A method and system are provided for determining a time-dependent, three-dimensional perfusion data set relating to the perfusion of at least one vessel and/or of tissue of an examination object. Projection images of the vessel and/or tissue are acquired in a plurality of recording geometries by an X-ray detector at a plurality of recording times in each case, which images describe detected intensities in a plurality of imaging regions of the X-ray detector. The perfusion data set is determined by associating a weighted sum of specified time-dependent base functions with each voxel of the perfusion data set.