10.3 Doppler Artifacts

Doppler Assumptions and Their Failures

Doppler instruments add assumptions of their own: pulses are transmitted at a fixed pulse repetition frequency (PRF), flow is sampled along a known beam-to-vessel angle, and every detected frequency shift comes from blood moving inside the sample gate. Each artifact below is recognizable by a characteristic display pattern and correctable with a specific control. The ARRT outline names aliasing, twinkling, and mirror image explicitly; the others appear as recognition-and-correction scenarios.

Aliasing

Aliasing occurs when the Doppler shift exceeds the Nyquist limit, defined as PRF/2 — the sampling rate is too slow to track the true shift, so the systolic peak is cut off and wraps around to the opposite side of the baseline. In color Doppler the fastest flow inverts to the opposite color (red wrapping to blue) without the black baseline separation of true flow reversal. A worked example: at a PRF of 5000 Hz the Nyquist limit is 2500 Hz; a true shift of 3200 Hz is displayed as a reversed signal at 1800 Hz below the baseline. Aliasing is typical in high-velocity jets at stenoses. Corrections, roughly in order:

1. Increase the scale (PRF) — this raises the Nyquist limit directly. Note that maximum PRF falls as sampling depth increases, because each pulse needs time to return before the next is sent.
2. Shift the baseline — reallocates the entire scale to one direction of flow.
3. Use a lower Doppler frequency — a lower transmitted frequency yields a smaller Doppler shift for the same blood velocity, keeping the shift under the Nyquist limit.
4. Switch to continuous wave (CW) Doppler — CW transmits and receives simultaneously with no PRF and therefore cannot alias, at the cost of losing range specificity (the exact depth of the high velocity is no longer known).
5. If still aliasing, reduce sampling depth or adjust the Doppler angle.

Spectral Mirror Image

With excess spectral gain (or a near-perpendicular beam), a weaker symmetric ghost waveform appears on the opposite side of the baseline — a spectral mirror image that makes flow look bidirectional. It is recognized as an imperfect, fainter copy of the real trace and is corrected simply by reducing the Doppler gain and optimizing the insonation angle.

Color Mirror Image

A strong specular reflector — classically the pleura at the lung apex behind the subclavian vessels — can bounce the Doppler beam so that a vessel's color (and even its spectral trace) appears duplicated on the far side of the reflector, suggesting a second vessel that does not exist. Confirm by changing the angle of approach: the phantom vessel disappears while the real one persists.

Twinkling Artifact

Twinkling is a rapidly fluctuating mosaic of color pixels appearing behind a rough, strongly reflecting surface — urinary tract stones, parenchymal calcifications, stents. It arises from machine phase jitter interacting with the irregular reflector, not from motion or true flow. It is a genuinely useful sign: twinkling can flag a small non-shadowing renal or ureteral stone that grayscale imaging misses. Raising color gain or write power makes it more conspicuous; if you need to suppress it, reduce the color gain or power.

Blooming and Flash

Blooming (color bleed) occurs when color gain is set too high: color pixels spill beyond the vessel walls, exaggerating lumen size and potentially hiding plaque or a filling defect. Reduce color gain until color just fills the lumen without overflowing. Flash artifact is a sudden burst of color across part or all of the image caused by transient tissue or transducer motion — respiration, bowel movement, or probe slip. Because power Doppler integrates the amplitude of the Doppler signal and is more sensitive to any motion, flash is worse in power mode. Control it with a steady hand, a patient breath-hold, and appropriate persistence settings.

Angle Dependence

The Doppler equation makes the detected shift proportional to the cosine of the beam-to-flow angle. At 90 degrees the cosine is zero: a perfectly patent vessel imaged perpendicular to the beam shows no color and no spectral signal — pseudo-absence of flow, easily mistaken for occlusion or avascularity. Always heel-and-toe the probe or steer the beam to 60 degrees or less before declaring a vessel avascular, and never report velocities measured at angles above 60 degrees, where small angle errors translate into large velocity errors.

Wall Filter

The wall (high-pass) filter strips out low-frequency, high-amplitude signals produced by vessel-wall and surrounding tissue motion. Set too high, it also deletes true slow flow — diastolic flow, venous signals, and the trickle of perfusion you are hunting in suspected testicular or ovarian torsion, transplant compromise, or a borderline renal artery. When slow flow is the clinical question, lower the wall filter together with the scale so genuine low-velocity signals survive.

Recognition Guide