The coil orientation dependency of the electric field induced by TMS for M1 and other brain areas

Abstract

Background: The effectiveness of transcranial magnetic stimulation (TMS) depends highly on the coil orientation relative to the subject's head. This implies that the direction of the induced electric field has a large effect on the efficiency of TMS. To improve future protocols, knowledge about the relationship between the coil orientation and the direction of the induced electric field on the one hand, and the head and brain anatomy on the other hand, seems crucial. Therefore, the induced electric field in the cortex as a function of the coil orientation has been examined in this study.

Methods: The effect of changing the coil orientation on the induced electric field was evaluated for fourteen cortical targets. We used a finite element model to calculate the induced electric fields for thirty-six coil orientations (10 degrees resolution) per target location. The effects on the electric field due to coil rotation, in combination with target site anatomy, have been quantified.

Results: The results confirm that the electric field perpendicular to the anterior sulcal wall of the central sulcus is highly susceptible to coil orientation changes and has to be maximized for an optimal stimulation effect of the motor cortex. In order to obtain maximum stimulation effect in areas other than the motor cortex, the electric field perpendicular to the cortical surface in those areas has to be maximized as well. Small orientation changes (10 degrees) do not alter the induced electric field drastically.

Conclusions: The results suggest that for all cortical targets, maximizing the strength of the electric field perpendicular to the targeted cortical surface area (and inward directed) optimizes the effect of TMS. Orienting the TMS coil based on anatomical information (anatomical magnetic resonance imaging data) about the targeted brain area can improve future results. The standard coil orientations, used in cognitive and clinical neuroscience, induce (near) optimal electric fields in the subject-specific head model in most cases.

Figures

Figure 1

Realistic head model: (A) A sagittal cut plane of the T2 weighted MRI showing the different skull layers. (B) The same sagittal cut plane of the manually corrected segmentation including skin, skull compacta, skull spongiosa, neck muscle, eyes and one compartment for inner skull (CSF, GM and WM, before segmentation with Freesurfer). (C) High resolution triangular surface meshes of GM (transparent) and WM (red), constructed with Freesurfer. (D) Sagittal cut plane of the final tetrahedral volume mesh created with TetGen. The different tissue types are represented with different colors.

Figure 2

Electric field for three cortical locations: The electric field distribution (V m−1), just within the cortex for three locations. On the top row the field strengths E→ for (A) the right motor cortex (MR), (B) the left premotor cortex (PML) and (C) the supplementary motor area 3 cm anterior to Cz (SM1) are displayed. In the bottom row the field strengths perpendicular to the CSF-GM boundary E⊥ are shown for (D) MR, (E) PML and (F) SM1. For the later scale, a positive value means directed inward and a negative means directed outward. The black dot indicates the location of the center of the TMS coil. The direction of the primary electric field directly under the coil center is indicated with the black arrow.

Figure 3

Electric field for five coil orientations over M1: The electric field distribution (V m−1), just within the cortex, for M1 stimulation with the standard coil orientation (1st column) and 4 other orientations (+40 (2nd column), +90 (3rd column), +150 (4th column) and +180 (5th column) degrees of clockwise rotation). The field strength E→ (top row) and the field strength perpendicular to the CSF-GM boundary E⊥ (bottom row) are shown. For the later scale, a positive value means directed inward and a negative means directed outward. The black dot indicates the location of the center of the TMS coil. The direction of the primary electric field directly under the coil center is indicated with the black arrow.

Figure 4

Mean electric field strength in target region M1: The mean electric field values for (A)E→ and (B) E⊥ within the target region M1. The standard coil orientation from literature is indicated separately in both panels (red circle with cross). The coil is rotated in steps of 10 degrees.

Figure 5

Optimal coil orientation for all target locations: The optimized electric field perpendicular to the CSF-GM boundary E⊥ (V m−1), just within the cortex, for all fourteen cortical target locations (Table 1). The cortical location index from Table 1 is shown in every right bottom corner. A positive value means directed inward and a negative means directed outward. The black dot indicates the location of the center of the TMS coil. The direction of the primary electric field directly under the coil center is indicated with the black arrow for the optimized coil orientation. The green arrow indicates direction of the primary electric field for the standard coil orientation.

Figure 6

Cortical column in sulcal wall: A simplified schematic representation of the cortical column in the sulcal wall. Included are neural elements (P2, P3, P5) that are possibly stimulated by the electric field component aligned with the axis of the cortical column. The electric fields perpendicular (Eperp) and tangential (Etan) to the sulcal wall are represented by red arrows.