Intelligent assessment and revision tools for OCR A Physics

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?

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