title: PET-based confirmation of orientation sensitivity of TMS-induced cortical activation in humans.
abstract: BACKGROUND
  Currently, it is difficult to predict precise regions of cortical activation in response to transcranial magnetic stimulation (TMS). Most analytical approaches focus on applied magnetic field strength in the target region as the primary factor, placing activation on the gyral crowns. However, imaging studies support M1 targets being typically located in the sulcal banks.
  
  
  OBJECTIVE
  To more thoroughly investigate this inconsistency, we sought to determine whether neocortical surface orientation was a critical determinant of regional activation.
  
  
  METHODS
  MR images were used to construct cortical and scalp surfaces for 18 subjects. The angle (θ) between the cortical surface normal and its nearest scalp normal for ~50,000 cortical points per subject was used to quantify cortical location (i.e., gyral vs. sulcal). TMS-induced activations of primary motor cortex (M1) were compared to brain activations recorded during a finger-tapping task using concurrent positron emission tomographic (PET) imaging.
  
  
  RESULTS
  Brain activations were primarily sulcal for both the TMS and task activations (P < 0.001 for both) compared to the overall cortical surface orientation. Also, the location of maximal blood flow in response to either TMS or finger-tapping correlated well using the cortical surface orientation angle or distance to scalp (P < 0.001 for both) as criteria for comparison between different neocortical activation modalities.
  
  
  CONCLUSIONS
  This study provides further evidence that a major factor in cortical activation using TMS is the orientation of the cortical surface with respect to the induced electric field. The results show that, despite the gyral crown of the cortex being subjected to a larger magnetic field magnitude, the sulcal bank of M1 had larger cerebral blood flow (CBF) responses during TMS.
authors.affiliation: Department of Biomedical Engineering, Illinois Institute of Technology, Wishnick Hall 314, 3255 S. Dearborn St., Chicago, IL 60616, USA.
authors.forename: Todd D
authors.initials: TD
authors.lastname: Krieg
authors.forename: Felipe S
authors.initials: FS
authors.lastname: Salinas
authors.forename: Shalini
authors.initials: S
authors.lastname: Narayana
authors.forename: Peter T
authors.initials: PT
authors.lastname: Fox
authors.forename: David J
authors.initials: DJ
authors.lastname: Mogul
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D009044:Motor Cortex / Q000033:anatomy & histology* / Q000000981:diagnostic imaging / Q000502:physiology*; D049268:Positron-Emission Tomography; D050781:Transcranial Magnetic Stimulation / Q000379:methods*
keywords: Cerebral blood flow; Cortex; MRI; Orientation; PET; TMS