Despite the success of the quantitative techniques, thus far described, in predicting breast cancer risk, neither the automated methods nor the PD measure has provided as strong a risk measure as does the subjective six-category classification system. The reason for this may be explained by Eqs. (1) and (2). Among many other factors, the optical density at a given point on the image depends on both the composition of the breast and the total thickness of tissue through which the X-rays must pass. Techniques relying solely upon the image brightness are unable to separate these factors.
A breast under compression can be considered to consist of two regions: a central area of approximately uniform thickness; and, a margin where the thickness is reduced. These are illustrated schematically in Fig. 7. Depending on the size and compressed thickness of the breast, the margin can represent a substantial portion ofthe projected area. In the central area, the image will vary in brightness according only to the relative composition of fat and glandular tissue. When a threshold, ¿DY, is set using the PD method, while considering the uniformly
FIGURE 7 Schematic illustration of the compressed breast. Under compression the breast is considered to consist of two regions, one of approximately uniform thickness referred to as the central region, and a margin where thickness varies. In the margin, variation in transmitted X-ray fluence occurs because of changes in both breast thickness and composition.
Was this article helpful?