With perpendicular incidence, approximately what percent of the incident beam is transmitted?

• A. 50%
• B. 75%
• C. 99%
• D. 100%

Answer: (C)

At normal incidence, the boundary between two media lets energy transmit most efficiently when their acoustic impedances are very similar. The amount reflected is governed by the normal-incidence reflection coefficient R = ((Z2 − Z1)/(Z2 + Z1))^2. For soft tissues, Z1 and Z2 are close (about 1.6 MRayls), so the mismatch is small and R is around 1% or less. Transmission is then roughly 1 − R, which is about 99%. So almost all the energy passes into the next medium, with only a tiny fraction reflected. That's why about 99% transmitted is the best answer. The remaining energy is absorbed or scattered within the tissue, and any slight mismatch prevents a full 100% transmission.