Real-time Correction Scheme for Calibration and Implementation of Microscope-based Image-guided Neurosurgery Hai Sun * , Hany Farid § , Alex Hartov *† , Karen E. Lunn * , David W.Roberts †‡ , Keith D. Paulsen *†‡ * Thayer School of Engineering, Dartmouth College, NH 03755 § Department of Computer Science, Dartmouth College, NH 03755 † Dartmouth Hitchcock Medical Center, Lebanon, NH, 03766 ‡ Norris Cotton Cancer Center, Lebanon, NH 03766 ABSTRACT Microscope-based image-guided neurosurgery can be divided into three steps: calibration of the microscope optics; registration of the pre-operative images to the operating space; and tracking of the patient and microscope over time. Critical to this overall system is the temporal retention of accurate camera calibration. 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, which remain 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 update the extrinsic camera parameters. The error function is minimized using a gradient descent. This correction procedure reduces RMS registration errors for cortical features on the surface of the brain by an average of 72%, or 1.5 mm. These errors were reduced to less than 0.6 mm after each correction during the entire surgical procedure. Keywords: Microscope-based image-guided neurosurgery, registration error, camera calibration 1. INTRODUCTION Pre-operative high-resolution magnetic resonance (MR) and computed tomography (CT) provide surgeons with an atlas for surgical navigation. During neurosurgery, a microscope is used to help the surgeon locate struc- tures on the surface of the brain. It is often desirable to establish an accurate correspondence between the real-time two-dimensional (2-D) microscope image and the pre-operative three-dimensional (3-D) MR or CT data. The registration between pre-operative data and the real-time microscope image allows the surgeon to access a surgical focus deep in the brain surface. Projections calculated from 3-D pre-operative data can be overlaid with corresponding microscope views. Such image overlays ensures the surgeon a more accurate surgical trajectory. 1, 2 Microscope-based image-guided neurosurgery can be divided into three steps 1–3 : calibration of the micro- scope optics; tracking of the patient and microscope over time; and registration of the pre-operative images to Further author information: (Correspondence should be addressed to H.S.) H.S.: E-mail: hai.sun@dartmouth.edu, Telephone: 1 603 646 1094, fax: 1 603 646 3856 H.F.: E-mail: hany.farid@dartmouth.edu, Telephone: 1 603 646 2761 A.H.: E-mail: alex.hartov@dartmouth.edu, Telephone: 1 603 646 3936 K.E.L: Email: Karen.E.Lunn@dartmouth.edu, Telephone: 1 603 646 1094 D.W.R.: E-mail: David.W.Roberts@hitchcock.org, Telephone: 1 603 650 8736 K.D.P.: E-mail: Keith.D.Paulsen@dartmouth.edu, Telephone: 1 603 646 2695