Influence of Brain–Scalp Distance on Focality of the Quadruple Butterfly Coil for Transcranial Magnetic Stimulation

Transcranial magnetic stimulation (TMS) is a neuromodulation technique that noninvasively depolarizes neurons in the brain. During TMS, a pulse (or multiple pulses) of a time-varying magnetic field (H) is delivered to the brain using specialized coils. The quadruple butterfly coil (QBC) is a novel coil design that shows increased focality of the induced electric field over that of the standard figure-of-eight (FoE) coil. Using 50 different head models created from magnetic resonance imaging (MRI) scans of healthy individuals, our research investigated the role that brain-scalp distance (BSD) plays in the brain's response to the magnetic fields generated by the QBC and FoE. The variability of BSD is an inherent characteristic of the human population. As the BSD increases, so does the distance between the brain and TMS coil. Therefore, the anatomical variation of BSD is an independent variable that may play a significant role in the intensity of the induced electric field produced in the brain. Our results show no significant difference in the QBC's focality to that of the FoE with respect to BSD.