Sound comparison of seven TMS coils at matched stimulation strength

Abstract

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 75-97 dB(A) for a 15 Hz pulse train. At 5 cm distance, these values were estimated to increase to 112-139 dB(Z) and 89-111 dB(A), respectively.Conclusions: The coils' sound was below, but near, relevant exposure limits for operators and may exceed some limits for the subject. Exposure standards may inadequately capture some risks to hearing. For persons near operating TMS coils we recommend hearing protection, and we consider it essential for the TMS subject. Highlights 1. Coil click varies by over 20 dB(Z) between TMS coils at matched stimulation strength. 2. Close to TMS coil, sound pressure level may reach nearly 140 dB(Z). 3. For rTMS, the continuous sound level can exceed 110 dB(A). 4. Hearing protection is recommended during TMS, especially for the subject.

Modular multilevel TMS device with wide output range and ultrabrief pulse capability for sound reduction

“…The coil current was measure with a commercial Rogowski current sensor. The electric field, E, was measured with a PCB-based single-turn search coil fixed on the MM-TMS coil [2]. The measurements were recorded with a digitizing oscilloscope.…”

“…Existing TMS devices, however, have several significant limitations. First, TMS pulse delivery is associated with a loud clicking sound that can be as high as 140 dB resulting from electromagnetic forces in the coil [2]. The loud noise presents a risk to the hearing of the TMS subject and the operator that necessitates the use of hearing protection [2][3][4].…”

“…First, TMS pulse delivery is associated with a loud clicking sound that can be as high as 140 dB resulting from electromagnetic forces in the coil [2]. The loud noise presents a risk to the hearing of the TMS subject and the operator that necessitates the use of hearing protection [2][3][4]. Further, by evoking an auditory response synchronized with the electromagnetic stimulus, the sound compromises the spatial localization of the stimulation effects, impacts neuromodulation, and complicates blinding, significantly impeding both basic research and clinical applications of TMS.…”

“…Further, by evoking an auditory response synchronized with the electromagnetic stimulus, the sound compromises the spatial localization of the stimulation effects, impacts neuromodulation, and complicates blinding, significantly impeding both basic research and clinical applications of TMS. A contributing factor to the coil sound is the typical biphasic pulse duration of 150-400 µs, which corresponds to a vibration spectral peak at 5-13 kHz due to the coil winding electromagnetic forces [2,5]. Second, existing TMS devices have limited adjustability of their pulse characteristics [6,7].…”

Large-scale analysis of interindividual variability in single and paired-pulse TMS data: results from the ‘Big TMS Data Collaboration’

“…We found that Nexstim machines were more powerful than MagPro machines for single pulse MEP amplitude, yet observed higher biphasic RMT and biphasic AMT for the Magstim Rapid machine than MagPro and Nexstim machines. Much of this effect is likely due to the use of Magstim Rapid machines for biphasic MT assessment prior to repetitive TMS protocols (delivered with biphasic pulses), which have a reduced power output in comparison to Magstim 200 2 (Kammer et al 2001), and MagPro X100 machines (Koponen et al 2020). These differential effects highlight the importance of the inclusion of TMS machine (and study location if applicable) as a covariate in statistical analyses on data that are pooled collaboratively using different machines.…”