How is the duty cycle defined in therapeutic ultrasound?

• A. on time/total time (%)
• B. peak intensity
• C. frequency of pulses
• D. duration of treatment

Answer: (A)

In therapeutic ultrasound, the duty cycle tells you how much of each ultrasound cycle is actually delivering energy. It’s calculated as on time divided by the total cycle time, and then expressed as a percentage. For example, if the transducer is emitting for 2 milliseconds and then off for 8 milliseconds, one cycle lasts 10 milliseconds, and the duty cycle is 2/10, or 20%. This measure controls heating: lower duty cycles (like 20%) limit heating and emphasize non-thermal, healing effects; higher duty cycles (toward 100% continuous) deliver more energy and increase tissue heating. The other factors—peak intensity, pulse frequency, or total treatment duration—do not define the duty cycle. Continuous emission corresponds to a 100% duty cycle.