title: Experimental Characterization of the Electric Field Distribution Induced by TMS Devices.
abstract: BACKGROUND
  In transcranial magnetic stimulation (TMS) a strong, brief current pulse driven through a coil is used for non-invasively stimulating the cortex. Properties of the electric field (E-field) induced by the pulse together with physiological parameters determine the outcome of the stimulation. In research and clinical use, TMS is delivered using a wide range of different coils and stimulator units, all having their own characteristics; however, the parameters of the induced E-field are often inadequately known by the user.
  
  
  OBJECTIVE
  To better understand how the use of a specific TMS device may affect the resulting cortical stimulation, our objective was to develop an instrument for automated measurement of the E-fields induced by TMS coils in spherically symmetric conductors approximating the head.
  
  
  METHODS
  We built a saline-free, robotized measurement tool based on the triangle construction. The 5-mm-wide measurement probe allows complete sampling of the induced E-field at the studied depth. We used the instrument to characterize TMS coils and stimulators made by two companies.
  
  
  RESULTS
  The measurements revealed that all tested stimulators performed as expected, but we also found significant differences between the different stimulators. Measurements of different coil specimens of the same stimulator models agreed with each other.
  
  
  CONCLUSIONS
  The presented TMS calibrator allows a straightforward characterization of the E-fields induced by TMS coils. By performing measurements using this kind of a tool helps in ensuring that an investigator knows the properties of the E-field.
authors: Department of Neuroscience and Biomedical Engineering, Aalto University School of Science, Espoo, Finland; BioMag Laboratory, HUS Medical Imaging Center, University of Helsinki and Helsinki University Hospital, Helsinki, Finland; Aalto TMS Laboratory, Aalto Neuroimaging, Aalto University, Espoo, Finland; Department of Psychiatry, University of Wisconsin, Madison, WI, USA. Electronic address: jaakko.nieminen@aalto.fi.
authors: Jaakko O
authors: JO
authors: Nieminen
authors: Department of Neuroscience and Biomedical Engineering, Aalto University School of Science, Espoo, Finland; BioMag Laboratory, HUS Medical Imaging Center, University of Helsinki and Helsinki University Hospital, Helsinki, Finland; Aalto TMS Laboratory, Aalto Neuroimaging, Aalto University, Espoo, Finland.
authors: Lari M
authors: LM
authors: Koponen
authors: Department of Neuroscience and Biomedical Engineering, Aalto University School of Science, Espoo, Finland; BioMag Laboratory, HUS Medical Imaging Center, University of Helsinki and Helsinki University Hospital, Helsinki, Finland; Aalto TMS Laboratory, Aalto Neuroimaging, Aalto University, Espoo, Finland.
authors: Risto J
authors: RJ
authors: Ilmoniemi
mesh_terms: D004574:Electromagnetic Fields; D006257:Head; D006801:Humans; D008962:Models, Theoretical; D050781:Transcranial Magnetic Stimulation / Q000379:methods*
keywords: Calibration; Coil; Electric field; Focality; Transcranial magnetic stimulation; Triangle construction