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. Results: 50-
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
