Conventional stereotactic surgery has evolved from a ring-based system with a simple software which calculated few parameters to frameless intraoperative localization systems that provide the surgeon with, real-time localization and correlation with several imaging modalities. The localization system described in this paper is an opto-electronic system that uses infrared emitters and three precalibrated CCD cameras. The system was chosen among others for the following reasons: it tracks target points defined by up to 256 miniature light emitting diodes, and its accuracy in locating the spatial positron of a diode marker in the operating volume 1.0 × 1.0 m at a distance of 2.0 m is 0.1 mm with a resolution better than 0.01 mm. Systems like this one can track and define the position and orientation of any object in the field view of the camera. This is done by attaching a few small infrared emitters (light emitting diodes) to the surface of each ''rigid body'' (surgical instrument) being tracked. Subsequently, through a calibration process a corresponding rigid body file is created. This rigid body file represents this particular object (i.e. surgical tool, microscope) and defines a local coordinate system that identifies each translation and orientation of that tool with respect to the camera coordinate system. This in turn is transferred into the computed tomo-graphic/magnetic resonance imaging coordinate system by a process referred to as coordinate matching. Fiducial markers are placed on the patient''s head prior to image scanning. Coordinate matching involves establishing the transformation matrix between the computed tomographic/magnetic resonance imaging coordinate system and the camera coordinate system using the fiducial markers as common points in both coordinate systems. This process is done during the initial calibration process. The system is integrated to a two-dimensional and three-dimensional multi-imaging neurosurgical software program (Neurosurgical Planning System, Detroit, Mich., 48201). By feeding in real-time the numerical data which represent the position and orientation of the surgical tool to the neurosurgical software, a cursor is displayed on the computer monitor indicating the position of the surgical tool with respect to the physical anatomy of the brain in several projections. In addition to the unique advantage of being able to track any surgical tool in real-time along with surgical preplanning and simulation, this system provides a complete frameless stereotaxis system that could be applied to the same extent as a frame-based system.

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