Conference Paper

Quiet transcranial magnetic stimulation: Status and future directions

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Abstract

A significant limitation of transcranial magnetic stimulation (TMS) is that the magnetic pulse delivery is associated with a loud clicking sound as high as 140 dB resulting from electromagnetic forces. The loud noise significantly impedes both basic research and clinical applications of TMS. It effectively makes TMS less focal since every click activates auditory cortex, brainstem, and other connected regions, synchronously with the magnetic pulse. The repetitive clicking sound can induce neuromodulation that can interfere with and confound the intended effects at the TMS target. As well, there are known concerns regarding blinding of TMS studies, hearing loss, induction of tinnitus, as well as tolerability. Addressing this need, we are developing a quiet TMS (qTMS) device that incorporates two key concepts: First, the dominant frequency components of the TMS pulse sound (typically 2-5 kHz) are shifted to higher frequencies that are above the human hearing upper threshold of about 20 kHz. Second, the TMS coil is designed electrically and mechanically to generate suprathreshold electric field pulses while minimizing the sound emitted at audible frequencies (<; 20 kHz). The enhanced acoustic properties of the coil are accomplished with a novel, layered coil design. We summarize a proof-of-concept qTMS prototype demonstrating noise loudness reduction by 19 dB(A) with ultrabrief pulses at conventional amplitudes. Further, we outline next steps to accomplish further sound reduction and suprathreshold pulse amplitudes.

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... This approach is further supported by the conventional hierarchy of hazard controls, in which personal protective equipment is considered the least effective, last-resort solution [33]. We have proposed a two-pronged approach to quiet TMS, involving improved electromechanical coil design and briefer pulses [34], [35]. In the present work we focus on the first part of this approach and demonstrate a TMS coil design with high electromagnetic output but substantially reduced acoustic emission for conventional TMS pulse waveforms. ...
... The DCC was designed to be compatible with ultra-brief TMS pulses, where the total duration of the biphasic pulse will be reduced by an order of magnitude, from 300 µs down to about 30 µs [35]. These pulses are expected to require comparable peak currents, which necessitates next-generation TMS devices with operating voltages in excess of 10 kV [35]. ...
... The DCC was designed to be compatible with ultra-brief TMS pulses, where the total duration of the biphasic pulse will be reduced by an order of magnitude, from 300 µs down to about 30 µs [35]. These pulses are expected to require comparable peak currents, which necessitates next-generation TMS devices with operating voltages in excess of 10 kV [35]. Such ultra-brief pulses are expected to further reduce the sound level by both pushing the spectral peak at two times the pulse current frequency outside the human hearing range and reducing the energy in the excited subharmonics by a factor of 10. ...
Preprint
Objective: This work aims to reduce the acoustic noise level of transcranial magnetic stimulation (TMS) coils. TMS requires high currents (several thousand amperes) to be pulsed through the coil, which generates a loud acoustic impulse whose peak sound pressure level (SPL) can exceed 130 dB(Z). This sound poses a risk to hearing and elicits unwanted neural activation of auditory brain circuits. Methods: We propose a new double-containment coil with enhanced winding mounting (DCC), which utilizes acoustic impedance mismatch to contain and dissipate the impulsive sound within an air-tight outer casing. The coil winding is potted in a rigid block, which is mounted to the outer casing by its acoustic nodes that are subject to minimum vibration during the pulse. The rest of the winding block is isolated from the casing by an air gap, and sound is absorbed by foam within the casing. The casing thickness under the winding center is minimized to maximize the coil electric field output. Results: Compared to commercial figure-of-eight TMS coils, the DCC prototype has 10-33 dB(Z) lower SPL at matched stimulation strength, whilst providing 22% higher maximum stimulation strength than equally focal commercial coils. Conclusion: The DCC design greatly reduces the acoustic noise of TMS while increasing the achievable stimulation strength. Significance: The acoustic noise reduction from our coil design is comparable to that provided by typical hearing protection devices. This coil design approach can enhance hearing safety and reduce auditory co-activations in the brain and other detrimental effects of TMS sound.
... This approach allowed reduced sound while having separation between the winding and the coil surface (4-6 mm) comparable to the upper range for conventional coils (2-5 mm). This coil design was part of our proposed two-pronged approach to "quiet TMS," involving improved electromechanical coil design and the use of briefer pulses [33], [34]. ...
... Some aspects of the DCC prototype were designed based on qualitative considerations and approximations for the coil design necessary for our quiet TMS framework, which aims to use high-amplitude ultra-brief pulses [34]. Consequently, the design was optimized to accommodate windings made of litz wire with higher voltage insulation (see DCC* version of the coil described in the Supplementary material). ...
Article
This work aims to reduce the acoustic noise level of transcranial magnetic stimulation (TMS) coils. TMS requires high currents (several thousand amperes) to be pulsed through the coil, which generates a loud acoustic impulse whose peak sound pressure level (SPL) can exceed 130 dB(Z). This sound poses a risk to hearing and elicits unwanted neural activation of auditory brain circuits. $Methods$ : We propose a new double-containment coil with enhanced winding mounting (DCC), which utilizes acoustic impedance mismatch to contain and dissipate the impulsive sound within an air-tight outer casing. The coil winding is potted into a rigid block, which is mounted to the outer casing through the block's acoustic nodes that are subject to minimum vibration during the pulse. The rest of the winding block is isolated from the casing by an air gap, and the sound is absorbed by polyester fiber panels within the casing. The casing thickness under the winding center is minimized to maximize the electric field output. $Results$ : Compared to commercial figure-of-eight TMS coils, the DCC prototype has 18–41 dB(Z) lower peak SPL at matched stimulation strength, whilst providing 28% higher maximum stimulation strength than equally focal coils. $Conclusion$ : The DCC design greatly reduces the acoustic noise of TMS while increasing the achievable stimulation strength. $Significance$ : The acoustic noise reduction from our coil design is comparable to that provided by typical hearing protection devices. This coil design approach can enhance hearing safety and reduce auditory co-activations in the brain and other detrimental effects of TMS sound.
... As the optimised coils require lower power, they might enable the use of smaller, and thus cheaper, power electronics in the future. Energy-efficient coils also make it easier to produce briefer TMS pulses, which would otherwise require higher voltage and peak power for equivalent stimulation intensity [26,27], and thus more complicated power electronics [28]. Briefer pulses would cause less coil heating and offer less perceptible coil click [28]. ...
... Energy-efficient coils also make it easier to produce briefer TMS pulses, which would otherwise require higher voltage and peak power for equivalent stimulation intensity [26,27], and thus more complicated power electronics [28]. Briefer pulses would cause less coil heating and offer less perceptible coil click [28]. ...
Article
Background: Transcranial magnetic stimulation (TMS) allows focal, non-invasive stimulation of the cortex. A TMS pulse is inherently weakly coupled to the cortex; thus, magnetic stimulation requires both high current and high voltage to reach sufficient intensity. These requirements limit, for example, the maximum repetition rate and the maximum number of consecutive pulses with the same coil due to the rise of its temperature. Objective: To develop methods to optimise, design, and manufacture energy-efficient TMS coils in realistic head geometry with an arbitrary overall coil shape. Methods: We derive a semi-analytical integration scheme for computing the magnetic field energy of an arbitrary surface current distribution, compute the electric field induced by this distribution with a boundary element method, and optimise a TMS coil for focal stimulation. Additionally, we introduce a method for manufacturing such a coil by using Litz wire and a coil former machined from polyvinyl chloride. Results: We designed, manufactured, and validated an optimised TMS coil and applied it to brain stimulation. Our simulations indicate that this coil requires less than half the power of a commercial figure-of-eight coil, with a 41% reduction due to the optimised winding geometry and a partial contribution due to our thinner coil former and reduced conductor height. With the optimised coil, the resting motor threshold of abductor pollicis brevis was reached with the capacitor voltage below 600 V and peak current below 3000 A. Conclusion: The described method allows designing practical TMS coils that have considerably higher efficiency than conventional figure-of-eight coils.
... Apparently, the perception of strength was not affected by the reduction of coil energy [20] and acoustic output [21] for briefer pulses, suggesting that these contributions may be insignificant. On the other hand, the different audible pitch of the pulses, which is higher for briefer widths [32], may have modulated the perception of stimulation, resulting in sharper sensation associated with briefer pulses. Alternatively or in conjunction, the sharper sensation may reflect the decreasing relative threshold of Aδ fibers compared to C fibers as the pulse width is reduced, due to larger membrane time constant of C fibers [7,8]. ...
... This could be done by manipulating the influence of the various factors (e.g., by using mechanically damping foam spacers, applying topical anesthetics to the scalp, delivering masking noise with earphones, comparing with electrical scalp stimulation), rating additional dimensions of the sensation, as well as electrophysiological recordings from the scalp. Future studies may also explore even briefer pulses where the mechano-acoustic emission of the coil is reduced further [10,32] and it is unclear if the tendency for increased discomfort would continue. ...
Article
Background: Scalp sensation and pain comprise the most common side effect of transcranial magnetic stimulation (TMS), which can reduce tolerability and complicate experimental blinding. Objective: We explored whether changing the width of single TMS pulses affects the quality and tolerability of the resultant somatic sensation. Methods: Using a controllable pulse parameter TMS device with a figure-8 coil, single monophasic magnetic pulses inducing electric field with initial phase width of 30, 60, and 120 µs were delivered in 23 healthy volunteers. Resting motor threshold of the right first dorsal interosseus was determined for each pulse width, as reported previously. Subsequently, pulses were delivered over the left dorsolateral prefrontal cortex at each of the three pulse widths at two amplitudes (100% and 120% of the pulse-width-specific motor threshold), with 20 repetitions per condition delivered in random order. After each pulse, subjects rated 0-to-10 visual analog scales for Discomfort, Sharpness, and Strength of the sensation. Results: Briefer TMS pulses with amplitude normalized to the motor threshold were perceived as slightly more uncomfortable than longer pulses (with an average 0.89 point increase on the Discomfort scale for pulse width of 30 µs compared to 120 µs). The sensation of the briefer pulses was felt to be substantially sharper (2.95 points increase for 30 µs compared to 120 µs pulse width), but not stronger than longer pulses. As expected, higher amplitude pulses increased the perceived discomfort and strength, and, to a lesser degree the perceived sharpness. Conclusions: Our findings contradict a previously published hypothesis that briefer TMS pulses are more tolerable. We discovered that the opposite is true, which merits further study as a means of enhancing tolerability in the context of repetitive TMS.
... Stimulation with pulses of different durations demonstrated that pulse shapes also affect a subject's or patient's perception of the pulse on the scalp, likely due to the different activation dynamics of nociceptors and other sensory fibers in the skin compared to the various cortical neurons [74]. Pulses with the majority of their spectral content in higher frequency ranges emit less sound, which is more than just a technical nuisance and artifact of TMS as it concur-rently stimulates auditory circuits [75][76][77][78][79][80]. The loud clicking sound of pulses could previously not be isolated from the electromagnetic stimulation and is always exactly in sync with it. ...
Preprint
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 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: Existing 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 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. MPS TMS can synthesize practically any pulse shape, including conventional ones, with fine quantization of the induced electric field.
... In addition to the direct effect of TMS on neural tissue, TMS pulses are always accompanied by loud clicking noises (up to 120 dB) due to the rapid expansion of coil windings caused by the electromagnetic force induced during pulse generation (Starck et al., 1996). These noises result in auditory stimulation that is synchronous to magnetic stimulation pulses, making it difficult to conduct blinded studies (Peterchev et al., 2015;Tiitinen et al., 1999). Activation of skin nerves and facial muscles due to TMS pulses represent another potential confounding factor that must be considered and addressed. ...
Article
Full-text available
The human brain is arguably one of the most complex systems in nature. To understand how it operates, it is essential to understand the link between neural activity and behavior. Experimental investigation of that link requires tools to interact with neural activity during behavior. Human neuroscience, however, has been severely bottlenecked by the limitations of these tools. While invasive methods can support highly specific interaction with brain activity during behavior, their applicability in human neuroscience is limited. Despite extensive development in the last decades, noninvasive alternatives have lacked spatial specificity and yielded results that are commonly fraught with variability and replicability issues, along with relatively limited understanding of the neural mechanisms involved. Here we provide a comprehensive review of the state-of-the-art in interacting with human brain activity and highlight current limitations and recent efforts to overcome these limitations. Beyond crucial technical and scientific advancements in electromagnetic brain stimulation, new frontiers in interacting with human brain activity such as task-irrelevant sensory stimulation and focal ultrasound stimulation are introduced. Finally, we argue that, along with technological improvements and breakthroughs in noninvasive methods, a paradigm shift towards adaptive closed-loop stimulation will be a critical step for advancing human neuroscience.
... The sound associated with a TMS pulse is impulsive and resembles a small explosion, such as shooting a firearm, in terms of its duration and peak sound pressure [13]. Consequently, measurement protocols and safety regulations for such sounds provide a good reference for TMS noise characterization. ...
Article
Full-text available
Background Accurate data on the sound emitted by transcranial magnetic stimulation (TMS) coils is lacking. Methods We recorded the sound waveforms of seven coils with high bandwidth. We estimated the neural stimulation strength by measuring the induced electric field and applying a strength–duration model to account for different waveforms. Results Across coils, at maximum stimulator output and 25 cm distance, the sound pressure level (SPL) was 98–125 dB(Z) per pulse and 76–98 dB(A) for a 20 Hz pulse train. At 5 cm distance, these values were estimated to increase to 112–139 dB(Z) and 90–112 dB(A), respectively. Conclusions The coils’ airborne sound can exceed some exposure limits for TMS subjects and, in some cases, for operators. These findings are consistent with the current TMS safety guidelines that recommend the use of hearing protection.
... MEG and EEG both offer noninvasive fast signal recording. TMS coils can be designed to introduce deeper (Zangen et al., 2005) (but see Deng et al., 2014 ;Guadagnin et al., 2016), quieter (Peterchev et al., 2015), and more focal (Huh et al., 2018) stimulation. However, diffusion tractography algorithms produce connectome that lack ground truth and produce many systematic false positive tracts (Maier-Hein et al., 2017) and do not estimate fiber directionality (van den Heuvel et Medaglia et al. ...
Article
In the era of “big data”, we are gaining rich person-specific information about neuroanatomy, neural function, and cognitive functions. However, the optimal ways to create precise approaches to optimize individuals' mental functions in health and disease are unclear. Multimodal analysis and modeling approaches can guide neuromodulation by combining anatomical networks, functional signal analysis, and cognitive neuroscience paradigms in single subjects. Our progress could be improved by progressing from statistical fits to mechanistic models. Using transcranial magnetic stimulation as an example, we discuss how integrating methods with a focus on mechanisms could improve our predictions TMS effects within individuals, refine our models of health and disease, and improve our treatments.
... Vacuum casing, as well as the use of shear coolants, were proposed Ilmoniemi, Ruohonen, Kamppuri, & Virtanen, 1997). A recent solution implemented a coil structure with dedicated layers for acoustic decoupling and dissipation, as well as pulses, with its spectral content exclusively in the ultrasound range (Goetz, Luber, Lisanby, & Peterchev, 2014a;Peterchev, Murphy, & Goetz, 2015b). For ultrasound, the sound impedances and absorption rates of most materials increase, the ear is less sensitive from a mechanical perspective, and safety levels are far higher (Howard, Hansen, & Zander, 2005;M€ oser & Kropp, 2010). ...
Article
Magnetic stimulation is a non-invasive neurostimulation technique that can evoke action potentials and modulate neural circuits through induced electric fields. Biophysical models of magnetic stimulation have become a major driver for technological developments and the understanding of the mechanisms of magnetic neurostimulation and neuromodulation. Major technological developments involve stimulation coils with different spatial characteristics and pulse sources to control the pulse waveform. While early technological developments were the result of manual design and invention processes, there is a trend in both stimulation coil and pulse source design to mathematically optimize parameters with the help of computational models. To date, macroscopically highly realistic spatial models of the brain, as well as peripheral targets, and user-friendly software packages enable researchers and practitioners to simulate the treatment-specific and induced electric field distribution in the brains of individual subjects and patients. Neuron models further introduce the microscopic level of neural activation to understand the influence of activation dynamics in response to different pulse shapes. A number of models that were designed for online calibration to extract otherwise covert information and biomarkers from the neural system recently form a third branch of modelling.
... In addition to a sham condition, the auditory artifacts may also be minimized by masking them with a continuous noise, such as that tailored with the same frequencies as the TMS click . Efforts are currently focused on developing a quiet TMS-coil (Peterchev et al., 2015), which would reduce the influence of auditory responses. Still, the challenge of sensory processing might be a methodological challenge that will take more time to solve. ...
Article
Biomarkers are essential for understanding the underlying pathologies in brain disorders and for developing effective treatments. Combined transcranial magnetic stimulation and electroencephalography (TMS-EEG) is an emerging neurophysiological tool that can be used for biomarker development. This method can identify biomarkers associated with the function and dynamics of the inhibitory and excitatory neurotransmitter systems and effective connectivity between brain areas. In this review, we outline the current state of the TMS-EEG biomarker field by summarizing the existing protocols and the possibilities and challenges associated with this methodology.
Article
Background Coupling transcranial magnetic stimulation with electroencephalography (TMS-EEG) allows recording the EEG response to a direct, non-invasive cortical perturbation. However, obtaining a genuine TMS-evoked EEG potential requires controlling for several confounds, among which a main source is represented by the auditory evoked potentials (AEPs) associated to the TMS discharge noise (TMS click). This contaminating factor can be in principle prevented by playing a masking noise through earphones. New method Here we release TMS Adaptable Auditory Control (TAAC), a highly flexible, open-source, Matlab®-based interface that generates in real-time customized masking noises. TAAC creates noises starting from the stimulator-specific TMS click and tailors them to fit the individual, subject-specific click perception by mixing and manipulating the standard noises in both time and frequency domains. Results We showed that TAAC allows us to provide standard as well as customized noises able to effectively and safely mask the TMS click. Comparison with existing methods Here, we showcased two customized noises by comparing them to two standard noises previously used in the TMS literature (i.e., a white noise and a noise generated from the stimulator-specific TMS click only). For each, we quantified the Sound Pressure Level (SPL; measured by a Head and Torso Simulator - HATS) required to mask the TMS click in a population of 20 healthy subjects. Both customized noises were effective at safe (according to OSHA and NIOSH safety guidelines), lower SPLs with respect to standard noises. Conclusions At odds with previous methods, TAAC allows creating effective and safe masking noises specifically tailored on each TMS device and subject. The combination of TAAC with tools for the real-time visualization of TEPs can help control the influence of auditory confounds also in non-compliant patients. Finally, TAAC is a highly flexible and open-source tool, so it can be further extended to meet different experimental requirements.
Preprint
Full-text available
Background Coupling transcranial magnetic stimulation with electroencephalography (TMS-EEG) allows recording the EEG response to a direct, non-invasive cortical perturbation. However, obtaining a genuine TMS-evoked EEG potential requires controlling for several confounds, among which a main source is represented by the auditory evoked potentials (AEPs) associated to the TMS discharge noise (TMS click). This contaminating factor can be in principle prevented by playing a masking noise through earphones. New method Here we release TMS Adaptable Auditory Control (TAAC), a highly flexible, open-source, Matlab-based interface that generates in real-time customized masking noises. TAAC creates noises starting from the stimulator-specific TMS click and tailors them to fit the individual, subject-specific click perception by mixing and manipulating the standard noises in both time and frequency domains. Results We showed that TAAC allows us to provide standard as well as customized noises able to effectively and safely mask the TMS click. Comparison with existing methods Here, we showcased two customized noises by comparing them to two standard noises previously used in the TMS literature (i.e., a white noise and a noise generated from the stimulator-specific TMS click only). For each, we quantified the Sound Pressure Level (SPL; measured by a Head and Torso Simulator - HATS) required to mask the TMS click in a population of 20 healthy subjects. Both customized noises were effective at safe (according to OSHA and NIOSH safety guidelines), lower SPLs with respect to standard noises. Conclusions At odds with previous methods, TAAC allows creating effective and safe masking noises specifically tailored on each TMS device and subject. The combination of TAAC with tools for the real-time visualization of TEPs can help control the influence of auditory confounds also in non-compliant patients. Finally, TAAC is a highly flexible and open-source tool, so it can be further extended to meet different experimental requirements.
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Magnetic stimulation is a standard tool in brain research and has found important clinical applications in neurology, psychiatry, and rehabilitation. Whereas coil designs and the spatial field properties have been intensively studied in the literature, the temporal dynamics of the field has received less attention. Typically, the magnetic field waveform is determined by available device circuit topologies rather than by consideration of what is optimal for neural stimulation. This paper analyzes and optimizes the waveform dynamics using a nonlinear model of a mammalian axon. The optimization objective was to minimize the pulse energy loss. The energy loss drives power consumption and heating, which are the dominating limitations of magnetic stimulation. The optimization approach is based on a hybrid global-local method. Different coordinate systems for describing the continuous waveforms in a limited parameter space are defined for numerical stability. The optimization results suggest that there are waveforms with substantially higher efficiency than that of traditional pulse shapes. One class of optimal pulses is analyzed further. Although the coil voltage profile of these waveforms is almost rectangular, the corresponding current shape presents distinctive characteristics, such as a slow low-amplitude first phase which precedes the main pulse and reduces the losses. Representatives of this class of waveforms corresponding to different maximum voltages are linked by a nonlinear transformation. The main phase, however, scales with time only. As with conventional magnetic stimulation pulses, briefer pulses result in lower energy loss but require higher coil voltage than longer pulses.
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Transcranial magnetic stimulation (TMS) is widely used for noninvasive activation of neurons in the brain for research and clinical applications. The strong, brief magnetic pulse generated in TMS is associated with a loud (>100 dB) clicking sound that can impair hearing and that activates auditory circuits in the brain. We introduce a two-pronged solution to reduce TMS noise by redesigning both the pulse waveform and the coil structure. First, the coil current pulse duration is reduced which shifts a substantial portion of the pulse acoustic spectrum above audible frequencies. Second, the mechanical structure of the stimulation coil is designed to suppress the emergence of the sound at the source, diminish down-mixing of high-frequency sound into the audible range, and impede the transmission of residual sound to the coil surface but dissipate it away from the casing. A prototype coil driven with ultrabrief current pulses (down to 45-μs biphasic duration) is demonstrated to reduce the peak sound pressure level by more than 25 dB compared to a conventional TMS configuration, resulting in loudness reduction by more than 14-fold. These results motivate improved mechanical design of TMS coils as well as design of TMS pulse generators with shorter pulse durations and increased voltage limits with the objective of reducing TMS acoustic noise while retaining the neurostimulation strength.
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Objective: This work aims at flexible and practical pulse parameter control in transcranial magnetic stimulation (TMS), which is currently very limited in commercial devices. Approach: We present a third generation controllable pulse parameter device (cTMS3) that uses a novel circuit topology with two energy-storage capacitors. It incorporates several implementation and functionality advantages over conventional TMS devices and other devices with advanced pulse shape control. cTMS3 generates lower internal voltage differences and is implemented with transistors with a lower voltage rating than prior cTMS devices. Main results: cTMS3 provides more flexible pulse shaping since the circuit topology allows four coil-voltage levels during a pulse, including approximately zero voltage. The near-zero coil voltage enables snubbing of the ringing at the end of the pulse without the need for a separate active snubber circuit. cTMS3 can generate powerful rapid pulse sequences (< 10 ms inter pulse interval) by increasing the width of each subsequent pulse and utilizing the large capacitor energy storage, allowing the implementation of paradigms such as paired-pulse and quadripulse TMS with a single pulse generation circuit. cTMS3 can also generate theta (50 Hz) burst stimulation with predominantly unidirectional electric field pulses. The cTMS3 device functionality and output strength are illustrated with electrical output measurements as well as a study of the effect of pulse width and polarity on the active motor threshold in ten healthy volunteers. Significance: The cTMS3 features could extend the utility of TMS as a research, diagnostic, and therapeutic tool.
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We compare half-bridge and full-bridge circuit topologies for controllable pulse parameter transcranial magnetic stimulation (cTMS) devices. The full bridge can generate two more distinct coil voltage levels for enhanced pulse shaping, does not need an active snubber circuit, and requires lower semiconductor switch voltage ratings, at the cost of necessitating four semiconductor switches and associated drivers compared to two in the half bridge. In cTMS devices it may be beneficial to use coils with higher number of turns and, hence, higher inductance than that optimal in conventional TMS, since this reduces the required capacitance, stored energy, coil current, and, potentially, conduction losses and heating. However, increasing the number of coil turns raises the required capacitor voltage which presents practical limits on the number of turns, especially for very brief pulses.
Article
Objective: To demonstrate the use of a novel controllable pulse parameter TMS (cTMS) device to characterize human corticospinal tract physiology. Methods: Motor threshold and input-output (IO) curve of right first dorsal interosseus were determined in 26 and 12 healthy volunteers, respectively, at pulse widths of 30, 60, and 120 μs using a custom-built cTMS device. Strength-duration curve rheobase and time constant were estimated from the motor thresholds. IO slope was estimated from sigmoid functions fitted to the IO data. Results: All procedures were well tolerated with no seizures or other serious adverse events. Increasing pulse width decreased the motor threshold and increased the pulse energy and IO slope. The average strength-duration curve time constant is estimated to be 196 μs, 95% CI [181 μs, 210 μs]. IO slope is inversely correlated with motor threshold both across and within pulse width. A simple quantitative model explains these dependencies. Conclusions: Our strength-duration time constant estimate compares well to published values and may be more accurate given increased sample size and enhanced methodology. Multiplying the IO slope by the motor threshold may provide a sensitive measure of individual differences in corticospinal tract physiology. Significance: Pulse parameter control offered by cTMS provides enhanced flexibility that can contribute novel insights in TMS studies.
Article
Magnetic stimulation pulse sources are very inflexible high-power devices. The incorporated circuit topology is usually limited to a single pulse type. However, experimental and theoretical work shows that more freedom in choosing or even designing waveforms could notably enhance existing methods. Beyond that, it even allows entering new fields of application. We propose a technology that can solve the problem. Even in very high frequency ranges, the circuitry is very flexible and is able generate almost every waveform with unrivaled accuracy. This technology can dynamically change between different pulse shapes without any reconfiguration, recharging or other changes; thus the waveform can be modified also during a high-frequency repetitive pulse train. In addition to the option of online design and generation of still unknown waveforms, it amalgamates all existing device types with their specific pulse shapes, which have been leading an independent existence in the past years. These advantages were achieved by giving up the common basis of all magnetic stimulation devices so far, i.e., the high-voltage oscillator. Distributed electronics handle the high power dividing the high voltage and the required switching rate into small portions.
Article
The characteristics of transcranial magnetic stimulation (TMS) pulses influence the physiological effect of TMS. However, available TMS devices allow very limited adjustment of the pulse parameters. We describe a novel TMS device that uses a circuit topology incorporating two energy storage capacitors and two insulated-gate bipolar transistor (IGBT) modules to generate near-rectangular electric field pulses with adjustable number, polarity, duration, and amplitude of the pulse phases. This controllable pulse parameter TMS (cTMS) device can induce electric field pulses with phase widths of 10-310 µs and positive/negative phase amplitude ratio of 1-56. Compared to conventional monophasic and biphasic TMS, cTMS reduces energy dissipation up to 82% and 57% and decreases coil heating up to 33% and 41%, respectively. We demonstrate repetitive TMS trains of 3000 pulses at frequencies up to 50 Hz with electric field pulse amplitude and width variability less than the measurement resolution (1.7% and 1%, respectively). Offering flexible pulse parameter adjustment and reduced power consumption and coil heating, cTMS enhances existing TMS paradigms, enables novel research applications and could lead to clinical applications with potentially enhanced potency.
An intense impulse noise artifact is generated by the coil used in extracranial magnetic stimulation (EMS) of the brain and cranial nerves. In this study we measured and analyzed the sound pressure level (SPL), spectral content, wave form, and time course of the magnetic coil acoustic artifact (MCAA) impulse noise in the sound field and in the ear canal of life-size models of the human cranium. Two different clinical magnetic stimulators and coils were used. Sound field measurements from both coils showed the MCAA to be a transient impulse noise with a rapid rise-time, brief duration, broad acoustic spectrum, and high intensity. Measurements made on models of the human head with the magnetic coils positioned at selected standard clinical positions for EMS, particularly the peripheral facial nerve, auricle and mastoid areas, indicated that the MCAA may reach sound pressure levels that exceed noise damage-risk criteria limits for sensorineural hearing loss. The maximum peak energy in the acoustic spectrum of the MCAA measured in the ear canal of the model heads was from 2 to 5 kHz, the range of highest sensitivity in human ears. Ear protectors were found to attenuate the SPL of the MCAA, reaching the ear canal of the model heads by 15-22 dB SPL, and were recommended for use by patients and subjects exposed to EMS.
We describe the first investigation into the effect on stimulation efficiency of varying the output of a commercial magnetic stimulator based on our original clinical design. Over the range of magnetic field waveforms considered, it is shown that the stored energy required to achieve stimulation, both cortically and in the periphery, varies by approximately 2:1. Greater efficiency is obtained by using shorter risetime magnetic fields. This results in more effective stimuli for the same stored energy, or, for the same stimulus, a decrease in energy storage, power dissipation and peak currents, thus simplifying hardware design. A novel method of processing the data obtained from different waveforms is presented which enables neural membrane time constant to be calculated. Data from normal subjects is presented showing both peripheral and neural time constants to be of order 150 microseconds. The cortical measurements represent the first non-invasive determination of cortical membrane time constant in man. Time constant measurements using magnetic stimulation may be clinically useful because they give information concerning the electrical properties of the nervous system not available from present techniques. Finally a method of quantifying the output of magnetic stimulators and coils is described which enables laboratory comparisons to be made, and takes into account magnetic field waveforms and coil geometry. The proposed symbol for this new measurement is Et150 with units volt seconds/meter.
Article
The noise generated by stimulating coils may jeopardize the hearing of the patients as well as the hearing of the examiner. To evaluate the potential risk caused by the impulse noise of stimulating coils, we examined the A-weighted peak sound pressure levels from five different types of magnetic stimulator coils. At a distance of 10 cm, with 100% stimulation intensity, the coils with Dantec and Magstim stimulators created maximum peak sound pressure levels of 110 dB. Correspondingly, Cadwell MES-10 created maximum peak sound pressure levels of 132 dB. The decrease in the peak levels followed the distance rule quite closely. At a distance of 40 cm, the decrease in peak level was on average 14 dB (range -1-(+)1 dB). Based on American Conference of Governmental Industrial Hygienists (ACGIH) threshold limits of impact noise, the permitted maximum daily number of magnetic stimuli would be 1000 to 10,000. The permitted number of daily stimuli may be difficult to exceed in clinical practice. We consider the risk as small for the patients that are being examined and the operator using magnetic stimulation. The potential risk can be further diminished by even very light weighted hearing protectors providing proper attenuation to the coil impulses.
Article
A novel transcranial magnetic stimulation (TMS) device with controllable pulse width (PW) and near-rectangular pulse shape (cTMS) is described. The cTMS device uses an insulated gate bipolar transistor (IGBT) with appropriate snubbers to switch coil currents up to 6 kA, enabling PW control from 5 mus to over 100 mus. The near-rectangular induced electric field pulses use 2%-34% less energy and generate 67%-72% less coil heating compared to matched conventional cosine pulses. CTMS is used to stimulate rhesus monkey motor cortex in vivo with PWs of 20 to 100 mus, demonstrating the expected decrease of threshold pulse amplitude with increasing PW. The technological solutions used in the cTMS prototype can expand functionality, and reduce power consumption and coil heating in TMS, enhancing its research and therapeutic applications.