Image Production · Equipment, AEC and QC3 min read5-question check
Quality Control Tests
QC tests confirm the unit delivers what you set. Know the key tolerances: ±2% of SID, ±5% kVp, 5% reproducibility, 10% linearity.
What QC is
Quality control (QC) is routine testing that confirms equipment works within set limits. It is one part of quality assurance, which also covers scheduling, repeat analysis and patient care. A qualified medical physicist does acceptance testing on new equipment and performance testing at least once a year. Radiographers and QC technologists handle routine checks, such as detector and monitor checks, visual inspections and repeat analysis. You need to know what each test checks and its usual limit.
The tests
Collimation: the light field must match the x-ray field within ±2% of SID, and the light field center must match the beam center within ±1% of SID. Beam alignment: the central ray must be within 1 degree of perpendicular to the receptor. kVp accuracy: commonly within ±5% of the set value, though some texts, including Bushong, allow up to ±10%. Exposure timer: within ±5% for exposures longer than 10 ms. Reproducibility: repeated identical exposures vary by no more than 5%. Linearity: output per mAs at adjacent mA stations within 10%. HVL confirms adequate filtration. NCRP 102 requires at least 2.5 mm aluminum total filtration above 70 kVp. The minimum HVL at 80 kVp is 2.3 mm Al in Bushong and 2.9 mm Al for units built since 2006 under FDA rules.
The numbers
Worked example: at 40 inches SID, 2% is 0.8 inch. If the light field edge is off by 0.3 inch on the left and 0.4 inch on the right, the total along that axis is 0.7 inch, so it passes. A kVp test set at 80 kVp allows 76 to 84 kVp. A reading of 83 passes, and 85 fails. For linearity, 200 mA at 0.1 second and 400 mA at 0.05 second are both 20 mAs and should give nearly the same output.
Detectors and schedule
Digital detectors need their own QC. Uniformity, or flat-field, images show uneven response, dead pixels and artifacts. DR detectors are recalibrated on the manufacturer's schedule. Display monitors are checked with a test pattern and for luminance against the DICOM grayscale standard display function (GSDF). Lead aprons and gloves are checked for cracks, often under fluoroscopy, at least once a year. Most generator tests are annual, while collimation may be checked twice a year and after service.
Clinical Example
A technologist notices anatomy cut off along one edge of several images even though the light field covered it. She reports the room, and a collimator test shows the light and x-ray fields are off by 3% of SID. The room is serviced before more patients are imaged.
Common Mistakes
- Confusing reproducibility (same settings, same output) with linearity (different mA, same mAs, same output).
- Applying the 2% collimator limit to the field size instead of the SID.
- Thinking radiographers perform annual physics testing; that is the medical physicist's job.
- Assuming digital rescaling hides equipment problems, so QC is less important.
Why It Matters
The 2027 specifications include equipment QC, and the registry asks for tolerances and which test fits a problem. Spotting a failing unit early protects patients from repeats.
Key Relationships
- Light field vs x-ray field → within ±2% of SID
- Set kVp vs measured kVp → within ±5%
- Same technique repeated → output within 5% (reproducibility)
- Adjacent mA stations at equal mAs → output within 10% (linearity)