The system described above provides a novel application for surgical navigation systems for epilepsy assessment and surgery. EEG investigation results are merged with MRI imaging, and 3D reconstruction, to offer a functional as well as structural map for guidance during epileptic focus resection.
For the evaluation of epilepsy cases, EEG remains the cornerstone for the localization of seizure foci [11]. Interictal as well as ictal epileptiform activity which include such abnormalities as sharp waves, spikes, focal slowing and asymmetries of beta activity are used for the localization [1]. However, it is common that the EEG results, clinical neuroimaging results and clinical findings are not congruent, in which case the procedure of subdural grid and strip evaluation become quite useful. These electrodes are used for the planning of a safe resection, especially if the source of epileptic activity is located near eloquent cortex. In such a case, functional mapping of the area surrounding the lesion using successive electrical stimulation of the electrodes is also conducted. This procedure has been conducted routinely [12] and although it is associated with risks (e.g., bleeding and infection), it is tolerated remarkably well by children [13].
Because of the limitations of the surgeons' views of the brain prior to and during the surgery, they have had to create a mental picture of the lesion(s) and their spatial arrangement to neighboring structures using pre-operative EEG results and MRI scan data. By integrating the above, a physical map of the child's brain can be created, both for surgical planning and intraoperative navigation. The 3D models that are created pre-operatively in which each structure can be phased in and out, rotated in any direction and enlarged according to the view that is desired, essentially provide a means for the neurosurgical team to "look inside the brain". Moreover, the application of these models for intraoperative use, contributes to a higher definition of a lesion's location and margins. In addition, the possibility of recording the location of each electrode on the subdural grids and strips, on a model in which the brain is made more transparent and in which the lesion is labeled in a separate color, represents a great leap from the conventional two dimensional X-ray visualization.
The increased use of 3D-reconstruction [6, 15-24] and surgical navigation [25-30,33-36] have substantially influenced neurosurgery. Although numerous navigation systems have been widely used for epilepsy surgery, the ability to establish a correspondence between the subdural electrodes that are placed on the brain surface and on the 3D model has not yet been described. Conventional methods of viewing these grids and strips include X-ray films [1] which in turn do not allow proper assessment of the underlying soft tissue.
We have developed the routine use of 3D reconstructions in surgical navigation for seizure focus removal in epilepsy surgery. Pre-operatively, the 3D model is used for surgical planning [30,16,17], facilitating the evaluation of the surgical approach. Intraoperatively, this system enables recording of the subdural grid and strip electrodes directly on the 3D model, providing an intuitive way to visualize the electrodes which can easily be translated into the surgical field.
The fusion of several techniques routinely used in the operating room for neurological evaluation with imaging algorithms provides an optimal array of resources available for seizure focus evaluation and resection. The registration process, for instance, in which multiple MRI modalities including T1 and T2-weighted, pre and post-gadolinium injection and MR angiogram scans, can be fused together to generate the final 3D model [36], substantially increases the precision of the 3D model and positioning of the focus with neighboring structures. The visualization of blood vessels surrounding the focus provides a useful reference point for localization. Furthermore, areas which cannot be seen on T1-weighted images but which are seen on T2-weighted slices, may be combined on the same model for a more accurate map for navigation and resection.
During pediatric epilepsy surgery, the selectivity of resection is crucial, especially in foci near eloquent cortex. The ability to visualize the brain anatomy, seizure focus location and grid and strip electrodes in 3D provides an additional tool for focus localization and margin determination both pre- and intraoperatively. The fusion of modalities including EEG evaluation results, MRI scans, subdural electrode stimulation results and 3D reconstruction increases the selectivity of abnormal versus normal brain tissue and as a result, increases the likelihood of a favorable surgical outcome.