The number of pulses that occur in one second is known as which quantity?

• A. Duty factor
• B. Frame rate
• C. Pulse repetition frequency
• D. Spatial pulse length

Answer: (C)

Pulses per second are measured by pulse repetition frequency. This quantity is defined as how many ultrasound pulses the machine emits each second and is the reciprocal of the time between the start of one pulse and the start of the next (the pulse repetition period). The imaging depth affects this because deeper targets require more round-trip travel time, so the pulse repetition period lengthens and the pulse repetition frequency decreases. Frame rate, by contrast, is about how many complete image frames are produced each second and depends on how many scan lines are in a frame, not solely on how often pulses occur. Duty factor is the fraction of time the transmitter is active within a cycle (pulse width divided by the period), and spatial pulse length is the physical length of the pulse in tissue (number of cycles times wavelength). So the quantity describing how many pulses occur per second is pulse repetition frequency.