Introduction

Although most children suffering from epilepsy have a good prognosis for remission, a small percentage of cases are resistant to conventional Anti-Epileptic Drugs. For these patients, surgical intervention for seizure focus removal to stop the seizures and to prevent further brain injury provides great hope [1]. Subdural grid and strip electrodes in chronic monitoring of the Electroencephalogram (EEG) have become more widely used as a means of accurately obtaining the necessary localization of the focus with a minimum morbidity [2]. Implanting subdural electrodes directly on the brain surface allows the epilepsy investigator to record a series of ictal and interictal recordings, with ample time for a complete and precise mapping [3]. In addition, cortical stimulation for localizing an epileptic focus also determines its relation to eloquent areas and surrounding cortical function [4].

Magnetic Resonance Imaging (MRI) is a highly reliable neuroimaging procedure in patients with partial or localized epilepsy [4]. For several years, we have been making computerized, 3D reconstructions of pre-operative MRI scan data for surgical planning [5] and intraoperative navigation. We have recently developed a novel neurosurgical navigator in which medical image registration and instrument tracking automatically establishes correspondence between the medical images from MRI scans, the three-dimensional model and the actual patient environment [6]. Using this intraoperative guidance, medical instruments acting on the patient are localized in the 3D coordinate frame of the MRI imagery. Consequently, the surgeon is allowed to view a patient and at the same time display, in exact alignment, that view and all other surrounding internal structures on both the reconstructed model and three views on the original MRI scans. Most importantly, any point may be recorded and displayed on the 3D model as an arrow or simple dot (see Figure 1).

  

Figure: The interface displayed on the computer monitor during surgery. The position of the LED probe is simultaneously displayed on the 3D model (white arrow). The position on the corresponding MRI splices is displayed in the axial, sagittal and coronal planes. The subdural electrodes are shown as black spheres.

We have combined functional diagnostic tools including EEG results and subdural strip and grid electrodes results, with real-time, intra-operative imaging and 3D reconstruction to increase the precision of localization and subsequent resection of seizure foci. The 3D models constructed from pre-operative MRI scans allow pre-operative surgical planning as well as intraoperative navigation. The latter is used as guidance for the surgeon to the correct location of the abnormality and to establish its boundaries for safe resection without affecting neighboring areas and decreasing the invasion of eloquent cortex. Most importantly, it allows the recording of the subdural grid and strip electrode positions on the 3D model and as a result, provides an accurate, 3D visualization. A precise functional and structural map of the patient's brain is thus created and used for pre-operative planning and intraoperative navigation, ultimately increasing the accuracy localization of the focus and decreasing the likelihood of post-operative neurological deficits.