Toward Detection of Transient Changes in Magnetic-Resonance Signal Intensity Arising From Neuronal Electrical Activities

Detection of weak magnetic fields arising from neuronal electrical activities using magnetic-resonance imaging (MRI) is a potentially effective method for functional imaging of the brain. The purpose of this study is to theoretically and experimentally assess the influence of neuronal magnetic fields on magnetic-resonance signals. The neuronal magnetic field was modeled by using a current dipole formula, and the resulting magnetic-resonance signals were calculated for a voxel located close to the dipole. A current dipole strength of 12 nAmiddotm resulted in the maximum signal drop of 0.6%. In the experiments, magnetic-resonance images of the rat brain were obtained by using a 4.7-T MRI system. A pair of electrodes was attached to the left sciatic nerve for electric stimulation. Images were obtained at ten time points of 0, 30, 60, ..., and 270 ms after the electric stimulation. In the right somatosensory cortex, a transient decrease in the signal intensity was found at 0-30 ms after the stimulation, which was attributed to the effects of neuronal magnetic fields. A neuronal activation map was generated by a comparison of signal intensities between the images obtained at 30-60 ms and 60-90 ms. The right somatosensory cortex exhibited a statistically significant difference in these two groups of images.