Which ultrasound frequency is typically used to heat deep tissues?

• A. 3 MHz
• B. 1 MHz
• C. 5 MHz
• D. 0.75 MHz

Answer: (B)

In therapeutic ultrasound, depth of heating is tied to frequency: lower frequencies penetrate deeper because they’re less attenuated as they travel through tissue, while higher frequencies deposit energy mainly at or near the surface. For heating deeper tissues, you want energy that reaches those deeper layers, and 1 megahertz is the standard choice because it provides enough penetration to affect tissues several centimeters beneath the skin. Higher frequencies like 3 and 5 megahertz heat the superficial layers more readily, which is why they’re not used for deep-tissue heating. A lower frequency such as 0.75 megahertz would reach even deeper in theory, but 1 megahertz is the typical clinical setting for deep-tissue heating.