Developing a more focal magnetic stimulator. Part II: Fabricating coils and measuring induced current distributions

First, our program for fabricating stimulator coils that are focal is reviewed. These are in the figure-eight shape, where the goal is to make them successively more focal, hence of successively smaller size. Although smaller coils require larger pulse currents, with resulting stress and heat problems, operating prototypes were readily fabricated with circle diameters down to 2.5 cm. However, for smaller diameters, the coil casings were fractured, and new casing techniques are being explored. Second, our program is reviewed for measuring the distribution of current induced in saline tanks by stimulator coils, to determine their focality. A coaxial probe measures the potential difference between two neighboring points in the tank, yielding the local current density. Measurements are presented due to several shapes of tanks and coil configurations. In a spherical tank, results confirm the theory that there can be neither radial current induced anywhere in the sphere nor any current at the sphere center. In tanks approximating a semi-infinite volume and the human limb, arrow-map distributions are shown, due to a commercial "pancake" coil and several figure-eight coils. In the semi-infinite tank, where the distribution can also be computed theoretically, the measured distribution agrees with the computations, thus validating the measurements. In the limb tank, the distribution is compressed and is somewhat more focal than in the semi-infinite tank, depending on the coil orientation.