Which parameter primarily determines the depth of heating in therapeutic ultrasound?

• A. Intensity
• B. Duration
• C. Frequency
• D. Wavelength

Answer: (C)

Frequency primarily controls how deep ultrasound heating occurs because attenuation in tissue increases with frequency. Higher frequency waves lose energy more quickly and deposit most of it near the surface, producing shallow heating. Lower frequency waves penetrate further before being absorbed, allowing deeper heating. In practice, a lower frequency like 1 MHz is used for deeper heating, while a higher frequency like 3 MHz targets superficial tissues. While intensity and duration affect the total amount of heat produced, they determine how much heat accumulates rather than how deep it travels; wavelength is tied to frequency and speed of sound but doesn’t by itself set the heating depth.