung ultrasoundComet tail
A short, tapering trail of closely spaced echoes behind a tiny strong reflector, such as metal, cholesterol crystals in the gallbladder wall or colloid in a thyroid nodule.
Comet tails in thyroid TI-RADSRing-down
A long, unbroken bright band behind small gas bubbles, from fluid trapped between them resonating. Often seen with gas in bowel or in an abscess.
Mirror images, side lobes and misplaced echoes
Mirror image
A strong curved reflector — usually the diaphragm — acts like a mirror, so the liver or spleen appears again above it. This is normal. Its loss, with dark fluid above the diaphragm, suggests a pleural effusion.
The chest views in eFASTSide lobes
Weaker beams to the side of the main beam pick up strong reflectors and paint false echoes into fluid — a fake layer of sludge in the gallbladder or debris in the bladder. They change or vanish when the probe angle changes.
Slice thickness (partial volume)
The beam has thickness, so tissue next to a small cyst is averaged into it and looks like debris. Scan in two planes and narrow the focus.
Refraction duplication
Sound bending through the muscles of the abdominal wall can double a structure in the midline — a known cause of a false “twin” gestational sac in early pregnancy.
Speed displacement
A step in a straight line, such as a needle or the diaphragm, where it passes behind fat.
Anisotropy in tendons, ligaments and nerves
Tendons and ligaments — and, to a lesser degree, nerves and muscle — look bright only when the beam meets them at a right angle. Tilt the probe a few degrees and a normal tendon turns dark, imitating a tear or tendinopathy.
- Rock or tilt the probe until the fibres light up before calling anything abnormal.
- Compare with the same structure on the other side.
- Use anisotropy on purpose: tilting makes a tendon go dark against bright fat, which helps pick it out from the tissue around it.
Read next MSK ultrasound machines and probesSeeing nerves for nerve blocks
Doppler artefacts
Aliasing
Colour wraps from red to blue, or a spectral trace wraps around, when flow is faster than the scale can show. Raise the scale or move the baseline.
Twinkling
A rapidly changing mix of colour behind a rough, strong reflector such as a kidney stone. It can help confirm a small stone.
Flash and blooming
Colour from patient or probe movement, and colour spilling outside the vessel when colour gain is set too high.
Colour and spectral DopplerHow settings change artefacts
| Setting | Effect on artefacts | When to change it |
|---|---|---|
| Tissue harmonic imaging | Cuts reverberation, side-lobe clutter and haze in fluid; sharpens cyst walls | Keep on for most abdominal and cardiac work; some lung protocols turn it off to see A-lines and B-lines as they are |
| Spatial compound imaging | Averages several beam angles: smoother image and less speckle, but weaker shadowing and enhancement | Turn it off to look for the shadow behind a small stone or calcification |
| Speckle reduction and smoothing | Less grain, but fine echoes and faint B-lines can blur | Lower it for lung and fine detail |
| Gain and TGC | Too much gain fills cysts with false echoes; badly set TGC can fake or hide enhancement | Set gain so fluid is black, then adjust TGC depth by depth |
| Focus and frequency | The beam is narrowest at the focus; higher frequency gives a thinner slice and less partial volume | Put the focus at the structure you are measuring |
| Probe angle and window | Moves mirror images, refraction, side lobes and anisotropy | Real anatomy stays in two planes; most artefacts move or disappear |
Read next Knobology: every control explained
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