Implementation of a stereotactic microscope using an optically coupled tracking system.

Image-directed neurosurgical systems allow the correlation of a point in the operative field to a corresponding point on the patient's CT/MR scans. We have modified a surgical microscope by mounting three light-emitting diodes (LEDs) on a small frame attached to the microscope. The operative field was registered to the MR scans by indexing four MR/patient visible fiducial points with a LED pointing device. To calibrate the microscope, the probe point was focused and centered in the microscopic field of view with the aid of cross hairs in the microscope ocular. The focal plane of the microscope could now be tracked in addition to the probe tip. During surgery the point of interest is centered in the cross hairs and brought into focus, and the corresponding point is indicated in three planar views on MR scans.