Mechanical Analysis of the Quadruple Butterfly Coil during Transcranial Magnetic Stimulation and Magnetic Resonance Imaging

Transcranial Magnetic Stimulation (TMS) is a tool for the treatment of psychiatric and neurological disorders. It involves using a transient magnetic field generated from electromagnetic coils in inducing an electric field (E-field) within the neurons of the brain. The induced E-field results in an increase in the brain membrane's electric potential, leading to polarization or depolarization of the neurons depending on the mode of treatment. There has been much development in TMS technology recently, with most research focusing on improving the performance of TMS coils at greater depths and achieving more localized stimulation. Another development has been the combination of TMS with other medical techniques such as Functional Magnetic Resonance Imaging (fMRI) and Electroencephalography (EEG) to enable accurate mapping of the brain's electrical activity during TMS. However, the TMS coils experience large forces in this new highly energized external magnetic field environment. Accurately determining the magnitude and location of the Lorentz force, torque, and stresses that the TMS coils experience in this environment becomes of utmost importance. In this chapter, the authors used finite element analysis to determine the magnitude and location of the Lorentz forces and stresses experienced by a novel TMS coil, Quadruple Butterfly Coil (QBC), in a TMS-fMRI operation. With the TMS-fMRI operation, the maximum values of the magnetic flux density, Lorentz force density, and von Mises stress were observed in the z-axis of the QBC orientation. They resulted in a 39.65 %, 38.94 %, and 94.59 % increase, respectively, from the typical TMS operation.