Abstract Objective: This study aimed to determine the optimal coil orientation for transcranial magnetic stimulation (TMS) of the hand motor area by integrating physiological and computational approaches. Approach: Resting motor thresholds (RMTs) were measured in 10 healthy volunteers for the first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles when stimulating the primary motor cortex (M1) with a coil set at several orientations ranging from 0° to 90°. Electric field (EF) distributions were estimated using individualized head models constructed from magnetic resonance imaging (MRI) data of the same 10 participants in the measurements, as well as additional 135 MRI-derived models. Simulations employed a scalarpotential finite-difference method to quantify the EF strength in the M1-hand region across orientations. Main Results: The lowest RMTs were obtained between 30° and 45° for both muscles, and the optimal angle depended on the target muscle (45° for FDI and 30° for ADM). EF simulations supported the above RMT findings. It showed a maximum EF strength in the same angle range across all head models, with consistent angular dependence of RT despite small coil displacements. Anatomical analysis revealed that the cortical surface orientations in the M1-hand area were frequently 30°-45° to the parasagittal plane. Significance: These findings support the current guidelines' recommendation of a ~45° orientation, but suggest that a 30°-45° range better aligns EF with cortical geometry. Individualized optimization can further improve the precision and efficacy of TMS.
Nagata et al. (2026) studied this question.