Revision 6 for 'Aliasing phenomenon (ultrasound)'

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Aliasing artefact (ultrasound)

Aliasing is a phenomenon inherent to doppler modalities which utilize intermittent sampling in which an insufficient sampling rate results in an inability to accurately record direction and velocity. 

Physics

Unlike continuous wave doppler, pulsed wave and color flow doppler modalities alternate between rapid emission of ultrasound waves (at a rate termed the pulse repetition frequency) and reception of incident ultrasound waves. The time an ultrasound wave travels, given a constant speed in soft tissue (c = 1540 meters/second) will correspond to the distance traveled. 

Pulsed wave doppler and color flow doppler operate on this presumption; when a location of interest is designated, and the ultrasound machine will record returning echoes during the interval that corresponds to the time necessary for wave egress and return along a linear path (at twice the distance to the location of interest). If doppler shifts occur at a frequency exceeding the maximum pulse interval (1/pulse repetition frequency) and detected phase shifts will be calculated based on incorrect assumptions.

The Nyquist limit defines the frequency at which aliasing and range ambiguity will occur, and is equal to the PRF/2.

Clinical use

Pulsed wave 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.

Color flow doppler

In color 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. Color 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.

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