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.

Trade-off between stimulation focality and the number of coils in multi-locus transcranial magnetic stimulation

“…In figure 4(A), we show a planar five-coil transducer with Focality 33-50 able to control the peak E-field within a 35-mm-diameter cortical target area (with 95% explained variance). This transducer has properties close to previously described fivecoil mTMS transducers [9,10]. In figure 4(B), we show a planar transducer that can control the location and orientation of the peak E-field within a 61-mm-diameter cortical region (Focality 67-100).…”

“…to design a planar five-coil mTMS transducer that can stimulate within a 60mm-diameter cortical region. Such a region would be considerably larger than the cortical region available for previously described five-coil mTMS transducers able to stimulate within a 30-mm-diameter region with more focal E-fields [9,10]. For a sphericalcap transducer able to control the stimulation target within an 87-mm-diameter region, the number of coils required dropped from 25 to 7 when the accepted E-field 71%-amplitude width was increased from 27 to 67 mm (figure 3).…”

“…However, when designing a curved transducer to be built, its geometry should preferably be based on the curvature of the scalp above the cortical target region to maximize the stimulation efficiency [6,17]. When an mTMS transducer is applied for brain stimulation, the E-fields are preferably calculated based on the individual cortical anatomy [10], regardless of the model applied in the transducer design, as E-field patterns in a realistic cortical geometry are rather complex [10,27].…”

“…In multi-locus TMS (mTMS), the stimulation is administered with a transducer consisting of several coils that allow controlling the induced E-field pattern electronically [9,10]. This enables one to stimulate neighboring cortical areas with millisecond-scale interstimulus intervals [11] and to implement closedloop stimulation paradigms [12,13].…”

“…In this case, the furthermost coils would require excessive current amplitudes due to their suboptimal coupling to the brain [6,9]. Previously described mTMS transducers are sufficient for controlling the stimulation within an approximately 30-mm-diameter region [9,10]; however, many applications would benefit from a larger targeting region.…”