Physics, Instrumentation and Radiation Safety
The Physics, Instrumentation and Radiation Safety domain tests how x-ray production, image formation and dose metrics translate into safe protocol choices across radiography, fluoroscopy, CT, MRI and ultrasound. Expect calculation-style items and questions that ask you to justify a protocol change.
Core physics and dose metrics
- mAs governs photon quantity and image noise; kVp governs beam quality, penetration and subject contrast.
- The 15% kVp rule: raising kVp by 15% roughly doubles detector exposure, so halve mAs to keep noise constant; lowering kVp raises contrast but increases dose unless mAs falls.
- Half-value layer (HVL) is the aluminium thickness halving beam intensity; added filtration and higher kVp raise HVL and lower skin dose.
- Inverse square law: intensity falls with 1/d squared.
- CT: CTDIvol (mGy) is dose per slice; DLP = CTDIvol x scan length (mGy cm); effective dose (mSv) = DLP x k, where k is about 0.015 mSv/mGy cm for abdomen, 0.014 chest and 0.002 head in adults.
Protection
Justification, optimisation and ALARA underpin every request. ICRP limits apply to workers and the public, not patients: 20 mSv/y averaged over five years (50 mSv in any single year), lens 20 mSv/y, public 1 mSv/y. Lead aprons at 0.5 mm Pb attenuate most diagnostic scatter; collimation and last-image hold reduce fluoroscopy time.
Worked example
A CT abdomen has CTDIvol 20 mGy over 30 cm. DLP = 20 x 30 = 600 mGy cm. Effective dose = 600 x 0.015 = 9 mSv. Reasoning: CTDIvol is per unit length, so multiplying by scanned length gives DLP; the k factor converts to stochastic risk. This is roughly 450 chest radiographs (0.02 mSv each), which justifies iterative reconstruction and reduced phases.
Traps
Confusing mGy (dose index) with mSv (effective dose); quoting worker limits for patients; forgetting that lowering kVp at fixed mAs raises dose; ignoring scan length when comparing protocols.