Abstract: A computer tomography (CT) imaging apparatus (2) for correlating images of the colon in prone and supine positions is disclosed. The apparatus comprises pairs of x-ray sources (6) and detectors (8) for generating 3 D image data representing at least one first location on the colon wall in the prone or supine position, and for generating 3 D image data representing a plurality of locations along the teniae coli extending along the colon wall in that position. The sources and detectors also generate 3 D image data representing the same locations along the teniae coli in the other of the prone or supine position. A computer (16) contains a processor (20) for determining a location in the second scanned image corresponding to the first location in the first scanned image.