What is absorption?

• A. transfer of energy to the tissue (conversion of mechanical energy into heat)
• B. transmission of energy through tissue without heating
• C. reflection of ultrasound at tissue boundary
• D. generation of electrical current in tissue

Answer: (A)

Absorption is the process by which ultrasound energy is taken up by tissue and converted into heat. As the wave traverses tissue, some of its energy is absorbed and dissipated as thermal energy, which is what raises tissue temperature during therapeutic ultrasound. This is a primary way energy is attenuated in tissue, contributing to heating and the overall attenuation of the beam. The other options describe different interactions: transmission is energy that passes through tissue with little or no heating, reflection is energy bouncing off a boundary, and generating an electrical current in tissue is not how ultrasound energy interacts with tissue.