• Aapo Nummenmaa, Jennifer A McNab, Peter Savadjiev, Yoshio Okada, Matti S Hämäläinen, Ruopeng Wang, Lawrence L Wald, Alvaro Pascual-Leone, Van J Wedeen, and Tommi Raij.
• MGH/MIT/HMS Athinoula A. Martinos Center for Biomedical Imaging, MA, USA; Harvard Medical School, MA, USA.
• Brain Stimul. 2014 Jan 1;7(1):80-4.

BackgroundTMS 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/MethodsHere, 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).ResultsAs 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.ConclusionsDiffusion MRI tractography with electromagnetic modeling may improve spatial specificity and efficacy of TMS.Copyright © 2014 Elsevier Inc. All rights reserved.

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