Measurement of Current Density Induced in Cat Brain During Transcranial Magnetic Stimulation

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This study measures in vivo current densities induced in the brains of fifteen cats during transcranial magnetic stimulation. This was essential for determining regions of excitation. Three magnetic stimulators were used in this study. Two of the stimulators were constructed in-house; one used a 5-cm diameter coil that was driven with a damped sinusoidal signal; the other used the same coil and was driven with an undamped sinusoidal signal. The damped system was operated at a frequency of 620 Hz and damping time constant of 0.912 ms. It generated a peak magnetic field of 140 gauss at the center of the coil. The undamped system was operated at a frequency of 3 KHz and generated a peak magnetic field of 28 gauss. The third system is a Cadwell MES-10 stimulator. Results showed that current distribution patterns of the laboratory-made stimulators were similar. Thus, a coil driven with a damped sinusoidal input induces the same current distribution as when it is driven with an undamped sinusoid. Also, current density induced in the tissue was found to be maximum near the edge of the coil and decreased as the edge of the coil moved away from the region of interest. This corresponds to mathematical models previously developed (Guidi, 1989; Ferguson, 1989; Roth, 1990; Reuter, 1988). Inducing maximum current at a region, however, does not necessarily imply tissue stimulation in that particular region. Neurophysiologic factors, such as nerve fiber diameter and orientation, infuence neural excitability. Thus, it is necessary to consider these parameters during magnetic stimulation.

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