Doppler spectral aliasing is most likely to occur when:

• A. PRF is too high
• B. PRF is too low
• C. Flow velocity is zero
• D. Depth is shallow

Answer: (B)

Doppler spectral aliasing happens when the sampling rate of the Doppler measurement is too low to capture the true Doppler shifts. The maximum unambiguous velocity is half the pulse repetition frequency (the Nyquist limit). If the blood flow produces a Doppler shift larger than this limit, the spectrum wraps around and appears misrepresented. A low PRF lowers the Nyquist limit, making aliasing more likely for higher velocities. A higher PRF reduces aliasing risk, zero flow would show no shift, and shallow depth actually increases PRF (reducing aliasing), while deeper depth lowers PRF and would raise aliasing risk.