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 by a computing device, 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 by the computing device, wherein the estimation parameter describes a non-periodic motion in a non-periodic motion model and is derived from a periodic 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 by the computing device; animating the three-dimensional image dataset based on the estimation parameter of the non-periodic motion model and the motion information by the computing device; breaking two of the projection images acquired at two different recording instants into orthogonal motion components on two projection planes, wherein the orthogonal motion components comprise motion parameters of the periodic motion model, wherein a motion parameter of the periodic motion model that is along a projection direction is missing at one of the two projection planes respectively, wherein the missing motion parameter of the periodic motion model is determined from the two-dimensional projection images, and wherein the periodic motion model described by the motion parameters is in agreement with the non-periodic motion model; and combining the orthogonal motion components of the two projection images for determining the motion information.