Which boundary produces the strongest reflection?

• A. soft tissue - blood
• B. soft tissue - fat
• C. air - soft tissue
• D. air - lung

Answer: (C)

The strongest reflection comes from boundaries with the greatest difference in acoustic impedance between the two media. Acoustic impedance is the product of density and the speed of sound in the tissue, and when two media have very different impedances, most of the incident ultrasound energy reflects back rather than transmits.

Air has a very low impedance, while soft tissue (and blood) has a high impedance. The mismatch between air and soft tissue is enormous, so the boundary between them reflects almost all of the ultrasound energy, producing the strongest echo. Boundaries where both sides have similar impedances, like soft tissue–blood or soft tissue–fat, reflect only a small fraction of the energy, giving weaker echoes.

The boundary between air and lung is also an air–tissue interface, but the lung itself is largely air-filled and its impedance is lower than that of solid soft tissue. This reduces the mismatch compared with air–soft tissue, so the reflection is not as strong as at an air–soft tissue boundary.