At an air–tissue interface, most of the ultrasound energy is reflected.

• A. Near 100%
• B. 0%
• C. 25%
• D. 50%

Answer: (A)

At an air–tissue interface, the boundary is almost perfectly reflective because the acoustic impedance of air is vastly different from that of soft tissue. Air is about 0.0004 MRayl, while tissue is around 1.6 MRayl. Using the normal-incidence reflection coefficient formula, the fraction of energy reflected is nearly one (about 99% or more), so the vast majority of the ultrasound energy bounces back toward the transducer and only a tiny portion is transmitted across the boundary. This is why interfaces with air produce strong echoes and imaging through air is difficult.