1. A data processing unit for an iterative reconstruction of cross-sectional images of an object in a given observation phase of a cyclic movement of the object, comprising: an input module for reception of projections of the object from different directions and of simultaneously measured movement parameters characterizing the cyclic movement of the object; a processing module for determining a time distance from a projection to the observation phase for one or more of the projections; a sorting module for sorting the projections into subsets such that the projections within a subset correspond to a predetermined time distance range to the observation phase; and a reconstruction module for the iterative reconstruction of a cross-sectional image with an algorithm that processes in each iteration all projections of at least one subset.