[Radiographic images: appearance and reality ( a method of measurement of the 3D coordinates of anatomic points)]

The catch of stereotypes X-ray introduces a conical projection phenomenon which introduce errors from 1 to 5 millimetres. To obtain the real position of anatomical point with precision, we show an original method of co-ordinate 3D calculation which takes account of system radiographic dimensions. A point of space is located compared to the frame of reference of center O (medium of the segment materialized by olives). The frontal stereotype is selected like image of reference after having placed, a 4 mms metal ball on midface of patient. The side stereotype is obtained while making turn the patient of 90 degrees. The axial stereotype is obtained by rotation of the head, around horizontal olives axis. We suppose that the Source-Stereotype unit turns around the patient's head (considered fixed). The protocol comprises a stage of catch of stereotypes X-ray, a stage of digitalization of the sights and initialization of the system and a stage of measurement itself. We describe the setting in equation of real co-ordinate point determination. We apply to the apparent coordinate, the correction formulas we obtain the real co-ordinates of the point.