Targeting of white matter tracts with transcranial magnetic stimulation.
BACKGROUND
TMS activations of white matter depend not only on the distance from the coil, but also on the orientation of the axons relative to the TMS-induced electric field, and especially on axonal bends that create strong local field gradient maxima. Therefore, tractography contains potentially useful information for TMS targeting.


OBJECTIVE
Here, we utilized 1-mm resolution diffusion and structural T1-weighted MRI to construct large-scale tractography models, and localized TMS white matter activations in motor cortex using electromagnetic forward modeling in a boundary element model (BEM).


RESULTS
As expected, in sulcal walls, pyramidal cell axonal bends created preferred sites of activation that were not found in gyral crowns. The model agreed with the well-known coil orientation sensitivity of motor cortex, and also suggested unexpected activation distributions emerging from the E-field and tract configurations. We further propose a novel method for computing the optimal coil location and orientation to maximally stimulate a pre-determined axonal bundle.


CONCLUSIONS
Diffusion MRI tractography with electromagnetic modeling may improve spatial specificity and efficacy of TMS.
MGH/MIT/HMS Athinoula A. Martinos Center for Biomedical Imaging, MA, USA; Harvard Medical School, MA, USA.
Aapo
A
Nummenmaa
MGH/MIT/HMS Athinoula A. Martinos Center for Biomedical Imaging, MA, USA; Harvard Medical School, MA, USA; Department of Radiology, Stanford University, CA, USA.
Jennifer A
JA
McNab
Harvard Medical School, MA, USA; Brigham and Women's Hospital, MA, USA.
Peter
P
Savadjiev
Harvard Medical School, MA, USA; Department of Neurology, Boston Children's Hospital, MA, USA.
Yoshio
Y
Okada
MGH/MIT/HMS Athinoula A. Martinos Center for Biomedical Imaging, MA, USA; Harvard Medical School, MA, USA; Harvard-MIT Division of Health Sciences and Technology, MA, USA.
Matti S
MS
Hämäläinen
MGH/MIT/HMS Athinoula A. Martinos Center for Biomedical Imaging, MA, USA; Harvard Medical School, MA, USA.
Ruopeng
R
Wang
MGH/MIT/HMS Athinoula A. Martinos Center for Biomedical Imaging, MA, USA; Harvard Medical School, MA, USA; Harvard-MIT Division of Health Sciences and Technology, MA, USA.
Lawrence L
LL
Wald
Harvard Medical School, MA, USA; Berenson-Allen Center for Noninvasive Brain Stimulation, Beth Israel Deaconess Medical Center, MA, USA.
Alvaro
A
Pascual-Leone
MGH/MIT/HMS Athinoula A. Martinos Center for Biomedical Imaging, MA, USA; Harvard Medical School, MA, USA.
Van J
VJ
Wedeen
MGH/MIT/HMS Athinoula A. Martinos Center for Biomedical Imaging, MA, USA; Harvard Medical School, MA, USA. Electronic address: raij@nmr.mgh.harvard.edu.
Tommi
T
Raij
D001931:Brain Mapping / Q000379:methods; D056324:Diffusion Tensor Imaging; D006801:Humans; D009044:Motor Cortex / Q000502:physiology*; D009413:Nerve Fibers, Myelinated / Q000502:physiology*; D009434:Neural Pathways / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation / Q000379:methods*
Coil orientation; DTI; Diffusion MRI tractography; Electromagnetic modeling; MRI; Navigation; TMS; Transcranial magnetic stimulation; diffusion tensor imaging; magnetic resonance imaging; transcranial magnetic stimulation