The comparison between left and right breasts is based on the fact
that both breasts have similar internal structure, as it is shown
in Figure , where mammograms
corresponding to left and right breasts of a woman are displayed
as radiologists are used to. A thoroughly evaluated method for
comparing mammographic images is known as bi-lateral
subtraction. Both images are first aligned and subsequently
subtracted. The alignment of both breasts is the critical
component in this process. There are two main approaches to this:
The advantage of the former techniques over the latters is that the distortion of the internal structures is taken into account, thus registering correctly the breast interior. It should be noted that imaging conditions (breast compression and exposure, X-rays energy, etc.) are likely to differ for the left and right mammograms.
There are alternatives to this alignment and subtraction approach. Kok-Wiles et al. [] represent the bright zones of the breast as a tree-like-structure, which forms the basis for the comparison process. Stamatakis et al. [] developed a Multiple Image Comparison approach. This method starts with one pair of mammograms, then the system computes eight new pairs of images using a set of grey-level features. Finally each of those pairs is bilaterally compared with each other and the resulting difference images are recombined into a final result.