The principal source of attenuation for diagnostic ultrasound in soft tissues is:

• A. Reflection
• B. Refraction
• C. Scattering
• D. Absorption

Answer: (D)

Attenuation in soft tissue is dominated by absorption, where the advancing acoustic energy is converted into heat through molecular relaxation processes in the tissue. This energy loss occurs continuously as the wave travels, and at diagnostic frequencies (2–15 MHz) absorption is the largest contributor to attenuation. Scattering and reflection also remove energy from the beam, but their overall impact on attenuation in soft tissue is smaller than absorption. Reflection occurs at impedance mismatches and mainly creates echoes at interfaces, while refraction changes direction. So the primary mechanism reducing the beam’s amplitude with depth in soft tissue is absorption.