1. An operating method for an X-ray machine for examining a patient by using a contrast agent, a preliminary examination method for determining a contrast agent curve being carried out according to the method comprising: setting a scanning position; carrying out a series of scans at defined scanning instants, there being acquired in relation to each scan an attenuation value by which a concentration of the contrast agent at the scanning position is represented; determining the contrast agent curve with respect to time on the basis of the acquired attenuation values; storing the determined contrast agent curve for a later follow-up examination of the patient; and accessing the contrast agent curve from the preliminary examination during the follow-up examination.