1. A method for imaging a heart of a patient, comprising: providing a 3D X-ray image data set of the heart assigned to a reference phase in an electrocardiogram of the heart; providing a plurality of 2D X-ray projection images of the heart assigned to a same phase with each other in the electrocardiogram of the heart; calculating a plurality of 2D reference projection images from the 3D X-ray image data set each corresponding to one of the 2D X-ray projection images for a same projection angle; calculating a plurality of 2D differential images each between one of the 2D X-ray projection images and the corresponding one of the 2D reference projection images; back projecting the 2D differential images; combining the back projections into a 3D differential image data set; deforming the 3D X-ray image data set using the 3D differential image data set; and imaging the heart of the patient using the deformed 3D X-ray image data set.