Radiation Physics and Radiation Biology
Core physics
Understand the X-ray tube as the source: thermionic emission from the tungsten filament, acceleration across the kVp potential, and the two spectra - bremsstrahlung (continuous, dominant at diagnostic energies) and characteristic peaks (K-shell, tungsten ~59–69 keV). Know that mA controls quantity and kVp controls both quality and quantity; exposure time and distance (inverse square law) modify intensity. Filtration (inherent + added, ~2.5 mm Al equivalent) removes low-energy photons, reducing dose without losing diagnostic information; collimation limits field size.
Interactions with matter
Photoelectric absorption dominates at low energy and high Z (bone, metal) and produces high contrast but higher dose; Compton scatter dominates at higher energy and in soft tissue and degrades image quality. Pair production is irrelevant at diagnostic energies. Attenuation follows exponential behavior (half-value layer).
Biology
Distinguish deterministic (threshold, severity-related: erythema, mucositis) from stochastic (no threshold, probability-related: carcinogenesis, heritable effects) responses. Recall the law of Bergonié and Tribondeau: rapidly dividing, undifferentiated cells (bone marrow, basal epithelium, lymphocytes) are most radiosensitive; neurons and muscle least. DNA is the critical target; sublethal damage repair and cell-cycle position (most sensitive in M and G2) explain fractionation effects. ALARA and dose–risk linear-no-threshold assumptions underpin practice.
Traps
Questions confuse kVp vs mA effects, misattribute characteristic vs bremsstrahlung origin, or misstate which interaction dominates in bone. Watch for stochastic/deterministic swaps and for claims that filtration increases dose.
Rule of thumb
If a stem asks what changes penetration/quality, answer kVp; if it asks number of photons/intensity, answer mA or time.