The invention relates to a medical x-ray imaging apparatus which includes a support construction (1) supporting a substantially ring-shaped structure (2), an O-arm (2), which in turn supports imaging means (21, 22) and which O-arm (2) is arranged with an examination opening (4). The imaging means (21, 22) are arranged movable within the O-arm (2). The examination opening (4) is arranged for its predominant portion substantially of the shape of an arch of a circle but to comprise an extension, within the sector covered by which extension the distance from the centre of said arch of the circle to the edge of the examination opening (4) is longer than within the prevailing portion of the examination opening (4) being substantially of the shape of an arch of a circle.