Abstracts

Rationale: Knowing where sources of electroencephalography (EEG) signals are located in the brain can help with diagnosis. High frequency oscillations (HFOs) have been studied as precise biomarkers of the seizure onset zone (SOZ) in epilepsy and can be classified as gamma activity (30-100 Hz), ripples (100-200 Hz), and fast ripples (200-500 Hz). However, these hallmarks are difficult to detect in conventional, noninvasive EEG. Tripolar EEG (tEEG) has been shown to better detect HFOs than conventional measures (1) and its increased spatial resolution lends itself to more focal localization results in distributed methods. Methods: In the present study, tEEG was used to non-invasively detect and localize HFOs in epilepsy patients. Subjects (n=4) were concurrently recorded with both conventional EEG and tri-polar concentric ring electrodes (TCREs) during periods of ictal and inter-ictal activity. Standard 10-20 locations were used for conventional disc electrodes and TCREs were placed directly behind said locations. An 8th order, Butterworth band-pass filter specific to each subject's HFO range was used to isolate their narrow-band, high-power activity. Surface potentials of these HFOs were localized on a case-by-case basis using distributed source methods on the ICBM152 head model derived from a non-linear average of MRI scans of the 152 subjects in the MNI152 database. In the open-source data analysis application Brainstorm, a linear L2-minimum norm estimates algorithm was used to localize sources to a source space constrained normal to the cortical surface, and subject results were normalized to Z-scores. Results: In the tEEG of two of the subjects, gamma activity was detected in the pre-ictal period at approximately 70-80 Hz and localized, ripples at 120-150 Hz in the third subject, and fast ripples of approximately 300-350 Hz were detected in the fourth subject and localized. However, these HFOs were not detected in the EEG data and thus could not be localized. The source regions of said HFOs in tEEG data for all four subjects fell within regions previously, independently defined as seizure onset zones (SOZs) or irritative zones (IZs) by three neurologists. Conclusions: HFOs are typically difficult to detect with conventional EEG and cannot be localized. Tripolar EEG allows for their detection and subsequent localization, using a noninvasive measure of brain activity. The localization appears to be useful for precise identification of the SOZ or IZ in epilepsy patients. References: 1. Besio W., Martinez-Juarez I., Oleksandr M., Giatanis J., Blum A., Fisher R., Medvedev A., "High-Frequency Oscillations Recorded on the Scalp of Patients with Epilepsy Using Tripolar Concentric Ring Electrodes", IEEE Journal of Translational Engineering in Health and Medicine, 2013 Funding: Christopher Toole is funded by the Science, Mathematics, and Research for Transformation (SMART) scholarship from the American Society for Engineering Education.