Curvature-based nonfiducial registration for the Frameless Stereotactic Operating Microscope.

The Frameless Stereotactic Operating Microscope permits information from CT and MRI scans to be displayed in the operating microscope in the proper scale and perspective without using a mechanical frame in the process, when the microscope is positioned over the surgical field. This registration is currently done using fiducials that must be present when the imaging scans are taken. A new technique, based on the theory of curvatures, has been developed as an alternative to the use of fiducials for the registration of a patient's head position during surgery with diagnostic images from CT or MRI. Surface curvatures are estimated from a trace of points of the surface of the skin, obtained from both the diagnostic images and an intraoperative nonimaging ultrasonic rangefinder. The point traces need not be identical or evenly spaced. Plots of the resulting curvature fields are compared visually and the alignment determined. Phantom testing, using a human skull, has resulted in a median alignment error of 1.95 mm. Testing using a human subject with clinically obtained data has resulted in alignment errors on the order of 7 mm.