1. A method for determining a presence distribution with a three-dimensional dataset for a substance in a vascular system of a patient, comprising: receiving a volume data set describing the vascular system with the three-dimensional dataset by a computer; receiving a sequence of groups of x-ray image locally acquired at a sequence of acquisition times by the computer, the x-ray image showing an actual presence distribution with a two-dimensional dataset for the substance in the vascular system at an acquisition time; assigning the acquisition times to the corresponding groups of the x-ray image by the computer; defining a start time for an initial presence distribution by the computer; simulating further presence distributions by iteratively resolving fluid dynamics movement equations for determination times from the initial presence distribution and the volume data set by the computer; and correcting the further presence distributions based on the corresponding groups of the x-ray image if the determination times correspond to the acquisition times by the computer.