Toward SQUID-Based Direct Measurement of Neural Currents by Nuclear Magnetic Resonance

Abstract

Modern high field (HF) MRI uses magnetic fields greater than 1.5 T to yield exquisite anatomical features. We have also seen an explosion in functional MRI in the last decade that measures hemodynamic responses that are ultimately sluggish (~one sec) and only indirectly related to electrophysiological processes. Magnetoencephalography (MEG) is a direct measure of the external fields generated by neuronal currents with exquisite temporal information (less than one msec). Spatial localization, however, is inferred from modeling priors, making MEG ldquoimagingrdquo only indirect at best. Ultra low field (ULF) MRI has recently been demonstrated with 2-3 mm resolution using fields in the microtesla regime. While the nuclear magnetic resonance (NMR) signal at ULF is dramatically weaker than at HF, we acquired high signal-to-noise measurements for a variety of samples at ULF using SQUID technology. Several researchers have proposed that electrophysiological activity may interact with the nuclear spins in a volume of interest, causing measurable variations in the NMR signal. We have developed a new approach to directly measuring neuronal activity with SQUID-based ULF-NMR techniques based on the hypothesis that interactions between the spin population and neural activity in cortex can be dominated by resonant mechanisms unique to ULF. We have experimentally demonstrated the feasibility of this approach via ULF-NMR using a single-channel SQUID system.