National Physical Therapy Examination (NPTE) Modalities Practice Exam

To determine the duty cycle for pulsed wave ultrasound, you need to know the duration of the "on" time compared to the total cycle time, which includes both the on and off times. The duty cycle is calculated with the formula:

Duty Cycle (%) = (On Time / Total Cycle Time) × 100

In this scenario, the on time is 2 milliseconds (msec) and the off time is 8 milliseconds (msec).

First, calculate the total cycle time:

Total Cycle Time = On Time + Off Time = 2 msec + 8 msec = 10 msec

Next, apply the values to the duty cycle formula:

Duty Cycle (%) = (2 msec / 10 msec) × 100 = 0.2 × 100 = 20%

This calculation confirms that the duty cycle of the pulsed wave ultrasound set to 1.2 W/cm² with the specified on and off times is indeed 20%. Therefore, the correct answer reflects a proper understanding of how to measure and calculate the duty cycle in the context of ultrasound therapy.