A disadvantage of Pulsed Wave Doppler is the _________ of the Doppler waveform.

• A. low frame rate
• B. signal distortion
• C. wrapping around (aliasing)
• D. color bleed

Answer: (C)

Pulsed wave Doppler is constrained by the Nyquist sampling limit because it samples velocity along the ultrasound beam at a finite pulse repetition frequency. The Doppler shift you measure must stay within plus or minus half of that PRF to be unambiguous. If the actual flow velocity is high enough that the Doppler shift exceeds this limit, the waveform wraps around and appears as a lower velocity in the opposite direction—this is aliasing. Aliasing distorts velocity information and can misrepresent the true hemodynamics, especially with high-velocity jets or deeper samples where the PRF is effectively lower. To reduce aliasing, you can increase the PRF (e.g., by sampling at shallower depths or using techniques to raise PRF), apply baseline shifts, or switch to continuous-wave Doppler when high velocities need to be measured without aliasing. Other potential issues like signal distortion or color bleed are separate artifacts and do not define this fundamental limitation of pulsed wave Doppler.