Radiation Protection
Framework
Every exposure must satisfy three ICRP principles: justification (the benefit outweighs the risk), optimisation (ALARA, taking economic and social factors into account), and dose limitation (which applies to occupational and public exposure, never to the patient's own medical exposure). Limits are constraints, not targets: occupational effective dose 20 mSv per year averaged over five defined years (100 mSv in that period, with no single year above 50 mSv); lens of the eye 20 mSv per year averaged; extremities and skin 500 mSv per year; public 1 mSv per year. A declared pregnancy applies a 1 mSv constraint to the foetus and demands careful justification of abdominal or pelvic examinations.
Practical controls
Time, distance and shielding. The inverse square law means dose falls with the square of distance, so doubling your distance quarters your scatter dose. Use lead aprons (0.25–0.5 mm Pb equivalent), thyroid collars and mobile screens; collimate to the region of interest; shield gonads and breast where anatomy allows; avoid repeats; use pulsed fluoroscopy and last-image hold; keep the image intensifier close to the patient to cut both patient dose and scatter. Tube leakage must not exceed 1 mGy/h at 1 m.
Traps
Questions conflate effective-dose limits with equivalent-dose limits, or wrongly apply limits to patients. Raising kVp at fixed receptor dose lowers patient dose but reduces contrast. Distance protects the operator only when measured from the patient, the scatter source, not from the tube.
Rule of thumb
Dose is proportional to mAs, roughly to kVp squared, and inversely to distance squared. Halving mAs halves dose; stepping from 1 m to 2 m cuts scatter to one quarter, which is why staff step back rather than increase output.