Intelligent assessment and revision tools for OCR A Physics

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?

• 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.
• B Keep 200 mAs; otherwise the detector counts would fall and subject contrast would increase.
• C Use about 400 mAs at 120 kV; if left at 200 mAs, detector counts would be roughly half as high and subject contrast would increase.
• D Use about 300 mAs at 120 kV; if left at 200 mAs, detector counts would be unchanged and subject contrast would decrease.