In pulsed-wave Doppler, the sampling rate is determined by which quantity?

• A. Ultrasound frequency
• B. Pulse repetition frequency
• C. Depth of sample volume
• D. Frame rate

Answer: (B)

In pulsed-wave Doppler, you sample the returning Doppler signal once for each transmitted pulse. The rate at which you can sample per second is the pulse repetition frequency, the number of pulses sent each second. Therefore the sampling rate is determined by how often those pulses are emitted. Depth of the sample volume affects the time between pulses (PRP) and thus can indirectly influence the PRF, but the fundamental determinant of the sampling rate is the pulse repetition frequency. The ultrasound frequency sets the range of Doppler shifts you can measure (via the Nyquist limit, which is half the PRF), while frame rate relates to imaging frames rather than spectral sampling.