Define pulse repetition period (PRP) and its relationship to pulse repetition frequency (PRF) and Doppler measurement.

• A. PRP is the time between transmitted pulses; PRF = 1/PRP; higher PRF increases velocity range but limits maximum measurable depth due to range ambiguity
• B. PRP is the speed of sound in tissue; PRF is the number of frames per second
• C. PRP is the depth of imaging; PRF is spatial resolution
• D. PRP and PRF are unrelated to Doppler

Answer: (A)

In pulsed ultrasound Doppler, PRP is the time interval between transmitted pulses, and PRF is its reciprocal. Doppler measurements rely on how often you sample the returned echoes, so the PRF sets the sampling rate for the Doppler shifts. The Doppler frequency shift you can measure unambiguously is limited by Nyquist to about PRF/2. That means higher PRF increases the maximum unaliased Doppler shift, and thus expands the velocity range you can quantify, but it also shortens the time window for echoes to return from deeper structures, leading to range ambiguity and a limit on the maximum measurable depth. This trade-off is exactly what the correct statement describes: PRP is the time between transmitted pulses; PRF = 1/PRP; higher PRF increases velocity range but limits maximum measurable depth due to range ambiguity.