Coil designs for localized and efficient magnetic stimulation of the nervous system

The goal of this research is to design and test coils for magnetic stimulation of nerves that efficiently induce localized electric fields with components parallel and perpendicular to the skin.; For stimulation of deep nerves with electric fields parallel to the skin, large flat butterfly coil designs with round wings are proposed. These coils induce localized electric fields with larger amplitudes than any other coil for a given coil inductance and depth inside a semi-infinite volume conductor. The optimal size of the butterfly coils increases with the stimulation depth, but it is limited by the size of the volume conductor. A 7 turn 6 cm radius flat butterfly coil was built and tested for stimulation of the deep nerves of dog bladder. Numerical calculations showed that this coil provides larger electric fields than any other round coil with the same inductance of 30 μH at a depth of 4 cm. Large increases in bladder pressure due to detrussor muscle contraction was obtained with dorsal stimulation above the sacrum and cauda equina.; Toroidal coils with high permeability (Supermendur) cores embedded in a conducting medium were designed to induce electric fields perpendicular to the skin. These coils reduce the current requirement for magnetic stimulation by three orders of magnitude, eliminate the problem of coil heating, and allow long periods of stimulation at higher frequencies than previously possible using simpler stimulators. Magnetic stimulation with toroidal coils is similar to surface electrical stimulation in terms of driving current requirements, stimulation localization and field orientation, but it eliminates the risk of tissue damage and reduces uncomfortable sensations because of the slower decay of the induced electric field magnitude with stimulation depth. In preliminary results, in-vivo stimulation of the Brachioradialis muscle in human with toroidal coils was achieved at frequencies ranging from 0.25 to 12 KHz with driving currents as low as 1.6 A and without uncomfortable sensations.; The coil designs presented in this thesis considerably improve the limitations affecting magnetic stimulation, and create new opportunities for this technique that can eventually allow its use as a reliable tool in the clinical environment.