Coil Orientation and Stimulation Threshold in Transcranial Magnetic Stimulation (TMS)

Transcranial Magnetic Stimulation (TMS) is used to treat mental disorders and explore brain function via applied electromagnetic fields generated by a high current coil placed on the subject's scalp. While TMS has been in clinical use for decades, and continues to be a rapidly expanding therapeutic modality, there are still many unknowns regarding how stimulation parameters may affect treatment efficacy and how they may be optimized. One key parameter that is readily accessible is coil orientation and its effects on TMS stimulation threshold. In this work, a multi-scale modeling approach addressed the effects of coil orientation on the TMS stimulation threshold for neuronal activation with several TMS coil current pulse shapes and widths. The modeling tool that was used incorporated an anatomically realistic macro scale model of the human brain cortex, and a micro scale neuronal model based on a representative layer 5 pyramidal brain cell. Simulations were performed for the left primary motor cortical area. Coil orientations associated with a minimum stimulation threshold for various current pulses were identified and then validated using data collected and published by another research team. This multi-scale modeling approach is versatile and can potentially be used for various current pulses and TMS coil locations to predict and map optimum TMS coil orientations for a wide range of treatment applications.