Which condition will result in aliasing of the Doppler frequency shift using continuous-wave Doppler?

Which condition will result in aliasing of the Doppler frequency shift using continuous-wave Doppler?

• A. PRF 10kHz, frequency shift 4kHz
• B. PRF 12kHz, frequency shift 8kHz
• C. PRF 5kHz, frequency shift 2.3kHz
• D. None of the above

Answer: (D)

Aliasing arises when sampling is too coarse to capture the signal’s highest frequency component. In pulsed Doppler ultrasound, the Doppler shift is sampled at the pulse repetition frequency, so if the Doppler frequency exceeds half the PRF, the frequency becomes aliased. Continuous-wave Doppler, however, does not sample along depth or at a fixed PRF; it continuously mixes the transmitted and received signals to produce a beat frequency equal to the Doppler shift, limited only by the system’s bandwidth. There’s no Nyquist limit tied to PRF in CW mode, so aliasing cannot occur. Therefore, none of the given conditions would cause aliasing with continuous-wave Doppler; aliasing would only be a concern in pulsed Doppler when the Doppler shift exceeds PRF/2.

Aliasing arises when sampling is too coarse to capture the signal’s highest frequency component. In pulsed Doppler ultrasound, the Doppler shift is sampled at the pulse repetition frequency, so if the Doppler frequency exceeds half the PRF, the frequency becomes aliased. Continuous-wave Doppler, however, does not sample along depth or at a fixed PRF; it continuously mixes the transmitted and received signals to produce a beat frequency equal to the Doppler shift, limited only by the system’s bandwidth. There’s no Nyquist limit tied to PRF in CW mode, so aliasing cannot occur. Therefore, none of the given conditions would cause aliasing with continuous-wave Doppler; aliasing would only be a concern in pulsed Doppler when the Doppler shift exceeds PRF/2.