Pulsed ultrasound with which duty cycle produces nonthermal effects?

• A. 50%
• B. 100%
• C. 20% or lower
• D. 75%

Answer: (C)

Nonthermal effects come from delivering ultrasound in a pulsed pattern with a very low duty cycle. The duty cycle is the fraction of time the transducer is actively emitting during each cycle. When this on-time is brief (20% or less) and the off-time is long, the tissue experiences minimal temperature rise, so thermal effects are minimized and mechanical phenomena—such as microstreaming and cavitation—dominate. This mechanical stimulation is thought to aid healing without heating the tissue. Higher duty cycles deliver more energy continuously, increasing heating and producing thermal effects rather than nonthermal ones. Therefore, a duty cycle of 20% or lower best produces nonthermal effects.