A2 Daily A Level Physics question
Which statement must be true when an additional 6.0 mm of aluminium filtration is inserted into an X‑ray beam that has a half‑value thickness (HVT) of 3.0 mm Al at a fixed kVp, with mA and exposure time unchanged? Consider two adjacent soft‑tissue regions in the image that differ by one soft‑tissue HVT in thickness at this kVp.
Answer
The correct answer is C.
Correct: C — Incident intensity falls to about 25%, and contrast between the two regions decreases. Adding 6.0 mm Al to a beam with HVT = 3.0 mm Al adds two HVTs, so the intensity before the patient is reduced to 1/2 × 1/2 = 1/4 (about 25%); the filtration hardens the beam, increasing the mean photon energy so soft tissues attenuate less per unit thickness, reducing their contrast. A … The 50% factor assumes only one HVT, and beam hardening would not increase soft‑tissue contrast. B … The beam intensity cannot remain unchanged when two HVTs of filtration are added; while the contrast decrease part is correct, the intensity claim is wrong. C … Correct for both the quantitative intensity change (two HVTs → 25%) and the qualitative contrast decrease due to higher mean energy. D … The 25% intensity is right, but beam hardening reduces, not increases, contrast between soft tissues of different thickness.