Measuring Transcranial Magnetic Stimulation-Induced Electric Fields in Anatomically and Conductively Accurate Rat Head Phantoms

The efficacy of neuromodulation techniques like transcranial magnetic stimulation (TMS), are highly dependent on the geometry and conductivity of the stimulated target of the brain. Although anatomically accurate head models are routinely used for computational simulations of TMS-induced electric (E) fields, there is still a need for realistic phantoms that mimic the anatomy and electrical conductivity of the head. Here, we present a realistic rat brain phantom constructed using advanced 3-d printing technologies. Our phantom allows validation of TMS-induced E-fields using embedded mutually orthogonal triaxial dipole probes (TDP) that can measure induced E-fields along three axes. We tested the TDP probes in the constructed phantom using four TMS coils with different core materials and core geometry. These measurements were then compared to computational simulations using the finite element method (FEM). The rat brain phantoms had a conductivity of roughly 0.5 S/m, which was mimicked in the FEM simulations. When the measured induced e-fields in the phantoms were compared to the simulated results, the measured results were in the same expected range and fairly close to one another with an average error of 5.1%. The peak E-fields (measured vs. simulated) close to the surface of the grey matter were: permender v-tip core (115.3 V/m - 110V/m), permender flat tip core (91.9V/m - 85 V/m), AISI 1010 v-tip core (94.7 V/m - 100 V/m), and AISI 1010 flat-tip core (85.9 V/m - 84 V/m) respectively.