Development of Anatomically Accurate Brain Model of Small Animals for Experimental Verification of Transcranial Magnetic Stimulation

Transcranial magnetic stimulation (TMS) is a neuromodulation therapy that is used to treat neurological and psychiatric conditions and has been approved by the Food and Drug Administration (FDA) for certain disorders. Since TMS trials with humans are not always feasible, brain phantoms have been developed as ways of testing the safety and feasibility of novel TMS coils and treatment procedures. Although a brain phantom has been developed for humans, a comparable brain phantom for rats or mice has not been reported for TMS or other neuromodulation techniques. An individualized brain phantom for rats is needed to accelerate the study of neuromodulation techniques, especially the measurement of induced electric field and voltages in the brain regions when stimulated by a time-varying magnetic field. The main objective of this article is to develop an individualized, anatomically accurate rat brain model for finite element simulation of stimulation strengths that are highly sensitive to complex structure and variation in brain anatomy of individual rats. We initially obtained magnetic resonance images (MRIs) and computed tomography (CT) images from public databases to create anatomically accurate brain models. The images were segmented with statistical parametric mapping (SPM) using the SIGMA brain atlases. The programs Meshmixer and Meshlab were then used to reduce the file size and fix any model errors. Maxwell 3-D module in ANSYS finite element simulation software was then used to simulate a TMS procedure with a coil designed for small animals. An overlapped Figure-of-8 coil was used on the top of the rat's head with 5 kA current supplied at 2.5 kHz to the coil. The peak magnetic flux density of 2.55 T and electric field of 106.94 V/m occur on the surface of the brain approximately below the center of the coil. Our model matches expected results from a Figure-of-8 coil.