Masaki Sekino

Osaka University, Suika, Ōsaka, Japan

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Publications (151)217.84 Total impact

  • No preview · Article · Jan 2016 · IEEE Transactions on Magnetics
  • Keita Yamamoto · Yu Miyawaki · Youichi Saitoh · Masaki Sekino

    No preview · Article · Jan 2016 · IEEE Transactions on Magnetics
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    ABSTRACT: The magnetic technique for sentinel node biopsy provides a radioisotope-free alternative for staging breast cancer. This technique requires refinement to reduce the 'residual iron content' at the injection site by maximising lymphatic uptake to prevent 'void artefacts' on magnetic resonance imaging (MRI), which could adversely affect clinical use. The impact of site and timing of injection of magnetic tracer was evaluated in a murine tumour model (right hind limb) in 24 wild type mice. Right-sided intratumoral and left sided subcutaneous injection of magnetic tracer and assessment of nodal iron uptake on MRI, surgical excision and histopathological grading at time frames up to 24 hours was performed. Rapid iron uptake on MRI, smaller 'void artefacts' (P<0.001) and a significant increase in iron content with time was identified in the subcutaneous injection group (r=0.937; P<0.001). A subcutaneous injection and increasing time-delay between tracer injection and surgery is beneficial for lymphatic iron uptake.
    No preview · Article · Dec 2015 · Nanomedicine: nanotechnology, biology, and medicine
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    ABSTRACT: We report a fabrication method for flexible and printable thermal sensors based on composites of semicrystalline acrylate polymers and graphite with a high sensitivity of 20 mK and a high-speed response time of less than 100 ms. These devices exhibit large resistance changes near body temperature under physiological conditions with high repeatability (1,800 times). Device performance is largely unaffected by bending to radii below 700 µm, which allows for conformal application to the surface of living tissue. The sensing temperature can be tuned between 25 °C and 50 °C, which covers all relevant physiological temperatures. Furthermore, we demonstrate flexible active-matrix thermal sensors which can resolve spatial temperature gradients over a large area. With this flexible ultrasensitive temperature sensor we succeeded in the in vivo measurement of cyclic temperatures changes of 0.1 °C in a rat lung during breathing, without interference from constant tissue motion. This result conclusively shows that the lung of a warm-blooded animal maintains surprising temperature stability despite the large difference between core temperature and inhaled air temperature.
    No preview · Article · Nov 2015 · Proceedings of the National Academy of Sciences
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    ABSTRACT: Phase map cross-correlation detection and quantification may produce highlighted signal at superparamagnetic iron oxide nanoparticles, and distinguish them from other hypointensities. The method may quantify susceptibility change by performing least squares analysis between a theoretically generated magnetic field template and an experimentally scanned phase image. Because characteristic phase recognition requires the removal of phase wrap and phase background, additional steps of phase unwrapping and filtering may increase the chance of computing error and enlarge the inconsistence among algorithms. To solve problem, phase gradient cross-correlation and quantification method is developed by recognizing characteristic phase gradient pattern instead of phase image because phase gradient operation inherently includes unwrapping and filtering functions. However, few studies have mentioned the detectable limit of currently used phase gradient calculation algorithms. The limit may lead to an underestimation of large magnetic susceptibility change caused by high-concentrated iron accumulation. In this study, mathematical derivation points out the value of maximum detectable phase gradient calculated by differential chain algorithm in both spatial and Fourier domain. To break through the limit, a modified quantification method is proposed by using unwrapped forward differentiation for phase gradient generation. The method enlarges the detectable range of phase gradient measurement and avoids the underestimation of magnetic susceptibility. Simulation and phantom experiments were used to quantitatively compare different methods. In vivo application performs MRI scanning on nude mice implanted by iron-labeled human cancer cells. Results validate the limit of detectable phase gradient and the consequent susceptibility underestimation. Results also demonstrate the advantage of unwrapped forward differentiation compared with differential chain algorithms for susceptibility quantification at high-concentrated iron accumulation.
    No preview · Article · Nov 2015 · Magnetic Resonance Imaging
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    ABSTRACT: Occupational exposure to the static magnetic field (SMF) is an arising problem in operators of magnetic resonance imaging (MRI) system. It has been well established that human body movements in the stray field from MRI system induces the electric current inside the body and this magnetically induced current causes the temporal sensational changes such as vertigo, headache, nausea and metallic taste etc... to MR workers [1]. Although the self-motivated motion control is recommended for MR workers to prevent these temporal effects [1], more feasible safe working procedure is required to achieve the compliance monitoring. The present study performed the trial of the safe working procedure among 3 T MRI system users and investigated its effectiveness in occupational SMF exposure levels and their work performances.
    No preview · Article · Jul 2015 · IEEE Transactions on Magnetics
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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&apos; 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&apos;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.
    No preview · Article · May 2015 · Journal of Applied Physics
  • Taeseong Woo · Dongmin Kim · Takao Someya · Masaki Sekino
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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.
    No preview · Article · Apr 2015 · Journal of Applied Physics
  • Yawara Eguchi · Seiji Ohtori · Masaki Sekino · Shoogo Ueno
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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.
    No preview · Article · Feb 2015 · Bioelectromagnetics
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    ABSTRACT: 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/m2 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.
    Preview · Article · Jan 2015
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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.
    Full-text · Article · Dec 2014 · Nature Communications
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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.
    No preview · Article · Nov 2014 · Bioelectromagnetics
  • M. Sekino · T. Someya · H. Ohsaki · S. Ueno
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    ABSTRACT: Mapping of electromagnetic fields and electric properties of the brain is helpful in understanding the fundamentals of brain function. This paper introduces recent progress in magnetic resonance imaging (MRI) techniques to obtain the electric properties of the biological tissues and weak magnetic fields arising from neuronal electrical activities. There are two approaches for impedance MRI: Application of external electric currents to the sample and observation of the resulting changes in images provide a straightforward way to estimate the electric properties of the sample. Inferring the tissue conductivity from the water diffusion coefficient provides a less invasive method to estimate the tissue conductivity. Considering that MRI is inherently sensitive to magnetic fields generated in the samples, detection of magnetic fields arising from neuronal electrical activities using MRI is an attractive approach for dramatically improving the temporal resolution of functional MRI. Evaluating the sensitivity of MRI to weak magnetic fields is crucial in realizing this detection. Some papers report that the neuronal magnetic fields are detectable using MRI.
    No preview · Article · Oct 2014
  • T. Woo · D. Kim · T. Sekitani · T. Someya · M. Sekino
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    ABSTRACT: Magnetic resonance imaging (MRI) of patients having a conductive implant causes interference with internal RF magnetic fields. The interference results in a signal loss around the implant. In this study, we propose an MRI acquisition method to recover the signal loss using the RF pulse for spatially designed excitation. Based on the measured distribution of RF magnetic field, the Bloch equation was inversely solved to design the waveform of RF pulse for improving the homogeneity of flipped magnetization. MRI acquisitions were performed with phantoms including metallic implants made from copper and aluminum. The results show that the designed waveform of RF pulse improves the homogeneity of signal intensity in MRI, and the signal loss around the metals was recovered.
    No preview · Article · Oct 2014
  • M. Sekino · M. Suyama · D. Kim · Y. Saitoh
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    ABSTRACT: Transcranial magnetic stimulation (TMS) is effective for treatment of neurological diseases such as neuropathic pain. We are newly developing magnetic stimulators for use at patient's home for realizing daily treatment. One of the technical challenges is a system for positioning the stimulator coil. In this study, we proposed a coil design which gives a high robustness to positioning error. We numerically evaluated the characteristics of the coil when changing the width, depth, height, and the number of turns of the coil. The results showed that the mostly influencing parameter on the broadening of the eddy current was the width of the coil, and the vertical length was influential on the current density. We found the proposed coil induces eddy currents in a wider range than conventional figure-eight coils.
    No preview · Article · Oct 2014
  • Y. Inoue · T. Sekitani · I. Saito · T. Isoyama · Y. Abe · T. Someya · M. Sekino
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    ABSTRACT: The purpose of this study is to investigate the effect of electrical stimulation on angiogenesis. We developed an implantable camera device which observes blood vessels in animal bodies with stimulating electrodes. This device was implanted under the goat skin for 60 days. Electrical stimulations were applied with a sinusoidal wave of 60 kHz and a current density of 300 uA/mm2. Angiogenesis was clearly observed in the non-stimulated chamber, while angiogenesis did not occur in the stimulated chamber. These results show that electrical stimulation of this condition has effective to suppress angiogenesis. © 2014, Japan Soc. of Med. Electronics and Biol. Engineering. All rights reserved.
    No preview · Article · Aug 2014 · Transactions of Japanese Society for Medical and Biological Engineering
  • Y. Takiyama · M. Suyama · D. Kim · T. Maruo · Y. Saitoh · M. Sekino
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    ABSTRACT: There are individual variations in the motor threshold (MT) of transcranial magnetic stimulation (TMS). These variations may result from individual brain anatomy.In this study, we built numerical brain models for 6 subjects, and calculated the distributions of induced eddy currents. The MRI data were segmented into the gray matter, white matter and cerebrospinal fluid. The location of the figure-eight stimulator coil was recorded using a binocular infrared camera. The eddy current distributions were obtained using an originally developed solver based on the scalar potential finite difference (SPFD) method.The results showed that the eddy current distributions in and around the precentral gyrus exhibited clear variations among individuals. The current density in the primary motor cortex was 17.19±6.83 A/m2 for the stimulus intensity corresponding to MT.Assessment of the relationship between the eddy current density, stimulus conditions, and bran anatomy would help understanding of the mechanism of the varying MT. © 2014, Japan Soc. of Med. Electronics and Biol. Engineering. All rights reserved.
    No preview · Article · Aug 2014
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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.
    Full-text · Article · Aug 2014 · Advanced Materials
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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.
    No preview · Article · Aug 2014
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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.
    No preview · Article · Aug 2014

Publication Stats

954 Citations
217.84 Total Impact Points


  • 2012-2015
    • Osaka University
      • Graduate School of Medicine
      Suika, Ōsaka, Japan
  • 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