VOXEL-BASED LOCALIZATION IN FRAME-BASED AND FRAMELESS STEREOTAXY AND ITS ACCURACY

The problem of accurate stereotactic localization and registration of targets in computed tomography (CT) data sets is addressed, in particu lar the effect of using a single transformation matrix to map voxel co ordinates onto stereotactic coordinates. An algebraic approach to the calculation of stereotactic target coordinates in tomographic data acq uired with conventional stereotactic localizers is presented. The volu me transformation matrix (VTM) is discussed, which is useful for the r egistration of volumetric data sets, and also corresponds to the rigid body transformation matrix used in many so-called frameless registrat ion methods. The VTM can lead to accuracy degradation, in particular d ue to patient movement during scanning. Simulations were performed and CT data sets acquired with patients fitted with the CRW or the GTC st ereotactic localizer were analyzed. Comparison of STM- and VTM-derived stereotactic coordinates shows an average overall registration error of 0.1 mm for anesthetized patients and in the range 0.6-1.4 mm for no nanesthetized patient. Accuracy maps are described that enable the use r to visualize the registration error in relation to the data. It is s hown that the effect of fiducial point localization error and patient movement for VTM-based localization is minimized when all available fi ducials in the region of interest are used. The significance of these results is discussed, and methods are proposed to minimize these effec ts for frame-based and frameless registration methods.