Impedance mismatch reflections are greatest at which interface?

• A. Water/air interface
• B. Soft tissue/bone interface
• C. Oil/water interface
• D. Steel/air interface

Answer: (D)

The amount of reflection at a boundary depends on how different the acoustic impedances of the two media are. A big mismatch makes most of the energy bounce back rather than transmit through. Air has an extremely low acoustic impedance, while steel has a very high one, so at the boundary between steel and air almost all of the ultrasound energy is reflected. That contrast is far larger than at a water/air boundary, a soft tissue/bone boundary, or an oil/water boundary, making steel/air the interface with the greatest reflection.