Which technique directly reduces Doppler aliasing by increasing the pulse repetition frequency?

• A. Use Continuous Wave Doppler
• B. Increase PRF
• C. Apply baseline shift
• D. Increase depth

Answer: (B)

Doppler aliasing happens when the Doppler shift from moving blood exceeds half of the pulse repetition frequency. In pulsed Doppler, the PRF determines how often velocity samples are taken, so the aliasing limit is set by Nyquist. Increasing the PRF raises that Nyquist limit, allowing larger Doppler shifts to be captured without wrapping around, which directly reduces aliasing. Keep in mind that raising PRF also reduces the maximum imaging depth at which unambiguous measurements can be obtained due to range ambiguity. The other techniques don’t address the sampling limit in the same way: continuous wave Doppler avoids pulsed sampling and thus aliasing in the usual sense, baseline shift changes how the spectrum appears without changing the sampling rate, and increasing depth lowers PRF and would tend to worsen aliasing rather than reduce it.