Effects of coil design on delivery of focal magnetic stimulation. Technical considerations

The localization of effects from magnetic coil stimulation is not immediately obvious. We measured the magnetic fields produced by several different coils and compared the results with theoretical calculations. Magnetic stimuli were delivered from a Cadwell MES-10 magnetic stimulator using 3 circular coils (one 9 cm in diameter; two with an angulated extension, 5 and 9 cm in diameter) and twin oval coils arranged in a butterfly shape (each coil approximately 4 cm in diameter) and from a Novametrix Magstim 200 using two circular flat-spiral coils (6.7 and 14 cm in diameter). Peak-induced strength of the magnetic field was recorded with a measuring loop (1 cm in diameter) at different distances from the center of the coil. When the measuring loop was moved in the same plane laterally from the center of the coil, for all coils except the butterfly-shaped coil, the field was highest in the center and fell off near the circumference of the coil. The field dropped progressively when measurements were made more distant from the plane of the coils. The electric field induced from the magnetic coil could be calculated from the coil geometry. For all coils except the butterfly-shaped coil, the largest electric field was at the circumference of the coils. The 6.7 cm flat-spiral coil induced currents similar to those induced by the larger coils but more focally. The butterfly-shaped coil induced the largest currents under its center, where the circumferences of the two component coils come together. The component of the electric field parallel to the wire in the center of this coil was the largest and most localized.