Automatic exposure control

Last revised by Lachlan McKay on 22 Feb 2024

Automatic exposure control (AEC) is incorporated into radiographic and mammographic imaging systems. Its function automatically terminates exposure when a preset amount of radiation has been detected.

Automatic exposure control systems help to provide a consistent optical density/signal-to-noise ratio between images, regardless of patient-centric factors such as size and density.

Automatic exposure control systems also help to reduce 'dose creep' that can occur with inadvertent radiation overexposure by the technologist.

The first generation of automatic exposure control systems are phototimers, which are now largely superseded by ionization chambers. These phototimers/ionization chambers terminate the exposure once the radiation detector receives a predetermined signal of radiation (which indirectly means the image receptor is sufficiently exposed).

In radiography, the automatic exposure control device is placed in front of the image receptor. In mammography, the automatic exposure control device is placed underneath the image receptor. This is mainly due to the low energy requirement for mammography; if the AEC device was placed in front of the receptor, the device would produce an image on the receptor, confounding the mammogram.

In chest radiography, typically three ionization chambers are used; a left, right and central chamber. The patient's position should align with the chambers such that the left and right are behind their lungs, whilst the central is behind the spinal column.

Related (though discrete) principles include automatic brightness control (ABC) in fluoroscopy and tube current modulation in CT.

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