How to compute the difference between tomographic images

In a narrow sense, the task of computerized tomography is to reconstruct images from projections. However, the end user may be interested in other tasks that are traditionally performed by post-processing of reconstructed images. An algorithm for estimating the difference between two tomography images is proposed if the two data vectors are obtained in nonoverlapping time intervals. The algorithm takes advantage of statistical independence of two vectors. The results are significantly different from the traditional approach in which the two images are reconstructed separately and then subtracted