1. An X-ray imaging device with an image processing device which is provided for visualizing blood flow in a coronary vascular tree of a patient such that the visualization is effected based on data which contain a first set of X-ray projection images of the vascular tree in various phases of a heart cycle, a first ECG of the patient recorded simultaneously with the first set, a second set of X-ray projection images recorded during or after administration of a contrast agent and a second ECG of the patient recorded simultaneously with the second set, wherein the image processing device comprises a program control which operates in accordance with the following method steps for determining a time-dependent concentration of the contrast agent within a three-dimensional structure of the vascular tree: reconstructing the three-dimensional structure of the vascular tree during the various phases of the heart cycle using the first set of X-ray projection images and splitting of the structure into a number of vascular segments; determining the time-dependent concentration of the contrast agent within the reconstructed three-dimensional structure of the vascular tree by a) assigning the second set of X-ray projection images to a respective phase of the heart cycle using the recorded second ECG; b) finding local image areas assigned to the vascular segments within the second set of X-ray projection images corresponding to spatial positions of the vascular segments in the respective phase of the heart cycle according to the three-dimensional structure of the vascular tree; c) determining the concentration of the contrast agent within the vascular segments by evaluating an X-ray absorption within the local image areas found in the method step b); and generating a visualization of flow of the contrast agent through the three-dimensional structure of the vascular tree according to the time-dependent concentration of contrast agent: wherein the recording of the first and second set of X-ray projection images is effected at a plurality of projection angles.