A NEW HIGH-FREQUENCY MAGNETIC STIMULATOR WITH AN OIL-COOLED COIL

A new high frequency magnetic stimulator, Labmag, is described which m ay be used in clinical neurophysiology and for the treatment of spasti city in multiple sclerosis. In order to avoid heating of the coil duri ng repetitive stimulation, an oil/air cooling system was employed. The magnetic stimulus waveform is a half cosine with a rise time of 200 m u s and a pulse width of 400 mu s induced by alternating capacitor vol tage. The Labmag product was compared with a commercially available hi gh frequency stimulator, MagPro. The electric fields induced by Labmag were half the size of those induced by MagPro at identical stimulus i ntensities. Using a leaky integrator to simulate the effects on neuron s, only minor differences in integrated electric fields were observed. There were no changes of the electric fields in relation to coil geom etry. A minor difference in electric fields (1.0 +/- 0.8%) was observe d when the polarity of the capacitor voltage alternated. Compound musc le action potentials (CMAPs) evoked by transcranial magnetic stimulati on were recorded from the first dorsal interosseous muscle (FDI) in ei ght subjects. Threshold intensities of CMAPs evoked by Labmag (58 +/- 7%) were significantly smaller (p < 0.05) than threshold intensities o btained 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.