Radiation Physics and Equipment
This domain tests the physics of X-ray production and the equipment that controls beam quantity, quality and geometry. Expect questions on tube design, generators, filtration, collimation, automatic exposure control and digital receptors.
Core rules
- Thermionic emission from the filament releases electrons; the tube current (mA) controls the number of electrons and therefore beam quantity.
- Tube voltage (kVp) controls electron acceleration and beam penetrability; increasing kVp raises transmission through tissue and usually lowers patient dose for the same receptor signal, but it reduces subject contrast and increases scatter.
- Exposure time and mA jointly determine mAs. mAs changes quantity, not penetration.
- Focal spot size affects spatial resolution and tube loading: a small focal spot improves sharpness but has a lower mA rating.
- Filtration removes low-energy photons; collimation limits field size and reduces scatter and dose.
- AEC terminates exposure when the receptor receives the preset signal; it does not guarantee correct kVp or positioning.
Worked example
A lumbar spine exposure at 70 kVp and 20 mAs is noisy. Raising to 80 kVp with the same mAs increases receptor signal and penetration, but contrast falls. The technologist should optimize kVp and use the lowest mAs that gives acceptable noise, then check whether positioning or collimation caused the original problem. Simply doubling mAs raises dose without correcting poor penetration.
Traps
Do not say kVp controls beam quantity. Do not equate small focal spot with higher tube rating. Do not assume AEC corrects patient position. In fluoroscopy, remember that dose rate and last-image hold differ from continuous exposure.