An image-based coordinate tracking system using afocal optics for surgical navigation

The optical tracker is the equipment for real-time 3D position and pose measurement of marker using camera. The general optical tracker uses the stereo camera and rigid body marker. When using in surgical application, the rigid body marker is attached to the surgical instrument or patient. Therefore, it is necessary to reduce the size of attached marker for the convenience of surgeon. But if the size of the attached marker reduces, the size shown in the camera image also reduces. Then the accuracy of calculated coordinates decreases. To solve this problem, a novel coordinate tracking method is proposed which uses the bokeh phenomenon. The afocal system which uses this method is used with a marker which has a fisheye lens in front, behind it a 10 mm x 10 mm glass pattern with 30 μm x 30 μm datamatrices printed on. If the camera lens is out of focused, these small datamartices are magnified and shown in the image took by the camera. The tracking method calculates the geometrical relation of the camera and the marker using the information of the patterns. Then the position and pose of the marker are calculated using the result of geometrical relations. Because the patterns are magnified, the accuracy of measurement is high. Also for the size of glass pattern is small, the total size of the marker also can be reduced. The result of experiments shows that the proposed algorithm is usable in image based surgical navigation.