Identification of a suitable current waveform for acute stroke imaging.

MFEIT (multi-frequency electrical impedance tomography) has the potential to provide a portable non-invasive neuroimaging method ideal for use in acute stroke. Skin perception has not previously occurred in MFEIT with injected frequencies above 2 kHz, but use in brain imaging requires applied current below 100 Hz, which could stimulate cutaneous nerve endings. The purpose of this work was to find the most suitable current pattern that could be employed in MFEIT measurements in the adult head with the UCLH Mk2.5 system, which applies currents from 20 Hz-1.6 MHz. Single frequency current waveforms of 0.28 mA peak-to-peak at 20 Hz-80 Hz were applied to the forearms of three volunteers; although the skin was abraded, none of these were perceived, which agrees with similar studies in the literature. When a full frequency pattern at 20 Hz-1.6 MHz was applied to the forearm or head in four healthy subjects, with the same current amplitude of 0.28 mA for each component, an unpleasant tingling sensation was perceived, due to summation of the applied currents. The sensation was reduced or abolished by attenuation or removal of frequencies below 100 Hz; the optimal compromise was a pattern with absence of 40 Hz, and with 80 and 20 Hz respectively reduced to 75% and 50%.