The most likely amount of reflection at a boundary between soft tissue is:

• A. 1%
• B. 22%
• C. 34%
• D. 99%

Answer: (A)

When ultrasound encounters a boundary, how much energy is reflected is governed by the mismatch in acoustic impedance between the two tissues. Soft tissues have very similar impedances, so only a tiny portion of the incident wave is reflected while most of it continues through the boundary. The reflection is proportional to the square of the relative impedance difference, and with soft tissues being close in impedance, that value is very small—about one percent. That’s why the boundary between soft tissues typically shows a very small reflection and most of the energy is transmitted. If the boundary involved a much larger impedance mismatch, such as air, the reflection would be nearly complete, but between soft tissues the reflection remains minimal.