Radiographic Physics
X-ray production
Electrons are emitted thermionically from a tungsten filament, accelerated by the tube potential, and strike the focal spot, where about 99% of energy becomes heat. Bremsstrahlung produces the continuous spectrum; characteristic radiation appears only when tube potential exceeds the K-shell binding energy of the target (about 70 keV for tungsten, giving emissions near 59 and 67 keV). Total filtration of at least 2.5 mm aluminium equivalent removes low-energy photons, raises the half-value layer and reduces skin dose.
Exposure factors
kVp governs beam quality (penetration and contrast) and also quantity; mAs governs quantity alone. The 15% rule: a 15% rise in kVp approximately doubles receptor exposure, so halve mAs to maintain density, accepting lower contrast. Heat units = kVp × mA × seconds × generator constant (1.0 single-phase, roughly 1.35–1.4 three-phase and high-frequency).
Interactions
In the diagnostic range, photoelectric absorption dominates in bone and contrast media (high atomic number, high attenuation, produces contrast); Compton scatter dominates in soft tissue at higher energies and degrades contrast; coherent scattering is negligible for dose.
Traps
Questions often claim mAs affects penetration, or that characteristic radiation appears at any kVp. Increasing kVp raises both quality and quantity. A longer SID reduces magnification and dose but demands more mAs by the inverse square law.
Worked example
An examination uses 60 kVp and 10 mAs. Applying the 15% rule, 69 kVp with 5 mAs gives similar receptor exposure with lower patient dose and wider latitude - acceptable for a barium study, but not where fine bony detail is required.