Why quality control?
A performance drift in an imaging device does not always show an obvious sign of failure. Often it appears to be "working" — an image comes out, the patient leaves — while in the background the tube output has shifted, the calibration has drifted, or the automatic exposure control is producing higher-than-needed dose output. None of this is visible to the naked eye.
This is exactly why quality control exists: to verify, with evidence, that a device works accurately and consistently. Otherwise the machine misleads with full confidence — either dosing the patient more than necessary, or quietly degrading diagnostic quality.
QA vs QC
The two terms are often confused:
- Quality assurance (QA): the broad framework covering the management of the whole process — staff, protocols, equipment, records — to ensure quality.
- Quality control (QC): the measurable, technical leg of that framework; the regular testing of equipment performance.
In short, QA is the whole system; QC is the part of that system that is verified with numbers.
What gets tested?
QC tests fall into two main types:1
- Acceptance testing: done to determine whether a newly installed component meets the desired performance specifications. The "zero point" before the device enters service.
- Routine constancy tests: tests repeated at regular intervals to catch the drift that is inevitable over time. They compare today's performance against the baseline from acceptance testing.
In practice, typical measured quantities include: tube output and kVp accuracy, beam quality (HVL), the consistency of automatic exposure control, and image quality. Parameters such as spatial resolution are usually assessed with test phantoms (e.g. line-pair phantoms).1
Where it is mandatory: mammography
Quality control is not always voluntary; in some areas it is a legal requirement. Mammography is the clearest example: in the US, the Mammography Quality Standards Act (MQSA) requires facilities to run a regular quality control program.12
The reason is plain: mammography tries to catch very low-contrast, very small structures (microcalcifications); even the smallest loss of quality can cause a finding to be missed. In a task this delicate, the device's performance cannot be left to chance.
Who does it?
Quality control is a team effort. A large share of the daily and weekly tests are carried out by radiologic technologists; the more comprehensive measurements, acceptance tests, and annual evaluations are the responsibility of the medical physicist. Together, they keep the device both safe and diagnostically adequate.
| Modality | Typical quality-control topics |
|---|---|
| Radiography | kVp accuracy, tube output, HVL, beam–image field alignment, AEC, detector artifacts |
| CT | CT number (HU) accuracy, uniformity, noise, slice thickness, spatial resolution, CTDI |
| Mammography | AEC, mean glandular dose, compression force, detector uniformity, artifacts, spatial resolution |
| Fluoroscopy | Dose rate, KAP/Ka,r indicators, image quality, collimation, shields |
| Nuclear medicine | Energy peak, uniformity, center of rotation (COR), collimator, dose-calibrator accuracy |
In the end, quality control is the invisible guardian of the dose–quality balance: when it does its job well, no one notices, because no problem ever arises.
References
- Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. The Essential Physics of Medical Imaging, 3rd ed. Lippincott Williams & Wilkins, 2011. Kabul/rutin testler (s.157), çözünürlük test fantomları (s.77), mamografide MQSA QC zorunluluğu (s.259). Atıflardaki sayfa numaraları bu baskıya aittir.
- U.S. FDA. Mammography Quality Standards Act (MQSA), 21 CFR 900 — mamografi tesisleri için zorunlu kalite kontrol programı. fda.gov
- IAEA Human Health Series — diagnostic radiology medical physics ve kalite yönetimi rehberleri. iaea.org