Abstract
Objective: Electrical impedance tomography (EIT) is capable of imaging fast compound electrical activity (compound action potentials, or CAPs) inside peripheral nerves. The ability of EIT to detect impedance changes (dZ) which arise from the opening of ion channels during the CAP is limited by the dispersion with distance from the site of onset, as fibres have differing conduction velocities. The effect is largest for autonomic nerves mainly formed of slower conducting unmyelinated fibres where signals cannot be recorded more than a few cm away from the stimulation. However, as CAPs are biphasic, monophasic dZ are expected to be detectable further than them; testing this hypothesis was the main objective of this study. Approach: An anatomically accurate FEM model and simplified statistical models of 50-fibre Hodgkin–Huxley and C-nociceptor nerves were developed with normally distributed conduction velocities; the statistical models were extended to realistic nerves. Main results: Fifty-fibre models showed that dZ could persist further than biphasic CAPs, as these then cancelled. For realistic nerves consisting of Aα or Aβ fibres, significant dZ could be detected at 50 cm from the onset site with signal-to-noise ratios (SNR, mean ± s.d.) of 2.7 ± 0.2 and 1.8 ± 0.1 respectively; Aδ and rat sciatic nerve—at 20 cm (1.6 ± 0.03 and 1.6 ± 0.06), rat vagus—at 10 cm (1.6 ± 0.05); C fibres—at 1–2 cm (2.4 ± 0.02). Significance: This study provides a basis for determining the distance over which EIT may be used to image fascicular activity in electroceuticals and suggests dZ will persist further than CAPs if biphasic.
Highlights
This study provides a basis for determining the distance over which Electrical impedance tomography (EIT) may be used to image fascicular activity in electroceuticals and suggests dZ will persist further than CAPs if biphasic
Electrical Impedance Tomography (EIT) allows imaging of impedance changes in neural tissue which arise from the opening of ion channels over milliseconds [1]–[6]
Dispersion has the effect of smoothing out these differences and so poses a challenge for recording EIT images in relation to the CAP in velocities and they occupy the majority of the nerve cross-sectional area
Summary
Electrical impedance tomography (EIT) is capable of imaging fast compound electrical activity (Compound Action Potentials, or CAPs) inside peripheral nerves. The ability of EIT to detect impedance changes (dZ) which arise from the opening of ion channels during the CAP is limited by the dispersion with distance from the site of onset, as fibres have differing conduction velocities. Significance: This study provides a basis for determining the distance over which EIT may be used to image fascicular activity in electroceuticals and suggests dZ will persist further than CAPs if biphasic. In Electroceuticals in humans, one aim might be to undertake fast neural EIT with a cuff around the cervical vagus nerve and modulate activity in abdominal organs which could be one metre distant. Fast neural EIT relies on imaging of impedance changes over time and so the differences over
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