1. An apparatus for correcting a cross sectional slice image for an image artifact, the slice image based on CT projection images taken of an object at different times and taken whilst a high-attenuation part of the object is in motion, the artifact caused by said motion, the apparatus comprising: an input unit; a processing unit; an output unit; the input unit configured to receive the slice image and to access the plurality of CT projection images; the processing unit comprising: a segmenter configured to segment in the slice an artifact region; an identifier configured to use the so segmented artifact region to identify in each of a plurality of the CT projection images a footprint corresponding to the artifact region; a matcher configured to match, in each of the plurality of CT projection images, the identified footprint in the CT projection image to a matching footprint in that CT projection image, the matching footprint representing the part's position at acquisition time of that CT projection image; an image reconstructor configured to reconstruct upon user requests a new slice image based on the plurality of matching footprints, thereby compensating for the part's motion during acquisition of the CT projection images, the segmenter, identifier and matcher then processing said new slice image instead of the slice image; and an interpolator configured to generate, for each of the plurality of CT projection images, an interpolated CT projection image by using the matching footprint in the CT projection image as a constraint for the interpolation in that CT projection image to so obtain a plurality of interpolated CT projection images; the image reconstructor configured to use the plurality of interpolated CT projection images to reconstruct a corrected version of the slice image; the output unit configured to output the corrected version of the slice image.