The invention relates to a method for determining a probability distribution with local three-dimensional resolution for a substance in a vascular system. An acquisition time is assigned to a group of x-ray images showing a presence distribution with local 2D resolution for a substance in the vascular system at the acquisition time. A first preliminary presence probability distribution is determined using the x-ray images assigned to the acquisition time and a volume data set of the vascular system. A second preliminary presence probability distribution is determined using the presence probability distribution for the preceding acquisition time and a migration probability distribution with the substance migrating from first vascular locations to second vascular locations. A high individual probability to vascular locations is assigned if both preliminary presence probability distributions have a high probability value for the respective vascular location. Otherwise a lower and greater than zero individual probability is assigned.