Preparation-Free Hydrogel EEG Electrodes for Concurrent EEG-fMRI-TMS

Transcranial magnetic stimulation (TMS) techniques provide a non-invasive tool to modulate cortical excitability by inducing charges at targeted brain areas and suppressing or enhancing local neural activity patterns. In neurological studies, it can be combined with other multimodal brain recording tools, such as Magnetic Resonance Imaging (MRI) or Electroencephalography (EEG), to provide important insight into how cells and neural circuits interact on a system level. Currently, only separate EEG-TMS and EEG-MRI systems are commercialized and clinically available. However, a concurrent EEG-fMRI-TMS system will allow EEG and fMRI to provide complementary spatial and temporal information of the transient and dynamic TMS-evoked network response and help advance neurological knowledge. One major challenge of such a system is that MRI and TMS environments are harsh on electronics and impose strong EMI due to the large switching magnetic fields in both RF ranges and EEG-relevant domains. Therefore, careful EEG material selection and electrode design are critical to ensure patients’ safety, minimize heating, improve EEG signal quality, and reduce interference on MR images and TMS operation.<br/><br/>In this work, we aim to develop preparation-free hydrogel EEG electrodes compatible with concurrent EEG-fMRI-TMS. Biocompatible physically-crosslinked alginate hydrogel and freeze-thawed PVA hydrogel were prepared with varying polymer concentrations, crosslinking densities, and conductive additives. Different conductive additives including salt, PEDOT:PSS, and CNT were tested to enhance the conductivity of the hydrogel. The hydrogel conductivity was characterized with an LCR meter for EEG relevant range (20Hz-2MHz) and a Vector Network Analyzer at MRI-relevant frequencies that are sources of RF noises and major heating. After integration with stencil-printed Ag/AgCl electrodes, we measured the contact impedance on the subjects’ forearms. Alginate/CaCl2/Ag/AgCl and PVA/NaCl/Ag/AgCl showed the most comparable performance to the gold standard wet gel and compatibility with MRI and TMS.<br/><br/>To validate the MRI compatibility of the EEG electrodes, we performed common MRI and fMRI sequences (T2*-weighted GRE scan and GRE EPI fMRI BOLD scan) with the electrodes on the subject’s head. No MR image distortions due to the EEG electrodes were observed, and no heating occurred during the sequences. The EEG data were recorded by a commercial MR-compatible amplifier and power unit. RF noises were removed by in-line filtering and the gradient noise can be canceled by synchronization with the scanner clock and signal processing methods. We also characterized artifacts from polarization during biphasic repetitive TMS pulses to test TMS compatibility. The electrodes were submerged in a physiological saline solution, wired out through a twisted pair, and aligned with the TMS coil in the worst-case scenario. Varying TMS intensity from 10% (14A/us) to 50% (70A/us), the hydrogel electrodes showed a settling time of less than 10us after the TMS pulses, and no significant heating was observed. Our results demonstrate that the proposed hydrogel EEG electrodes are safe and suitable for application in harsh MRI and TMS environments with performance comparable to the traditional wet electrode, and without complex skin preparation and clean-up afterward. This work demonstrates an important step towards a fully integrated multimodal concurrent EEG-fMRI-TMS system.