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 are sometimes implanted with intracerebral electrodes to localize their seizure onset zone. These electrodes are limited by a "tunnel vision" effect. We developed an optical anchor bolt (OAB), which integrates near infrared 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, provided a signal detected by an OAB with a maximal depth dependent on the optical properties of white matter. 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 white matter due to their impact on ifNIRS.

Functional near-infrared spectroscopy (fNIRS) is a non-invasive method for monitoring brain activity by detecting hemodynamic changes. Studies have shown that it can identify ictal and pre-ictal hemodynamic variations, supporting its potential use as a complement to electroencephalography (EEG) in epilepsy monitoring. This study explores an expanded illumination and detection approach utilizing wide-based optodes and increased emitter–detector separation (EDS) to enhance fNIRS sensitivity to cortical hemodynamic changes while minimizing scalp contamination. A Monte Carlo simulation was designed to assess signal amplitude and sensitivity of fNIRS with varying emitter and detector diameters (1–15 mm) and EDS (30–50 mm). Signal strength, grey matter to scalp path ratio (GSPR), and percentage signal change per absorption coefficients (AC) variation were analyzed. Sensitivity to changes in AC of superficial and deep grey matter (SGM, DGM) and scalp was assessed. Increasing emitter and detector diameters substantially increased total detected photon packet weights, enabling practical use at larger EDS. Sensitivity to SGM AC changes tripled at 50 mm EDS, while GSPR increased by 80%, indicating reduced signal contamination from the scalp. Sensitivity to deep cortical hemodynamic changes also improved. Therefore, wide-based fNIRS optodes with increased EDS can enhance seizure-related hemodynamic detection, potentially improving epilepsy diagnostics.

Introduction: Functional Near-Infrared Spectroscopy (fNIRS) is widely used to monitor cerebral hemodynamics, however, it is limited by shallow penetration depth and susceptibility to hemodynamic noise from the scalp. A novel intracranial fNIRS (ifNIRS) system, featuring depth optrodes (optode-electrodes) and optical anchor bolts (OABs), has been proposed to address these limitations. This study investigates the feasibility of ifNIRS in a swine model under controlled interventions. Methods: Three animals were implanted with ifNIRS. Each animal with three OABs, with depth optrodes (DO) inserted into two of the OABs. Hemodynamic changes were recorded using OAB-to-OAB (OAB-OAB) and DO-to-OAB (DO-OAB) channels. Two interventions were performed to generate hemodynamic changes: rapid infusion of hypotonic saline to induce cerebral edema and blood withdrawal. Postmortem assessment for tissue damage and hemorrhage was performed. Hemoglobin concentration changes were analyzed using the Beer-Lambert equation. Results: A decrease in total hemoglobin (tHb) levels during blood withdrawal was observed in all channel configurations that displayed relevant signals. During hypotonic saline infusion, variable patterns of tHb were observed. Postmortem findings showed minor extra-axial hemorrhages near OABs, but no intracerebral or heat-related injuries. Discussion: This study demonstrates the feasibility of the ifNIRS system in detecting hemodynamic changes in vivo . While technical refinements are needed, ifNIRS shows promise for improving cerebral hemodynamic monitoring and enhancing diagnostic accuracy in invasive monitoring of patients with epilepsy.