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Introduction Focal resection for refractory epilepsy is an extremely effective treatment, but diagnosis of its focus often requires invasive intracranial electrode placement. We developed an intravascular EEG electrode (EP01) as a minimally invasive method, and reported that it is possible to diagnose focus lateralization in vivo, and that an FIH study showed that it was possible to obtain EEG recordings with higher sensitivity than scalp EEG, and that multiple electrodes up to 6 could be placed into venous sinuses.Aim of Study In order to verify whether EP01 is capable of diagnosing focus lateralization in the same way as conventional intracranial electrodes, we started a multicenter prospective single-arm study (EPSILON IE) in March 2024 in Japan.Method Patients with refractory focal epilepsy aged 15 to 70 years who undergo conventional intracranial electrode placement and have appropriate vascular anatomy. EP01 is placed in the bilateral cavernous sinuses, bilateral transverse sinuses, and superior sagittal sinus at the same time as conventional intracranial electrodes, and video-EEG recording is performed for up to 2 weeks. PE was defined as the concordance rate between EP01 with non-invasive testing for focal lateralization diagnosis and that of conventional intracranial electrodes. The planned number of cases is 37.Results 17 cases had enrolled by March 2025.Conclusion The EPSILON IE trial is progressing smoothly and is scheduled to complete enrollment within a year.Conflict of Interest Yes Yuji Matsumaru is officer and stock holder of Epsilon Medical