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 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 scanned values of at least one of the first and second arrays onto a prescribed time pattern; fitting a theoretical parameterized contrast agent profile in the second region of the organ to the contrast agent profile measured in the second region of the organ by minimizing differences between the theoretical and measured contrast agent profiles over a multiplicity of times in the measurement interval by fitting function parameters; and determining at least one perfusion parameter from the function parameters in a case of minimal deviation between the theoretical and measured contrast agent profiles; wherein a function v t ⁡ ( t ) = v 0 + I i ⁢ ⁢ n ⁢ ∫ - ∞ t ⁢ a ⁡ ( t ′ - t i ⁢ ⁢ n ) ⁢ ⅆ t ′ - I out ⁢ ∫ - ∞ t ⁢ a ⁡ ( t ′ - t out ) ⁢ ⁢ ⅆ t ′ , which has ν 0 , t in , t out , I in , I out as parameters which need to be fitted, is used as the theoretical contrast agent profile, and a(t) corresponds to an arterial course of the contrast agent.