ARDMS Ultrasound Physics & Instrumentation Practice Exam

As imaging depth increases, what must happen to the pulse repetition frequency?

1. It must decrease

2. It must increase

3. It must remain constant

4. It must alternate

The correct answer is: It must decrease

As imaging depth increases, the pulse repetition frequency (PRF) must decrease. This is due to the relationship between depth and the time it takes for the ultrasound pulse to travel to the deeper tissues and return to the transducer. When imaging at greater depths, the ultrasound wave has a longer round-trip time, which is the time taken for the pulse to travel to the depth and back. Since the PRF is defined as the number of pulses emitted per second, an increased imaging depth leads to longer return times, which consequently reduces the number of pulses that can be sent out in a given second. Therefore, to accommodate the increased time required for deeper imaging, the system must lower the PRF to ensure that it does not transmit a new pulse before the previous pulse has returned. This mechanism maintains accurate imaging without overlaps or loss of data. In summary, as imaging depth increases, the need to accommodate longer pulse travel times directly leads to a decrease in pulse repetition frequency, making this the correct answer.