The present invention relates to a device for reconstructing an X-ray tomography image, the device comprising a reconstruction module, which is configured to utilize an ordered subset maximum likelihood optimization with a diagonal paraboloid approximation of a cost function for the reconstructing of the X-ray tomography image; and a calculation module, which is configured to calculate a pre-computable denominator term for the cost function for a plurality of subsets of projection data based on a distribution of diagonal denominator terms over the plurality of the subsets of the projection data.