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Acoustic Impedance Reflection — FRCA Primary MCQ

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HardPhysics & Clinical MeasurementAcoustic Impedance ReflectionFRCA Primary

A peripheral nerve lies immediately deep to a tissue interface with a large acoustic-impedance mismatch. What change in the returning ultrasound signal is expected at that interface?

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Correct answer: B — A larger reflected fraction and a smaller transmitted fraction

The best answer is “A larger reflected fraction and a smaller transmitted fraction”. The fraction reflected at an interface increases with the difference in acoustic impedance between the two media, leaving less energy to continue into deeper tissue. Frequency is set by the transducer and does not double at an ordinary boundary. Large mismatches, particularly with air or bone, can obscure deeper anatomy.

Reference: Gupta et al., Potential role of ultrasound in anesthesia and intensive care: https://pmc.ncbi.nlm.nih.gov/articles/PMC4173359/