1. registration algorithm - centerline registration algorithm (Sintef, Trondheim) A catheter equipped with a position sensor records the travelled path of the instrument, which is assumed to be close to the centerline of the aorta. From the preoperative computer tomography scan (CT) the aorta is segmented and vessel centerline is extracted. The centerline from the position sensor recordings is registered to the extracted centerline of the preoperative CT.