The invention relates to a medical x-ray imaging apparatus which is specifically designed for enabling three-dimensional imaging of relatively small volumes from the area of extremities of a patient. The apparatus 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). A positioning support (8) is arranged in connection with said examination opening (4) whose location with respect to the O-arm (2) is arranged adjustable. Preferably the apparatus comprises a control system comprising a control function for moving the positioning support (8) in a first direction at a first speed within the area of the examination opening (4) and for simultaneously moving the O-arm (2) with respect to its support construction (1) substantially at said first speed in substantially opposite direction than said first direction.