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

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.

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.