A method is disclosed for determining a PET image data record of a target area from PET image data recorded during a period using a combined PET device provided with an additional imaging modality, an attenuation map describing the absorption of photons in the imaging area being used for attenuation correction of the PET image data. In at least one embodiment of the method, as a function of at least one change in the attenuation in the imaging area during the period, the PET image data are divided into at least two PET image data groups of different time domains of the period and are evaluated differently with respect to consideration of the attenuation.