Real-time Correction Scheme for Calibration and Implementation of Microscope-Guided Neurosurgery

H. Sun, H. Farid, A. Hartov, K.E. Lunn, D.W. Roberts, and K.D. Paulsen

SPIE's International Symposium on Medical Imaging, San Diego, CA, 2002

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Microscope-guided neurosurgery can be divided into three steps: calibration of the microscope optics; registration of the preoperative images to the operating space; and tracking of the patient and microscope over time. Critical to this overall system is the retention of accurate camera calibration over time. Classic calibration algorithms are routinely employed to find both intrinsic and extrinsic camera parameters. The accuracy of this calibration, however, is quickly compromised due to the complexity of the operating room, the long duration of a surgical procedure, and the inaccuracies in the tracking system. To compensate for the changing conditions, we have developed an adaptive procedure which responds to accruing registration error. The approach utilizes miniature fiducial markers implanted on the bony rim of the craniotomy site. These markers are always in the field of view of the operating microscope. A simple error function that enforces the registration of the known fiducial markers is used to continually update the extrinsic camera parameters. The error function is minimized using a gradient descent minimization. This correction procedure reduces RMS registration errors for cortical features on the surface of the brain by an average of 65%, or 2 mm. These errors were less than 1.5 mm after each correction during the entire surgical procedure.