Aapo Nummenmaa1, Jennifer A McNab2, Peter Savadjiev3, Yoshio Okada4, Matti S Hämäläinen5, Ruopeng Wang1, Lawrence L Wald5, Alvaro Pascual-Leone6, Van J Wedeen1, Tommi Raij7. 1. MGH/MIT/HMS Athinoula A. Martinos Center for Biomedical Imaging, MA, USA; Harvard Medical School, MA, USA. 2. MGH/MIT/HMS Athinoula A. Martinos Center for Biomedical Imaging, MA, USA; Harvard Medical School, MA, USA; Department of Radiology, Stanford University, CA, USA. 3. Harvard Medical School, MA, USA; Brigham and Women's Hospital, MA, USA. 4. Harvard Medical School, MA, USA; Department of Neurology, Boston Children's Hospital, MA, USA. 5. 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. 6. Harvard Medical School, MA, USA; Berenson-Allen Center for Noninvasive Brain Stimulation, Beth Israel Deaconess Medical Center, MA, USA. 7. MGH/MIT/HMS Athinoula A. Martinos Center for Biomedical Imaging, MA, USA; Harvard Medical School, MA, USA. Electronic address: raij@nmr.mgh.harvard.edu.
Abstract
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/ METHODS: 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.
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/ METHODS: Here, we utilized 1-mm resolution diffusion and structural T1-weighted MRI to construct large-scale tractography models, and localized TMSwhite 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.
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