Dynamic MRI of Small Electrical Activity

Abstract

Neuroscience methods entailing in vivo measurements of brain activity have greatly contributed to our understanding of brain function for the past decades, from the invasive early studies in animals using single-cell electrical recordings, to the noninvasive techniques in humans of scalp-recorded electroencephalography (EEG) and magnetoencephalography (MEG), positron emission tomography (PET), and, most recently, blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI). A central objective of these techniques is to measure neuronal activities with high spatial and temporal resolution. Each of these methods, however, has substantial limitations in this regard. Single-cell recording is invasive and only typically records cellular activity in a single location; EEG/MEG cannot generally provide accurate and unambiguous delineations of neuronal activation spatially; and the most sophisticated BOLD-based fMRI methods are still fundamentally limited by their dependence on the very slow hemodynamic responses upon which they are based. Even the latest neuroimaging methodology (e.g., multimodal EEG/fMRI) does not yet unambiguously provide accurate localization of neuronal activation spatially and temporally. There is hence a need to further develop noninvasive imaging methods that can directly image neuroelectric activity and thus truly achieve a high temporal resolution and spatial specificity in humans. Here, we discuss the theory, implementation, and potential utility of an MRI technique termed Lorentz effect imaging (LEI) that can detect spatially incoherent yet temporally synchronized, minute electrical activities in the neural amplitude range (microamperes) when they occur in a strong magnetic field. Moreover, we demonstrate with our preliminary results in phantoms and in vivo, the feasibility of imaging such activities with a temporal resolution on the order of milliseconds.