A 3-axis coil design for multichannel TMS arrays

Abstract

Purpose: Multichannel Transcranial Magnetic Stimulation (mTMS) arrays enable multiple sites to be stimulated simultaneously or sequentially under electronic control without moving the system’s stimulation coils. Here, we build and characterize the performance of a novel modular 3-axis TMS coil that can be utilized as a unit element in large-scale multichannel TMS arrays. Methods: We determined the basic physical characteristics of the 3-axis TMS coil x -, y - and z -elements using a custom 2-channel programmable stimulator prototype. We mapped the temporal rate-of-change of the induced magnetic field ( dB/dt ) on a 2D plane parallel to the coil surface (including an extended line for full spatial coverage) and compared those values with predictions from magnetic field simulations. Temperature measurements were carried out to assess the incorporated air-cooling method. We measured the mutual and self-inductances of the x/y/z -elements to assess coupling between them. Additionally, we measured and calculated the coupling between z -elements in the array configuration. Finally, we performed electric field simulations to evaluate the stimulation intensity and focality of the coil and compared the results to conventional TMS coils as well as demonstrated suitability of the 3-axis coil for a multichannel array configuration. Results: The experimentally obtained dB/dt values validated the computational model of the 3-axis coil and therefore confirmed that both the coil and stimulator system are operating as intended. The air-cooling system was effective for brief high-frequency pulse trains and extended single- and paired-pulse TMS protocols. The electromagnetic simulations suggested that an array of the 3-axis coils would have comparable stimulation intensity to conventional TMS coils, therefore enabling clearly suprathreshold stimulation of the human brain. The recorded coil coupling between the x/y/z -elements was < 1% and the maximal coupling between z -elements in the array configuration was 1.8% and 3.4% for the measured and calculated values, respectively. Conclusion: We presented a 3-axis coil intended for multichannel TMS arrays. The electromagnetic measurements and simulations verified that the coil fabrication met the desired specifications and that the inductive coupling between the elements was negligible. The air-cooled 3-axis TMS coil appears suitable to be used as an element in multichannel TMS arrays.

Multi-locus transcranial magnetic stimulation system for electronically targeted brain stimulation

“…With mTMS, distinct cortical targets can be stimulated with sub-millisecond interstimulus intervals (ISIs) [10] and physiological feedback can be utilized in a closed loop to automate stimulation protocols [11,12]. Others have also taken steps towards implementing multi-coil TMS [13], with Navarro de Lara et al presenting a prototype concept based on a 3-axis coil [14].…”

Devices and Technology in Transcranial Magnetic Stimulation: A Systematic Review

“…Although the FoE coil has demonstrated its applicability for focused TMS with few required improvement and optimization [ 88 ], it is not very useful when the therapy protocol requires deeper or full-head stimulation. This could be partially solved by using an array of circular coils together providing a complex magnetic stimulation of the cortical regions [ 42 , 70 , 82 , 107 , 108 ] with capabilities of multiple stimulation patterns [ 62 ] and complex optimization algorithms [ 108 ]. Although the endeavor of placing a large number of coils is not an important factor to consider if they are fixed in a chassis, the determination of their effects after superposing the fields produced by these coils is not simple.…”

Rapid computation of TMS-induced E-fields using a dipole-based magnetic stimulation profile approach

“…The MSP approach may be particularly useful for such application where the computational resources available for the real-time step are limited by the Augmented Reality (AR) headset hardware and a lightweight and simple ‘linearized’ implementation is preferred. We also anticipate the ongoing multichannel TMS technology development ( Han et al, 2004 ; Jiang et al, 2013 ; Navarro de Lara et al, 2020 ) to benefit from the proposed approach as the E-fields of several coils need to be rapidly evaluated for computational targeting. Future work includes optimizing the MSP basis set selection such that it provides maximal accuracy with the smallest number of basis functions as well as further improvements on the numerical efficiency of the BEM-FMM algorithm.…”