Investigating Evoked Action Potential in Human Optic Nerves using MRI

Abstract

This study aims to investigate the presence of evoked action potentials in the human optic nerve using magnetic resonance imaging (MRI). The detection method is based on the effect of the evoked action potentials, which produce the axonal current flowing through the optic nerve. It produces a minute axonal magnetic field around the optic nerve. The axonal current is expected to be generated at the same frequency as the evoked action potentials, which are at the same frequency as the visual stimulus during imaging. This study attempted to detect the axonal magnetic field variation which interacts with the MR main magnetic field, B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> , and produces signal modulation during image acquisition. Consequently, there will be intra-voxel dephasing within the region of interest (ROI) in the optic nerve. The signal variation can be found by converting the time series signal into the frequency domain using a fast Fourier transform (FFT). The checkerboard visual stimulus was projected to the subject in synchronization with MRI using a gradient echo – echo planar imaging (GE-EPI) sequence. A total of five healthy volunteers and five optic neuritis patients were recruited for this study. The visual stimulus response was only found in one out of the five healthy volunteers, with an estimated axonal field of 7 nT. No response was found in the optic neuritis patients as expected due to the effects of the disease on optic nerve signaling. Clinical Relevance – This study measured the evoked action potentials in the optic nerve which will allow further study into the effects of optic neuritis on optic nerve signaling.