Masaki Sekino

The University of Tokyo, Tōkyō, Japan

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Publications (137)188.4 Total impact

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    ABSTRACT: 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.
    Journal of Applied Physics 05/2015; 117(17):17A318. DOI:10.1063/1.4914876 · 2.19 Impact Factor
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    ABSTRACT: Metallic implants can result in considerable inhomogeneity in the signal intensity of magnetic resonance imaging (MRI), because the implant generates a shielding effect to the applied radio-frequency (RF) magnetic fields. In this study, we propose an acquisition method to mitigate the signal inhomogeneities using an adaptive RF pulse waveform. The effectiveness of the method was investigated using both numerical simulations and experiments. The RF pulse waveform was calculated based on inverse analyses of the Bloch equation incorporating the measured RF field distribution within the object. A simulation was carried out using a simplified numerical model of RF field inhomogeneity assumed at the center of model. An RF pulse waveform was designed to recover the attenuated signal region in the given model, and we show a significant improvement in the signal homogeneity compared with that obtained using a conventional pulse. We implemented the proposed method on a 7T-MRI system to show the efficacy experimentally. Test samples were fabricated from agarose gel with inserted copper or aluminum implants of different thicknesses. The RF pulse for selective excitation was calculated after mapping the RF field distribution of each imaging object. The acquired images exhibit an improvement in the homogeneity at the region of metallic implants. These results indicate that the proposed method is effective for MRI measurements of objects containing metallic implants.
    Journal of Applied Physics 04/2015; 117(17). DOI:10.1063/1.4908298 · 2.19 Impact Factor
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    ABSTRACT: We investigated the effectiveness of using magnetically aligned collagen (after exposure to a maximum 8-T magnetic field) for nerve regeneration in both an in vitro and in vivo model. Neurite outgrowth from embryonic chick dorsal root ganglion (DRG) neurons was significantly greater on magnetically aligned collagen gel than on control gel, and was dependent on magnetic field strength. Silicone tubes (15 mm length) filled with collagen gel formed bridges between severed rat sciatic nerves. We prepared tubes for four groups: collagen gel only (COL), magnetically aligned collagen gel (M-COL), collagen gel mixed with Schwann cells (S-COL), and magnetically aligned collagen gel mixed with Schwann cells (M-S-COL). The ratio of infiltrating regenerated nerves was higher in the M-COL group compared to the COL group at 8 weeks post-operation. There were no significant differences between the two groups with and without Schwann cells. Compound action potentials showed higher amplitude and shorter latency in the M-COL than COL group at 12 weeks post-operation. The number and diameter of regenerated axons increased significantly in the M-COL compared with the COL group at 12 weeks post-operation. Here we demonstrated that magnetically orientated collagen promoted nerve regeneration using both an in vitro and in vivo model. Bioelectromagnetics. 9999:1-11; 2014. © 2015 Wiley Periodicals, Inc. © 2015 Wiley Periodicals, Inc.
    Bioelectromagnetics 02/2015; 36(3). DOI:10.1002/bem.21896 · 1.86 Impact Factor
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    ABSTRACT: To measure electrophysiological signals from the human body, it is essential to establish stable, gentle and nonallergic contacts between the targeted biological tissue and the electrical probes. However, it is difficult to form a stable interface between the two for long periods, especially when the surface of the biological tissue is wet and/or the tissue exhibits motion. Here we resolve this difficulty by designing and fabricating smart, stress-absorbing electronic devices that can adhere to wet and complex tissue surfaces and allow for reliable, long-term measurements of vital signals. We demonstrate a multielectrode array, which can be attached to the surface of a rat heart, resulting in good conformal contact for more than 3 h. Furthermore, we demonstrate arrays of highly sensitive, stretchable strain sensors using a similar design. Ultra-flexible electronics with enhanced adhesion to tissue could enable future applications in chronic in vivo monitoring of biological signals.
    Nature Communications 12/2014; 5:5898. DOI:10.1038/ncomms6898 · 10.74 Impact Factor
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    ABSTRACT: 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 and 78 A/µs, respectively. The results indicate that the eccentric coil consistently requires a lower driving current to reach the motor threshold than the concentric coil. Future development of compact magnetic stimulators will enable the treatment of some intractable neurological diseases at home. Bioelectromagnetics. © 2014 Wiley Periodicals, Inc.
    Bioelectromagnetics 11/2014; 36(1). DOI:10.1002/bem.21886 · 1.86 Impact Factor
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    ABSTRACT: We developed an MRI-compatible, ultra-thin, flexible stimulator array for the rat brain and performed functional MRI (fMRI) acquisition during direct electrical stimulation of the brain. This technique measured brain activity evoked by direct stimulation of the motor and the somatosensory cortex. In order to avoid MR signal loss due to interferences with the main static field and RF field in the MRI system, the stimulator array was made from a non-magnetic gold electrode of 100-nm thickness on a 2-um-thick parylene substrate. By using this stimulator array, MR images without signal loss around conducting electrode pads were acquired, and fMRI acquisition during concurrent electrical stimulation of the cerebral cortex was achieved. Neuronal activity propagated to distant brain areas from the stimulated motor cortex. Positive blood oxygenation level dependent (BOLD) signals were observed with direct stimulation of the motor cortex, while negative BOLD signals were observed with direct stimulation of the somatosensory cortex. Interestingly, the pattern of brain activity evoked by direct stimulation of the somatosensory cortex was different from that evoked by electrical stimulation of the forepaw.
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    ABSTRACT: Measuring the local pressure in blood vessels is valuable in the postoperative monitoring of aneurysms. However, implanting a conventional pressure sensor equipped with power and signal cables causes difficulties during the operative procedure and carries a risk of infection after the implantation. In this study, we developed a wireless, implantable, and flexible pressure sensor. A magnetic resonance imaging (MRI) system reads out the sensor output. The proposed wireless sensor is based on an LC resonant circuit with a spiral coil and a pressure-sensitive capacitor. The pressure-dependence of the capacitance affects the magnetic field produced by the spiral coil, changing the magnetization of the nearby sample that can be observed as a signal variation by MRI. We fabricated a prototype sensor using a capacitor with a silicone elastomer as the dielectric and a spiral coil made of gold. The maximum change in the capacitance was 8% under an external pressure of 20 kPa. A change in the thickness of the dielectric elastomer caused the capacitance to change, resulting in a signal variation detectable by MRI.
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    ABSTRACT: A unique form of adaptive electronics is demonstrated, which change their mechanical properties from rigid and planar to soft and compliant, in order to enable soft and conformal wrapping around 3D objects, including biological tissue. These devices feature excellent mechanical robustness and maintain initial electrical properties even after changing shape and stiffness.
    Advanced Materials 08/2014; 26(29). DOI:10.1002/adma.201400420 · 15.41 Impact Factor
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    ABSTRACT: A 64-channel surface electromyogram (EMG) measurement sheet (SEMS) with 2 V organic transistors on a 1 μ m-thick ultra-flexible polyethylene naphthalate (PEN) film is developed for prosthetic hand control. The surface EMG electrodes must satisfy the following three requirements; high mechanical flexibility, high electrode density and high signal integrity. To achieve high electrode density and high signal integrity, a distributed and shared amplifier (DSA) architecture is proposed, which enables an in-situ amplification of the myoelectric signal with a fourfold increase in EMG electrode density. In addition, a post-fabrication select-and-connect (SAC) method is proposed to cope with the large mismatch of organic transistors. The proposed SAC method reduces the area and the power overhead by 96% and 98.2%, respectively, compared with the use of conventional parallel transistors to reduce the transistor mismatch by a factor of 10.
    IEEE Transactions on Biomedical Circuits and Systems 06/2014; 8(6). DOI:10.1109/TBCAS.2014.2314135 · 3.15 Impact Factor
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    ABSTRACT: A wet sensor, which detects the presence or absence of liquid, is an important tool for biomedical, nursing-care, and elderly-care applications such as the detection of blood in bandages, sweat in underwear, and urination in diapers. A wet sensor should be a thin, mechanically flexible, large-area, and low-cost device with wireless power and data transmission, because constant monitoring with a rigid and wired wet sensor placed on human skin is annoying. Moreover, the wet sensor should be disposable from a hygiene perspective. In order to meet these requirements, an organic transistor based flexible wet sensor sheet (FWSS) with wireless power and data transmission using 13.56MHz magnetic resonance is developed to detect urination in diapers.
    2014 IEEE International Solid- State Circuits Conference (ISSCC); 02/2014
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    ABSTRACT: Magnetic susceptibility is the inherent property of materials. Magnetic resonance imaging as a kind of non-invasive testing technique may measure the magnetic field distortion induced by the imaging object and consequently calculate its magnetic susceptibility distribution. Conventionally, magnetic susceptibility mapping is performed in three-dimensional mode with the requirement of multi-slice scanning. In the condition of long objects that have uniform distribution along one direction, two-dimensional phase image of the cross section might be enough to estimate the susceptibility. In this study, we adjust susceptibility mapping algorithm into a two-dimensional mode and evaluate the possibility of using a single-slice image to estimate the magnetic susceptibility. Simulation was performed to validate the algorithm in both ideal and noise-imposed conditions. MRI experiment was performed by using copper, graphite and iron oxide at different concentrations. The results suggest that it is possible to measure the susceptibility of a long object by a two-dimensional image.
    16th International Symposium on Applied Electromagnetics and Mechanics; 01/2014
  • IEEJ Transactions on Fundamentals and Materials 01/2014; 134(4):266-272. DOI:10.1541/ieejfms.134.266
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    ABSTRACT: MRI systems have technical challenges in interference with metals and high cost because of the use of strong magnetic field. Ultralow-field MRI is an emerging technique enabling measurement of samples containing metals due to low resonant frequencies, as well as saving cost due to its simplified coil system. We developed a compact SQUID system for ultralow-field MRI equipped with a magnetic shield box of 1 x 1 x 1.5 m. Oscillating magnetic fields were applied to the pick-up coil of SQUID from an external coil with a magnetic flux density equivalent to magnetic resonance signals from water of 10 ml. The SQUID successfully detected external magnetic fields of 3.08 x 10(-12) T.
    16th International Symposium on Applied Electromagnetics and Mechanics; 01/2014
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    ABSTRACT: PURPOSE: To evaluate the accuracy of an equilibrium magnetization (M(0) ) map obtained using a two-dimensional (2D) spoiled gradient-recalled echo (SPGR) pulse sequence with variable flip angle (VFA). MATERIALS AND METHODS: Single-slice 2D SPGR images of 4% agar gel phantoms with different gadolinium-diethylene triamine pentaacetic acid (Gd-DTPA) concentrations (0-1 mM) were obtained with a VFA (2-30°). The 2D SPGR-VFA data were acquired with different repetition times (TRs; 7.8-117.2 ms), Gaussian and sinc RF pulses, and different field strengths (4.7, 7, and 9.4 Tesla). M(0) and T(1) maps were calculated from the 2D SPGR-VFA data. M(0) and T(1) values were compared with those calculated from free-relaxed 2D gradient-recalled echo (GRE) images and inversion recovery-prepared 2D SPGR images. The M(0) and T(1) slice profiles were also investigated. RESULTS: Consistent M(0) values were obtained, regardless of the different Gd concentrations, TRs, and pulse sequences. The M(0) slice profiles calculated from the sliced SPGR-VFA data quantitatively reproduced those calculated from the free-relaxed sliced GRE. In contrast, the T(1) values calculated from the 2D SPGR-VFA data were underestimated at a high Gd concentration, short TR, and Gaussian RF pulse. CONCLUSION: M(0) values calculated from 2D SPGR-VFA images are highly quantitative. J. Magn. Reson. Imaging 2013;. © 2013 Wiley Periodicals, Inc.
    Journal of Magnetic Resonance Imaging 11/2013; 38(5). DOI:10.1002/jmri.24023 · 2.79 Impact Factor
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    ABSTRACT: Neuroinflammation is associated with many conditions that lead to dementia, such as cerebrovascular disorders or Alzheimer's disease. However, the specific role of neuroinflammation in the progression of cognitive deficits remains unclear. To understand the molecular mechanisms underlying these events we used a rodent model of focal cerebral stroke, which causes deficits in hippocampus-dependent cognitive function. Cerebral stroke was induced by middle cerebral artery occlusion (MCAO). Hippocampus-dependent cognitive function was evaluated by a contextual fear conditioning test. The glial neuroinflammatory responses were investigated by immunohistochemical evaluation and diffusion tensor MRI (DTI). We used knockout mice for P2Y1 (P2Y1KO), a glial ADP/ATP receptor that induces the release of proinflammatory cytokines, to examine the links among P2Y1-mediated signaling, the neuroinflammatory response, and cognitive function. Declines in cognitive function and glial neuroinflammatory response were observed after MCAO in both rats and mice. Changes in the hippocampal tissue were detected by DTI as the mean diffusivity (MD) value, which corresponded with the cognitive decline at 4 days, 1 week, 3 weeks, and 2 months after MCAO. Interestingly, the P2Y1KO mice with MCAO showed a decline in sensory-motor function, but not in cognition. Furthermore, the P2Y1KO mice showed neither a hippocampal glial neuroinflammatory response (as assessed by immunohistochemistry) nor a change in hippocampal MD value after MCAO. In addition, wild-type mice treated with a P2Y1-specific antagonist immediately after reperfusion did not show cognitive decline. Our findings indicate that glial P2Y1 receptors are involved in the hippocampal inflammatory response. The findings from this study may contribute to the development of a therapeutic strategy for brain infarction, targeting the P2Y1 receptor.
    Journal of Neuroinflammation 07/2013; 10(1):95. DOI:10.1186/1742-2094-10-95 · 4.90 Impact Factor
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    ABSTRACT: Objective: To assess the pain-relieving effects of motor cortex electrical stimulation (MCS) and the predictive factors retrospectively.
    Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 07/2013; 2013:6163-6166. DOI:10.1109/EMBC.2013.6610960
  • Dongmin Kim, Takao Someya, Masaki Sekino
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    ABSTRACT: We developed an experimental setup for magnetic resonance imaging (MRI) of rat brain slices maintained in a hemoglobin-free medium and showed that the MRI system has a sensitivity to magnetic fields of 10(-11) T. The originally developed non-magnetic sample holder consisted of a microelectrode array for recording neuronal potentials and perfusing channels for the medium. Because of the hemoglobin-free condition, the magnetic fields could be distinguished from the baseline signal fluctuations due to hemoglobin. A theoretical estimation of the signal-to-noise ratio showed a sensitivity of 3.3 × 10(-10) T. Parameter optimization using a 7-T MRI system with the developed sample holder resulted in an experimental sensitivity of 4.0 × 10(-10) T. These MRI sensitivities potentially enable us to detect weak magnetic fields arising from neuronal activities, and are sufficiently high for detecting neuronal magnetic fields of 1.4 × 10(-11) T generated in rat brain slices by averaging signals 810 times.
    Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 07/2013; 2013:1370-1373. DOI:10.1109/EMBC.2013.6609764
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    ABSTRACT: The identification of the sentinel lymph nodes that cause tumor metastasis is important in breast cancer therapy. The detection of magnetic fluid accumulating in the lymph nodes using a magnetic probe allows surgeons to identify the lymph nodes. In this study, we carried out numerical simulations and experiments to investigate the sensitivity and basic characteristics of a magnetic probe consisting of a permanent magnet and a small magnetic sensor. The measured magnetic flux density arising from the magnetic fluid agreed well with the numerical results. In addition, the results helped realize an appropriate probe configuration for achieving high sensitivity to magnetic fluid. A prototype probe detected magnetic fluid located 30 mm from the probe head.
    Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 07/2013; 2013:5485-5488. DOI:10.1109/EMBC.2013.6610791
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    ABSTRACT: A superconducting power cable is a prospective application of high-temperature superconductors. Although power cables using Bi2223 or YBCO wires have been developed in the world so far mainly for utility grid application, we focus attention on railway application of a superconducting power cable. We have studied how to introduce superconducting power cables effectively and economically to a dc electric railway. Numerical analysis models based on the MATLAB-Simulink are constructed for a dc electric feeder system of singe- and double-track railways. Tractive force and train resistance characteristics are modeled. Energy saving by the superconducting power cable is investigated with the influences of the number of substations, the train operation interval, etc., taken into account. The results show that the superconducting power cables can improve regeneration rate and system energy saving, and reduce the substation capacity very effectively. It will be also an advantage that if the transportation capacity of a railway line needs to be increased, the introduction of superconducting cables could achieve it without changing the existing substations along the line.
    IEEE Transactions on Applied Superconductivity 06/2013; 23(3):3600705-3600705. DOI:10.1109/TASC.2013.2237874 · 1.32 Impact Factor
  • Y. Terao, M. Sekino, H. Ohsaki
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    ABSTRACT: This paper deals with the comparison of electromagnetic characteristics of 10 MW class direct-drive wind turbine generators for offshore wind farms. Four kinds of wind turbine generators including a conventional permanent magnet generator, a salient/nonsalient pole superconducting generator, and a fully superconducting generator are investigated. Electromagnetic characteristics for these generators have been investigated with FEM analysis. The results show that a fully and a salient pole type superconducting wind turbine generators have a potential for 10 MW class offshore wind turbine generators from the aspect of size and economical points.
    IEEE Transactions on Applied Superconductivity 06/2013; 23(3):5200904-5200904. DOI:10.1109/TASC.2012.2237223 · 1.32 Impact Factor

Publication Stats

706 Citations
188.40 Total Impact Points

Institutions

  • 2002–2015
    • The University of Tokyo
      • • Department of Electrical Engineering and Information Systems
      • • Graduate School of Frontier Sciences
      • • Department of Advanced Energy
      • • Department of Radiology and Biomedical Engineering
      • • Department of Medical Engineering
      Tōkyō, Japan
  • 2014
    • Japan Science and Technology Agency (JST)
      Edo, Tōkyō, Japan
  • 2012–2014
    • Osaka University
      Suika, Ōsaka, Japan
  • 2011
    • National Institute of Occupational Safety and Health JAPAN
      Edo, Tōkyō, Japan
  • 2004
    • Kumamoto University
      Kumamoto, Kumamoto, Japan