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 cross-section of an outer cover (3) of the O-arm (2) in a direction perpendicular with respect to the central axis of the O-arm is arranged for its predominant portion substantially of the shape of an arch of a circle but to comprise a cut, within the sector covered by which cut the distance from the center of said arch of the circle to the edge of the outer cover (3) is shorter than within the portion of the outer cover (3) being substantially of the shape of an arch of a circle.