Diagnostic Radiology Physics and Protection
Core concepts
Understand X-ray production (bremsstrahlung, characteristic), tube voltage/current, filtration, collimation, and the inverse-square law. Know how kVp, mAs, and filtration affect dose and image quality. Understand fluoroscopy dose management (pulsed fluoro, last-image hold, dose spreading), and the role of DAP and air kerma. Know patient shielding (gonadal, thyroid) and the principle that shielding should not compromise diagnostic information. Understand automatic exposure control (AEC) and grid use.
Traps
Questions test that increasing kVp reduces dose but lowers contrast, and that mAs controls quantity of photons. Don't confuse half-value layer (HVL) with filtration adequacy. Watch for the inverse-square law applied to distance from the source. Remember that fluoroscopy time and patient size drive dose; collimation reduces both dose and scatter. A common trap is that added filtration hardens the beam and reduces patient skin dose but may increase tube load.
Worked example / rule of thumb
Doubling distance from 1 m to 2 m reduces dose by 1/2² = 1/4. Rule of thumb: dose ∝ 1/d²; double distance, quarter the dose.