Most mammograms are made using Automatic Exposure Control (AEC). The AEC system measures the exposure that reaches the receptor after penetrating the breast and turns the exposure off when the necessary exposure has been delivered to produce the expected film density.
While AEC is a valuable function for producing optimum film density and visibility, it does not always produce the "perfect" exposure. There are several potential sources of error that must be considered as illustrated here. Two are associated with the set-up and calibration of the system by the engineers, and two are under the control of the technologist/radiographer.
AEC Calibration (Physicist and Engineer Function)
The AEC must be calibrated by the engineering staff to produce the desired film density. The calibration is verified by a medical physicist who specifies a density value that is optimum for the specific clinical facility. The calibration is specified in terms of the film density (a value of 1.6 is illustrated here) produced when imaging a test device (phantom) of uniform thickness.
AEC Tracking (Physicist and Engineer Function)
Tracking is the ability of the AEC to maintain correct calibration over the range of receptor types, KV values, and exposure times used in a clinical facility. This is generally not a problem with modern mammography equipment but is evaluated periodically by a medical physicist in the context of the QA program.
AEC Sensor Position (Radiographer Function)
Typical mammography systems have multiple radiation sensors, or at least multiple sensor positions, for measuring the exposure reaching the receptor. The film will be exposed to the calibrated density in the anatomical area over the selected sensor or sensor position.
The appropriate sensor position should be selected by the radiographer based on breast characteristics, especially the presence of dense areas.
An incorrect sensor location can result in an exposure error (too light or too dark) to the film.
Density Control Setting (Radiographer Function)
The radiographer can adjust the "Density" control to change the film density from that produced by the AEC normal setting. Typical settings of the Density control are: (N)ormal, +1, +2, +3, -1, -2, -3,etc. Although there is no standard relationship, changing the Density control by one unit will generally increase or decrease the exposure about 15%.
The Density control is useful when it appears that the AEC (N)ormal setting does not produce the appropriate film density (until it can be recalibrated by the engineer) and when certain breast conditions are better visualized with lighter or darker films.
Associated with the Density control is a function for indicating which receptor (film/screen combination) is being used.
The AEC must have this information to make the correct exposure.