The pulse repetition frequency (PRF) is the number of pulses of a repeating signal in a specific time unit, normally measured in pulses per second. The term is used within a number of technical disciplines, notably radar.
https://en.wikipedia.org/wiki/Pulse_repetition_frequency

PRF - (Pulse Repetition Frequency)
Definition
HINT:
Units Determined By Changed by Sonographer
PRF is the number of pulses that occur in one second. PRF is the reciprocal of pulse repetition period.
PRF is determined by depth of view, PRF is entirely unrelated to frequency.
Hertz, hz, per second Sound source
Yes

Typical values Notes
In clinical imaging, from 1,000–10,000Hz (1-10kHz)
• The PRF depends only upon imaging depth. It is unrelated to the transducer frequency.
• As imaging depth increases, PRF decreases (inverse relationship).
• In most cases, only one pulse of US travels in the body at one time. Thus, as imaging depths changes, PRF changes.
Since depth of view is changed by the sonographer, the sonographer changes the PRF.
Similarly, the operator also adjusts the pulse repetition period.
Shallow image, higher PRF. Deeper image, lower PRF.
https://www.scahq.org/sca3/events/2013/echo/fundamentals/sub...