The most widely used technique for brain functional imaging is functional Magnetic Resonance Image (fMRI). The spatial resolution of fMRI is high. However, fMRI signals are highly influenced by the vasculature in each voxel and can be affected by capillary orientation and vessel size. Functional MRI analysis may, therefore, produce misleading results when voxels are nearby large vessels. Another problem in fMRI is that hemodynamic responses are slower than the neuronal activity. Therefore, temporal resolution is limited in fMRI. Furthermore, the correlation between neural activity and the hemodynamic response is not fully understood. fMRI can only be considered an indirect method of functional brain imaging.
Another MR-based method of functional brain mapping is neuronal current magnetic resonance imaging (ncMRI), which has been studied over several years. However, the amplitude of these neuronal current signals is an order of magnitude smaller than the physiological noise. Works on ncMRI include simulation, phantom experiments, and studies in tissue including isolated ganglia, optic nerves, and human brains. However, ncMRI development has been hampered due to the extremely small signal amplitude, as well as the presence of confounding signals from hemodynamic changes and other physiological noise.
Magnetic Resonance Electrical Impedance Tomography (MREIT) methods could have the potential for the detection of neuronal activity. In this technique, small external currents are applied to a body during MR scans. This current flow produces a magnetic field as well as an electric field. The altered magnetic flux density along the main magnetic field direction caused by this current flow can be obtained from phase images. When there is neural activity, the conductivity of the neural cell membrane changes and the current paths around the neurons change consequently. Neural spiking activity during external current injection, therefore, causes differential phase accumulation in MR data. Statistical analysis methods can be used to identify neuronal-current-induced magnetic field changes.
We describe a secondary analysis of an in vitro experiment that supports the capabilities of a relatively new imaging technique known as functional Magnetic Resonance Electrical Impedance Tomography (fMREIT) to detect conductivity changes in neural tissue caused by activity. Methods: Magnetic Resonance (MR) phase data of active Aplysia ganglia tissue in artificial seawater (ASW) were collected before and after exposure to an excitotoxin using two different imaging current strengths, and these data were then used to reconstruct conductivity changes throughout the tissue. Results: We found that increases in neural activity led to significant increases in imaged conductivity when using high imaging currents, but these differences in conductivity were not seen in regions that did not contain neural tissue nor in data where there were no differences in neural activity. Conclusion: We conclude that the analysis presented here supports fMREIT as a contrast technique capable of imaging neural activity in live tissue more directly than functional imaging methods such as BOLD fMRI.