Abstract
Electrical impedance tomography (EIT) could provide images of fast neural activity in the adult human brain with a resolution of 1 ms and 1 mm by imaging impedance changes which occur as ion channels open during neuronal depolarization. The largest changes occur at dc and decrease rapidly over 100 Hz. Evoked potentials occur in this bandwidth and may cause artefactual apparent impedance changes if altered by the impedance measuring current. These were characterized during the compound action potential in the walking leg nerves of Cancer pagurus, placed on Ag/AgCl hook electrodes, to identify how to avoid artefactual changes during brain EIT. Artefact-free impedance changes (δZ) decreased with frequency from −0.045 ± 0.01% at 225 Hz to −0.02 ± 0.01% at 1025 Hz (mean ± 1 SD, n = 24 in 12 nerves) which matched changes predicted by a finite element model. Artefactual δZ reached c.300% and 50% of the genuine membrane impedance change at 225 Hz and 600 Hz respectively but decreased with frequency of the applied current and was negligible above 1 kHz. The proportional amplitude (δZ (%)) of the artefact did not vary significantly with the amplitude of injected current of 5–20 µA pp. but decreased significantly from −0.09 ± 0.024 to −0.03 ± 0.023% with phase of 0 to 45°. For fast neural EIT of evoked activity in the brain, artefacts may arise with applied current of >10 µA. Independence of δZ with respect to phase but not the amplitude of applied current controls for them; they can be minimized by randomizing the phase of the applied measuring current and excluded by recording at >1 kHz.
Highlights
IntroductionElectrical impedance tomography (EIT) is a medical imaging method which has the potential to yield images of neuronal depolarization in the brain, by imaging changes in impedance which occur as ion channels open (Holder 1987, Gilad et al 2009)
The compound action potential (CAP) diminished from 6 ± 2 mV (n = 216 in 12 nerves, p < 0.001) at electrode 3 to 0 ± 0.01 mV at 12 (n = 216 in 12 nerves, p > 0.05)
Significant artefactual changes occurred in recording impedance changes during the CAP if voltages were recorded either side of the current measuring electrodes, as the distal voltage electrode recorded a CAP which was altered by the injecting current
Summary
Electrical impedance tomography (EIT) is a medical imaging method which has the potential to yield images of neuronal depolarization in the brain, by imaging changes in impedance which occur as ion channels open (Holder 1987, Gilad et al 2009). In EIT, a set of electrodes is applied to the surface of the excitable tissue (brain or nerve). The technique is applied in time difference mode i.e. voltages are measured differentially with respect to the baseline, or initial time point, which results in voltage changes with respect to time. Images of the internal complex conductivity are reconstructed to yield tomographic images of the activity. Images are produced during repeated evoked activity, usually physiologically evoked responses in response to somatosensory, auditory or visual stimuli. Averaging is usually employed to increase the effective signal-to-noise ratio (SNR)
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