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    Neural Engineering

    (J-387) Electric field characteristics of transcranial rotating permanent magnet stimulation

    Friday, October 13, 2023
    9:30 AM – 10:30 AM PDT
    Location: Exhibit Hall - Row J - Poster # 387

      Presenting Author(s)

    • Pei Robins, BA

      Post Baccalaureate Trainee
      NIMH
      Bethesda, Maryland, United States

      • Last Author(s)

    • ZD

      Zhi-De Deng

      Staff Scientist
      NIMH, United States

    Introduction:: Low-field synchronized transcranial magnetic stimulation (sTMS) is a promising noninvasive neuromodulation device that is being explored for its potential therapeutic benefits in patients with psychiatric and neurological disorders. When applied directly to the scalp, the sTMS device generates a time-varying magnetic field using mechanically rotating permanent magnets, resulting in a time-varying electric field in the brain. This study aims to characterize the electric field strength and distribution of sTMS for different magnetic configurations using finite element method analysis and measurements.

    Materials and Methods:: Finite element models were implemented in COMSOL Multiphysics using a sphere head (r=8.5 cm) and an IEEE Specific Anthropomorphic Mannequin (SAM) phantom, both with a uniform, isotropic electrical conductivity of 0.33 S/m and a relative permeability of 1, to simulate the electric field for ten different permanent magnetic configurations. Eight of these configurations featured single permanent magnets with different magnetization directions, while the remaining two configurations involved multiple permanent magnets. The configurations were rotated at 10, 13.3, and 400 Hz fixed frequencies. Electric field strengths were measured using a hollow sphere (r=9.2 cm) filled with a 0.9% sodium chloride solution in deionized water. The electric field was measured with a custom-made silver-chloride dipole probe for two of the single permanent magnet configurations. One magnet (A) had a diameter of 1 inch, an inner diameter of 1/4 inch, and a length of 1 inch, was diametrically magnetized with a surface field intensity of 6909 Gauss and a residual flux density of 1.32 Tesla, while the other magnet (B) had a diameter of ¼ inch and length of 3/8 inch, was axially magnetized with a surface field intensity of 7020 Gauss and a residual flux density of 1.48 Tesla. Electric field measurements were also taken with the MagVenture TMS coil (figure-8, cooled B65 coil) at 100% maximum stimulator output.

    Results, Conclusions, and Discussions::

    Factors that influence the shape of the electric field include dimensions of the magnet, number of poles and direction of the magnetization, and axis of rotation, number of magnets and placement, while E-field strength depends on the rotational frequency and surface field of the magnets. The induced electric field strength on the surface of the head ranged between 0.0092 and 0.59 V/m. In the full sTMS configuration within the SAM head model, the electric field distribution exhibited broad spread over midline frontal polar, medial frontal, and parietal regions, with maximum induced electric field strengths of approximately 0.06 V/m on the head's surface and 0.02 V/m in the cortex. The experimental electric field measurements for magnet A and B were 0.39 V/m and 0.082 V/m. Additionally, the electric field strength for the TMS coil was measured to be approximately 401.5 V/m.




    The finite element models and experimental measurements reveal that the sTMS system, for both single and multiple magnet configurations, induces electric field strengths that are approximately an order of magnitude lower than those delivered by conventional transcranial magnetic stimulation. Future work involves simulating sTMS on anatomically realistic head models derived from individual brain scans, further advancing our understanding of its potential therapeutic applications.




    Acknowledgements (Optional): :

    This project is supported by the National Institute of Mental Health (NIMH) Intramural Research Program (ZIAMH002955). This work utilized resources of the NIH Section on Instrumentation and computational resources of the NIH HPC Biowulf cluster (https://hpc.nih.gov).



    References (Optional): :