Cortical Surface Tracking Using a Stereoscopic Operating Microscope

H. Sun, D.W. Roberts, H. Farid, Z. Wu, A. Hartov and K.D. Paulsen

Neurosurgery, 56:86-97, 2005

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OBJECTIVE: In order to measure and compensate for soft tissue deformation during image-guided neurosurgery, we have developed a novel approach to estimate the three-dimensional (3-D) topology of the cortical surface and track its motion over time. METHODS: We employ stereopsis to estimate the 3-D cortical topology during neurosurgical procedures. To facilitate this process, two CCD cameras have been attached to the binocular optics of a stereoscopic operating microscope. Prior to surgery, this stereo imaging system is calibrated to obtain the extrinsic and intrinsic camera parameters. During surgery the 3-D shape of the cortical surface are automatically estimated from a stereo pair of images and registered to the preoperative image volume to provide navigational guidance. This estimation requires robust matching of features between the images, which, when combined with the camera calibration, yields desired 3-D coordinates. After estimating the 3-D cortical surface from stereo pairs, its motion is tracked by comparing the current surface to its prior locations. RESULTS: We are able to estimate the 3-D topology of the cortical surface with an average error less than 1.2mm. Executing on a 1.1 GHz Pentium machine, the 3-D estimation from a stereo pair of 1024 x 768 resolution images requires approximately 60 seconds of computation. By applying stereopsis over time, we are able to track the motion of the cortical surface including the pulsatile movement of the cortical surface, gravitational sag, tissue bulge as a result of increased intracranial pressure, and the parenchymal shape changes associated with tissue resection. The results from ten surgical cases are reported. CONCLUSION: We have demonstrated that a stereo vision system coupled to the operating microscope can be used to efficiently estimate the dynamic topology of the cortical surface during surgery. The 3-D surface can be co-registered to the preoperative image volume. This unique intraoperative imaging technique expands the capability of the current navigational system in the OR and increases the accuracy of anatomical correspondence with preoperative images through compensation for brain deformation.