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Comparative Study
. 2020 May-Jun;13(3):873-880.
doi: 10.1016/j.brs.2020.03.004. Epub 2020 Mar 12.

Sound comparison of seven TMS coils at matched stimulation strength

Lari M Koponen  1 , Stefan M Goetz  2 , Debara L Tucci  3 , Angel V Peterchev  4
Affiliations
Comparative Study

Sound comparison of seven TMS coils at matched stimulation strength

Lari M Koponen et al. Brain Stimul. 2020 May-Jun.

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 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.

Keywords: Coil click; Hearing safety; Sound; Stimulation strength; TMS; Transcranial magnetic stimulation.

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Conflict of interest statement

Declaration of competing interest L. M. Koponen, S. M. Goetz, and A. V. Peterchev are inventors on patents and patent applications on TMS technology including TMS devices with reduced acoustic noise. S. M. Goetz has received research funding from Magstim Inc. Related to TMS technology, A. V. Peterchev has received research and travel support as well as patent royalties from Rogue Research; research and travel support, consulting fees, as well as equipment donation from Tal Medical/Neurex; patent application and research support from Magstim; equipment loans and hardware donations from MagVenture; and expert witness compensation from Neuronetics.

Figures

Fig. 1.
Fig. 1.
E-field and sound characteristics of various measured TMS coils. (A) Peak E-field value has approximately linear relationship with stimulator output. (B) Pulse durations of air-core coils are approximately constant, whereas pulse duration of NeuroStar iron-core coil starts to drop after about 30% MSO. (C) Stimulation strength estimated with neural model, where 50.3% MSO of Magstim 70mm Double Coil is set to correspond to 100% RMT. (D) Peak sound pressure level (SPL) with Z-weighting, i.e., flat response curve. (E) Duration for which sound is within 20 dB of peak value. (F) Simulated sound level of a 20 Hz rTMS pulse train with A-weighting (slow time weighting).
Fig. 2.
Fig. 2.
1/3-octave spectra of tested coils at estimated 100% RMT (solid line) and at 100% MSO (dash-dotted line). Solid black line shows ambient sound spectrum. The spectra below 300 Hz are affected by the acoustics of the soundproof chamber (for more detail, see Supplementary material). For all coils, spectra power peak is between 1000 and 10,000 Hz, i.e.,within hearing range (20e20,000 Hz). Spectra at 100% RMT were produced from closest measured stimulator output by assuming that sound waveform is proportional to pulse energy. Due to the 0.2 s time window for each sample, the sound level corresponds to that of a 5 Hz rTMS pulse train.
Fig. 3.
Fig. 3.
For each tested coil, the rTMS pulse repetition rate corresponding to an estimated airborne sound level of 85 dB(A) for the subject as a function of stimulation strength (in % of average RMT).
See this image and copyright information in PMC

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