1. X-ray imaging device with computer means which is provided for visualizing the 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 the 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 the administration of a contrast agent and a second ECG of the patient recorded simultaneously with the second set, which computer means comprise a program control which operates in accordance with the following method steps for determining the time-dependent concentration of contrast agent within the three-dimensional structure of the vascular tree: Reconstruction of 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 contrast agent within the reconstructed three-dimensional structure of the vascular tree by aa) Assignment of the X-ray projection images of the second set to a respective phase of the heart cycle using the recorded second ECG; bb) Finding local image areas assigned to the individual vascular segments within the X-ray projection images of the second set corresponding to the spatial positions of the vascular segments in the respective phase of the heart cycle according to the three-dimensional structure of the vascular tree; cc) Determining the concentration of contrast agent within the vascular segments by evaluating the X-ray absorption within the local image areas found in the method step bb); Visualization of the flow of contrast agent through the three-dimensional structure of the vascular tree according to the time-dependent distribution of contrast agent