Because the PRF sets the Nyquist limit (1/2 PRF), higher velocities can be measured without aliasing by increasing the ______.

• A. PRF
• B. Depth
• C. Frame rate
• D. Angle

Answer: (A)

The key idea is the Nyquist limit: the highest Doppler frequency you can measure without aliasing is half the PRF. So if you want to capture higher velocities without their signals wrapping around, you raise the Nyquist limit by increasing the PRF. In ultrasound, PRF is tied to how fast you sample per second, and higher PRF expands the unaliased velocity range. Depth affects PRF (deeper imaging lowers PRF), so going deeper actually makes aliasing more likely unless you adjust other settings. Angle changes the measured velocity magnitude via cos θ but doesn’t change the unambiguous velocity range. Frame rate relates to image updates, not the Doppler Nyquist limit.