Which statement describes what happens when you lower the pulse repetition frequency (PRF) in Doppler color imaging?

Which statement describes what happens when you lower the pulse repetition frequency (PRF) in Doppler color imaging?

• A. The frame rate is increased
• B. The wall filter is increased
• C. The Nyquist limit is decreased
• D. The Doppler frequency is decreased

Answer: (C)

The key idea is how PRF sets the sampling limit for Doppler data. The Nyquist limit, which determines the highest Doppler frequency you can sample without aliasing, is equal to half the PRF. So when you lower PRF, the Nyquist limit decreases. That means the range of velocities you can measure without aliasing shrinks, making high-velocity flows more susceptible to aliasing. The actual Doppler frequency produced by the moving blood doesn’t change with PRF; it’s determined by velocity and the ultrasound beam angle. Wall filter settings or frame rate aren’t the direct consequence of changing PRF in this context, so they don’t describe what happens to Doppler sampling here.

The key idea is how PRF sets the sampling limit for Doppler data. The Nyquist limit, which determines the highest Doppler frequency you can sample without aliasing, is equal to half the PRF. So when you lower PRF, the Nyquist limit decreases. That means the range of velocities you can measure without aliasing shrinks, making high-velocity flows more susceptible to aliasing. The actual Doppler frequency produced by the moving blood doesn’t change with PRF; it’s determined by velocity and the ultrasound beam angle. Wall filter settings or frame rate aren’t the direct consequence of changing PRF in this context, so they don’t describe what happens to Doppler sampling here.