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. X-ray termination is done by the same human operator or an exposure control device such as an electronic timer or an AEC device, which ever comes first. The intention of the AEC termination device is to provide consistent x-ray film appearance.
An AEC system uses a physically thin radiation ionization detector "AEC detector" which is positioned between the patient being x-rayed and the x-ray film cassette. X-rays passing through the patient also pass through this "AEC detector" before they strike the x-ray film.
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 exam densities. 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 good consistent x-ray film densities compared to non AEC x-ray exposures.
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.