Picture two patients: one thin, one large. Taken at the same mAs, the thin patient's image is over-exposed and the large patient's under-exposed. Instead of setting the value by hand for each patient, the technologist can hand the job to physics: AEC (automatic exposure control) automatically ends the exposure once the radiation reaching the receptor reaches the right level. The result: a consistent image regardless of patient size, and an optimized dose.
What is AEC?
Also known as the phototimer, AEC is often used in radiography instead of setting the exposure time by hand.1 Its core logic: the system measures the actual amount of radiation reaching the image receptor and stops x-ray production when the proper exposure is obtained.1 So patient thickness and attenuation differences are compensated at the moment of imaging; the technologist need not know the time in advance.
How does it work?
An AEC system consists of: one to three ionization chambers (photocells) placed in front of the receptor, an amplifier, a comparator/integrator circuit, a termination switch, and a backup timer safety switch.1 X-rays transmitted through the patient (and the grid, if present) generate a signal in the ion chambers; this signal is amplified and integrated. When the accumulated signal equals a preselected reference value, an output pulse terminates the exposure.1 A density/SNR selector on the panel fine-tunes the total exposure by raising or lowering this reference by about 10–15% per step.1 There are usually three photocells, and the technologist selects which to use for the projection (e.g. the outer cells for a PA chest). If a photocell or circuit fails, the backup timer cuts the beam at a preset time to ensure safety.1
Dose & patient size
AEC's dose value is twofold. On one hand it provides consistency: the dose reaching the receptor (and hence image quality) stays constant regardless of patient size; needless extra dose in a thin patient and under-exposure (and a repeat) in a large patient are both avoided. On the other hand AEC is not blind: a wrong photocell selection (e.g. if a metal prosthesis or a dense structure lies over the sensor) can mislead the system into over- or under-exposure. So correct photocell selection and positioning are a precondition for AEC to work as an ALARA tool.
References
- Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. The Essential Physics of Medical Imaging, 3rd ed. Lippincott Williams & Wilkins, 2011. §6 (Phototimer — Automatic Exposure Control): fotometre, alıcıya ulaşan gerçek radyasyonu ölçer ve uygun pozlamaya ulaşıldığında X-ışını üretimini sonlandırır; hasta kalınlığı ve zayıflatma farkları görüntüleme anında telafi edilir; 1–3 iyon odası, yükselteç, karşılaştırıcı/integratör, sonlandırma anahtarı, yedek zamanlayıcı; yoğunluk/SNR seçici adım başına ~%10–15; üç fotosel seçilebilir (Şekil 6-28, s.218). Sayfa numaraları bu baskıya aittir.
- İlişkili: Işınlama Parametreleri (kVp, mAs) · Radyografide Kalite Kontrol · ALARA Prensibi