The method involves providing a first 3D image data set for determining the position of a relevant area of the object (5). The x-ray system is registered with the data. After inserting the instrument (6) into the vein etc. (9) a 2D x-ray image is recorded at a given projection geometry and a 2D position of a first position of the instrument is detected. At this position a projection line is placed through the 3D data set and a 3D position is determined from the intersection of the line with the structure. DE102005007893A1
