Optimal coil orientation in transcranial magnetic stimulation of the hand motor area: integration of experimental and computational analyses

<i>Objective.</i>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.<i>Approach.</i>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 scalar-potential finite-difference method to quantify the EF magnitude in the M1-hand region across orientations.<i>Main results.</i>The lowest RMTs were obtained between 30° and 45° for both muscles, and the optimal angle depended on the target muscle (44.3 ± 9.8° for FDI and 37.9 ± 10.3° for ADM). Calculated EF magnitudes correlated negatively with the measured RMT values. Analysis of the additional 135 MRI-derived head models showed that coil orientations of 30°-45° most frequently produced high EF, and this coil-angle dependence remained stable even when the coil position was slightly displaced. Anatomical analysis revealed that the cortical surface orientations in the M1-hand area were frequently 30°-45° to the parasagittal plane.<i>Significance.</i>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.