Optimized Monophasic Pulses with Equivalent Electric Field for Rapid-Rate Transcranial Magnetic Stimulation

Objective. Transcranial magnetic stimulation (TMS) with monophasic pulses achieves greater changes in neuronal excitability but requires higher energy and generates more coil heating than TMS with biphasic pulses, and this limits the use of monophasic pulses in rapid-rate protocols. We sought to design a stimulation waveform that retains the characteristics of monophasic TMS but significantly reduces coil heating, thereby enabling higher pulse rates and increased neuromodulation effectiveness.Approach. A two-step optimization method was developed that uses the temporal relationship between the electric field (E-field) and coil current waveforms. The model-free optimization step reduced the ohmic losses of the coil current and constrained the error of the E-field waveform compared to a template monophasic pulse, with pulse duration as a second constraint. The second, amplitude adjustment step scaled the candidate waveforms based on simulated neural activation to account for differences in stimulation thresholds. The optimized waveforms were implemented to validate the changes in coil heating.Main results. Depending on the pulse duration and E-field matching constraints, the optimized waveforms produced 12% to 75% less heating than the original monophasic pulse. The reduction in coil heating was robust across a range of neural models. The changes in the measured ohmic losses of the optimized pulses compared to the original pulse agreed with numeric predictions.Significance. The first step of the optimization approach was independent of any potentially inaccurate or incorrect model and exhibited robust performance by avoiding the highly nonlinear behavior of neural responses, whereas neural simulations were only run once for amplitude scaling in the second step. This significantly reduced computational cost compared to iterative methods using large populations of candidate solutions and more importantly reduced the sensitivity to the choice of neural model. The reduced coil heating and power losses of the optimized pulses can enable rapid-rate monophasic TMS protocols.

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“…The code and data that support the findings of this study are openly available at the following URL/DOI [46]: https://doi.org/10.7924/r4wd41f7d.…”

Section: Data Availability Statementmentioning

confidence: 68%

Optimized monophasic pulses with equivalent electric field for rapid-rate transcranial magnetic stimulation

Wang

Zhang

et al. 2023

J. Neural Eng.

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“…Beyond the physiology, optimized pulse shapes allow reducing the heating of TMS coils, which traditionally limits repetitive TMS protocols and has to be kept within certain limits for safety reasons [83][84][85][86][87].…”

Section: Introductionmentioning

confidence: 99%

Transcranial Magnetic Stimulation Tech-nology with Freely Adjustable Pulse Shape and Sequence

Li¹,

Zhang²,

Peterchev³

et al. 2023

Preprint

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The temporal shape of a pulse in transcranial magnetic stimulation (TMS) influences which neuron populations are activated preferentially as well as the strength and even direction of neuromodulation effects. Furthermore, various pulse shapes differ in their efﬁciency, coil heating, sensory perception, and clicking sound. However, the available TMS pulse shape repertoire is still very limited to a few biphasic, monophasic, and polyphasic pulses with sinusoidal or near-rectangular shapes. Monophasic pulses, though found to be more selective and stronger in neuromodulation, are generated inefﬁciently and therefore only available in simple low-frequency repetitive protocols. Despite a strong interest to exploit the temporal effects of TMS pulse shapes and pulse sequences, waveform control is relatively inflexible and only possible parametrically within certain limits. Previously proposed approaches for flexible pulse shape control, such as through power electronic inverters, have signiﬁcant limitations: The semiconductor switches can fail under the immense electrical stress associated with free pulse shaping, and most conventional power inverter topologies are incapable of generating smooth electric ﬁelds or existing pulse shapes. Leveraging intensive preliminary work on modular power electronics, we present a modular pulse synthesizer (MPS) technology that can, for the ﬁrst time, flexibly generate high-power TMS pulses (one-side peak ~ 4,000 V, ~ 8,000 A) with user-deﬁned electric ﬁeld shape as well as rapid sequences of pulses with high output quality. The circuit topology breaks the problem of simultaneous high power and switching speed into smaller, manageable portions, distributed across several identical modules. In consequence, MPS TMS can use semiconductor devices with voltage and current ratings lower than the overall pulse voltage and distribute the overall switching of several hundred kilohertz among multiple transistors. MPS TMS can synthesize practically any pulse shape, including conventional ones, with ﬁne quantization of the induced electric ﬁeld (≤ 17% granularity without modulation and ~300 kHz bandwidth). Moreover, the technology allows optional symmetric differential coil driving so that the average electric potential of the coil, in contrast to conventional TMS devices, stays constant to prevent capacitive artifacts in sensitive recording ampliﬁers, such as electroencephalography (EEG). MPS TMS can enable the optimization of stimulation paradigms for more sophisticated probing of brain function as well as stronger and more selective neuromodulation, further expanding the parameter space available to users.

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Optimisation of asymmetric field pulses for transcranial magnetic stimulation

Ma¹,

Goetz²

2023

Preprint

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Transcranial magnetic stimulation (TMS) is a widely-used noninvasive brain stimulation technique through electromagnetic induction. Nowadays commercial TMS devices routinely use conventional biphasic pulses for repetitive TMS protocols and monophasic pulses for single-pulse stimulation. They respectively generate underdamped or damped cosinusoidal electric field pulses that have been proven to be power-inefficient. Recently, symmetric field pulses with near-rectangular electric fields show great potential in terms of energy loss and coil heating, but only limited studies have investigated asymmetric field pulses with different asymmetry levels for the induced electric field shape. Thus, this paper optimises and searches a wide range of potential waveforms with the goal of minimising energy loss and coil heating. The optimised results demonstrated that asymmetric field pulses with near-rectangular electric fields have significantly lower power consumption than conventional ones. These optimised waveforms commonly consist of an initial falling phase followed by rapidly rising and falling phases, trailed by a slow decay to zero. Interestingly, the initial phase has a decay time constant around 260 μs and introduces a pulse-duration-dependent negative bias for the current baseline to minimise the energy loss and coil heating. Thus, it is possible to directly design asymmetric field pulses with various asymmetry ratios by using several prediction equations rather than running optimisation. These results also suggest that introducing such an initial current phase could likely significantly reduce the coil heating of most TMS pulse shapes to improve their power efficiencies.

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Optimized Monophasic Pulses with Equivalent Electric Field for Rapid-Rate Transcranial Magnetic Stimulation

Cited by 3 publications

References 90 publications

Optimized monophasic pulses with equivalent electric field for rapid-rate transcranial magnetic stimulation

Optimized monophasic pulses with equivalent electric field for rapid-rate transcranial magnetic stimulation

Transcranial Magnetic Stimulation Tech-nology with Freely Adjustable Pulse Shape and Sequence

Optimisation of asymmetric field pulses for transcranial magnetic stimulation

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