Physics, Instrumentation, and Radiation Protection
Physics, Instrumentation, and Radiation Protection
This domain tests X-ray and CT physics, image-chain performance, MRI and ultrasound safety, and radiation protection. Questions often combine a calculation with a practical optimization decision.
Core rules
- X-ray photons arise from Bremsstrahlung and characteristic emission. kVp governs beam quality and penetration; mAs governs photon quantity. Filtration removes low-energy photons and increases HVL.
- CT dose: CTDIvol (mGy) is dose per volume for the selected protocol; DLP = CTDIvol × scan length (mGy·cm); effective dose ≈ DLP × k, where k is body-region specific.
- Dose limits: occupational 20 mSv/year averaged over 5 years, with 50 mSv in any single year; public 1 mSv/year; lens 20 mSv/year; fetus 1 mSv. ALARA applies below limits.
- MRI safety zones I–IV: zone IV is the scanner room; ferromagnetic objects and inadequately screened implants are the main hazard.
Worked example
A CT protocol reports CTDIvol 20 mGy over 30 cm. DLP = 20 × 30 = 600 mGy·cm. For abdomen, k ≈ 0.015 mSv/(mGy·cm), so effective dose ≈ 9 mSv. If the scan length falls to 20 cm at the same CTDIvol, DLP falls to 400 mGy·cm and effective dose to about 6 mSv. This shows why collimation and scan length matter as much as mA. Increased filtration raises HVL but lowers tube output, so exposure factors may need adjustment.
Traps
Equating CTDIvol with patient effective dose; forgetting that pitch changes dose and DLP; mixing annual and cumulative limits; assuming all implanted devices are MRI-safe; ignoring that filtration changes both beam quality and output.