Which frequency is absorbed faster by tissue?

• A. 1 MHz
• B. 3 MHz
• C. 0.5 MHz
• D. 2 MHz

Answer: (B)

In therapeutic ultrasound, how quickly tissue absorbs ultrasound energy depends on the frequency: higher frequency becomes attenuated more rapidly as it travels through tissue, so energy is deposited—and heat is produced—over a shorter distance. The attenuation in soft tissue is roughly proportional to frequency (about 0.5 dB per cm per MHz), so a higher frequency like 3 MHz loses energy faster per centimeter than lower frequencies (0.5, 1, or 2 MHz). This means the tissue near the surface heats up more quickly with 3 MHz, while deeper tissues receive less energy. The trade-off is that deeper penetration is achieved with lower frequencies. Therefore, the frequency absorbed fastest by tissue among the options is 3 MHz.