Aliasing phenomenon (ultrasound)
Aliasing is a phenomenon inherent to Doppler modalities which utilize intermittent sampling in which an insufficient sampling rate results in an inability to record direction and velocity accurately.
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. Factors such as higher velocities of target structures and increasing depth of the region of interest insonated will result in aliasing and consequent range ambiguity. Other potential causative factors include:
- use of higher frequency transducers
- inappropriate angle of insonation
- large sampling volume
A specific use for aliasing in echocardiography is the calculation of the effective regurgitation orifice area in the assessment of valvular regurgitation, most commonly involving the mitral valve. With color Doppler interrogation of a mitral regurgitant jet, a hemispheric flow convergence forms surface area tapering to form the vena contracta before entering the left atrium. The hemispheric area (PISA) is calculated and the product of PISA and aliasing velocity yields regurgitant flow. The quotient with calculated MR regurgitant VTI yields EROA.
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.
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.