Neuronal current imaging: sensitivity of magnitude and spatial distribution to changes in current timing

Introduction: Direct neuronal current imaging is a method that could potentially provide direct access to neuronal activity with an excellent spatial (mm) and temporal resolution (ms) [1-3]. However, so far it has not been convincingly demonstrated that MRI has adequate sensitivity to detect neuronal currents in vivo. In order to pin down the most influential factors in the signal formation, with the goal of eventually optimizing them for the MR detectable signal change, Pell et al. used a simple conducting wire model with various scanning and field parameters to investigate signal magnitude changes [1]. The experimental results indicate not only significant changes in signal magnitude but also in spatial pattern. Specifically, the current can create a very localized Gibbs ringing in the image. We show both experimentally and analytically that the timing of the current within the pulse sequence completely determines the width and the magnitude of the ringing. Since currently we are on the limit of neuronal-current MR sensitivity, this is a significant finding as it can help us both tune the parameters of the acquisition to achieve maximal sensitivity as well as understand the effects of arbitrary neuronal currents on the MR signals.