Calibration of tracking systems in a surgical environment

The purpose of this paper was to assess to what extent an optical tracking system (OTS) used for position determination in computer-aided surgery (CAS ) can be enhanced by combining it with a direct current (dc) driven electro magnetic tracking system (EMTS), The main advantage of the EMTS is the fact that it is not dependent on a free line-of-sight. Unfortunately, the accur acy of the EMTS is highly affected by nearby ferromagnetic materials. We ha ve explored,to what extent the influence of the metallic equipment in the o perating room (OP) Can be compensated by collecting precise information on the nonlinear local error in the EMTS by using the OTS for setting up a cal ibration look-up table. After calibration of the EMTS and registration of t he sensor systems in the OR we have found the average euclidean deviation i n position readings between the de tracker and the OTS reduced from 2.9 +/- 1.0 mm to 2.1 +/- 0.8 mm within a half-sphere of 530-mm radius around the magnetic held emitter. Furthermore we have found the calibration to be stab le after re-registration of the sensors under varying conditions such as di fferent heights of the OR table and varying positions of the OR equipment o ver a longer time interval. These results encourage the further development of a hybrid magnetooptical tracker for computer-aided surgery where the el ectromagnetic tracker acts as an auxiliary source of position information f or the optical system. Strategies for enhancing the reliability of the prop osed hybrid magnetooptic tracker by detecting artifacts induced by mobile f erromagnetic objects such as surgical tools are discussed.