X-ray image data of an examination subject is acquired for an acquisition period by an X-ray detector of an X-ray system simultaneously with a decay process of a radioactive material taking place in or on the examination subject. The X-ray image data includes information relating to an X-ray attenuation distribution of the examination subject and information relating to the decay process. Correction image data representing the information relating to the decay process in the X-ray image data for the acquisition period is determined. Corrected X-ray image data for the acquisition period is generated using the X-ray image data and the correction image data.
