Fluoroscopy is radiology's "live broadcast": the image forms in real time and an interventional procedure can run for minutes or even hours. That duration sets fluoroscopy apart from other modalities — because cumulative doses to the patient's skin can reach the threshold for deterministic effects (erythema, transient epilation). So fluoroscopy QC has to hold both ends: the dose rate must not exceed the legal limit, yet the image must be good enough to remain diagnostic.
Why is fluoroscopy special?
Other exams are "single frames"; fluoroscopy is continuous. Tens of frames per second accumulate over minutes. In interventional work the beam may stay on the same skin region for a long time. The result: a cumulative skin dose far above a single radiograph. So QC verifies both that the equipment limits the dose rate and that this dose yields an adequate image.
Dose-rate limits
Fluoroscopy's most tightly regulated parameter is the maximum entrance air-kerma rate. Under US FDA regulations, the limit for normal fluoroscopy is 87.3 mGy/min (10 R/min), and for the specially activated (high-level) mode 175 mGy/min (20 R/min).1 These rates are measured at specified positions: for an under-table tube, 1 cm above the table; for a C-arm, 30 cm from the image receptor toward the source along the central axis.1 Typical entrance rates are 8.7–17 mGy/min (1–2 R/min) for thin body parts and 26–44 mGy/min (3–5 R/min) for the average patient; they can be much higher for oblique/lateral projections and obese patients.1
AERC
In fluoroscopy the exposure rate is set automatically: automatic exposure-rate control (AERC) — formerly automatic brightness control (ABC) — strives to keep the image's signal-to-noise ratio (SNR) constant.1 When the system pans from a thin to a thick region, fewer photons reach the detector; the AERC sensor detects this and signals the generator to raise the exposure rate. In continuous fluoroscopy it changes mA and kV; in pulsed fluoroscopy it changes pulse width/height and kV.1 How mA and kV change with patient thickness directly sets the dose–image-quality compromise — so AERC behavior is checked in QC.
Phantom dose measurement
Dose rates are measured with a tissue-equivalent PMMA phantom placed in the field and an ionization chamber in front of it.1 The measurement is made in all typical modes (magnification, low-dose/high-contrast, cine, DSA) and at different PMMA thicknesses (e.g. 10, 20, 30 cm), so entrance dose is derived as a function of "patient" thickness. To measure the maximum dose rate, a lead sheet is placed between the PMMA and the detector; this drives the AERC to its highest output so the maximum tabletop exposure rate can be measured.1
Image quality
Dose alone is not enough; the image that dose buys is also checked. Fluoroscopy QC covers high- and low-contrast resolution, image lag, geometric distortion (pincushion distortion in image intensifiers), contrast and the last-image hold function. Last-image hold leaves the final frame on screen after the beam is off, allowing review with no dose — one of the most practical ways to lower dose — so its operation is verified in QC.
Patient skin dose
In interventional fluoroscopy the real risk is the cumulative skin dose. Modern systems display the reference-point air kerma and the kerma-area product (KAP); QC checks that these indicators are correct (calibrated). In long procedures one must ensure the peak skin dose does not exceed deterministic thresholds. How skin dose, KAP and deterministic effects are defined is covered separately: Dose in Fluoroscopy.
Who, how often?
Fluoroscopy QC is layered. Technologists/operators do the daily checks and visual quality review, and use the dose-reduction practices (collimation, pulsed mode, last-image hold). Medical physicists handle the dose-rate-limit measurements, AERC performance, image quality and geometric tests — especially at acceptance/commissioning, periodic review and after service. For interventional systems the physicist also verifies the calibration of patient dose indicators and the skin-dose tracking protocol.
References
- Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. The Essential Physics of Medical Imaging, 3rd ed. Lippincott Williams & Wilkins, 2011. Bölüm 9 (Floroskopi): otomatik doz hızı kontrolü (AERC), eski adıyla otomatik parlaklık kontrolü (ABC) — SNR'yi sabit tutmak için pozlama hızını düzenler (s.292); maksimum giriş hava kerma hızı — normal floroskopi 87,3 mGy/dk (10 R/dk), özel/yüksek seviye 175 mGy/dk (20 R/dk); ölçüm konumları — masa altı tüpte masadan 1 cm yukarı, C-kollu sistemde dedektörden kaynağa doğru 30 cm (s.304–305); tipik giriş hızları 8,7–17 mGy/dk (ince), 26–44 mGy/dk (ortalama hasta) (s.305); PMMA fantom + iyon odasıyla giriş doz hızı ölçümü ve AERC'yi maksimuma sürmek için kurşun levha (Şekil 9-16, s.305). Sayfa numaraları bu baskıya aittir.
- U.S. FDA, 21 CFR 1020.32 — floroskopik ekipman performans standartları ve giriş hava kerma hızı sınırları. accessdata.fda.gov
- Kesin tolerans değerleri ve test sıklıkları ulusal düzenlemeler ile uluslararası protokoller (IAEA, IEC 60601-2-43, AAPM) ve kurum prosedürlerince belirlenir. Hasta cilt dozu, KAP ve deterministik etkiler için bkz. Floroskopide Doz.