Abstracts

RATIONALE: Traditional equivalent dipole models of EEG spikes or seizures are unrealistic because they have no spatial extent, unlike true cerebral sources. Point-like dipoles attempting to model extended sources are also usually located deep to the generator cortex in underlying white matter. Both tend to make traditional dipole models unappealing and more difficult to interpret.
METHODS: We have developed and tested a new dipole model that has an adjustable spatial extent in eight surgical candidates with complex partial seizures . This [dsquote]dipole patch[dsquote] is composed of many individual dipoles that are constrained to follow the surface of the patient[ssquote]s own cortex (derived from 3-D MRI reconstructions) in both position and orientation (surface normals). Dipole patch solutions for EEG spikes or seizures were compared to single dipole models and to the location and extent of cerebral sources recorded with intracranial electrodes.
RESULTS: Dipole patch solutions with a physiological goodness of fit were obtained for spikes or initial seizure rhythms in all patients. The location of dipole patches as compared to underlying cerebral sources was accurate at a sublobar level as validated by intracranial EEG. Dipole patches of 1.5 to 3 cm radius most accurately simulated the spatial extent of real cortical sources. Single dipole solutions usually fell within the boundary of the dipole patch or its underlying white matter, but often they did not lie at the geometrical center of the patch.
CONCLUSIONS: The dipole patch model of EEG spikes and seizure rhythms is probably the most realistic representation of actual cortical sources currently available. In this technique, the location accuracy achieved with a dipole model using a realistic boundary element head model is enhanced by the spatial extent and realism of an extended genertor area that is constrained to cortical anatomy. Other extended source models are not limited to realistic source areas nor are they constrained to anatomically contiguous cortex. The dipole patch model offers not only the most realistic characterization of spike or seizure foci, but it also provides the best EEG source model for comparisons with other functional imaging technologies. A realistic functional representation of spike and seizure sources is a significant improvement in non-invasive localization of epileptogenic foci.
(Disclosure: Salary - Neuroscan, Inc.)