A new high-frequency magnetic stimulator with an oil-cooled coil

A new high frequency magnetic stimulator, Labmag, is described which may be used in clinical neurophysiology and for the treatment of spasticity in multiple sclerosis. In order to avoid heating of the coil during repetitive stimulation, an oil/air cooling system was employed. The magnetic stimulus waveform is a half cosine with a rise time of 200 microseconds and a pulse width of 400 microseconds induced by alternating capacitor voltage. The Labmag product was compared with a commercially available high frequency stimulator, MagPro. The electric fields induced by Labmag were half the size of those induced by MagPro at identical stimulus intensities. Using a leaky integrator to simulate the effects on neurons, only minor differences in integrated electric fields were observed. There were no changes of the electric fields in relation to coil geometry. A minor difference in electric fields (1.0 +/- 0.8%) was observed when the polarity of the capacitor voltage alternated. Compound muscle action potentials (CMAPs) evoked by transcranial magnetic stimulation were recorded from the first dorsal interosseous muscle (FDI) in eight subjects. Threshold intensities of CMAPs evoked by Labmag (58 +/- 7%) were significantly smaller (p < 0.05) than threshold intensities obtained with MagPro (63 +/- 8%). Peak-to-peak amplitudes, P-Pamps, of CMAPs were significantly smaller after single stimulations with Labmag at stimulation intensities of 130 and 140% of threshold intensity.