Angle Dependency of Intraoral Coil for Magnetic Stimulation Targeting the Base of the Brain

Tsutomu Ando, Toshihiro Nozaki, Daisuke Katayama, Masaki Sekino, Kaechang Park
Vol. 11 (2022) p.142-150

Non-invasive magnetic stimulation has the potential to be an effective treatment for Alzheimer’s disease, which inevitably involves neurodegeneration of the hippocampus at the base of the brain. In this study, we fabricated an excitation coil designed to be mounted inside the oral cavity, i.e., an intraoral coil, to stimulate the base of the brain. The oral cavity has an advantage of being in closer proximity with greater transitability to the base compared with the convexity of the brain. We determined the optimum angle of the intraoral coil targeting the base to achieve the largest magnetic flux density. In the experiments, the coil angle was changed and the z-component of the magnetic flux density was measured at positions corresponding to three regions at the base of the brain: hippocampus, thalamus, and hypothalamus. Here, a single pulse current of approximately 1920 A at 3 kHz was applied to the intraoral coil with a power supply device used in actual transcranial magnetic stimulation (TMS). A maximum magnetic flux density of 10.3–17.2 × 10−3 T was obtained at the base of the brain when the coil angle was between 35 and 40 degrees. Next, we conducted a numerical analysis using a numerical human head model. The z-component of the maximum magnetic flux density was obtained at coil angles between 35 and 40 degrees, and the result was consistent with the experimental result. Additionally, we evaluated the induced current density at the base of the brain by numerical analysis. The maximum value at the hippocampus was 2 A/m2 when the coil angle was 90 degrees. This is the angle at which the central axis of the designed intraoral coil (a solenoid coil) is horizontal to the ground. Specifically, the central axis of the present intraoral coil needs to be tilted to 60 degrees from the target.