Which boundary between media produces the highest reflection rate?

• A. Tissue-Air
• B. Tissue-Bone
• C. Tissue-Water
• D. Tissue-Fat

Answer: (A)

Reflection at a boundary depends on the impedance difference between the two media. Acoustic impedance is basically density times the speed of sound in a medium, and a larger mismatch means more energy is bounced back rather than transmitted. Air has an extremely low impedance compared with tissue, so the boundary between tissue and air creates a huge impedance difference. That massive mismatch makes nearly all of the ultrasound energy reflect back, producing the highest reflection rate at that boundary. In contrast, tissue–water has a much smaller difference and lets most energy pass through, while tissue–bone does reflect a substantial portion but not as dramatically as tissue–air, and tissue–fat yields a smaller reflection still. So the tissue–air interface shows the strongest echo because of the extreme impedance mismatch.