The parameter used to define when aliasing will occur in pulsed wave Doppler is?

• A. Doppler shift
• B. Waveform sampling
• C. Velocity aliasing
• D. Nyquist limit

Answer: (D)

Aliasing in pulsed wave Doppler is governed by the Nyquist limit. This limit is half the pulse repetition frequency (PRF). Because the Doppler frequency shift you measure is sampled at discrete intervals, any true Doppler frequency higher than the Nyquist limit cannot be represented correctly and wraps around to appear as a lower velocity. That’s why aliasing occurs: the signal’s Doppler frequency exceeds what the sampling rate can accurately capture.

PRF itself depends on depth: deeper imaging reduces PRF and lowers the Nyquist limit, increasing the chance of aliasing. To prevent aliasing, you can raise PRF (by imaging shallower, using a higher PRF setting, or adjusting the velocity scale), or switch to continuous-wave Doppler, which isn’t limited by the Nyquist criterion.