Which medium presents a significant obstacle to ultrasound transmission due to impedance mismatch?

• A. Water
• B. Blood
• C. Air
• D. Soft tissue

Answer: (C)

Ultrasound works best when the two media have similar acoustic impedances, because energy crossing a boundary is transmitted rather than reflected. Acoustic impedance Z = ρc, so the degree of reflection at an interface depends on how different the impedances are. At an air–tissue boundary, Z_air is about 400 Rayls while tissue is around 1.6 million Rayls, a huge mismatch. The reflection is nearly complete (the reflection coefficient is close to 1), so almost no ultrasound energy enters air. Water, blood, and soft tissue all have closely related impedances, so transmission between them is efficient with only modest reflections. Thus, air presents the significant obstacle to ultrasound transmission due to the large impedance mismatch.