Comparison of Coil Designs for Transcranial Magnetic Stimulation of a Pig Model

Transcranial Magnetic Stimulation (TMS) is a modulation tool that is non-invasive and used to treat neuropsychiatric disorders. Over the last decade, TMS has been approved by the United States Food and Drug Administration (FDA) for the treatment of Major Depressive Disorders (MDD) and Obsessive-Compulsive Disorder (OCD). TMS is based on Faraday's law of electromagnetic induction, involving the generation of time-varying magnetic fields from electromagnetic coils when intense pulses of current flow through the coils. This transient magnetic field, in turn, induces an electric field within the brain, which results in excitation or inhibition of the brain's neurons. Several coil designs have been proposed for achieving targeted stimulation at great depth within the brain. With the advancement in TMS technology, there is a need for preclinical studies and testing of proposed coil designs. Using animal models to conduct these preclinical studies becomes of utmost importance, especially since a successful animal trial precedes a human clinical trial. In this research, the authors model six different coil designs for an anatomically heterogeneous adult pig model. The magnetic field intensity, H (A/m), and electric field intensity, E (V/m), were calculated and compared for each coil configuration. The maximum induced electric field in the scalp and brain (grey matter) were compared for all the different coil configurations. The electric field distribution as a function of depth within the brain was also compared for the different coil configurations.Clinical Relevance— This study will be beneficial to TMS coil designers and researchers to treat neuropsychiatric disorders and in the preclinical development of TMS coils. Results from studies with pig models are easy to compare with that of humans, and this will help to guide our understanding of the mechanism of TMS.