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A2 Daily A Level Physics question

2026-03-19 OCR A Medical imaging II: Ultrasound; acoustic impedance & matching; reflection/transmission at boundaries 6.3.2 Ultrasound: acoustic impedance Z, reflection and transmission at boundaries, A-/B-scan interpretation, safety (qual.)

An ultrasound A-scan sends the same normal-incidence pulse through soft tissue (acoustic impedance 1.6 MRayl) toward two planar boundaries in a phantom: (i) soft tissue to bone (7.0 MRayl) and (ii) soft tissue to muscle (1.7 MRayl). Ignore absorption between the transducer and the boundary. Which statement about the reflected intensity fractions is most consistent with these values?

  1. A Roughly 40% is reflected at the soft tissue–bone boundary but less than 0.1% at the soft tissue–muscle boundary, so the bone echo is hundreds of times stronger. (correct)
  2. B About 63% is reflected at soft tissue–bone and about 3% at soft tissue–muscle, using the impedance difference over the sum directly for the intensity fraction.
  3. C Both boundaries reflect a similar fraction because the pulse frequency is the same; reflection mainly depends on frequency rather than impedance mismatch.
  4. D Using more coupling gel at the skin would increase the reflection from both internal boundaries by a similar percentage, making both echoes stronger.

Answer

The correct answer is A.

Correct: A — Roughly 40% is reflected at the soft tissue–bone boundary but less than 0.1% at the soft tissue–muscle boundary, so the bone echo is hundreds of times stronger. A Using the amplitude ratio at normal incidence (difference over sum of impedances) gives r ≈ (7.0−1.6)/(7.0+1.6) ≈ 0.63 for bone, so intensity reflection ≈ 0.63^2 ≈ 0.40; for muscle r ≈ (1.7−1.6)/(1.7+1.6) ≈ 0.03, so intensity reflection ≈ 0.0009 ≈ 0.09%. B Treats the amplitude ratio as the intensity fraction and so overestimates both reflections (63% and 3% are amplitudes, not intensities). C Reflection at normal incidence depends primarily on impedance mismatch, not on the shared frequency; the near-matched tissue–muscle interface reflects very little. D Coupling gel reduces reflection at the skin by removing the air gap and improving matching; it does not increase reflections from deeper internal boundaries.

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