1. A method of processing a 3-dimensional Computed Tomography Colonography data set, comprising a plurality of voxels, to remove tagged material, the method comprising: (i) classifying voxels in the data set as corresponding to air, tagged material or colon tissue, wherein each voxel lies within an associated intensity range; (ii) identifying tagged material voxels at a colon tissue/tagged material interface having an intensity value below a predetermined intensity value; (iii) applying an edge recovery algorithm to tagged material voxels identified in step (ii); and (iv) selectively applying to the data set the output of the edge recovery algorithm of step (iii) to provide a plurality of edge recovered voxels.