Optimal positioning of TMS coil using DTI

This paper is aimed to determine the optimal direction of Transcranial magnetic stimulation (TMS) coil using computer simulation. We used T1-weighted structural MR and diffusion tensor images of a healthy subject to construct a realistic anisotropic head model. Using this model for 16 different coil directions, equations governing TMS were solved for stimulating primary motor cortex. The pyramidal tract fibers were extracted from DTI images using fiber tracking and the distribution of electric field was obtained for them. Based on amplitude and direction of electric field vector in voxels corresponding to pyramidal tract neurons, the optimal coil orientation was determined. To the best of our knowledge, this is the first study that determines the optimal coil orientation using fibers extracted from DTI and computer simulation. Our findings show that by directing TMS coil in the range of 67.5°±11.25°, we can obtain the maximum stimulation in primary motor cortex. For validating our results, we compared them with results obtained by other experimental studies. We show that our simulation results have a large overlap with the experimental results.