Positron emission tomography, possibly in combination with computed tomography, allows in addition to medical diagnostic imaging the quantitative determination of various parameters. Quantitative measurements using tomographs exhibit a severe and unavoidable dependency on the imaging properties of the respective tomograph, which makes quantitative assessment of the results difficult. This relates particularly to multicentric medical studies, which obligatorily require quantitative comparability of the data measured by the participating centers. The methods claimed herein include the definition of a virtual tomograph with defined imaging properties. The claimed methods also cover determination of the imaging properties of different tomographs on the basis of suitable reference measurements and possibly by using a calibration phantom. Based on the definition of the virtual tomograph and the determination of the imaging properties of different tomographs, the methods according to the invention then allow conversion and subsequently standardized and quantitatively comparable representation of the image data recorded by the different tomographs or systems as if all measurements were acquired equally by the virtual system. The method according to the invention therefore supports the quantitative evaluation of image data in multicentric studies.