an introduction to magnetohydrodynamcs by Roberts

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36. Cohen D, Cuffin BN. Developing a more focal magnetic stimulator. Part I. Some basic principles. J Clin Neurophysiol 1991;8:102–111. Activation of Peripheral Nerve and Nerve Roots 3 Marcela Panizza and Jan Nilsson Magnetic stimulation of excitable tissue has been studied for more than a century,1–7 and probably one of the first human studies was the observation of phosphenes by D’Arsonval in 1896. 8,9 A real revolution in clinical neurophysiology came in 1985, when Barker and coworkers10 demonstrated painless transcranial magnetic stimulation applied to the scalp over the motor area and recording from the abductor digiti minimi muscle in humans.

2 ms Center of the coil D Motor recording Sensory recording Figure 3–1 A: Physical characteristics of an 8-shaped coil. B: Induced-current waveform for a monophasic stimulator. C: Induced-current waveform for a biphasic stimulator. D: Positioning of the 8-shaped coil and recording electrodes on the arm and hand. (From Nilsson J, Panizza M, Roth BJ, et al. Determining the site of stimulation during magnetic stimulation of a peripheral nerve. ) Activation of Peripheral Nerve and Nerve Roots Figure 3–2 X-ray films of different magnetic coils.

14 Stimulation occurs during the first rapid phase of the induced waveform, and if the direction of the current is clockwise in the stimulating coil, the resulting induced current will be counterclockwise near the nerve. By flipping the coil, the direction of the induced current is reversed. The oscillatory waveforms induce currents that have a very rapid rise time (still within 5 s) and typically a decay of about 80 s to the zero crossing of the first phase. The following phases are usually smaller in amplitude and have a longer decay.

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