In therapeutic ultrasound, which parameter primarily determines the depth of tissue targeted?

• A. Frequency (MHz)
• B. Power (W)
• C. Duty cycle
• D. Treatment duration

Answer: (A)

The depth of tissue affected by therapeutic ultrasound is determined by the frequency. Higher frequencies are absorbed more quickly by tissues, so the energy penetrates only shallowly. Lower frequencies travel farther before being attenuated, reaching deeper tissues. In practice, about 1 MHz targets deeper structures, while around 3 MHz is used for superficial tissues. Power, duty cycle, and treatment duration affect how much energy is delivered or how it's delivered, but they don’t set how deep the energy can reach.