Previously we proposed an eccentric figure-eight coil that can cause threshold stimulation in the brain at lower driving currents. In this study, we performed numerical simulations and magnetic stimulations to healthy subjects for evaluating the advantages of the eccentric coil. The simulations were performed using a simplified spherical brain model and a realistic human brain model. We found that the eccentric coil required a driving current intensity of approximately 18% less than that required by the concentric coil to cause comparable eddy current densities within the brain. The eddy current localization of the eccentric coil was slightly higher than that of the concentric coil. A prototype eccentric coil was designed and fabricated. Instead of winding a wire around a bobbin, we cut eccentric-spiral slits on the insulator cases, and a wire was woven through the slits. The coils were used to deliver magnetic stimulation to healthy subjects; among our results, we found that the current slew rate corresponding to motor threshold values for the concentric and eccentric coils were 86 A/μs and 78 A/μs, respectively. The results indicate that the eccentric coil consistently requires a lower driving current to reach the motor threshold than does the concentric coil. Future development of compact magnetic stimulators will enable the treatment of some intractable neurological diseases at home.
Transcranial magnetic stimulation (TMS) has recently been used as a method for the treatment of neurological and psychiatric diseases. Daily TMS sessions can provide continuous therapeutic effectiveness, and the installation of TMS systems at patients' homes has been proposed. A figure-eight coil, which is normally used for TMS therapy, induces a highly localized electric field; however, it is challenging to achieve accurate coil positioning above the targeted brain area using this coil. In this paper, a bowl-shaped coil for stimulating a localized but wider area of the brain is proposed. The coil's electromagnetic characteristics were analyzed using finite element methods, and the analysis showed that the bowl-shaped coil induced electric fields in a wider area of the brain model than a figure-eight coil. The expanded distribution of the electric field led to greater robustness of the coil to the coil-positioning error. To improve the efficiency of the coil, the relationship between individual coil design parameters and the resulting coil characteristics was numerically analyzed. It was concluded that lengthening the outer spherical radius and narrowing the width of the coil were effective methods for obtaining a more effective and more uniform distribution of the electric field.
Prototypes of the dipole and the fast steering magnet for the VSX project have been fabricated and measured. The field mapping and the end-shim correction were carried out for the dipole, and the frequency response was tested up to 2 kHz for the fast steering. The design of the magnets and measured results are presented.
There are individual variations on the motor threshold (MT) and therapeutic effect in clinical treatment using transcranial magnetic stimulation (TMS)�These variations may result from the difference of individual brain anatomies. In this study, we built numerical brain models individually from six subjects, and calculated the distributions of eddy currents induced by TMS. The brain models were built from individual MRI data with segmenting into gray matter, white matter, and cerebrospinal fluid. The location of the figure-eight stimulator coil was recorded using a binocular infrared camera when the stimulation response of twitch observed over 50 % of trials. The eddy current distributions were obtained using an originally developed solver based on the scalar potential finite difference (SPFD) method. The results showed different distributions of the eddy current density between each brain models. The average eddy current density in the primary motor cortex was 17±6.9 A/m 2 for the stimulus intensity corresponding to the MT. Assessment of the relationship between the eddy current density, stimulus conditions, and brain anatomy would help understanding of the mechanism of the varying MT. The developed model enabled us to compare the numerical results with experiments. Experiments have shown that a displacement of stimulator coil from the appropriate location causes an increase in the MT. This phenomenon was observed also in our simulations.
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