1. A method for generating a four-dimensional representation of a target region subject to a periodic motion, comprising: reconstructing a three-dimensional image dataset from a plurality of two-dimensional projection images recorded from different projection directions at different recording instants, wherein the three-dimensional image dataset represents a static reconstruction based on all the plurality of two-dimensional projection images for a specific instant; determining an estimation parameter based on the projection images, wherein the estimation parameter describes a non-periodic motion in a non-periodic motion model and is derived from a motion model that is formulated independently of a motion phase with respect to the recording instants; determining motion information that is missing in the estimation parameter due to the two-dimensionality of the projection images using a boundary condition describing the periodicity of the motion; and animating the three-dimensional image dataset based on the estimation parameter of the non-periodic motion model and the motion information.