Optical Anchor Bolt for Simultaneous Stereo-EEG and Intracranial Functional Near Infrared Spectroscopy: Concept, Phantom Experiment, and Monte Carlo Simulation

Functional near infrared spectroscopy (fNIRS) is used to measure cerebral hemodynamics and can be applied to detect seizures but has limited penetration depth and is sensitive to noise caused by scalp hemodynamics. Patients with drug resistant epilepsy (DRE) are sometimes implanted with intracerebral electrodes to localize their seizure onset zone (SOZ). These electrodes are limited by a “tunnel vision” effect. We developed a optical anchor bolt (OAB), which integrates near infrared (NIR) optodes positioned in the skull, as part of an implanted fNIRS (ifNIRS) sensor in order to overcome these limitations. The feasibility and utility of ifNIRS were assessed using a slab model of the head in a Monte Carlo (MC) simulation and a tissue-mimicking phantom model. Positioning the emitter and detector inside the OAB yielded a signal >30 higher than standard scalp positioned fNIRS at a 3-cm distance and a number of photons >6 times higher than the limit of detection at a 5-cm separation, with a greatly reduced sensitivity to noise from the scalp. In both the phantom model and the MC simulation, emitter optodes embedded inside a depth electrode, positioned several centimeters deep in the brain, and provided a signal detected by an OAB with a maximal depth dependent on the optical properties of white matter (WM). These findings demonstrate the advantages of the OAB and ifNIRS and potential future directions of development. Further in vivo studies are required to assess the feasibility and safety of the system and the optical properties of WM due to their impact on ifNIRS.