Abstracts

Rationale: In majority of the EEG source localization methods, a piecewise homogenous head model is used to represent the physical properties of the human head volume conductor. This model usually consists of three compartments (brain, skull and scalp). The conductivities of different tissues are assigned to each compartment. It is usually assumed that the scalp has the same conductivity as the brain while the skull has a much lower conductivity. In addition, relative conductivities are considered to be important when source localization is concerned. Although many simulation studies had been carried out, to our knowledge, no previous experimental study concentrated on exploring the relationship between conductivity uncertainties and EEG source localization accuracy. In this study, we evaluated the influence of different brain-to-skull conductivity ratios (BSCRs) on EEG source localization from epileptiform activity. Methods: High density EEG recordings utilizing a 76-electrode montage were obtained in a cohort of seven patients undergoing surgical evaluation for the treatment of medically intractable partial epilepsy. The patients were studied under a protocol approved by the Institutional Review Boards at the University of Minnesota and the Mayo Clinic (Rochester, MN). In order to evaluate the relationship between conductivity uncertainties and EEG source localization accuracy, we employed seven previously used BSCRs: 8, 15, 20, 25, 40, 80, and 120. For each BSCR, we performed the source reconstruction of interictal spike activities by using the LORETA algorithm. The accuracy of EEG source localization was assessed by comparing the estimated source activity to the resection area of brain which was regarded as the actual epileptiform source. The 8-18 interictal spikes were analyzed in this manner for each patient. Results: In Patient 1, the post-operative MR images revealed a left frontal lobectomy. Eight interictal spikes were selected from the interical recordings. The source activity of LORETA analysis was localized to the epileptiform foci in the left frontal lobe of this patient. In addition, the source localization results by different BSCRs were similar to each other. The mean localization errors of all interictal spikes for different BSCRs for this patient ranged from 5.65 to 8.37 mm. The ANOVA statistical analysis indicated that there was no significant difference among seven BSCRs for the localization error (p = 0.746). In the remaining six patients, the same procedure of the source localization analysis was performed. The features of the source localization results by different BSCRs in these six patients were similar to those in Patient 1. Conclusions: In this study, we have performed an experimental study in 7 epilepsy patients to investigate the relationship between conductivity uncertainties and EEG source localization accuracy. The present results suggest that the variability of the BSCRs does not significantly affect EEG source localization accuracy when comparing the estimated interictal epileptiform activity with the resection area or lesion of brain in epilepsy patients.