A2 Daily A Level Physics question
A CT technologist considers raising the tube potential from 80 kV to 120 kV for a quality‑control phantom. In water, the half‑value thickness increases from 3.0 cm at 80 kV to 4.5 cm at 120 kV. For a ray path of 9.0 cm water‑equivalent material, 200 mAs at 80 kV gives suitable detector counts. Which statement must be true about the mAs needed at 120 kV to keep the detector counts the same, and about subject contrast between bone and soft tissue if the mAs were mistakenly left at 200 mAs?
Answer
The correct answer is A.
Correct: A — Use about 100 mAs at 120 kV; if left at 200 mAs, detector counts would be roughly twice as high and subject contrast would decrease. Over 9.0 cm, 80 kV gives three half‑values (1/8 transmitted) while 120 kV gives two (1/4 transmitted); to keep counts the same, mAs scales by 1/8 ÷ 1/4 = 1/2, so 200 mAs → 100 mAs, and higher kV reduces differential attenuation so subject contrast decreases. B is wrong because keeping 200 mAs at higher kV increases transmission, so counts rise, and higher kV reduces (not increases) subject contrast. C is wrong because mAs should be reduced (not increased) and counts would not halve at fixed mAs; they would roughly double. D is wrong because the required mAs is not 300 mAs and, at fixed 200 mAs, counts would not be unchanged—they would be higher at 120 kV.