1. An apparatus for correcting an X-ray tomographic image, where the tomographic image is created by measuring an X-ray transmission coefficient at each of a plurality of X-ray beam ray paths through an object to enable reconstructing a tomographic image of said object, comprising: means for selecting a plurality of volume averaged voxels along at least one said X-ray beam ray path, and for placing contiguous volume averaged voxels in a runlength of volume averaged voxels, to make a plurality of run lengths of volume averaged voxels; means for dividing said plurality of run lengths of volume averaged voxels into a plurality of subslices; means, responsive to said plurality of subslices and responsive to said plurality of run lengths of volume averaged voxels, for computing a corrected value of X-ray attenuation coefficient for said X-ray beam ray path; means for reconstructing a correction image using said corrected X-ray attenuation coefficient for each said X-ray beam ray path; means for obtaining an improved image by combining said tomographic image with said correction image.