Quad Bufferfly Coil (QBC) is based on the work of P. Rastogi, E. G. Lee, R. L. Hadimani, and D. C. Jiles at Iowa State University. 

 

This coil uses four windings with each pair at 90° to each other. The two larger windings are complemented by smaller windings to increase stimulation focality.

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[https://aip.scitation.org/doi/10.1063/1.4973604]

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