Electrical impedance tomography of human brain activity with a two-dimensional ring of scalp electrodes

Previously, electrical impedance tomography (EIT) has been used to image impedance decreases in the exposed cortex of rabbits during brain activity. These are due to increased blood volume at the site of the stimulated cortex; as blood has a lower impedance than brain, the impedance decreases. During human brain activity similar blood flow changes have been detected using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). If blood volume also changes then the impedance of human cortex will change during brain activity; this could theoretically be imaged with EIT. EIT data were recorded from a ring of 16 scalp electrodes in 34 recordings in 19 adult volunteers before, during and after stimulation with (1) a visual stimulus produced by an 8 Hz oscillating checkerboard pattern or (2) sensory stimulation of the median nerve at the wrist by a 3 Hz electrical square wave stimulus. Reproducible impedance changes, with a similar timecourse to the stimulus, were seen in all experiments. Significant impedance changes were seen in 21 +/- 5% (n = 16, mean +/- SEM) and 19 +/- 3% (n = 18) of the electrode measurements for visual and somatosensory paradigms respectively. The reconstructed 2D EIT images showed reproducible impedance changes in the approximate region of the stimulated cortex in 7/16 visual and 5/18 somatosensory experiments. This demonstrates that reproducible impedance changes can be measured during human brain activity. The final images contained spatial noise; the reasons for this and strategies to reduce this in future are discussed.