Imaging of the compound action potential (CAP) in fascicles in peripheral nerves could help avoid side effects in neuromodulation by selective stimulation of identified fascicles. Existing methods have low resolution, limited imaging depth, or are invasive. We propose fast neural electrical impedance tomography (EIT), which allows fascicular CAP imaging with a high resolution of ∼200 μm, <1 ms. This uses a non-penetrating flexible cuff electrode array with 14 circumferential electrodes. This has been validated in rat sciatic nerve by comparison to micro-computed tomography (microCT) and histology with fluorescent dextran tracers (n=5). With EIT, there were reproducible localized changes in tissue impedance in response to stimulation of individual fascicles (tibial, peroneal and sural). The reconstructed EIT images corresponded to microCT scans and neural tracer histology, with significant separation between the fascicles (p<0.01), and no significant difference between techniques. The standard deviation from the mean fascicle position for EIT was 86 μm (6% of nerve diameter). This suggests fast neural EIT can reliably image the functional fascicular anatomy of the nerves and so aid selective neuromodulation.
A feasibility study of a rotary planar electrode array for electrical impedance mammography using a digital breast phantom