Aliasing occurs when the frequency shift exceeds which?

Aliasing occurs when the frequency shift exceeds which?

• A. PRF
• B. 1/2 PRF
• C. Depth
• D. Gain

Answer: (B)

Aliasing in Doppler ultrasound follows the Nyquist principle: you must sample at a rate at least twice the highest frequency you want to measure. The pulse repetition frequency (PRF) sets that sampling rate. The maximum Doppler frequency shift that can be unambiguously measured is PRF divided by two. If the Doppler shift exceeds this limit, the spectrum folds back into the opposite side of zero frequency, causing aliasing. So, a shift larger than half the PRF will appear as a lower (or opposite) frequency on the display. For example, with a PRF of a few kilohertz, shifts above half that value alias. Depth and gain don’t determine this limit; depth relates to penetration and timing, while gain affects amplitude, not the sampling of frequency shifts.

Aliasing in Doppler ultrasound follows the Nyquist principle: you must sample at a rate at least twice the highest frequency you want to measure. The pulse repetition frequency (PRF) sets that sampling rate. The maximum Doppler frequency shift that can be unambiguously measured is PRF divided by two. If the Doppler shift exceeds this limit, the spectrum folds back into the opposite side of zero frequency, causing aliasing. So, a shift larger than half the PRF will appear as a lower (or opposite) frequency on the display. For example, with a PRF of a few kilohertz, shifts above half that value alias. Depth and gain don’t determine this limit; depth relates to penetration and timing, while gain affects amplitude, not the sampling of frequency shifts.