What is the number of ultrasound pulses emitted per second?

• A. Pulse duration
• B. Wavelength
• C. PRF
• D. PRP

Answer: (C)

The number of ultrasound pulses emitted per second is the Pulse Repetition Frequency. It’s a rate, measured in hertz, that tells you how often a pulse is sent out each second. PRF controls how quickly you can acquire frames because echoes from a given pulse must be received before the next pulse is emitted; sending pulses more frequently is limited by the need to listen for echoes, especially at greater depths.

Pulse duration is how long a single emitted pulse lasts, not how often pulses occur. Wavelength is the distance a single cycle travels and depends on sound speed and frequency. PRP, the pulse repetition period, is the time between pulses and is the reciprocal of PRF (PRP = 1/PRF). So the rate at which pulses are emitted per second is PRF.

For example, a PRF of 1500 Hz means 1500 pulses are emitted each second, with a 1/1500 second (about 0.667 ms) interval between pulses.