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Figure 5a.  Preprocessing with suppressing and enhancing filters. (a) Head CT scan illustrates the appearance of an image prior to filtering. (b) Same image as in a after application of a smoothing filter. Note that noise is suppressed in regions of uniform intensity, but edges are also blurred. (c) Same image as in a after application of an edge-enhancing filter. Note that regions of uniform intensity are unenhanced because the gradient in these regions is small. However, the boundaries (especially of skin and bone) are enhanced.