A Doppler waveform displays aliasing when the waveform wraps around the baseline.

A Doppler waveform displays aliasing when the waveform wraps around the baseline.

• A. Aliasing
• B. Spectral broadening
• C. Mirror image
• D. Bidirectional flow

Answer: (A)

Aliasing happens when the Doppler sampling rate (the pulse repetition frequency) isn’t high enough to capture very fast blood velocities. In a Doppler spectrum, velocity information is plotted within a limited range set by the Nyquist limit, which is half the PRF. If the actual flow velocity produces a Doppler shift beyond that limit, the signal folds back into the displayed spectrum and appears on the opposite side of the baseline. This wrap-around is what looks like the waveform jumping from the top end back near the baseline, creating the aliasing artifact. So the waveform wrapping around the baseline is the hallmark of aliasing. Spectral broadening is simply the widening of the spectral peak due to several factors and doesn’t involve wrapping around. A mirror image is a different Doppler artifact, and bidirectional flow describes the presence of flow in two directions rather than an artifact of the display. To reduce aliasing, you can increase the PRF (adjust the velocity scale), use a shallower sample depth, shift the baseline, or switch to continuous-wave Doppler, which does not alias.

Aliasing happens when the Doppler sampling rate (the pulse repetition frequency) isn’t high enough to capture very fast blood velocities. In a Doppler spectrum, velocity information is plotted within a limited range set by the Nyquist limit, which is half the PRF. If the actual flow velocity produces a Doppler shift beyond that limit, the signal folds back into the displayed spectrum and appears on the opposite side of the baseline. This wrap-around is what looks like the waveform jumping from the top end back near the baseline, creating the aliasing artifact.

So the waveform wrapping around the baseline is the hallmark of aliasing. Spectral broadening is simply the widening of the spectral peak due to several factors and doesn’t involve wrapping around. A mirror image is a different Doppler artifact, and bidirectional flow describes the presence of flow in two directions rather than an artifact of the display.

To reduce aliasing, you can increase the PRF (adjust the velocity scale), use a shallower sample depth, shift the baseline, or switch to continuous-wave Doppler, which does not alias.