Transcranial Magnetic Stimulation: The Effect of Age and Other Factors on the Intensity of the Quadruple Butterfly Coil

Transcranial magnetic stimulation (TMS) is a non-invasive technique approved by the US Food and Drug Administration (FDA) to treat certain neurological disorders. When comparing TMS to techniques with similar desired outcomes, such as conventional brain surgery, TMS has far fewer risks due to its non-invasive nature. TMS works by inducing an electric field (
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-field) that polarizes or depolarizes brain tissues depending on treatment mode. The human brain reaches 90% of its adult volume at about age six and does not reach full maturity until approximately 25 years. The differences in the brain volume from adolescents to adults could alter the effectiveness of TMS, primarily because the coils have been designed for use on adult patients. Therefore, comparing the efficacy of TMS coils across age groups to determine whether the difference in brain volume is enough to warrant specific TMS treatment protocols for adolescents is of the essence in advancing TMS research. In this research, the authors compare the use of a novel TMS coil design, quadruple butterfly coil (QBC), across different age groups to test the effectiveness of the coil on various stages of brain development. Simulations were run using a finite element analysis software, Sim4Life. Besides age and brain (gray matter) volume, the authors also considered other factors such as the brain-scalp distance (BSD), cerebrospinal fluid (CSF) thickness, scalp thickness, skull thickness, and skull volume of the human models in the analysis. This study provides an understanding of the various factors contributing to the response from TMS. The maximum
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-field intensity on the scalp (
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-Max scalp), maximum
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-field intensity on the brain (
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-Max brain), and the proportion of the
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-Max brain to
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-Max scalp were parameters considered in this analysis.