Imaging fast neural circuit activity in brain and nerve with Electrical Impedance Tomography (fnEIT) : review and neurophysiological applications

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 neurones, with thousands of synaptic connections, and functions with millisecond-scale graded depolarisation; there are substantial limitations in sampling this adequately, even with modern technology. Electrical Impedance Tomography (EIT) is a novel medical imaging technique which has the capability to image fast electrical activity in the brain or nerve over milliseconds at a mesoscopic scale of millimetres 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 which occur as ion channels open during neuronal depolarisation. A small, safe, insensible electrical current at a few kHz 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, it is accomplished using 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 msec 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 it to real-time non-invasive imaging of fast neural activity in humans using multiple frequency current injection with frequency division multiplexing and recording with atomic magnetometers.