When ultrasound energy encounters air, approximately what percentage is reflected?

When ultrasound energy encounters air, approximately what percentage is reflected?

• A. 99.9%
• B. 50%
• C. 0.1%
• D. 75%

Answer: (A)

A large acoustic impedance mismatch at the boundary with air makes almost all of the ultrasound energy reflect back. When the incident wave hits air, the difference between air’s impedance and tissue’s impedance is so extreme that the reflection coefficient is essentially one, so nearly all the energy is reflected and only a tiny fraction enters the air. In practical terms, about 99.9% of the energy is reflected at an air-tissue boundary. This explains why air-filled structures produce strong echoes and why ultrasound doesn’t penetrate air well. The other numbers imply more transmission than actually occurs at an air boundary, which doesn’t fit how impedance mismatch behaves here.

A large acoustic impedance mismatch at the boundary with air makes almost all of the ultrasound energy reflect back. When the incident wave hits air, the difference between air’s impedance and tissue’s impedance is so extreme that the reflection coefficient is essentially one, so nearly all the energy is reflected and only a tiny fraction enters the air. In practical terms, about 99.9% of the energy is reflected at an air-tissue boundary. This explains why air-filled structures produce strong echoes and why ultrasound doesn’t penetrate air well. The other numbers imply more transmission than actually occurs at an air boundary, which doesn’t fit how impedance mismatch behaves here.