Modelling of the Electromagnetic Field Distributions Induced by Different Transcranial Magnetic Stimulation Coil Configurations

As a brain modulation technology, transcranial magnetic stimulation(TMS) was favored by its particularly advantages such as non-contact, painless, variety stimulation options. There are two TMS coil configurations which was widely used in the clinical research about neurological and psychiatric disorders, and they are the conventional circular coil and figure-8 coil. Based on the same crafts, six different coil configurations with different loop offset distances between the two were proposed in this paper. For the characterization of magnetic and electric field distributions, finite element method was used in an isotropic human brain model and the penetration depth and stimulation focality were regarded as the two most important indicators. Our results support that the figure-S coil has better focality and penetration depth than conventional circular coil in the circumstance where same current pulse was delivered. As the increasing of loop offset distance, there is an obvious law in the focality of induced electric field. The coil which loop offset distance equals to 20 mm has the best focality. A tradeoff between penetration depth and stimulation focality of electric field induced by the six coil configurations was discovered. For the selection of coil setting and the design of novel coil configuration, the evaluation between these two different needs is considerable.