At an air–tissue interface, what fraction of the incident ultrasound energy is reflected?

At an air–tissue interface, what fraction of the incident ultrasound energy is reflected?

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

Answer: (A)

When ultrasound hits an interface where the two media have vastly different acoustic impedances, most of the energy is reflected back. The amount reflected at normal incidence is governed by the impedance contrast: R = [(Z2 − Z1)/(Z2 + Z1)]^2. Air has an acoustic impedance that is tiny compared with tissue, so (Z2 − Z1)/(Z2 + Z1) ≈ 1 and R ≈ 1. That means essentially all the incident energy is reflected, with only a negligible fraction transmitted into the tissue. This is why air gaps prevent effective ultrasound transmission and why a coupling gel is used.

When ultrasound hits an interface where the two media have vastly different acoustic impedances, most of the energy is reflected back. The amount reflected at normal incidence is governed by the impedance contrast: R = [(Z2 − Z1)/(Z2 + Z1)]^2. Air has an acoustic impedance that is tiny compared with tissue, so (Z2 − Z1)/(Z2 + Z1) ≈ 1 and R ≈ 1. That means essentially all the incident energy is reflected, with only a negligible fraction transmitted into the tissue. This is why air gaps prevent effective ultrasound transmission and why a coupling gel is used.