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A2 Daily A Level Physics question

2026-05-20 OCR A Medical Imaging (M6) OCR-A 6.3.1 X-rays: attenuation and half-value thickness (qualitative, simple ratios) OCR-A 6.3.2 Diagnostic imaging: image contrast and dose (qualitative)

In a simple X-ray projection, adjacent region S is 6.0 cm of soft tissue only; region B is 4.0 cm of soft tissue plus 2.0 cm of bone. The half-value thickness (HVT) is 3.0 cm for soft tissue and 1.5 cm for bone. The detector records transmitted intensities IS and IB. A radiographer then adds a uniform 3.0 cm soft‑tissue‑equivalent pad in front of both regions without changing exposure. Which statement must be true about the contrast at the detector, defined as the ratio IB/IS?

  1. A The ratio IB/IS is unchanged; the added 3.0 cm halves both signals, so the contrast by ratio stays the same. (correct)
  2. B The ratio IB/IS decreases, because the extra soft tissue reduces IB more than IS since the B-path already contains bone.
  3. C The ratio IB/IS increases, because although both intensities fall, the absolute difference between them shrinks.
  4. D The ratio IB/IS halves, because both IB and IS are halved by the added pad.

Answer

The correct answer is A.

Correct: A — The ratio IB/IS is unchanged; the added 3.0 cm halves both signals, so the contrast by ratio stays the same. A — An equal extra soft-tissue layer applies the same 1/2 factor to IS and IB, so their ratio is unaffected; in the limit of many equal HVTs added, both signals tend to zero but IB/IS remains unchanged in this ideal model. B — Wrong because the uniform front layer acts equally on both paths regardless of what lies deeper, so the extra 1/2 factor cancels in the ratio. C — Wrong because a smaller absolute difference does not imply a changed ratio when both quantities are scaled by the same factor. D — Wrong because halving numerator and denominator leaves their ratio unchanged, not halved.

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