Rao, Anling; (2000) Electrical Impedance Tomography of brain activity: Studies into its accuracy and physiological mechanisms. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

Electrical Impedance Tomography (EIT) is an imaging method designed for non-invasive imaging of physiological processes in the body. It is based on computerised reconstruction of multiple impedance measurements made with a ring of electrodes. It had previously been used to image experimental cerebral ischaemia and cortical spreading depression in which increases in impedance were over 25 - 200%. Purposes of the present work were to assess whether EIT could accurately image less than 10% change in cerebral impedance in vivo, to image the functional brain activity with cortical electrodes during somatosensory evoked responses (SEPs), visual evoked responses (VEPs) and experimental focal epileptic activity, and furthermore, to investigate its possible physiological mechanism. Functional brain images were acquired by a single frequency EIT system with 16 electrodes placed directly on the exposed superior surface of the brain of anaesthetised rabbits. SEPs were produced by repetitive electrical stimulation of the median nerve. VEPs were produced by repetitive binocular photic stimulation. Electrical stimulation on the cortex was used to induce epileptic activity. Decreases in cerebral impedance of 4.5 ± 0.9% (Mean ± S.E.M, 9 measurements in 3 animals) and 2.7 ± 0.8% (12 measurements in 5 animals) induced by somatosensory and visual evoked responses, respectively, could be effectively imaged by the Sheffield Mark 1 EIT system with cortical or subdural electrodes. Increases in cerebral impedance of 7.1 ± 0.8% (53 measurements in 9 animals) and 5.5 ± 0.8% (10 measurements in 9 animals) following electrically induced focal and generalised epileptic activity could also be imaged with cortical electrodes by EIT. These changes were at the sites where the evoked potentials were the greatest or the epilepsy was initiated. Further work comprised development of a system for calibrating multifrequency EIT systems in vitro with biological materials - cucumber in potassium chloride solution and polyurethane sponge in packed blood -in order to simulate capacitance. This work has shown, for the first time, that small impedance changes during physiologically evoked responses or electrically induced epileptic activity may be reliably imaged by EIT with cortical electrodes. This lends support to the view that it will be possible to image brain function with scalp electrodes by EIT.

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