Focality of rectangular planar butterfly coils with square vs. semicircular wing tips: a numerical comparison

Advances in neuromagnetic stimulation depend significantly on improved coil designs. Such improvements require a better understanding of the relationships between coil design parameters and electric field distributions. Coils with sharp angulations have been thought to offer improved focality over circular coils. The authors compute the electric field distribution created by square wing tip rectangular butterfly coils and compare it to the field distribution of circular wing tip butterfly coils. The positive and negative peaks of the electric field of the rectangular butterfly coil are found to have a close spatial relationship to the ends of the central leg of the butterfly coil, a relationship which is lost when circular wing tips are used. The authors conclude that square wing tip rectangular butterfly coils are better than semicircular wing tip rectangular butterfly coils when distal peripheral nerve stimulation with precise control of the locus of stimulation are desired