Transcranial Magnetic Stimulation: Investigations for Novel Coil Designs

Abstract

Transcranial Magnetic Stimulation (TMS) is a non-invasive neuromodulation technique that applies electric field pulses to the brain via a stimulation coil placed over the scalp. TMS is approved by the Food and Drug Administration (FDA) to treat neuropsychiatric disorders such as depression, migraine, and obsessive-compulsive disorder. For targeted brain stimulations, traditional TMS coils follow a conventional depth-spread tradeoff rule. Since many diseases originate in the sub-cortical areas, significant TMS depth-spread performance improvement is essential. Moreover, to understand both the disease and treatment mechanisms, multi-site simultaneous or sequential stimulations may be required. Existing TMS coils, like figure-8 and circular coils, occupy a large footprint. It is also difficult to operate simultaneously more than two coils on one human head and reach the desired multiple stimulation locations with close proximity for comprehensive multi-site stimulation. In this work, a novel coil design is developed to improve the spread and penetration depth and occupy a much smaller footprint than existing commercial coils by creating a more elliptical emitted field distribution from the coil. The proposed angle-tuned and multi-stacked ring coils (AT coils) were theoretically designed and evaluated through FEM analysis. This design allows TMS tools to accomplish significantly improved depth-spread performance for deep and targeted brain stimulation. We have also developed TMS tools with a significantly small stimulation spot size using ferromagnetic core-based coils with in-vivo tests on an anesthetized rat. Multiple AT coils were implemented theoretically and compared with multi-site conventional coils for multi-site stimulation. The results demonstrated promising capabilities from the proposed coils for this purpose due to their smaller footprint and improved performance. Utilizing AT coils for multi-site stimulation can help comprehend different brain systems and enhance the treatment of neural disorders. In addition, Multiple AT coils were used to form composite coil structures to further improve the TMS coils' performance. Using multiple AT coils with opposite polarities can create a symmetric structure and produce a more elliptical field distribution. A wide variety of coil arrangements were studied using FEM simulations. The designed coils were experimentally implemented and tested using 3-D vector field measurements to verify their performance.