The ______ describes the relationship of beam intensities with time.

• A. Duty factor
• B. Temporal factor
• C. Pulse factor
• D. Time constant

Answer: (A)

In pulsed ultrasound, how long the beam is on within each cycle shapes how the intensity behaves over time. That relationship is described by the duty factor. It’s the fraction of time the beam is active during one complete cycle, calculated as the pulse duration divided by the total period (often shown as a percentage). This figure directly governs the time-averaged energy delivered: a higher duty factor means more energy over time and a higher average intensity, while a lower duty factor means less. For example, if a pulse lasts 2 microseconds and the period is 100 microseconds, the duty factor is 2%, so the average intensity is only a small fraction of the peak during the pulse.

The other terms aren’t standard descriptors of how beam intensity changes with time in this context. Time constants refer to exponential processes in circuits, and the notions of temporal or pulse factors aren’t the established way to express the on/off time relationship in ultrasound.