Automatic exposure control

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Automatic Exposure Control (AEC) is an X-ray exposure termination device. A medical radiography x-ray exposure is always initiated by a human operator but an AEC detector system may be used to terminate the exposure when a predetermined amount of radiation has been received.[1] The intention of AEC is to provide consistent x-ray image exposure, whether to film or a digital detector. AEC systems may also automatically set exposure factors such as the X-ray tube tube current and voltage.[2]

Operation[edit]

An AEC system uses one or more physically thin radiation ionization detectors (the "AEC detector") which is positioned between the patient being x-rayed and the x-ray film cassette. Where low energy x-rays are used such as in mammography the AEC detector is placed behind the image receptor to avoid creating a shadow.[3]:106

In a simple AEC system a weak ionization signal from the AEC detector is integrated as a ramp shaped voltage waveform. This ramp signal rises until it matches a pre-set threshold. At this point the x-ray exposure is terminated.[4] AEC devices are calibrated to ensure that similar exams have linearity in optical density.[5] This is due to the fact that a milliamperage station is no longer selected and instead relies upon the ionization within the selected chambers.

Advantages[edit]

Because patients vary in size and shape, an AEC device is very useful in achieving consistent x-ray film densities, which can be difficult when manually setting exposure factors without AEC.[3]:130

Disadvantages[edit]

AEC devices are susceptible to operator error (usually due to mispositioned anatomy or having the incorrect AEC chamber selected).[6] Prosthetic devices such as total hip hardware can also cause the selected ionization chamber to overexpose the image receptor. This is due to the absorption of the x-ray beam into the metal of the hardware as opposed to exposing the ionization chamber.[7]

References[edit]

  1. ^ Sterling, S (1988). "Automatic exposure control: a primer.". Radiologic technology. 59 (5): 421–7. PMID 3290991. 
  2. ^ "Automatic exposure control devices". IAEA Human Health Campus. Retrieved 16 December 2016. 
  3. ^ a b Dance, D R; Christofides, S; Maidment, A D A; McLean, I D; Ng, K H (2014). Diagnostic radiology physics : a handbook for teachers and students. Vienna: International Atomic Energy Agency. ISBN 978-92-0-131010-1. 
  4. ^ Webb, S (2009). The physics of medical imaging (2nd ed.). London: Taylor & Francis. p. 79. ISBN 9780750305730. 
  5. ^ Doyle, P; Martin, C J (7 November 2006). "Calibrating automatic exposure control devices for digital radiography". Physics in Medicine and Biology. 51 (21): 5475–5485. doi:10.1088/0031-9155/51/21/006. PMID 17047264. 
  6. ^ Walsh, C; Larkin, A; Dennan, S; O'Reilly, G (November 2004). "Exposure variations under error conditions in automatic exposure controlled film–screen projection radiography". The British Journal of Radiology. 77 (923): 931–933. doi:10.1259/bjr/62185486. PMID 15507417. 
  7. ^ Carroll, Quinn B. (2014). Radiography in the digital age: physics, exposure, radiation biology (2nd ed.). Springfield, IL: Charles C Thomas. p. 415. ISBN 9780398080976. 

Further reading[edit]