Aliasing artifact description in PW Doppler when frequency shift is greater than half the Doppler PRF.

Aliasing artifact description in PW Doppler when frequency shift is greater than half the Doppler PRF.

• A. Aliasing
• B. Propagation speed error
• C. Refraction
• D. Multipath

Answer: (A)

The key idea is the Nyquist limit in pulsed-wave Doppler. The Doppler frequency shift you can measure is limited to plus or minus half the Doppler PRF. If the true shift from blood flow is larger than that, the signal wraps around and an aliasing artifact appears. On the display, high velocities can show up as lower velocities or even reversed direction because the measured frequency has folded back into the observable range. This aliasing is a sampling issue, not caused by changes in sound speed, refraction, or multipath. To reduce aliasing, you can increase the Doppler PRF (for example, by adjusting the depth or settings), shorten the sampling range, or use continuous-wave Doppler, which does not alias.

The key idea is the Nyquist limit in pulsed-wave Doppler. The Doppler frequency shift you can measure is limited to plus or minus half the Doppler PRF. If the true shift from blood flow is larger than that, the signal wraps around and an aliasing artifact appears. On the display, high velocities can show up as lower velocities or even reversed direction because the measured frequency has folded back into the observable range. This aliasing is a sampling issue, not caused by changes in sound speed, refraction, or multipath. To reduce aliasing, you can increase the Doppler PRF (for example, by adjusting the depth or settings), shorten the sampling range, or use continuous-wave Doppler, which does not alias.