Which boundary will create the strongest reflection?

Which boundary will create the strongest reflection?

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

Answer: (A)

The strongest reflection comes from the interface with the largest acoustic impedance mismatch. Air has a very low impedance, so when ultrasound encounters a boundary with body tissue, the difference in impedance is enormous. Among the given interfaces, air against soft tissue produces the greatest mismatch because soft tissue has a high impedance relative to air, leading to an almost complete reflection and a very bright echo. Interfaces with air and blood or air and lung also reflect strongly, but their impedance differences are slightly smaller than air–soft tissue, so the echoes are not as intense. The boundary between soft tissue and blood has a small impedance difference, giving a much weaker reflection.

The strongest reflection comes from the interface with the largest acoustic impedance mismatch. Air has a very low impedance, so when ultrasound encounters a boundary with body tissue, the difference in impedance is enormous. Among the given interfaces, air against soft tissue produces the greatest mismatch because soft tissue has a high impedance relative to air, leading to an almost complete reflection and a very bright echo.

Interfaces with air and blood or air and lung also reflect strongly, but their impedance differences are slightly smaller than air–soft tissue, so the echoes are not as intense. The boundary between soft tissue and blood has a small impedance difference, giving a much weaker reflection.