Holder, David S.; (1991) An assessment of some possible neurological applications of electrical impedance tomography. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Electrical impedance tomography (EIT) is a recently developed technique which produces reconstructed images of the internal distribution of the impedance of an object from measurement with external electrodes. It was assessed for its possible application in imaging intra-cranial disorders non- invasively with the use of scalp electrodes. Cerebral impedance increases of 12-55% were measured by a four electrode method at 50 kHz during global cerebral ischaemia or cortical spreading depression (CSD) in anaesthetized rats. Measured with scalp electrodes in two pairs 2-26 mm apart, impedance increased by 1.8-5.9% during global cerebral ischaemia for 5 or 15 min; the changes correlated in duration with cortical impedance changes, but increased more gradually. Increases of about 2% still were observed when the effects of variations in temperature and local scalp impedance were excluded. A finite element model was used to predict the attenuation of a signal due to cerebral anoxic depolarization by the extracerebral layers. These residual impedance changes were compatible with this, but their cause by other mechanisms related to the method of production of cerebral ischaemia could not be ruled out. An unexpected decrease of 0.8% was observed during CSD with electrodes 0.5mm apart on the scalp. This became undetectable when scalp temperature was kept constant. The model predicted that impedance changes of about 1% could be measured during CSD with scalp electrodes spaced further apart. Images were then collected using a prototype EIT system operating at 51 kHz during the same model of cerebral ischaemia. Test objects in a medium of constant resistivity could be accurately localized, but spatial resolution of intracranial impedance changes was substantially degraded when recorded with scalp electrodes. EIT has the potential for imaging various cerebral physiological or pathological changes, but improvements to the reconstruction algorithm are needed if regional intracerebral changes are to be discriminated during recording with scalp electrodes.

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