Abstracts

Rationale: While several studies demonstrate the advantages of high-density EEG (HD-EEG) in epilepsy monitoring and focus localization, clinical systems continue to use 20-40 electrodes. This work overcomes several practical challenges in the use of HD-EEG in clinic, including those identified in [Chu, Clinical Neurophysiology, 126 (3): 433–34] by enabling low setup time, automated monitoring and maintenance, and long-term recordings, while also enabling simultaneous stereo EEG (sEEG) and HD scalp EEG recordings. The benefit of simultaneous sEEG and scalp EEG recordings is the ability to get whole-scalp lower resolution recordings (using scalp EEG) in conjunction with the high resolution, but limited, sampling provided by sEEG recording. Methods: Several advances were made by the team to enable the system. i) Novel conductive sponge electrodes were developed to enable high-quality recordings even when the gel or saline dries out. The sponge is made by incorporating carbon nanofibers into hydrophilic polyurethane foams, which are molded into electrodes during the curing process; ii) A complementary electrode rehydration system was developed for long-term recordings. The system measures electrode-skin impedance and repeatedly rehydrates the electrodes with saline solution when the impedance rises; iii) A modular EEG system that can be placed to circumvent sutures, injuries, or stereo EEG probes. The modular system is made of silicone straps, designed for standard EEG  montages like the 10-20 system. These straps are affixed and spaced by hard resin rings and acrylic clips. The straps can be removed and replaced with patches that will accomodate uniform high density while respecting the standard 10-20 spacing. The modular system was tested for enabling simultaneous stereo EEG recordings: a representative surgical placement of sEEG probes was replicated on a styrofoam head (see attached Figure). Then, the modular EEG system was placed to provide good coverage while keeping distance from sEEG probes. Results: i) Sponge electrode maintains low electrode-skin impedance even when the electrode is dry. Also the electrode succeeds in conforming around hair, providing low impedance in presence of hair; ii) The rehydration system succeeds in maintaining impedance comparable to a gold-cup electrode over several hours;  iii) Modular EEG system successfully provided good coverage while avoiding pins around placed at the sEEG probe locations Conclusions: While the benefits of HD-EEG are widely acknowledged in research studies, practical issues limit their use in the clinical setting, particularly simultaneous application with SEEG recordings.  However, simultaneous intracerebral recordings and HD-EEG are an essential step toward clinical validation of HD-EEG systems for localization of not only epileptic foci, but other EEG biomarkers.  This technique represents a significant methodologic advancement that will overcome existing barriers to allow initial clinical validation of HD-EEG by direct intracerebral recordings. Further clinical testing may shed light on other clinical uses for HD-EEG. Funding: NSF WiFiUS award, NSF STTR Phase I #1843859, Chuck Noll Foundation for Brain Injury Research, PITA, Phil and Martha Dowd Fellowship.