The heating of metal electrodes during rapid-rate magnetic stimulation: a possible safety hazard

The temperature of electrodes and metal disks positioned close to a coil was measured during rapid-rate magnetic stimulation. The temperature rise ranged from a fraction of a degree to almost half a degree per stimulus pulse and increased with the electrical conductivity of the metal, the square of the electrode radius and the square of the stimulus strength, and was independent of the electrode thickness. During a brief high-frequency train, the temperature increase from each pulse added; during a long, high-frequency train the temperature increase approached a steady state. After the stimulus ended, an electrode on the arm cooled with a time constant of about 45 sec. A standard silver EEG electrode on the surface of the skin did not increase in temperature enough to induce a skin burn if the stimulating rate was below 0.4 Hz or the total number of stimuli was less than 20. Heating was reduced by cutting gaps in the electrode.