Abstracts

Subdural grid electrodes are routinely used for localization of seizure foci. Once subdural grid electrodes are placed, electrical cortical stimulation mapping and intracranial electrode recordings may be performed to delineate eloquent cortex in patients undergoing a presurgical seizure work up. The results from these studies not only provide clinically useful information for the individual patient studied, but can contribute significantly to our knowledge about the functional neuroanatomy of the brain if compared across groups. Meaningful group comparisons require the use of a common coordinate space. Functional imaging studies typically use the Talairach system for this purpose. We therefore devised a method enabling us to derive Talairach coordinates for subdural electrode locations. Five patients (4 males) underwent left temporal subdural electrode implantation for localization of their seizure focus and language mapping. All subjects had pre-operative 3D MRI scans and post-operative 3D CT scans showing the location of the implanted electrodes. Curry software (www.neuroscan.com) was used to integrate the 3D CT with the MRI, visually displaying the electrodes on the patient[apos]s own 3D brain model.
Using SPM2 (http://www.fil.ion.ucl.ac.uk/spm/software/spm2), the 3D MR images and 3D CT images of each patient were co-registered and then normalized via the MR image into standard stereotactic space (Montreal Neurological Institute, MNI). The grid position of each electrode in question was verified by visual inspection and cross-referenced with data obtained from the 3D reconstruction with Curry software. Coordinates in MNI space for each electrode were noted and Talairach coordinates for each electrode were calculated. Talairach coordinate locations were looked up in the Talairach Daemon Database (http://ric.uthscsa.edu/resources). In all cases the automated co-registration performed with SPM2 was comparable to the co-registration performed with Curry software establishing the robustness of our method. Talairach coordinates of electrodes of interest localized them correctly in language areas. Our combined use of SPM2 and Curry software allowed us to successfully transform subdural grid electrode positions into Talairach space. To our knowledge this is the first description of a method allowing group analysis of data from electrical cortical stimulation mapping and intracranial electrode recordings in Talairach space.