Feasibility of imaging evoked activity throughout the rat brain using electrical impedance tomography

Abstract

A B S T R A C TElectrical Impedance Tomography (EIT) is an emerging technique which has been used to image evoked activity during whisker displacement in the cortex of an anaesthetised rat with a spatiotemporal resolution of 200 μm and 2 ms. The aim of this work was to extend EIT to image not only from the cortex but also from deeper structures active in somatosensory processing, specifically the ventral posterolateral (VPL) nucleus of the thalamus. The direct response in the cortex and VPL following 2 Hz forepaw stimulation were quantified using a 57-channel epicortical electrode array and a 16-channel depth electrode. Impedance changes of À0.16 AE 0.08% at 12.9 AE 1.4 ms and À0.41 AE 0.14% at 8.8AE1.9 ms were recorded from the cortex and VPL respectively. For imaging purposes, two 57-channel epicortical electrode arrays were used with one placed on each hemisphere of the rat brain. Despite using parameters optimised toward measuring thalamic activity and undertaking extensive averaging, reconstructed activity was constrained to the cortical somatosensory forepaw region and no significant activity at a depth greater than 1.6 mm below the surface of the cortex could be reconstructed. An evaluation of the depth sensitivity of EIT was investigated in simulations using estimates of the conductivity change and noise levels derived from experiments. These indicate that EIT imaging with epicortical electrodes is limited to activity occurring 2.5 mm below the surface of the cortex. This depth includes the hippocampus and so EIT has the potential to image activity, such as epilepsy, originating from this structure. To image deeper activity, however, alternative methods such as the additional implementation of depth electrodes will be required to gain the necessary depth resolution.

Imaging fast electrical activity in the brain during ictal epileptiform discharges with electrical impedance tomography

“…The presented approach holds potential to image a variety of electroencephalographic seizure patterns, provided that the electrode arrangement for EIT current injection offers an adequate sensitivity for imaging in the region of interest. At present, the depth penetration of EIT with epicortical electrode arrays extends to the dorsal hippocampus ( Faulkner et al, 2018 ); this setup can therefore be used to image seizure activity throughout the cortex and in superficial hippocampal regions. The initial discharges within the seizure are of particular importance for localising the ictal onset zone; it would thus be desirable to specifically image the seizure onset pattern.…”

Imaging fascicular organization of rat sciatic nerves with fast neural electrical impedance tomography

“…It can also serve as a means to produce high-resolution tomographic images of activity in excitable neural tissue in brain and nerve with a millisecond and submillimeter resolution, "fast neural EIT." Our group has developed this for imaging neuronal depolarization in the brain during normal and epileptic activity with a resolution of 1 ms and <200 µm in the rat cerebral cortex 23,24 . The principle is that impedance in neuronal membranes falls as ion channels open during evoked activity 17 .…”

Tactile Sensor for Minimally Invasive Surgery Using Electrical Impedance Tomography

“…Reducing the point spread function of the reconstruction would improve the localisation accuracy and shape estimation. This could be achieved through reconfiguration of the FPC to include internal electrodes [12]- [14] or a full grid [10], [15], [16] to better distribute the sensitivity across the plane of the electrodes. However, these may not sufficiently address the decrease in out of plane sensitivity towards the deformable contact surface.…”