Direct methods for functional neural imaging are critical to advancements in understanding neural behavior, plasticity, connectivity and pathology. If we can directly image active neurons we will have the ability to examine neural activity more precisely than is presently the case with fMRI. We have developed an MRI-based conductivity imaging technique, Magnetic Resonance Electrical Impedance Tomography (MREIT) that can reconstruct conductivity maps with near-MRI resolution. In MREIT, small external currents are applied to an object. The MR magnetic flux density patterns created by current flow may be converted to conductivity or current density slice images. We developed this technique and have refined it to the stage of producing electrical conductivity images of animal brains in vivo, using relatively low applied currents. The large changes in membrane conductance that occur during activity cause dynamic changes in paths taken by externally applied currents. Changes in spiking activity during external current application will cause differential phase accumulation in MR data that will increase the longer current is applied. Neural activity therefore becomes visible as an increase in apparent conductivities of voxels coincident with active intracellular areas. Because the contrast controlling MREIT signals, conductivity, may only acquire positive values, phase accumulations cannot be cancelled by the presence of opposite polarity or opposingly oriented signals. This may give MREIT an advantage compared with other MRI-based methods for imaging neural currents that are based on perturbations of phase or main magnetic fields caused principally by summed axonal current flows. Thus, MREIT has the potential to detect activity in complex structures including gray matter. In this proposal, we will investigate the ability of functional MREIT (fMREIT) to detect activity-related conductivity changes in neural tissue. We will develop fMREIT techniques to image neural activity in vitro, in a several standard neural preparations, while progressively refining our methods to detect and locate active cells at high signal to noise ratio and using main magnetic field strengths conveniently used in vivo. In isolated preparations, our method has the potential to enable detailed analyses of single cell mechanisms. The method could thus be considered as a non-invasive extension of patch clamping techniques, and could stand alone for this purpose. However, ultimately we wish to image activity in vivo and our final study in this program will include a tentative exploration of fMREIT in a live animal model as a precursor to further research in this area. In summary, this study will establish the basis for functional MREIT (fMREIT) techniques. This method could ultimately be used to visualize effects of more general neural behavior and enable more fundamental analyses of neural behavior in vivo than is available with existing techniques such as fMRI.
Non-invasive functional neuroimaging is a critical element of attempts to understand structure and function in the brain. Functional MRI has produced revolutionary insight into brain activity, but suffers from the disadvantage that its contrast mechanism is correlated with blood flow and not directly related to neural activity. In this study we will establish the feasibility of a technique that uses a more robust and direct contrast mechanism than fMRI to examine neural activity.
|Keim, Steven F; Fu, Fanrui; Sadleir, Rosalind J (2015) External current application in a bidomain model of active neural tissue. Conf Proc IEEE Eng Med Biol Soc 2015:2303-6|