Mechanisms and developments of magnetic source MRI

Magnetic source MRI (msMRI) has been developed recently for direct detections of neuronal magnetic fields to map brain activity. However, whether MRI can be used for direct detection of neuronal activity is a matter of debate. Controversial theoretical and experimental results have been reported. In this work, theoretical modeling and experimental validation have been presented to demonstrate that the neuronal current signal is in the detectable range and could be detected by MRI. In the theoretical modeling section, we present an improved current-dipole model to compute magnetic field generated by neural firing and to calculate MRI signal changes resulting from the neuronal magnetic field. Neuronal magnetic field were estimated based on a synchronized activity of multiple neurons. Our results show that neuronal magnetic field can potentially generate up to a few percent changes in MRI magnitude signals. Phases of MRI signal tend to be destructively added and are insensitive to neuronal magnetic field in the activated region when the distribution of the activated dendrites is symmetrical. Our modeling implies that direct MRI detection of neuronal activity is possible. In the experimental validation section, a rapid median nerve stimulation paradigm has been used to detect the neuronal activity. The experiments were performed on six normal human participants to investigate the temporal specificity of the effect, as well as inter- and intra-subject reproducibility. Significant activation of contra-lateral primary sensory cortex (S1) was detected 80ms after stimulation onset (corresponding to the P80 evoked potential peak). The 80 ms latency S1 activation was observed over 3 independent sessions for one subject and for all 6 participants. The magnitude of the signal change was 0.2% - 0.3%. Coinciding with our expectations, no S1 activation was found when MRI data acquisitions were targeted at the N20 and P30 peaks because of mutual cancellation of magnetic fields generated by those peaks. The results demonstrated good reproducibility of S1 activations and indicated that the S1 activations most likely originated from neuronal magnetic field rather than hemodynamic response.