The number of pulses that an ultrasound system transmits into the body each second.

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

Answer: (B)

The number of pulses transmitted per second is the pulse repetition frequency. This is the rate at which the system begins a new transmit event, measured in hertz. It’s essentially how often the ultrasound machine starts a pulse each second.

PRF is the reciprocal of the time between starts of pulses (the pulse repetition period). That interval includes the actual pulse duration plus the listening time for echoes. So when you image deeper, you need more listening time, the pulse repetition period gets longer, and the PRF drops.

Frame rate is related but not the same thing. Frame rate tells you how many image frames are produced each second, and it depends on PRF as well as how many scan lines are used to form each frame. Pulse duration is the length of a single pulse, not how many pulses occur per second. Duty cycle is the fraction of time the system is actively transmitting, which equals PRF times pulse duration.