The invention relates to a medical x-ray imaging apparatus including a support structure (1) which supports a substantially ring-shaped structure, an O-arm (2) supporting imaging means (21, 22), to which O-arm (2) is arranged an examination opening (4). The ring-shaped structure (2) supporting the imaging means is arranged movable with respect to said support construction (1) at least in vertical direction. A patient stool (30) is arranged to be positioned in connection with the apparatus, as well as positioning means (32, 33, 34) in connection with said examination opening (4) for positioning the patient stool (30) in the examination opening (4).