The PRF is high when systems image only to a ______ depth.

• A. Deep
• B. Moderate
• C. Shallow
• D. Varying

Answer: (C)

Pulse repetition frequency is the number of ultrasound pulses emitted per second. It goes up when you image only to a shallow depth because the echoes return quickly. The system must wait for the farthest echo to arrive before sending the next pulse, and with a shallow maximum depth that waiting window is short, so more pulses can be sent each second. In practice, PRF is roughly proportional to the speed of sound divided by twice the maximum imaging depth, so decreasing depth raises PRF. Imaging deeper would require a lower PRF to avoid range ambiguity and overlapping echoes. So a high PRF at shallow depth makes sense because the round-trip time is short.