In a 3D image of a patient supported on a patient table, the 3D image incorporates depth information about the outline of the patient. The 3D image is received and image information based on the 3D image is displayed on a screen, embedded into a graphical user interface. A first recording area is selectable particularly precisely by the selection being based on the inputting of a first start position and also a first end position in the displayed image information via the graphical user interface. The first recording area is displayed as a graphically highlighted first zone. Furthermore, a first position of the first recording area is determined relative to a recording unit based on the depth information and also based on the selection of the first recording area. This results in the first position being determined rapidly and particularly reliably, in particular in the vertical direction.