Augmenting Transcranial Magnetic Stimulation Coil with Magnetic Material: An Optimization Approach

Transcranial magnetic stimulation (TMS) is a neuromodulation technique that has been approved by the U.S. Food and Drug Administration for several neuropsychiatric disorders, including major depression and obsessive-compulsive disorder. However, the therapeutic efficacy of TMS treatment has been modest, despite decades of research. While there are many potential reasons as to why, one of the most obvious is the limitations of current technologies. One prominent example is the penetration depth-focality tradeoff of existing TMS coils. The most widely used figure-of-8 coils stimulate brain regions just superficially under the coil, missing deep brain regions known to be critically involved in psychiatric disorders; while ring-type coils can stimulate deep into the brain, but stimulate a large brain volume (lack of focality). A new coil design strategy is proposed: magnetic materials encompassing the human head are optimized to shape the electromagnetic field generated by the primary coil. Specifically, a mathematical model was developed to describe the physical problem; the magnetic materials were discretized into unit blocks; Newtons gradient descent method was applied to iteratively optimize the spatial distribution of the unit blocks to achieve a desired electric field distribution inside a head model. Results reveal that the proposed design achieves a coil penetration depth equal to or better than state-of-the art commercial coils, while improving the depth-focality tradeoff by a factor of 2.2 to 2.7. As a proof-of-concept, a prototype coil and a spherical head model were constructed; the spatial distribution of the induced electric field inside the head model was mapped. Results validated the proposed coil design. TMS coils based on this novel design strategy could potentially lead to better therapeutic outcome.