Abstract
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.
Highlights
The understanding of complex neuronal networks in the human brain requires the study of connections in different pathways, as well as the projections that the synaptic activity of one region may have in another one
This way we found the AISI 1010 low-carbon steel to be an appropriate material for our ferromagnetic cores, having a saturation magnetization (SM) ≅ 2 T with a magnetic flux intensity (H) of 4×104 A/m, an initial relative permeability of μr=667.75 and standardized for relatively low complexity machining, given the low carbon composition
To accurately predict the induced E-field that it would be obtained in practical implementations, we identified the location of the pyramidal neurons of the layers V and VI (Fig. 4a) in the M1 region of the motor cortex, using the rat brain atlas6,7 in stereotaxic coordinates
Summary
The understanding of complex neuronal networks in the human brain requires the study of connections in different pathways, as well as the projections that the synaptic activity of one region may have in another one. This is the case for the motor pathway, a complex association of neurons in a network responsible for the motion of the musculoskeletal system. As utilized scitation.org/journal/adv in humans, TMS allows to discreetly alter the synaptic activity of selected regions in small animal brains, applying an external timevarying magnetic field that produces an associated electric field, according to the Maxwell-Faraday’s Induction Law.
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