1. Data processing unit for the iterative reconstruction of cross-sectional images of an object in a given observation phase of its cyclic movement, comprising: a) an input module for the reception of projections of the object from different directions and of simultaneously measured movement parameter characterizing the associated cyclic movement of the object; b) a sorting module for sorting the projections into subsets such that the projections within each subset correspond to a similar phase of the cyclic movement; c) a reconstruction module for the iterative reconstruction of a cross-sectional image with an algorithm that processes in each iteration step all projections of at least one subset.