Detection of cortical spreading depression in the anaesthetised rat by impedance measurement with scalp electrodes: implications for non-invasive imaging of the brain with electrical impedance tomography

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Abstract
The possible use of impedance measurement with scalp electrodes to detect intracranial events non-invasively was investigated by measuring the localised impedance changes during cortical spreading depression (CSD) in anaesthetised rats. Impedance was measured over ipsilateral fronto-parietal cortex by four electrode method operating at 50 kHz with electrodes spaced 0.5 mm apart. Cortical impedance increased by 39% of the resting level during CSD. With scalp electrodes placed on abraded skin, an unexpected impedance decrease of 0.8% occurred, which correlated spatially and temporally with CSD. CSD was accompanied by a small rise in temperature; when scalp temperature was held constants warming the scalp, no impedance change greater than baseline variability (+or-0.1% of the resting impedance level) was observed. The non-invasive detection of CSD in migraine in humans may be possible by measuring the characteristic temperature-relate-impedance changes with electrical impedance tomography and scalp electrodes.