Figure 5. Relationship between the mean optical density (OD) in the region of interest (ROI) corresponding to the automatic exposure control (AEC) detector position and the area of the main breast ROI. The position of the AEC detector used clinically is indicated by the data point symbol. The corresponding ODs for a semi-circular block of PMMA are shown as a solid curve. The PMMA data are from a different X-ray system (X-ray set 1) and are shown together with the clinical data to demonstrate the similarity in the shape of the relationship between the mean OD in the ROI corresponding to the AEC detector position and the area of the main breast.