Coils are not created equal: Effects on TMS thresholding

摘要

Transcranial magnetic stimulation (TMS) operators commonly use different figure-of-8 coils for assessing cortical excitability (e.g., motor evoked potential (MEP) and resting motor threshold (rMT)) and delivering therapeutic treatment (e.g., repetitive TMS (rTMS)). Distinct assessment and treatment coils with compatible stimulators are designed to balance coil cooling needs with maneuverability. While many factors (e.g., current direction, waveform, coil size) are known to alter rMT values across coil-stimulator settings [1Brasil-Neto J.P. et al.Optimal focal transcranial magnetic activation of the human motor cortex: effects of coil orientation, shape of the induced current pulse, and stimulus intensity.J Clin Neurophysiol. 1992; 9: 132-136Crossref PubMed Google Scholar, 2Pascual-Leone A. Cohen L.G. Brasil-Neto J.P. Hallett M. Non-invasive differentiation of motor cortical representation of hand muscles by mapping of optimal current directions.Electroencephalogr Clin Neurophysiology Evoked Potentials Sect. 1994; 93: 42-48Abstract Full Text PDF PubMed Scopus (74) Google Scholar, 3Davila-Pérez P. Jannati A. Fried P.J. Cudeiro Mazaira J. Pascual-Leone A. The effects of waveform and current direction on the efficacy and test-retest reliability of transcranial magnetic stimulation.Neuroscience. 2018; 393: 97-109Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar, 4Delvendahl I. et al.The role of pulse shape in motor cortex transcranial magnetic stimulation using full-sine stimuli.PLoS One. 2014; 9e115247Crossref PubMed Scopus (24) Google Scholar, 5Kammer T. Beck S. Thielscher A. Laubis-Herrmann U. Topka H. Motor thresholds in humans: a transcranial magnetic stimulation study comparing different pulse waveforms, current directions and stimulator types.Clin Neurophysiol. 2001; 112: 250-258Crossref PubMed Scopus (329) Google Scholar, 6Makarov S.N. et al.Degree of improving TMS focality through a geometrically stable solution of an inverse TMS problem.Neuroimage. 2021; 241118437Crossref Scopus (5) Google Scholar], it remains unclear the extent to which these differences should be accounted for when using both assessment and treatment equipment in the same protocol, given that the rMT is necessary to determine parameters for treatment. We sought to determine if a set of standard conversion factors could be used when coils and associated stimulator setup are switched within the same session. We investigated within-subject rMT differences between assessment and treatment equipment from two of the most widely used TMS systems in research and clinical settings, MagStim (MagStim, Whitland, UK) and MagVenture (MagVenture A/S, Farum, Denmark). Thirty-eight healthy participants were recruited and enrolled. Sixteen (mean age±SD = 26.9 ± 4.0; 10 females) were assessed with MagStim system (“MagStim” group); seventeen (mean age±SD = 25.1 ± 1.8; 11 females) with MagVenture system (“MagVenture” group) and five (mean age±SD = 29.6 ± 2.8; 2 females) with both MagStim and MagVenture systems (“Crossover” group). Power analysis for the primary comparison was based on pilot MagStim data to estimate the sufficient sample size within a TMS system (Supplementary Materials). The Crossover group was a sample of convenience. Inclusion and exclusion criteria were based on standard TMS safety guidelines [[7]Rossi S. et al.Safety and recommendations for TMS use in healthy subjects and patient populations, with updates on training, ethical and regulatory issues: expert Guidelines.Clin Neurophysiol. 2021; 132: 269-306Crossref PubMed Scopus (400) Google Scholar]. The study was approved by the Mass General Brigham Institutional Review Board and all participants provided written informed consent. During TMS procedures, participants were seated comfortably in a semi-reclined position. Neuronavigation was used to ensure consistent coil placement (Supplementary Table 1). As magnetic resonance imaging scans were not collected, a template T1 was co-registered