Functional brain imaging using MREIT and EIT: Requirements and feasibility

We may consider the electrical conductivity of a head tissue as a passive material property affecting the volume conduction process inside the head. The electrophysiology and bioelectromagnetism of neural activities suggest that the conductivity could be also a direct and instantaneous biomarker of a neural activity. In this paper, we review two conductivity imaging methods of Electrical Impedance Tomography (EIT) and Magnetic Resonance Electrical Impedance Tomography (MREIT) in the context of neuro-imaging. EIT produces cross-sectional images of a conductivity distribution inside the human body with a high temporal resolution and a low spatial resolution, whereas MREIT visualizes the conductivity distribution with a high spatial resolution and a low temporal resolution. EIT is based on boundary measurements of induced voltages subject to externally injected currents from 10 Hz to 500 kHz, for example. MREIT utilizes internal measurements of induced magnetic flux density distributions subject to externally injected currents at frequencies below 1 kHz. MREIT requires an MRI scanner as a tool to acquire induced magnetic flux density data, whereas one may implement EIT as an inexpensive portable electronic device. The state of the art in EIT and MREIT indicates that neither of them is readily available for direct fast functional neuro-imaging of the brain. We summarize what kinds of technical breakthroughs are needed for EIT and/or MREIT to be a clinically useful direct functional neuro-imaging method. We also propose a multi-modal approach including EIT, EEG, MREIT and MRI.