The percentage of the incident beam that is reflected is related to ________.

• A. The difference in the densities of the tissues
• B. The difference in the impedances of the tissues
• C. The boundary angle
• D. The speed of sound in each medium

Answer: (B)

Reflection at an interface is governed by how different the acoustic impedances are between the two media. Acoustic impedance is Z = ρc (density times speed of sound). When ultrasound encounters a boundary, the incident energy splits into reflected and transmitted parts, and the fraction reflected depends on the mismatch between Z in the two tissues. The larger the difference in impedances, the more energy is reflected, following the reflection coefficient formula R ≈ [(Z2 − Z1)/(Z2 + Z1)]^2 at normal incidence. The boundary angle mainly affects the direction and mode of the reflected and transmitted waves rather than the amount reflected at normal incidence. Since impedance embodies both density and speed of sound, it’s the difference in impedances that best determines the percentage reflected.