In therapeutic ultrasound, how is duty cycle defined?

• A. Average energy delivered per second.
• B. Ratio of on time to total time.
• C. Frequency of the ultrasound waves.
• D. Total duration of the treatment.

Answer: (B)

Duty cycle tells you how much of the treatment time the ultrasound is actually on. It’s the ratio of the on time to the total time of one cycle (on plus off), usually expressed as a percentage. This is how you control how much energy is delivered and how much heating occurs. For example, if the device is on for 2 milliseconds and off for 8 milliseconds, the total cycle is 10 milliseconds, so the duty cycle is 20%. Higher duty cycles deliver more energy and can increase heating; lower duty cycles reduce heating and emphasize non-thermal effects. This isn’t the same as average energy per second (that would be power), nor is it the frequency of the waves, nor the total duration of the entire treatment.