The value of a tube voltage of an x-ray tube, used to acquire x-ray projections of a patient in order to generate at least one image in each phase of a multiphase examination, is determined from a contrast-to-noise ratio that establishes the desired image quality of the image in each phase, and the tube current for each tube voltage (U) for each phase is determined given a constantly maintained contrast-to-noise ratio for different tube voltages (U). For each phase, a value for the dose of x-ray radiation is defined for each different tube voltage (U) and the associated determined tube current. From among the different tube voltages, a tube voltage (U) is set that causes the total dose of x-ray radiation applied to the patient (P) to be as low as possible for all phases of the examination.