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date: 04 June 2020

(p. 1) Physics and Biophysics of TMS

Throughout history, whenever a new magnetic or electrical device has been discovered, physicians have immediately applied it to the human body in hopes of better understanding or of cures for devastating disease. The urge to use new techniques therapeutically may at times outrun a full knowledge of their scientific basis. But the principles of electromagnetism that underlie transcranial magnetic stimulation (TMS) were well known more than a century before its introduction by Barker and colleagues, and the failure to develop TMS sooner was due primarily to lack of the necessary high-power electronics. In contrast, direct-current transcranial stimulation was performed decades before but was seldom subjected to controlled trials. High-voltage transcranial pulse stimulation was also introduced prior to TMS, but found application in the operating room only afterwards.

Electromagnetic theory is both elegant and arcane. This section freely dispenses with most of it to concentrate on the portions that are most relevant—and, hopefully, most likely to be comprehended—for users of TMS. The basic circuitry of magnetic stimulators is much simpler than that of transistor radios. Understanding it helps a great deal in comprehending why, for example, only a limited range of TMS coils and waveforms are widely used, and the possible tradeoffs among different waveforms and coil shapes. It is our hope that such comprehension will, in turn, foster more informed and effective use in clinical and research applications. (p. 2)