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Current functional MRI techniques are essentially based on detection of hemodynamic changes induced by neuronal activation. This study is an exploration of the feasibility of direct detection of neuronal current induced NMR phase and/or magnitude changes. Our analysis was based on the approximation that neurons exist in an infinite homogeneous conducting medium, and are represented by an infinitely long cylindrical conductor, carrying uniform current. Neuronal activation was modeled by a current dipole. Simulations were performed to evaluate the effects of the local neuronal magnetic fields on the MRI signal at 3T. The magnetic field changes and the corresponding phase changes induced from a current range of 2 nA to 100 (mu) A were estimated. The conductor diameter was varied from 10 micrometer to 1 mm corresponding to the sizes ranging from that of a single axon to that of a patch of functionally similar cortex. Current induced magnetic field effects were assessed as resolution and neuronal orientation with respect to Bo were modulated. Simulation results were compared with measurements obtained from a current phantom at 3T, for a current range of 10 - 100 (mu) A. Based on the dipole model formulated we found that neuronal currents can produce magnetic fields on the order of 1.3 pT; 0.0006 degree(s) to 9 nT; 4 degree(s) depending on the neuronal bundle diameter, orientation, and current capacity. The mechanism for signal changes is that of Bo shifts. This is fundamentally similar to that of BOLD contrast, but caused by current changes rather than susceptibility changes from changes in blood oxygenation. If the bundles are of random orientations within a voxel, the estimated fields and NMR magnitude changes (decrease in T2*) are on the order of present system detection levels of approximately 1nT; 0.5 degree(s) or 0.01% signal change. Conversely, if the bundle orientation is homogeneous, then neuronal current effects are above system detection levels. The spatial scale of the current distribution also determines the net phase shift and magnitude change. The simulation and phantom experiment results demonstrated the feasibility of using MRI to directly detect local magnetic field perturbations that can result from neuronal currents on the order of a few (mu) A. This study provides a simple model for the evaluation of the feasibility to directly measure neuronal currents with MRI. Additionally it gives a starting point for the design of appropriate imaging methods towards detecting low signal level neuronal currents. A more extensive modeling of cortical and neuronal geometry, tissue inhomogeneity, timing mechanisms, and current distributions, will provide further insight in the development of MRI experimental techniques.
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