Targeting of white matter tracts with transcranial magnetic stimulation

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.

Keywords: Coil orientation; DTI; Diffusion MRI tractography; Electromagnetic modeling; MRI; Navigation; TMS; Transcranial magnetic stimulation; diffusion tensor imaging; magnetic resonance imaging; transcranial magnetic stimulation.