When is impedance greatest during the reflection of an ultrasound wave?

• A. Water/air interface
• B. Tissue/air interface
• C. Steel/air interface
• D. Tissue/bone interface

Answer: (C)

The main idea is that ultrasound reflection is driven by how different the acoustic impedances are on either side of a boundary. Acoustic impedance is the product of density and speed of sound in a medium (Z = ρc). The fraction of the wave that reflects at normal incidence is determined by the mismatch: R = |(Z2 − Z1) / (Z2 + Z1)|. The larger the difference between the two impedances, the greater the reflection.

Air has a very low impedance, while steel has a very high impedance, so the mismatch at a steel/air boundary is enormous. This makes the reflection nearly complete, more than at interfaces like water/air, tissue/air, or tissue/bone, where the impedance differences are substantial but not as extreme. Therefore, the greatest impedance mismatch—and thus the strongest reflection—occurs at the steel/air interface.