The invention relates to the use of 2D projection images which belong to a specific common heart phase. A 3D image data set can be used to generate a reference projection image for the same projection angle for each of the 2D projection images and a differential image can be derived from the reference projection image and 2D projection image. The differential images are back-projected and combined in one 3D differential image data set and, by using this, a deformed 3D image data set is obtained from the previously recorded 3D image data set. Iterations guarantee that the deformed 3D image data set ensues with the smallest possible distance from the 2D projection images for the existing common heart phase. Finally, a 3D image data set is available for a different heart phase other than the reference heart phase and the possibilities for imaging a patient's heart are extended.