Coil Design for Neuromuscular Magnetic Stimulation Based on a Detailed 3-D Thigh Model

Magnetic stimulation is gaining importance as an alternative to electrical stimulation in neurorehabilitation because it offers deep penetration and low pain. However, presently available equipment is not ideal for this purpose and sometimes even unsuitable. Furthermore, it is not known what physical conditions a coil has to provide for efficient stimulation. To solve these two problems, we set up a detailed 3-D computational model of the thigh to evaluate various coil designs with previously reported experimental performance. Comparison of the stimulation results with known experimental performance shows that a high absolute electric field seems to be sufficient for effective stimulation. Coil-generated field gradients, in contrast, do apparently not play a role. As a more appropriate metric, the electromagnetic coupling between different coil designs and the muscles of the thigh is found to be characteristic and explains the previously reported experimental performance differences. Furthermore, it is a good predictor for the achievable muscle torque (more than 99% correlation), whereas the gradient fails in this context. Accordingly, the model is able to test the performance of coils virtually and reveal both general relationships as well as design rules. In addition, the coupling factor formalism predicts a theoretical maximum level of the possible field induction and thus a guideline for potential improvements in the future.