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Transcranial magnetic stimulation (TMS) is a non-invasive neuromodulation technique investigated for the treatment of various neuropsychiatric disorders such as major depressive disorder and obsessive-compulsive disorder. Coil design and analysis has been a popular topic in TMS studies, primarily as the coil geometry significantly affects the induced electric field ($E$ -field) distribution. In this work, we compare the distribution of the induced $E$ -field of 16 different TMS coils by finite element analysis. The coils are positioned at two head locations: the vertex and the dorsolateral prefrontal cortex (DLPFC). Due to the coil geometry, only nine of the 16 coils are positioned at the DLPFC. Notably, the authors use 50 heterogeneous head models derived from MRI scans of healthy patients in this analysis. As a result, the sensitivity of the $E$ -field intensity and focality to anatomical variations is investigated. The maximum $E$ -field intensity induced in the brain ($E$ -Max brain), and the volume in the brain exposed to the $E$ -field with at least half the $E$ -Max ($V$ -Half) were metrics used to assess the intensity and focality of the induced $E$ -field. It was observed that some of the coils induced high $E$ -field intensity and were highly sensitive to anatomical variations. Other coils exhibited high focality and lower sensitivity to anatomical variations at the same time. For the coils positioned at the DLPFC, less variability was observed when compared to the vertex location. This study provides an understanding of the effect of coil geometry and anatomical variation on the induced $E$ -field during TMS. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]