In its Diagnostic X-ray Performance Standard, FDA defines an automatic exposure control (AEC) as a device that automatically controls one or more technique factors in order to obtain a desired quantity of radiation at a pre-selected location. Such a device would automatically terminate the exposure when the selected quantity of radiation had been delivered. The AEC may control the selection of target material, focal spot, filter material, time, mA, mAs, kVp or a combination of any or all of these factors.
AEC mode refers to the type of AEC being used. Typically available AEC modes can range from fixed kVp and mA (where the kVp and mA are selected by the operator and the time is varied by the AEC), to fixed kVp (where the kVp is selected by the operator and the mAs is varied by the AEC), to various AEC modes in which all factors are varied by the AEC. Some of the more automated AEC modes are known by brand names such as BACE, OPDOSE, AUTO FILTER and AOP.
Mean Optical Density (MOD) means the average of the optical densities measured on the images produced with a given equipment configuration during the AEC performance test using 2, 4, and 6 centimeter thicknesses of a homogeneous material.
For AEC testing purposes, the only equipment configurations that need to be tested are the contact configuration, the magnification configuration (if used clinically), and the various image receptor sizes. Due to advances in AEC design, the example of a target-filter combination as an equipment configuration given in the regulations is no longer applicable. Therefore, we will not enforce testing a target-filter combination as a separate equipment configuration.