General radiography and digital radiography
A high-voltage generator drives an X-ray tube for a short exposure; a flat-panel detector turns the transmitted radiation into a digital projection image.
Principle
The generator converts mains power into a regulated high voltage (in general radiography roughly 40-150 kV) and tube current, applied to an X-ray tube for a timed exposure measured in mAs. Electrons from a heated cathode filament strike a rotating anode, producing bremsstrahlung and characteristic X-rays. Filtration removes low-energy photons and the collimator limits the field to the area of interest. Radiation passing through the patient is attenuated differently by bone and soft tissue; an anti-scatter grid suppresses scattered photons, and a detector records the remaining pattern. In digital radiography (DR) this is a flat-panel detector: either indirect (a scintillator such as caesium iodide or gadolinium oxysulfide plus an amorphous-silicon photodiode/TFT array) or direct (amorphous selenium converting X-rays straight to charge). Older systems use computed radiography (CR) storage-phosphor plates read in a separate reader. An automatic exposure control (AEC) chamber can terminate the exposure when the required dose at the detector is reached. Image processing software then applies corrections (offset, gain, defect map) and display processing.
The chain, block by block
| Block | What it does | How it fails |
|---|---|---|
| Mains supply and generator input | Supplies the generator, which on most modern units is high-frequency (inverter) type, sometimes with a capacitor or battery bank for mobile units. | Insufficient mains capacity or high line impedance causes kV drop, exposure aborts or generator errors on high-power exposures; mobile units fail on worn batteries. |
| High-voltage generator and HV cables | Inverter and HV transformer/tank produce regulated kV and mA; HV cables carry it to the tube. | kV/mA regulation errors, HV arcing in cables or connectors, failed inverter components; exposures inconsistent or refused. |
| X-ray tube and housing | Rotating-anode tube with filament, stator for anode rotation and oil-filled housing for insulation and cooling. | Filament burnout (no mA), stator or bearing faults (anode not up to speed, noise), arcing from vacuum degradation, overheating with heavy workload, oil leaks. |
| Collimator and filtration | Defines the field size with lead shutters and a light field showing the X-ray field; added filtration shapes the beam. | Light/X-ray field misalignment, failed lamp, shutter motor faults, field-size sensing errors. |
| Automatic exposure control (AEC) | Ionisation chambers in front of the detector measure dose and terminate the exposure. | Over- or under-exposed images, inconsistent exposures between chambers, exposures ending on back-up timer. |
| Anti-scatter grid and Bucky | Grid (stationary or moving) absorbs scattered radiation before it reaches the detector. | Grid lines on images, wrong grid for the source-to-image distance (cut-off, uneven density), grid drive faults. |
| Flat-panel detector (or CR plate) | Converts X-ray pattern into a digital image; wireless panels have batteries and Wi-Fi link, tethered panels use cable. | Dead lines or pixel clusters, ghosting, calibration drift, cracked panel after a drop, battery and wireless link failures, detector not ready. |
| Mechanics: tube stand, table, wall stand | Positioning of tube and detector with locks, counterweights, motors and distance indicators. | Brakes and locks slipping, cable wear in ceiling suspensions, counterweight or cable failures (a safety issue), alignment between tube and detector centre lost. |
| Acquisition workstation, processing and PACS | Applies offset/gain/defect correction and image processing, then sends images to PACS. | Processing artefacts after software changes, wrong anatomical program selected, network/DICOM problems. |
Symptom → where to look first
| Symptom | Suspect first |
|---|---|
| No exposure, generator error | Generator and supply first: interlocks (door, detector ready), mains capacity, filament circuit, anode rotation feedback. |
| Images too dark/light or inconsistent between patients | AEC chambers and calibration, then generator kV/mA accuracy, then detector calibration. |
| Lines or stripes across the image | Grid (stationary grid lines or moving grid not moving) or detector line defects; check with and without grid. |
| Anode noise, slow run-up or 'rotor' errors | Tube stator, bearing or starter circuit. |
| Edges of field cut off or off-centre | Collimator and light-field alignment, tube-to-detector centring, grid focus distance. |
| Detector not ready / disconnects | Wireless detector battery, Wi-Fi link, docking/charging contacts, or tethered cable. |
| Ghost images of previous exposure | Detector lag or calibration; recalibrate and check exposure timing between images. |
| Exposure aborts on high-power settings only | Mains supply impedance or capacity, generator power stage, tube heat load. |
Safety
Site and infrastructure
- Mains supply rated for the generator's peak power with adequate cable size and low line impedance; protective earth verified.
- Room shielding built to the approved radiation shielding design; door interlocks, warning lights and signage working.
- Ceiling rails, wall and floor fixings for tube stand and wall stand installed per manufacturer requirements and load-rated.
- Room temperature and humidity within the detector manufacturer's limits; detectors are sensitive to heat and condensation.
- Wireless detector network and charging stations available; PACS and worklist connectivity.
- Operator protected area (screen or control booth) with view of the patient.
Preventive maintenance basics
- Check kV accuracy, exposure time and mAs linearity and reproducibility with a calibrated non-invasive meter; compare with acceptance values.
- Check light field / X-ray field alignment and centring of the beam to the detector.
- Test AEC consistency across chambers and thicknesses.
- Run detector calibration (offset/gain) per manufacturer schedule and inspect for dead pixels, lines and ghosting; check wireless detector batteries and housing.
- Inspect HV cables, connectors and tube housing for oil leaks or damage; listen to anode run-up.
- Inspect mechanical safety: ceiling suspension cables, counterweights, brakes, locks, collision protection, emergency stop.
- Verify radiation safety devices: door interlocks, warning lights, exposure switch behaviour.
- Review error logs and exposure counters.
Safety
- Ionising radiation: make test exposures only from the protected area or behind shielding, with personal dosimeter, and with the room cleared; never point the beam at people.
- High voltage: generator tanks, inverters and HV cables carry lethal voltage and stored charge; isolate, lock out and discharge before opening.
- Mobile units with capacitor or battery banks can remain energised after unplugging; follow discharge procedures.
- Ceiling suspensions and counterweighted stands can drop if cables or brakes fail; secure the load before working on them.
- Insulating oil in tube housings and HV tanks: handle hot oil carefully and dispose of it as required.
Manufacturers
Siemens Healthineers · GE HealthCare · Philips · Canon Medical Systems · Fujifilm · Carestream Health · Agfa HealthCare · Shimadzu · Konica Minolta · Samsung Medison · DRGEM · Mindray