Electrical impedance tomography with cortical or scalp electrodes during global cerebral ischaemia in the anaesthetised rat.

The performance of a prototype impedance imaging (Electrical Impedance Tomography, EIT) system using cortical or scalp electrodes has been assessed in a model of global cerebral ischaemia in the anaesthetised rat. In preliminary calibration experiments using a circular array of electrodes around a tank of saline, the centre of a polythene rod could be localised with a mean error of 4% of the tank diameter, and two such rods could be discriminated when separated by 22% of the tank diameter. Cerebral ischaemia was produced by diathermy of the vertebral arteries and reversible occlusion of the common carotid arteries for 15 min. A bimodal impedance increase of about 50-200% was recorded with a ring of cortical electrodes in the fronto-occipital plane. With a similarly oriented ring of scalp electrodes, a unimodal impedance increase of about 10% was observed. In both cases, impedance reversed after cerebral reperfusion. Similar but irreversible changes were observed post mortem. As the reconstruction algorithm assumes constant initial resistivity, this suggests that modifications to allow for initial conditions would be needed for more accurate imaging with scalp electrodes.