title: The coil orientation dependency of the electric field induced by TMS for M1 and other brain areas.
abstract: BACKGROUND
  The effectiveness of transcranial magnetic stimulation (TMS) depends highly on the coil orientation relative to the subject's head. This implies that the direction of the induced electric field has a large effect on the efficiency of TMS. To improve future protocols, knowledge about the relationship between the coil orientation and the direction of the induced electric field on the one hand, and the head and brain anatomy on the other hand, seems crucial. Therefore, the induced electric field in the cortex as a function of the coil orientation has been examined in this study.
  
  
  METHODS
  The effect of changing the coil orientation on the induced electric field was evaluated for fourteen cortical targets. We used a finite element model to calculate the induced electric fields for thirty-six coil orientations (10 degrees resolution) per target location. The effects on the electric field due to coil rotation, in combination with target site anatomy, have been quantified.
  
  
  RESULTS
  The results confirm that the electric field perpendicular to the anterior sulcal wall of the central sulcus is highly susceptible to coil orientation changes and has to be maximized for an optimal stimulation effect of the motor cortex. In order to obtain maximum stimulation effect in areas other than the motor cortex, the electric field perpendicular to the cortical surface in those areas has to be maximized as well. Small orientation changes (10 degrees) do not alter the induced electric field drastically.
  
  
  CONCLUSIONS
  The results suggest that for all cortical targets, maximizing the strength of the electric field perpendicular to the targeted cortical surface area (and inward directed) optimizes the effect of TMS. Orienting the TMS coil based on anatomical information (anatomical magnetic resonance imaging data) about the targeted brain area can improve future results. The standard coil orientations, used in cognitive and clinical neuroscience, induce (near) optimal electric fields in the subject-specific head model in most cases.
authors.affiliation: Department of Neurology, Radboud University Medical Centre, Donders Institute for Brain, Cognition and Behaviour, Reinier Postlaan 4, 6525 CG, Nijmegen, The Netherlands. Arno.Janssen@radboudumc.nl.
authors.forename: Arno M
authors.initials: AM
authors.lastname: Janssen
authors.affiliation: Department of Cognitive Neuroscience, Radboud University Medical Centre, Donders Institute for Brain, Cognition and Behaviour, Nijmegen, The Netherlands. T.Oostendorp@donders.ru.nl.
authors.forename: Thom F
authors.initials: TF
authors.lastname: Oostendorp
authors.affiliation: Department of Neurology, Radboud University Medical Centre, Donders Institute for Brain, Cognition and Behaviour, Reinier Postlaan 4, 6525 CG, Nijmegen, The Netherlands. Dick.Stegeman@radboudumc.nl.
authors.forename: Dick F
authors.initials: DF
authors.lastname: Stegeman
mesh_terms: D001921:Brain / Q000502:physiology; D020342:Finite Element Analysis; D006801:Humans; D050781:Transcranial Magnetic Stimulation / Q000379:methods*