Related Experiment Video
Updated: May 6, 2026
Localizing Function-specific Targets for Transcranial Magnetic Stimulation in the Absence of Navigation Equipment
Published on: May 23, 2025
Targeting of white matter tracts with transcranial magnetic stimulation
Aapo Nummenmaa1, Jennifer A McNab2, Peter Savadjiev3
1MGH/MIT/HMS Athinoula A. Martinos Center for Biomedical Imaging, MA, USA; Harvard Medical School, MA, USA.
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.
