The number of pulses emitted by the transducer in one second.

• A. Pulse duration
• B. Pulse repetition frequency
• C. Frame rate
• D. Duty factor

Answer: (B)

Pulse repetition frequency is the number of ultrasound pulses the transducer emits each second. That makes it the exact term for counting how many pulses occur per unit time. The PRF is determined by the time between pulses (the pulse repetition period): PRF equals 1 divided by PRP. As you increase depth or need to listen longer for echoes, that period grows and PRF drops, since fewer pulses can be sent per second.

Understanding the other terms helps keep the idea clear: pulse duration is how long a single pulse lasts, not how often pulses occur. Frame rate is how many complete image frames are shown each second, which depends on PRF and the number of scan lines, not on the pulses per second alone. Duty factor is the fraction of time the transducer is actively emitting within one PRP, effectively pulse duration divided by PRP.

For a quick intuition, if a pulse lasts 2 microseconds and the time between pulses is 98 microseconds, the PRP is 100 microseconds, giving a PRF of 10,000 pulses per second, and a duty factor of 2/100 = 2%.