| Over the past three decades, transcranial magnetic stimulation (TMS) has emerged as an effective tool for many research, diagnostic and therapeutic applications in humans. TMS delivers highly localized brain stimulations via non-invasive externally applied magnetic fields. This non-invasive, painless technique provides researchers and clinicians a unique tool capable of stimulating both the central and peripheral nervous systems. However, a complete analysis of the macroscopic electric fields produced by TMS has not yet been performed. In this dissertation, we present a thorough examination of the total electric field induced by TMS in air and a realistic head model with clinically relevant coil poses. In the first chapter, a detailed account of TMS coil wiring geometry was shown to provide significant improvements in the accuracy of primary E-field calculations. Three-dimensional models which accounted for the TMS coil's wire width, height, shape and number of turns clearly improved the fit of calculated-to-measured E-fields near the coil body. Detailed primary E-field models were accurate up to the surface of the coil body (within 0.5% of measured values) whereas simple models were often inadequate (up to 32% different from measured). In the second chapter, we addressed the importance of the secondary E-field created by surface charge accumulation during TMS using the boundary element method (BEM). 3-D models were developed using simple head geometries in order to test the model and compare it with measured values. The effects of tissue geometry, size and conductivity were also investigated. Finally, a realistic head model was used to assess the effect of multiple surfaces on the total E-field. We found that secondary E-fields have the greatest impact at areas in close proximity to each tissue layer. Throughout the head, the secondary E-field magnitudes were predominantly between 25% and 45% of the primary E-fields magnitude. The direction of the secondary E-field was primarily in opposition to the primary E-field, however there are some locations (i.e. going from high to low conductivity) where the secondary E-field adds to the primary E-field. Thus the total E-field vector may change in magnitude and direction. These findings show that realistic head geometries should be used when modeling the total E-field. In the third chapter, we addressed the importance of the secondary electric field (E-field) in a realistic head model using the boundary element method at clinically relevant coil positions and orientations (ex. primary motor cortex) during transcranial magnetic stimulation (TMS). The effective E-fields produced at each clinical orientation were then correlated with electromyographic (EMG) recordings using the total E-field with the cortical column cosine model. Some TMS coil orientations led to total E-fields as much as 40% lower than the primary E-fields at sites located on the TMS coil's main axis of stimulation. Effective E-field values at the cortical level, were highly correlated (r = 0.9644, P < 0.01) with EMG responses indicating that both local biological characteristics (such as tissue geometry and electrical conductivity) and the total E-field induced by the TMS coil may provide a means for predicting the optimum coil position/orientation to consistently produce neuronal activations. |
