When depth resolution is not required, which Doppler modality can be used to avoid aliasing entirely?

• A. Color Doppler
• B. Power Doppler
• C. Continuous-wave Doppler
• D. Spectral Doppler

Answer: (C)

Aliasing comes from sampling the Doppler signal at a finite rate in pulsed Doppler techniques. If a high velocity shifts the frequency beyond half the sampling rate, the display folds the value back, causing aliasing. Continuous-wave Doppler avoids this entirely because it transmits and receives continuously along the beam with no range-resolved sampling. There’s no depth gating and effectively no Nyquist limit for velocity measurement, so you can measure high velocities without aliasing. The trade-off is you lose depth information, since you don’t know where along the beam the signal originates. So, when depth resolution isn’t needed, continuous-wave Doppler is the best choice to avoid aliasing.