Which statement about reflection is true?

• A. The energy wave is bent as it strikes an interface between two different tissue layer densities.
• B. The energy wave may be partially or fully reflected by the tissue layer.
• C. Energy can be absorbed by one layer reducing energy to deeper tissues.
• D. Any energy that is not reflected or absorbed passes through to the next layer.

Answer: (B)

Reflection occurs when an ultrasound wave hits a boundary between tissues with different acoustic impedances. At that interface, part of the incident energy can bounce back toward the source and part can continue forward into deeper tissues. The amount that is reflected can be partial or even complete, depending on how great the impedance difference is. That is why the statement that the energy wave may be partially or fully reflected by the tissue layer is the best description of reflection. The other ideas describe related phenomena: bending or change in direction (refraction) happens when the wave’s speed changes; absorption is energy taken up by the tissue as heat; and the remainder that isn’t reflected or absorbed transmits to the next layer.