Investigation of axonal magnetic fields in the human corpus callosum using visual stimulation based on MR signal modulation

Purpose: To investigate the possibility of detecting visually-evoked axonal currents in the splenium of the human corpus callosum using a 3.0T MRI system. Materials and Methods: Axonal currents produce weak and transient magnetic fields, and the components of these that lie parallel to the 130 field of the MRI system can potentially modulate the MR signal, which can be detected as an integrated effect over time. A fast gradient-echo echo-planar imaging (GE-EPI) sequence with short TR and intermediate TE was employed in an attempt to detect such axonal currents using light-emitting diode (LED) visual stimulation paradigms. Results: The mean magnitude signal change, expressed relative to the fully relaxed equilibrium signal calculated from the measured value using the known T1 of white matter, was 0.014 +/- 0.004% at TE = 30 msec. This corresponded to a mean axonal field of 0.11 +/- 0.03 nT, according to the hypothesis that the axonal currents create a Lorent-zian field distribution within an imaging voxel. Conclusion: Measured frequency spectra and statistical mapping using the general linear model (GLM) showed evidence of the stimulus localized within the splenium of the corpus callosurn, which was not thought to be due to motion artifacts or physiological responses.