In another aspect, the present invention relates to a method for determining the position and orientation of an anchor attached to the skull of a patient from a topographic image of the anchor acquired therefrom. In one embodiment, the method includes the steps of simulating an image of the anchor that matches the acquired topographic image of the anchor, determining the position and orientation of the anchor from the simulated image of the anchor, and determining the position and orientation of the anchor in the acquired topographic image of the anchor from the position and orientation of the anchor determined from the simulated image of the anchor. The simulating step comprises the step of modeling the anchor with an object that has a geometric shape simpler than that of the anchor.