Experimental demonstration of induction by means of a transcranial magnetic stimulator coil immersed in a conducting liquid

The full potentialities of deep-brain transcranial magnetic stimulation (TMS) are presently limited by the so-called surface discontinuity effect. This effect is responsible for a strong reduction of the capability of TMS systems to reach satisfactorily deep-brain regions with biostimulatory and/or bioinhibitory purposes. Consequently, a large number of neuropathologies that could potentially profit from TMS remain either unknown, or yield clinical trials with inconclusive results. Previous simulation studies have pointed to the fact that the surface discontinuity effect may be reduced very strongly when the coil and the patient head are immersed in a conducting liquid. In this work we provide experimental evidence of this fact. For that, we have constructed a TMS system capable of inducing quasi-monophasic currents in saline solution of up to 1.5 A/m² delivered in less than 200µs. Such current density and temporization are those typically used for TMS. We present results measured in an NaCl solution with a conductivity of ∼0.11 S/m (0.05%w/v), i.e. three times smaller than the brain conductivity. This saline solution was inserted in a cylindrical container with a diameter of 125mm, therefore representing tentatively a small brain. A surface-to-center induced current ratio of −81% was measured when the brain-like container was stimulated with the TMS system surrounded by air. When both the stimulating coil and the brain-like container where immersed in a saline solution with a conductivity of 5.5 S/m (3.33%w/v), the surface-to-center induced current ratio dropped only −24%, therefore confirming qualitative expectations from previous simulation work. This experimental confirmation opens new possibilities for deep-brain TMS in neurology.