in a computed tomography apparatus and operating method , a radiation source and radiation detector are rotated around a system axis , and a patient support plate and diaphragm elements of a diaphragm associated with the x - ray source are also movable in the direction of the system axis . movement of the patient support plate and the diaphragm plates between respective end positions is coordinated during a dynamic computed tomography examination of a subject so as to reduce and homogenize the dose of x - ray radiation to which the subject is exposed during the examination .