MRI‐Compatible Fiber Ion Sensors Enable Simultaneous Monitoring of Extracellular Ion Fluctuation and Whole‐Brain fMRI

Combining functional magnetic resonance imaging (fMRI) with real‑time monitoring of neurochemicals, such as ions, offers powerful opportunities for multimodal investigation of brain function through simultaneous acquisition of local neurochemical dynamics and whole-brain neural activation patterns. However, current combination approaches predominantly rely on fluorescent calcium imaging, which cannot effectively capture diverse extracellular ion dynamics in deep brain regions, thereby constraining comprehensive studies of various neurological disorders and physiological state transitions. Here, implantable MRI-compatible fiber ion sensors (MISs) are developed to realize concurrent extracellular ion monitoring and fMRI. The MIS employs a simplified architecture with an ion-selective membrane conformally coated on a conductive polymer fiber that simultaneously serves as both an ion-to-electron transduction interface and an electrical conductor. This design enables stable, high-sensitivity monitoring of calcium and potassium ions while maintaining negligible imaging artifacts during 7 T MRI. As a proof of concept, concurrent monitoring of extracellular ion fluctuations via the implanted MIS and whole-brain fMRI is realized during anesthesia transitions in rats, establishing the MIS as an effective tool for multimodal exploration of neural activity.