Automatic exposure control
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. 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.
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.: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. AEC devices are calibrated to ensure that similar exams have linearity in optical density. This is due to the fact that a milliamperage station is no longer selected and instead relies upon the ionization within the selected chambers.
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.:130
AEC devices are susceptible to operator error (usually due to mispositioned anatomy or having the incorrect AEC chamber selected). 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.
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