Which statement about ultrasound frequency is correct?

• A. They are absorbed equally at 1 and 3 MHz
• B. 3 MHz is absorbed more rapidly than 1 MHz
• C. Absorption is independent of frequency
• D. 1 MHz is absorbed more rapidly than 3 MHz

Answer: (B)

Attenuation of ultrasound in tissue increases with frequency, so higher frequency waves lose energy more quickly as they travel. In soft tissue, the attenuation coefficient is roughly proportional to frequency, meaning 3 MHz experiences more absorption per centimeter than 1 MHz. That makes the statement that higher frequency is absorbed more rapidly the best description. This also explains why higher-frequency ultrasound has a shallower penetration depth and is used for superficial heating, while lower frequencies reach deeper tissues. The other statements conflict with the known frequency dependence because absorption does change with frequency, and it does not favor the lower frequency.