Equipment Operation and Quality Control
This domain covers the x-ray tube and generator, automatic exposure control, and the periodic quality-control tests that keep a room within tolerance. Expect numerical tolerances, heat-unit calculations, and questions about which component fails to produce a given artifact.
Tube and generator
- Cathode filament is tungsten with about 1-2% thorium; the rotating anode target is tungsten-rhenium on a molybdenum or graphite disc turned by an induction motor.
- Smaller focal spots improve spatial resolution but limit mA because heat is concentrated.
- Heat units: HU = kVp x mAs x generator factor. Single-phase 1.0, three-phase six-pulse 1.35, twelve-pulse 1.41, high-frequency about 1.4.
- The tube rating chart gives the maximum single exposure; the anode cooling chart gives heat capacity over repeated exposures. They answer different questions.
- Rectification: single-phase ripple 100%, three-phase six-pulse 13%, twelve-pulse 4%, high-frequency under 1%. Constant potential raises quantity and quality for the same technique.
Automatic exposure control
- An ionization chamber or solid-state detector terminates exposure at a preset receptor dose; density or exposure controls shift the target.
- The backup timer prevents overexposure if the chamber fails; minimum response time limits very short exposures.
- A chamber positioned over the wrong anatomy terminates at the wrong exposure, producing a light or dark image that the technologist must recognize.
QC tolerances worth memorizing
- kVp accuracy within 5%; timer accuracy within 5% (or about 1 ms at short times); mA linearity within 10%; exposure reproducibility coefficient of variation 0.05 or less.
- Collimator and light-field alignment within 2% of SID.
- Half-value layer measured at least annually and compared with the state minimum, commonly 2.5 mm Al at 80 kVp.
- Digital systems: flat-field uniformity, ghosting, erasure completeness, exposure index consistency, and monitor luminance with a test pattern.
Worked example
A high-frequency generator is set to 85 kVp, 300 mA, 0.20 s. HU = 85 x 300 x 0.20 x 1.4 = 7,140. The same technique on a single-phase generator would be 85 x 60 x 1.0 = 5,100 HU. Reasoning: heat units scale with the waveform because ripple changes the area under the voltage curve.
Traps
- Using mAs in place of mA times seconds in the heat formula.
- Confusing the tube rating chart with the anode cooling chart.
- Expecting AEC to correct for a mispositioned patient or a contrast-filled structure over the chamber.
- Treating reproducibility and linearity as the same test.