Quintuple AISI 1010 carbon steel core coil for highly focused transcranial magnetic stimulation in small animals

Transcranial magnetic stimulation (TMS) is a non-invasive neuromodulation technique used to regulate the synaptic activity of neurons in the brain, improving the functionality of connecting regions and bringing effective treatment to different neurological and psychiatric disorders. The TMS induced E-field needs to be focal enough to avoid unwanted side effects caused by stimulation of the regions adjacent to the target. Attempts at TMS in small animals like rodents are highly constrained, since most of these studies use commercial equipment intended for humans, with power and coil geometries not designed for small animals. Using finite element modeling in ANSYS Maxwell, the present work shows the design and evaluation of customized arrays of two and five dual-winding solenoids, including a ferromagnetic core, to restrict the stimulation to areas as small as 1 mm2. Each solenoid is made with 50 turns of a wire with thickness = 1 mm, height = 25.4 mm and elliptical top-view cross section. Ferromagnetic cores with V-shape tip sharpening were included, using AISI 1010 carbon steel of 2 T of saturation flux density (Bsat) at 4×104 A/m, and an initial relative permeability µr=667.75. Electric fields and magnetic flux densities were calculated around 4.00 mm below the coil (vertical distance from the top of the scalp to the cortical layer 5/6 in adult rats) with peak currents of 10kA, in a single non-repetitive pulse at 2.5kHz. The achieved 100V/m in a small area of 1 mm2 suggests the suitability of the coil for in vivo experimentation in rodents. Future works will seek to improve the duration of the pulses for repetitive TMS with pulse shaping techniques and validate the novel coil with in vivo experiments in rat models.