Spectral Doppler aliasing occurs when which condition is met?

• A. When fD < PRF/2
• B. When fD > PRF/2
• C. When fD equals PRF
• D. When fD equals 0

Answer: (B)

Aliasing in spectral Doppler shows up due to the Nyquist limit: you can unambiguously measure Doppler shifts only up to half the pulse repetition frequency (PRF). When the true Doppler shift fD is greater than PRF/2, the measured frequency wraps around into the opposite part of the spectrum, so high velocities appear as lower velocities and in the wrong direction. That’s why aliasing occurs for fD > PRF/2. Values at or below PRF/2 stay within the display range and do not alias, and a Doppler shift of zero yields no frequency shift at all.