Transcranial Magnetic Stimulation Coil with Electronically Switchable Active and Sham Modes

Blinded studies with transcranial magnetic stimulation (TMS) require a valid sham condition. A wide range of sham approaches have been implemented but they have various limitations including residual electric field in the brain, inadequate reproduction of auditory and cutaneous sensations, and/or need for electrical stimulation with scalp electrodes. We propose a quadrupole TMS coil configuration that can be electronically switched between active and sham modes. In active mode, the quadrupole coil has electric field characteristics similar to a conventional figure-8 coil. In sham mode, the quadrupole coil compared to the reverse-current sham figure-8 coil has 50% less electric field penetration depth, is 97% more focal, produces 35% less intense field in the brain, and induces scalp electric field characteristics closer to those of active TMS.