Radiologic Physics, Equipment and Maintenance
Radiologic Physics
Production of the beam
X-rays are produced when electrons are accelerated from a heated filament to an anode. Most energy becomes heat. Bremsstrahlung arises from nuclear-field deceleration and creates a continuous spectrum. Characteristic radiation arises when an inner-shell vacancy is filled and produces discrete peaks whose energies depend on the anode material, not on kVp. Raising kVp increases photon energy and also output; raising mAs increases the number of photons with essentially the same spectrum.
Interactions
At diagnostic energies, photoelectric absorption dominates in bone and iodine, varies with atomic number and roughly as 1/E^3, and creates subject contrast. Compton scatter dominates in soft tissue as kVp rises and contributes noise rather than contrast. Coherent scatter is minor. Attenuation is exponential; the half-value layer (HVL) is the thickness that halves intensity and is a beam-quality measure.
Relationships
Inverse-square law: I1/I2 = (d2/d1)^2. mAs reciprocity means mA × time controls quantity. The 15% rule approximates that a 15% kVp increase roughly doubles receptor exposure while lowering contrast.
Traps
Characteristic peak energies do not shift when kVp changes. Photoelectric is not the dominant soft-tissue interaction at higher kVp. HVL describes quality, not quantity.
Worked example: An exposure rate is 400 μGy/h at 1 m. At 3 m it is 400/3^2 ≈ 44 μGy/h.
Equipment and Maintenance
Tube components
A rotating-anode tube contains a cathode (filament and focusing cup) and an anode (target, focal spot, rotor/stator), enclosed in an evacuated glass or metal envelope with oil and a window. Thermionic emission releases electrons; space charge limits current until saturation. The line-focus principle makes the effective focal spot smaller than the actual focal spot. A small focal spot improves spatial resolution but restricts mA; a steep anode angle gives a smaller effective spot but a shorter field. The heel effect makes the anode side less intense and the cathode side more intense.
Generators
Single-phase units have 100% voltage ripple, three-phase 6-pulse about 13%, 12-pulse about 4%, and high-frequency units usually under 1%. Higher ripple lowers effective energy for a given kVp.
Beam control and AEC
Collimation reduces field size, scatter, and patient dose. Grids improve contrast but can cause cutoff when off-level, off-center, or off-focus. AEC terminates exposure at a preset detector dose; chamber selection and calibration affect results, and a backup timer prevents runaway exposure.
Maintenance and QC
Warm-up procedures, tube rating charts, anode cooling, focal-spot check, kVp/mAs/timer accuracy, collimation, HVL, reproducibility, linearity, and AEC testing are routine. Faults are logged and referred, not ignored.
Traps
A small focal spot is not always usable at high mA. The heel effect is normal, not a fault. AEC does not measure patient thickness. High frequency is not the same as three-phase.
Rule of thumb: Place the thicker part of the anatomy toward the cathode side to use the more intense portion of the beam.