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bioRxiv · 10.1101/300616

Design of Transcranial Magnetic Stimulation Coils with Optimal Trade-off between Depth, Focality, and Energy

Gomez, L.·Goetz, S.·Peterchev, A. V.
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Abstract

BackgroundTranscranial magnetic stimulation (TMS) is a noninvasive brain stimulation technique used for research and clinical applications. Existent TMS coils are limited in their precision of spatial targeting (focality), especially for deeper targets.\n\nObjectiveThis paper presents a methodology for designing TMS coils to achieve optimal trade-off between the depth and focality of the induced electric field (E-field), as well as the energy required by the coil.\n\nMethodsA multi-objective optimization technique is used for computationally designing TMS coils that achieve optimal trade-offs between stimulation focality, depth, and energy (fdTMS coils). The fdTMS coil winding(s) maximize focality (minimize stimulated volume) while reaching a target at a specified depth and not exceeding predefined peak E-field strength and required coil energy. Spherical and MRI-derived head models are used to compute the fundamental depth-focality trade-off as well as focality-energy trade-offs for specific target depths.\n\nResultsAcross stimulation target depths of 1.0-3.4 cm from the brain surface, the stimulated volume can be theoretically decreased by 42%-55% compared to existing TMS coil designs. The stimulated volume of a figure-8 coil can be decreased by 36%, 44%, or 46%, for matched, doubled, or quadrupled energy. For matched focality and energy, the depth of a figure-8 coil can be increased by 22%.\n\nO_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=153 SRC=\"FIGDIR/small/300616_fig8.gif\" ALT=\"Figure 8\">\nView larger version (14K):\norg.highwire.dtl.DTLVardef@af0b5org.highwire.dtl.DTLVardef@4118d8org.highwire.dtl.DTLVardef@1c61f80org.highwire.dtl.DTLVardef@3e23fa_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOFigure 8.C_FLOATNO Energy, W, vs. spread, S1/2, curves for hand knob stimulation for a conventional figure-8 coil and optimized fdTMS coil on a 32 x 32 cm square plane.\n\nC_FIG ConclusionComputational design of TMS coils could enable more selective targeting of the induced E-field. The presented results appear to be the first significant advancement in the depth-focality trade-off of TMS coils since the introduction of the figure-8 coil three decades ago, and likely represent the fundamental physical limit.\n\nDECLARATION OF INTERESTThe fdTMS technology described in this paper is subject to a provisional patent application by Duke University with the authors as inventors. Additionally: L.J.G. is inventor on a patent pertaining to the design of focal multicoil TMS systems. S.M.G. is inventor on patents and patent applications; he has received royalties from Rogue Research, TU Muenchen, and Porsche; furthermore, he has been provided with research support and patent fee reimbursement from Magstim Co. A.V.P. is inventor on patents and patent applications, and has received research and travel support as well as patent royalties from Rogue Research; research and travel support, consulting fees, as well as equipment loan from Tal Medical / Neurex; patent application and research support from Magstim; and equipment loans from MagVenture, all related to technology for transcranial magnetic stimulation.

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Gomez, L., Goetz, S., Peterchev, A. V.. 2018-04-13. Design of Transcranial Magnetic Stimulation Coils with Optimal Trade-off between Depth, Focality, and Energy. https://doi.org/10.1101/300616

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