Imaging Physics and Radiation Safety

X-rays and their interactions

• Röntgen, 8 Nov 1895. Radio → microwave → IR → visible → UV increases photon energy. X-rays and gamma rays occupy overlapping energy ranges and are usually distinguished by production mechanism; all photons travel at c in vacuum.
• Tube: tungsten filament cathode → thermionic emission; tungsten–rhenium rotating anode (molybdenum/rhodium for mammography). ~99% heat, ~1% X‑rays.
• Bremsstrahlung (continuous) usually supplies most diagnostic X-rays; characteristic (discrete K lines) a variable component exploited in mammography.
• Photoelectric (inner shell, total absorption, ∝ Z³/E³) = contrast. Compton (outer shell, scatter) = most common, fog and staff dose. Pair production ≥ 1.02 MeV.
• Five densities: gas < fat < soft tissue/fluid < bone < metal. An edge exists only where different densities meet (silhouette sign).
• kVp = penetration, inversely related to contrast. mAs = photon number, noise and dose.

Radiation dose and protection

• Units: activity Bq (Ci), absorbed Gy (rad), equivalent/effective Sv (rem), exposure C/kg (R).
• Bergonié–Tribondeau: dividing + undifferentiated = sensitive. G2/M most sensitive, late S least.
• Deterministic (threshold, severity ↑): ARS, skin, cataract (~0.5 Gy). Stochastic (no threshold, probability ↑): cancer, heritable effects.
• ARS thresholds: haematopoietic 1–2 Gy, GI 6–10 Gy, CNS ≥ 20 Gy.
• ICRP principles: justification, optimisation (ALARA), limitation (not for patients). Staff: time, distance (inverse square), shielding.
• Limits: public 1 mSv/yr; occupational 20 mSv/yr (5‑yr average; AERB max 30 in one year); ICRP lens 20 mSv/yr.
• Doses: CXR 0.02, abdomen XR 0.7, CT head ~2, CT chest ~7, CT abdomen ~8–10, Ba enema ~7, PET‑CT 10–25 mSv.
• Pregnancy: enquire/test according to examination risk and local policy; justify and optimise essential imaging. Fetal risk depends on absorbed dose and developmental stage. A dose below 100 mGy alone does not justify termination on radiation grounds; this is not a zero-risk threshold. Cycle-based screening policies vary.

CT, ultrasound and MRI

• HU = 1000 × (μ − μwater)/μwater. Air −1000, lung ~ −800, fat ~ −100, water 0, fresh blood +50 to +80, bone ~ +1000.
• Generations: 1st pencil beam (EMI, Hounsfield); 3rd rotate–rotate (modern basis); 4th fixed ring; 5th electron beam (historical cardiac design). Helical MDCT standard.
• Windows: brain 40/80, lung −600/1500, bone ~500/2000. Phases: arterial (HCC), portal venous (routine), delayed (washout), excretory (urography). Stones and blood: non‑contrast.
• US: > 20 kHz; diagnostic 2–18 MHz; PZT crystal; c = 1540 m/s; B‑mode most common; A‑mode ophthalmic; M‑mode cardiac. Shadowing = calculus/bone; enhancement = fluid.
• Doppler Δf = 2f₀v cosθ/c; angle ≤ 60°; 90° = zero axial Doppler frequency shift; power Doppler most sensitive; BART colour map. TI (thermal), MI (mechanical, limit 1.9).
• MRI: T1 spin–lattice (63% recovery); T2 spin–spin (37% left). Water white on T2; fat bright on T1. FLAIR nulls CSF, STIR nulls fat, DWI restricts in acute infarct/abscess (ADC dark), SWI blooms with blood.
• MRI: check the exact device and conditions. MR Unsafe implants and ferromagnetic intraocular foreign bodies/aneurysm clips prevent scanning while unsafe. Selected non-MR-conditional CIED patients may be scanned under specialist protocols. Cochlear implants are model-specific; non-contrast MRI can be justified in pregnancy.