Automatic Exposure Control

The automatic exposure control (AEC) timer measures remnant radiation coming through the patient to terminate the exposure when a pre-set amount of remnant radiation has been accumulated. One type of AEC, the phototimer, is located after the image receptor. A fluorescent screen produces light when exposed to x-rays and charges the photomultiplier; when a predetermined charge is reached, the exposure is terminated. Another type of AEC, an ionization chamber, is located between the table top and the image receptor; when the predetermined amount of ionization of air has been reached, the exposure is terminated.

The phototimer or ionization chamber location corresponds to the location of the photocell/sensor. AEC units have several photocells/sensors to choose from depending on the anatomy to be imaged and its location in the radiograph. The area of interest must be positioned directly above the photocell/sensor. Misalignment can result in an under or over exposed radiograph. A backup timer (manual timer) protects the patient from excessive exposure if the AEC were to fail.

An additional feature that AEC units have is density control (+1 or -1). Using this feature correctly can help to compensate for pathologies. The + or – density settings will make adjustments to the density of the image in increments of 25%. If the area of interest requires more than a 25% change in density, then a change in mA station is a more appropriate adjustment.