Magnetically compatible and fiberless fNIRS enables simultaneous multimodal imaging with optically pumped magnetometer MEG.

Simultaneous acquisition of functional near-infrared spectroscopy (fNIRS) and magnetoencephalography (MEG) provides complementary hemodynamic and electrophysiological information for studies of neurovascular coupling and has previously been demonstrated using fiber-based fNIRS implementations. Compared with fiber-based systems, fiberless fNIRS is lightweight and eliminates fiber-induced mechanical constraints; however, its integration with MEG remains challenging due to stringent magnetic compatibility requirements. Here, we present a magnetically compatible and fiberless fNIRS system enabling flexible and non-invasive multimodal imaging with optically pumped magnetometer (OPM) MEG. We developed magnetically compatible source/detector optodes and implemented a multipole moment flexible printed circuit design that suppresses driving-current-induced magnetic fields by more than 1000-fold. The optodes and cables generated less than 1 nT of magnetic field at ∼1 cm from the OPM sensor, with no measurable impact on OPM sensitivity. Simultaneous fiberless fNIRS and OPM-MEG acquisition was demonstrated in a somatosensory paradigm, capturing concurrent hemodynamic and evoked magnetic responses, thereby demonstrating the feasibility and robustness of our integrated multimodal system. By addressing a key magnetic-compatibility barrier between fiberless fNIRS and OPM-MEG, this work paves the way for neurovascular coupling studies using flexible multimodal platforms, and supports future developments in wearable multimodal neuroimaging and multimodal brain-computer interface systems.