In an apparatus for displaying reference images of patients and slices to be measured in a displayed reference image for assisting the positioning of slices in preparation for a slice-by-slice measurement and a computer software product and corresponding method, a storage device stores at least one measured reference image of a current patient, a display screen displays a stored reference image, an input device allows entry of commands for displaying and positioning slices to be measured in a displayed reference image, and a processing device processes the entered commands and correspondingly controls the display of the reference image and the slices. The processing device, depending on the entry of commands via the input device by a user, generates a rotated representation of the reference image and a spatial representation—corresponding to the rotation of the reference image—of the slices and displays them on the screen.