Filtering, to increase the intensity of the edges,  is  done  through  the  convolution  of the image with a filtering function. This can be imagined as similar to sharpening the focus of a digital photo by using image processing software.    In contrast to photography, where this focus sharpening is done for purely aesthetic reasons, the folding step is mathematically necessary in tomography.

See in image above  the two steps of preparation and filtering.  The radiographic images (a) are converted into the image of the attenuated coefficients (b) by normalization, taking the logarithm, and linearization.  They are then filtered in order to accentuate the edges (c).