Aliasing phenomenon (ultrasound)

Aliasing is a commonly encountered artefact in colour and spectral Doppler ultrasound, which is caused by blood flow velocity exceeding the upper limits of the scale set by the user ¹. The maximum velocity that can be displayed is limited by the pulse repetition frequency (PRF). If the PRF is too low, this results in a "wraparound" effect. Generally, the pulse repetition frequency must be twice as high as the highest Doppler frequency shift (the Nyquist frequency) to avoid the artefact.

In spectral Doppler

In case of spectral Doppler the velocity peak is cut off at the peak of the scale, and the peak is displayed at the bottom of the scale, often overlapping with the rest of the curve. The artifact can be quickly remedied by lowering the baseline (if display of flow away from the transducer is not required), or increasing the PRF. Less frequently, increasing the Doppler angle might also become necessary ².

In colour Doppler

In colour Doppler aliasing is encountered as red to blue hues immediately adjacent to each other in a vessel, which is - unlike in case of true flow reversal - not separated by a black region of no flow. The artefact immediately disappears if the upper margin of the velocity scale is increased above the peak flow velocity. Colour aliasing is useful for detecting foci of increased flow (e.g. stenosis, arteriovenous fistula). Note that aliasing does not occur with power Doppler, as it does not display velocity ².