Quality control (QC) is the set of measurements and operational actions used to keep an imaging system within defined performance requirements. Quality assurance (QA) is broader: it assigns responsibility and includes QC, written procedures, staff training, image-quality and dose review, reject analysis, audits, corrective action, and periodic evaluation of the program. Passing a QC test does not guarantee that every clinical image will be diagnostic; positioning, protocol selection, processing, display conditions, and interpretation also matter.
A defensible program establishes performance baselines at acceptance, monitors stability, and links every action limit to a named source. In the United States, FDA’s 21 CFR Part 1020 is principally a federal performance standard for manufacturers and specified components. Facility operation, inspection frequency, personnel qualifications, and record retention for general radiography are largely governed by state or local rules and the facility’s accreditation or hospital program. Mammography has a separate federal framework under MQSA and is outside this general-radiography guide.
Do not treat “daily, weekly, monthly, and annual” as a national schedule. For each system, reconcile the manufacturer’s instructions, applicable state or local regulation, accreditation or hospital requirements, and the qualified medical physicist’s (QMP’s) protocol. Use the most restrictive applicable requirement and document its source.
The following is a framework, not a universal mandate. The written local program should name the test method, device, baseline, frequency, action level, responsible person, and response to failure. Testing is also appropriate after installation, detector or generator replacement, software changes that can affect image formation or exposure indication, major service, relocation, and whenever clinical images suggest a fault.
| When | Examples | Important qualification |
|---|---|---|
| Before use or at a locally specified operator interval | Safety and mechanical inspection; locks, brakes and interlocks; cables and detector housing; collimator lamp; obvious detector, plate, grid, processing, annotation and display artifacts; manufacturer self-test | Only call a check “daily” when the governing program or manufacturer does. A visual collimator-lamp check does not measure light/radiation-field congruence. |
| At a locally specified periodic interval | Uniformity/artifact test; CR plate and reader checks; exposure-indicator trends; reject/repeat analysis; display test pattern; phantom image-quality constancy | Compare quantitative results with an acceptance baseline using the same acquisition and processing conditions. |
| Acceptance and recurring QMP evaluation | Generator accuracy and reproducibility; output and HVL; AEC; collimation, SID and beam/receptor alignment; focal spot or resolution; detector response, noise, uniformity, lag and defective elements; grid; dose and technique charts; displays | The recurring interval and pass/fail criteria come from the applicable program, not from this example list. |
Before clinical use, follow the manufacturer’s startup and calibration instructions and inspect accessible parts for damage or contamination. Confirm that required locks, brakes, interlocks, indicators, detector communication, image orientation, patient/exam annotation, and transfer to PACS work as intended. A failing safety interlock, exposed electrical damage, unstable tube support, or clinically significant artifact warrants removal or restriction under the facility’s escalation policy.
For DR, a uniform exposure or automated calibration may reveal nonuniformity and uncorrected detector elements. The manufacturer defines calibration conditions, defective-pixel rules, and whether service is required; there is no universal “3–5 adjacent pixels” rule. Do not recalibrate merely to hide a new artifact before the cause is investigated and documented. For CR, identify cassettes/plates, inspect and clean them with approved materials, and investigate scratches, dust, cassette damage, plate-reader banding, and residual images. Erasure thoroughness is tested by giving a plate the protocol’s specified exposure, erasing it, and then reading it under specified conditions—not by simply making an unexplained second exposure.
Displays used for acquisition QC and diagnosis belong in the imaging chain. Check for gross artifacts, missing data, geometric distortion and unsuitable ambient light with an appropriate test pattern. Luminance response, contrast, uniformity and related quantitative tests should follow the display class, manufacturer, QMP protocol and applicable standard. AAPM TG 270 recommends acceptance testing and risk-based ongoing QA; it does not prescribe one universal monthly or annual interval for every display.
Repeat a test only after checking setup and the measuring instrument. Record the original result; do not erase it or repeatedly recalibrate until it passes. Apply the prewritten action level: this may mean restricting a detector or projection, stopping use, notifying the supervisor/QMP/service, and performing documented post-correction verification. A numerical failure does not automatically imply the same response for every defect; the QMP and responsible clinical lead should assess immediate patient and image-quality risk.
Test clinically relevant kVp, mA/mAs and exposure-time settings with instruments suitable for the generator waveform. Under 21 CFR 1020.31(a)(4), technique-factor deviation for covered equipment may not exceed the limits stated in the manufacturer’s compliance information; the federal rule does not set a general ±5% kVp rule or the timer percentages sometimes copied into QC worksheets. Numerical limits in a state rule or QMP protocol may be more specific.
The federal reproducibility requirement is precise: for a selected technique combination, the coefficient of variation of air kerma must be no greater than 0.05, based on 10 consecutive measurements within one hour under the specified test conditions. This is a federal equipment-performance test, not a claim that all facilities must perform 10 exposures every month.
For equipment with independent mA selection, the federal linearity criterion compares consecutive tube-current settings (or continuous settings no more than a factor of two apart): |X1 − X2| ≤ 0.10(X1 + X2), where X is average air kerma per indicated mAs. Covered mAs-selector equipment manufactured after May 3, 1994 has the analogous requirement. The compliance method uses 10 exposures at each setting. Calling this simply “±10% across all stations” changes both the comparison and the denominator.
AEC evaluation should use clinically relevant beam qualities, attenuation, chambers and processing. It may include repeatability; selected-chamber consistency; detector air kerma or calibrated exposure-indicator response; density-control steps; minimum response time; and compensation across thickness and kVp. Exposure index is not patient dose, and different vendors’ proprietary indices are not interchangeable. IEC 62494-1 standardizes the exposure index (EI), target exposure index (EIT) and deviation index (DI) framework for applicable digital systems, but the facility must establish appropriate EIT values for examinations and detector operating conditions.
21 CFR 1020.31 requires specific AEC safety and indication features, including selection indication, minimum exposure-time capability under stated conditions, an exposure limit (generally 60 kWs or 600 mAs at 51 kVp or more), a visible limit-termination signal, and manual reset. It does not create a universal annual frequency or an “EI within ±5%” calibration criterion.
Measure air kerma output under reproducible geometry, document distance and backscatter conditions, and compare with applicable limits and acceptance baseline. Output changes can arise from the generator, tube, filtration, measurement setup, or calibration; a 10% baseline action level is sometimes adopted locally but is not a universal federal limit. Patient-equivalent phantom measurements and representative examinations can be used to validate technique charts and compare dose indicators with facility diagnostic reference or achievable levels where available.
HVL measures beam quality. The FDA minimum in 21 CFR 1020.30(m), Table II varies with designed operating range and measured kVp. For systems designed to operate above 70 kVp, examples are 2.1 mm Al at 71 kVp, 2.3 at 80, 2.5 at 90, 2.7 at 100, 3.0 at 110, and 4.1 at 150 kVp. Therefore, “2.5 mm Al above 70 kVp” and “3.0 mm Al above 80 kVp” are incorrect shortcuts. Use the complete table, its test conditions, the manufacturer specification, and any stricter governing rule.
For covered mobile, portable and stationary general-purpose systems, 21 CFR 1020.31(d) says the total light/radiation-field edge misalignment along either the length or width may not exceed 2% of the distance from the source to the center of the visually defined field. This is not “each edge ±2%.” Stationary general-purpose systems also must provide alignment of the field center to the receptor center within 2% of SID, indicate SID within 2%, and meet field-size indication requirements under the specified conditions. Positive beam limitation has different 3%/4% field-size criteria. Test the requirement that actually applies to the equipment.
Inspect stationary and removable grids for damage and artifacts. Evaluate beam centering, angulation, focal distance and orientation with the intended geometry; off-level, off-center, off-focus or upside-down use can cause grid cutoff. Acceptance criteria should come from the grid/system manufacturer and QMP protocol. Grid problems are not reliably diagnosed from brightness alone because digital processing can mask exposure differences.
Acceptance testing creates a quantitative baseline before clinical use. Depending on receptor type and available methods, a QMP evaluation can include dark signal, response versus air kerma, calibrated EI behavior, uniformity, noise, defective detector elements, lag/ghosting, spatial resolution or modulation transfer function, low-contrast performance, artifacts, image processing, and DICOM/PACS integrity. Tests should use unprocessed or “for processing” data when the method requires it; processed clinical images can conceal detector behavior.
A bar pattern can provide a visual limiting-resolution check, but there is no universal requirement that all general DR systems resolve 2.5–3.5 lp/mm. Pixel pitch, sampling, geometry, focal spot, scatter, processing and viewing conditions affect the result. Compare the same validated method with the acceptance baseline and the system’s intended clinical tasks. Likewise, contrast-detail testing is method- and dose-dependent; the CDMAM phantom is designed for mammography and should not be presented as a standard general-radiography phantom.
Use a test object specified by the validated method; a brand name alone does not define the acquisition, analysis, dose, processing, or tolerance. Record enough detail to reproduce the measurement. Trend numerical data rather than relying only on memory or visual appearance, and retain representative images in a controlled location.
| Tool or data source | What it can assess | Caution |
|---|---|---|
| Uniform exposure / manufacturer calibration | DR gain uniformity, artifacts and defective elements | Use specified beam quality, dose, SID and calibration mode; service thresholds are detector-specific. |
| Unexposed and uniformly exposed CR plates | Dark noise, erasure, plate/reader artifacts and uniformity | Control plate history, delay, orientation, exposure and reader processing. |
| Edge, slit or bar device | MTF or visual limiting resolution; focal-spot testing with the proper tool | A bar pattern does not by itself estimate focal spot size, and visual lp/mm is not interchangeable with MTF. |
| Contrast-detail or multipurpose phantom | Constancy of task-based or composite image quality | Results depend strongly on phantom, dose, processing, observer and display; compare only like with like. |
| Calibrated dosimeter and attenuator | Output, reproducibility, linearity, HVL, AEC response and detector air kerma | Maintain calibration traceability and use appropriate energy response and geometry. |
| Display test pattern and photometer | Artifacts, geometric response, luminance response, luminance ratio and uniformity | Use the applicable display standard and control ambient illumination. |
| Reject and DICOM exposure metadata | Reject causes, trends, DI/EI distributions and protocol opportunities | Validate data definitions; deletion, merged series and misclassified rejects can bias rates. |
Digital processing can make an overexposed image look acceptable, so visual density is not a reliable exposure monitor. Review EI/DI distributions by examination, projection, detector and room against locally established targets. Investigate systematic shifts, broad distributions and outliers; do not use an EI from a poorly positioned or incorrectly segmented image as a detector-dose measurement without considering field recognition and anatomy.
Reject analysis is a QA process, not merely a percentage target. Define the denominator (for example, rejected acquisitions divided by total acquisitions), use consistent reason categories, and stratify by examination, projection, room/detector and cause. Review trends at the interval in the written program, provide nonpunitive feedback, and verify whether corrective action worked. A low rate can reflect under-reporting, while a high rate may reflect complex case mix; no single universal “acceptable reject rate” applies to every service.
Determine which rules actually apply to the facility and equipment. FDA certification requirements in 21 CFR 1020.30–1020.31 are not a facility QC calendar. States may require registration, inspections, equipment evaluations, specific test intervals, or particular qualifications. The Joint Commission’s equipment-management requirements apply to organizations it accredits. ACR’s current modality accreditation list does not include a general radiography accreditation program, so do not cite nonexistent “ACR Digital Radiography QC guidelines” as a universal requirement. ACR requirements for other accredited modalities, including mammography, cannot simply be transferred to general radiography.
The facility’s responsible leadership should approve the QA program. A QMP should design or approve quantitative methods and action levels, perform or oversee tests requiring physics expertise, interpret trends, and verify major corrective actions as required by the governing program. Trained technologists may perform assigned operational checks and must report failures. Service personnel repair and calibrate equipment; a service report is not necessarily an independent physics evaluation or authorization to resume clinical use.
For each required activity, retain the information specified by the governing rule and local policy:
Do not assume “life of equipment plus 5–10 years.” Retention periods differ among state rules, accreditation or hospital requirements, record type, contracts and legal policy. Put the exact period and authority in the facility’s retention schedule.
Try these ARRT-style multiple choice questions based on this article. Click an option to check your answer — correct answers turn green, wrong ones turn red.