1. A method for determining at least one perfusion parameter in tomography, in which a contrast agent bolus, comprising a contrast agent, is dispensed to a patient while the patient is being scanned by a scanner, tomographic image series are reconstructed from a start time to an end time using absorption data scanned by the scanner, and at least one perfusion parameter, relating to diffusion of the contrast agent bolus, are calculated from the reconstructed tomographic image series using numerical modeling, the method comprising: measuring a first temporally variable contrast agent concentration in a first arterial region by determining a change in the scanned values occurring there over a multiplicity of measurement times and saving the measured first temporally variable contrast agent concentration as an arterial contrast agent profile in a first array; measuring at least a second temporally variable contrast agent concentration in a second region of interest in an organ by determining a change in the scanned values occurring there over a multiplicity of measurement times and saving the measured at least a second temporally variable contrast agent concentration as a contrast agent profile in at least a second array; temporally synchronizing the first and second arrays by interpolating the values of at least one of the first and second arrays onto a prescribed time pattern; fitting a theoretical parametrized contrast agent profile in the second region of the organ to the at least one contrast agent profile measured in the second region of the organ by minimizing the differences between the theoretical and measured contrast agent profiles over a multiplicity of times in the measurement interval by fitting the function parameters; and determining at least one perfusion parameter from the obtained function parameters in the case of minimal deviation between the theoretical and measured contrast agent profiles.