Which of the following interactions of sound and tissue decreases the intensity of the transmitted beam?

• A. Absorption
• B. Reflection
• C. Scattering
• D. All of the above

Answer: (D)

Energy loss of a forward-propagating ultrasound beam in tissue arises from absorption, scattering, and reflection. Absorption converts part of the acoustic energy into heat, so less energy remains to continue forward. Scattering sends energy in many directions; energy redirected away from the forward path reduces the intensity that continues ahead. Reflection occurs at tissue interfaces with impedance mismatch, sending a portion of the energy back toward the transducer and away from the forward direction. Because each of these interactions can decrease the energy that travels forward, the transmitted beam’s intensity is diminished by all of them.