Imaging fast neural circuit activity in brain and nerve with electrical impedance tomography: review and...

Neural engineering has become topical with widespread publicity for projects such as implanted devices for recording and intelligent stimulation of the cerebral cortex. Unfortunately, the brain has many billions of neurons, with thousands of synaptic connections, and functions with millisecond-scale graded depolarization; there are substantial limitations in sampling this adequately, even with modern technology. Electrical impedance tomography (EIT) is a novel medical imaging technique that has the capability to image fast electrical activity in the brain or nerve over milliseconds at a mesoscopic scale of millimeters in the rat or several in the human brain. This enables tomographic images of the electrical properties of a subject to be produced with electrode arrays, by imaging the impedance changes that occur as ion channels open during neuronal depolarization. A small, safe, insensible electrical current at a few kilohertz is applied, usually serially, to pairs of electrodes. Images are reconstructed from the many transfer impedances recorded by measuring resulting voltages at other electrodes. Currently, this is accomplished with intracranial or perineural electrode arrays and requires averaging to a repeated trigger over several minutes. With this, images of neural activity with a resolution of 200 µm and 1 ms can be produced during evoked activity or epileptic seizures in rat cerebral cortex or fascicular compound action potentials in pig or human vagus nerve. Work is in progress to extend this to real-time noninvasive imaging of fast neural activity in humans using multiple-frequency current injection with frequency division multiplexing and recording with atomic magnetometers.