PW Doppler aliasing occurs when?

• A. Velocity is zero
• B. Frame rate is low
• C. Doppler shift exceeds half the PRF
• D. Spectral broadening occurs

Answer: (C)

Aliasing in pulsed-wave Doppler comes from how the signal is sampled. The Doppler shift is measured within a limited range set by the pulse repetition frequency (PRF). If the true Doppler shift exceeds half of the PRF, the sampled frequencies wrap around and appear as lower frequencies or on the opposite side of zero. This Nyquist limit—±PRF/2—determines when aliasing occurs, so high-velocity flow or low PRF settings are more prone to it. To avoid aliasing, raise the PRF (or sample depth to increase PRF) or use continuous-wave Doppler, which isn’t limited by the Nyquist range. Note that zero velocity or spectral broadening by itself does not cause aliasing.