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Koponen, L. M.

Publications and source records attributed to Koponen, L. M..

2 recordsLinked to original sources
bioRxiv2019

Sound comparison of seven TMS coils at matched simulation strength

BackgroundAccurate data on the sound emitted by transcranial magnetic stimulation (TMS) coils is lacking.\n\nMethodsWe 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.\n\nResultsAcross 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.\n\nConclusionsThe 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.\n\nHighlights O_LICoil click varies by over 20 dB(Z) between TMS coils at matched stimulation strength.\nC_LIO_LIClose to TMS coil, sound pressure level may reach nearly 140 dB(Z).\nC_LIO_LIFor rTMS, the continuous sound level can exceed 110 dB(A).\nC_LIO_LIHearing protection is recommended during TMS, especially for the subject.\nC_LI

neuroscience↗
bioRxiv2019

Real-time computation of the TMS-induced electric field in a realistic head model

BackgroundTranscranial magnetic stimulation (TMS) is often targeted using a model of TMS-induced electric field (E). In such navigated TMS, the E-field models have been based on spherical approximation of the head. Such models omit the effects of cerebrospinal fluid (CSF) on the E-field, leading to potentially large errors in the computed field. So far, realistic models have been too slow for interactive TMS navigation. ObjectiveWe present computational methods that enable real-time solving of the E-field in a realistic head model that contains the CSF. MethodsUsing reciprocity and Geselowitz integral equation, we separate the computations to coil-dependent and -independent parts. For the coil-dependent part of Geselowitz integrals, we present a fast numerical quadrature. Further, we present a moment-matching approach for optimizing dipole-based coil models. We verify the new methods using simulations in a realistic head model that contains the brain, CSF, skull, and scalp. ResultsThe new quadrature introduces a relative error of 1.1%. The total error of the quadrature and coil model was 1.43% and 1.15% for coils with 38 and 76 dipoles, respectively. The difference between our head model and a simpler realistic model that omits the CSF was 29%. Using a standard PC and a 38-dipole coil, our solver computed the E-field in 84 coil positions per second in 20000 points on the cortex. ConclusionThe presented methods enable real-time solving of the TMS-induced E-field in a realistic head model that contains the CSF. The new methodology allows more accurate targeting and precise adjustment of intensity during experimental or clinical TMS mapping.

neuroscience↗