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

Abstract

Background Transcranial magnetic stimulation (TMS) allows non-invasive stimulation of the cortex. In multi-locus TMS (mTMS), the stimulating electric field (E-field) is controlled electronically without coil movement by adjusting currents in the coils of a transducer. Objective To develop an mTMS system that allows adjusting the location and orientation of the E-field maximum within a cortical region. Methods We designed and manufactured a planar 5-coil mTMS transducer to allow controlling the maximum of the induced E-field within a cortical region approximately 30 mm in diameter. We developed electronics with a design consisting of independently controlled H-bridge circuits to drive up to six TMS coils. To control the hardware, we programmed software that runs on a field-programmable gate array and a computer. To induce the desired E-field in the cortex, we developed an optimization method to calculate the currents needed in the coils. We characterized the mTMS system and conducted a proof-of-concept motor-mapping experiment on a healthy volunteer. In the motor mapping, we kept the transducer placement fixed while electronically shifting the E-field maximum on the precentral gyrus and measuring electromyography from the contralateral hand. Results The transducer consists of an oval coil, two figure-of-eight coils, and two four-leaf-clover coils stacked on top of each other. The technical characterization indicated that the mTMS system performs as designed. The measured motor evoked potential amplitudes varied consistently as a function of the location of the E-field maximum. Conclusion The developed mTMS system enables electronically targeted brain stimulation within a cortical region.

Closed-loop optimization of transcranial magnetic stimulation with electroencephalography feedback

“…In this work, we determined the stimulation location and intensity manually based on visual inspection of averaged TEPs [ 29 ]. We believe that the whole procedure can, however, be automated by combining the presented closed-loop algorithm, for example, with a 5-coil multi-locus TMS system that allows adjusting the stimulation location and orientation without coil movement [ 54 ]. An advanced algorithmic implementation may thus make visual evaluation and manual assessment redundant in the future.…”

TMS with fast and accurate electronic control: measuring the orientation sensitivity of corticomotor pathways

“…The 2-coil transducer has a size similar to that of commercial figure-of-eight TMS coils and only half the size (150 mm) of our other mTMS transducers: the 2-coil transducer for shifting the stimulation locus within a line segment [12] and the 5-coil mTMS transducer that can shift and rotate the peak E-field within a cortical region [14]. The recently developed 5-coil mTMS 21 transducer comprises a similar combination of two perpendicular figure-of-eight coils to rotate the E-field orientation electronically; however, the large size (300 mm × 300 mm) prevents its use in cortical areas with restricted space.…”

“…If desired, this difference can be eliminated in future models by designing the bottom coil so that its focality matches the other coil's pattern [44]. The possibility to use 3D-printed formers might ease the manufacturing of transducers with more complicated winding patterns, such as those reported by Deng et al [45] and mTMS transducers with more coils [12,14].…”

“…Electronic control of the stimulus location and orientation on the cortex can be achieved by independently generated TMS pulses with coils placed on the scalp in a multi-coil TMS (mTMS) configuration [12][13][14]. The mTMS concept was introduced by Ruohonen et al [13,15]; however, the originally proposed coil arrays would require many coils to fine control the peak induced Efield [15] and be associated with excessive power electronics requirements.…”

“…To overcome the limitations, we developed a minimum-energy algorithm to design optimal TMS coils [16] and 4 developed an mTMS transducer comprising two overlapping coils (a figure-of-eight and an oval coil) capable of electronically shifting the stimulation locus along a line segment in the cortex [12]. We have also developed an mTMS system with a large general-purpose 5-coil transducer [14]. Previous studies have also combined two figure-of-eight coils to produce a rotating E-field in a single TMS pulse capable of reducing the orientation sensitivity of neuronal excitation [17,18].…”