Characteristics of bowl-shaped coils for transcranial magnetic stimulation

Abstract

Transcranial magnetic stimulation (TMS) has recently been used as a method for the treatment of neurological and psychiatric diseases. Daily TMS sessions can provide continuous therapeutic effectiveness, and the installation of TMS systems at patients' homes has been proposed. A figure-eight coil, which is normally used for TMS therapy, induces a highly localized electric field; however, it is challenging to achieve accurate coil positioning above the targeted brain area using this coil. In this paper, a bowl-shaped coil for stimulating a localized but wider area of the brain is proposed. The coil's electromagnetic characteristics were analyzed using finite element methods, and the analysis showed that the bowl-shaped coil induced electric fields in a wider area of the brain model than a figure-eight coil. The expanded distribution of the electric field led to greater robustness of the coil to the coil-positioning error. To improve the efficiency of the coil, the relationship between individual coil design parameters and the resulting coil characteristics was numerically analyzed. It was concluded that lengthening the outer spherical radius and narrowing the width of the coil were effective methods for obtaining a more effective and more uniform distribution of the electric field.

Devices and Technology in Transcranial Magnetic Stimulation: A Systematic Review

“…However, this design was not effectively implemented by others due to concerns of producing important eddy currents, increased heat in the core, or even lower efficiencies for high repetition rates due to core saturation. Along the same path, other designs using iron-core materials and windowed shielding plates with improved focality [ 97 , 98 ] and water-cooling stimulation (WCS) coils [ 60 ] have been also proposed. Applications of these coils designs are those detailed for the FoE coil.…”

“…Other options for deep-brain TMS are the cone coil and double cone (also called biconical or twin circular coils), which could produce deeper stimulations controllable with their respective angles [ 46 , 91 , 109 ]. Moreover, a bowl-shaped coil, intended for superficial TMS, can make the treated region significantly wider [ 98 ]. However, more full-head designs with more controllable capabilities would allow deeper magnetic stimulation with reduced effects at surface regions.…”

“…Those methods are present in an important part of the literature on TMS modeling and are used to determine realistic electromagnetic distributions [ 58 ]. Publications about modeling and optimization in TMS can be grossly divided into three main branches, (1) modeling with the finite element method (FEM) [ 43 , 52 , 82 , 92 , 98 , 99 , 102 , 115 , 131 , 132 ], (2) modeling with the boundary element method (BEM) [ 133 ], and (3) modeling using the finite differences method (FDM) [ 42 , 58 ]. Adaptations of these procedures for fast computing and optimization, such as the improvement of BEM with the fast multipole method (BEM-FMM) [ 38 , 55 ] or the use of the inverse boundary element method for coil calculation [ 40 , 59 , 134 ], were presented.…”

“…There are important aspects to consider when analyzing the modeling paradigms for TMS, for example, the required computer capabilities, the use of more reliable 3D modeling software based on medical images, and the required spatial/temporal resolutions of the final model. Current strategies propose optimization techniques for certain coils in order to provide increased focality with larger field intensities [ 69 ] by using geometry variations [ 98 ] and coils shielding (RQ5) [ 44 , 100 ]. The current most used approach for 3D modeling is based on the finite element method [ 43 , 53 , 88 ], which consumes large amounts of computational resources.…”

Review on biophysical modelling and simulation studies for transcranial magnetic stimulation

“…Wei et al 2017Homogeneous sphere (8.5 cm) (Deng, Lisanby and Peterchev, 2013;Guadagnin et al, 2016;Gomez, Goetz and Peterchev, 2018;Gomez-Tames et al, 2020) Focality Area (A1/x) Cortical area where EF is larger than Emax/x (Im and Lee, 2006;Salvador et al, 2009;Koponen, Nieminen and Ilmoniemi, 2015;Yamamoto et al, 2015;Rastogi et al, 2017) Volume (VΩ )…”