Quadruple Butterfly Coil With Passive Magnetic Shielding for Focused Transcranial Magnetic Stimulation

Coil design for transcranial magnetic stimulation (TMS) is a critical factor that is constantly being investigated to provide more focused stimulation of the brain. The design of focal coils is important because TMS uses time-varying magnetic fields generated from electromagnetic coils to stimulate a specific region of interest of the brain non-invasively. TMS is a useful tool in both diagnostic and therapeutic purposes for identifying and treating psychiatric and neurological disorders. Currently, TMS is approved by the U.S. Food and Drug Administration for the treatment of chronic depression and is being researched for the treatment of stroke, obsessive compulsive disorder, post-traumatic stress disorder, and Parkinson's disease. In this paper, the authors are proposing to further improve the focality of the quadruple butterfly coil (QBC) by using a high-permeability ferromagnetic material acting as passive magnetic shield. Analyses of the resulting field profiles with 50 heterogeneous head models using a single and double magnetic shield has been conducted at two different positions in the brain: the vertex and the dorsolateral prefrontal cortex. Comparison of the results has been illustrated with the help of QBC and figure-8 coil by using parameters of interest such as maximum electric field intensities in both the brain and the entire head, the volume of the brain receiving strong stimulation intensities, and the surface area of the brain receiving strong stimulation intensities. There is a significant amount of improvement found in focality when the QBC is used with a magnetic shield. With a shield, the coil is still able to maintain a comparable maximum electric field intensity in the targeted region of the brain to that of the original QBC. Although both shielding solutions show significant improvement, no notable difference has been found in the results between single and double shields with the QBC.