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Akimasa Hirata - One of the best experts on this subject based on the ideXlab platform.

  • group level analysis of Induced Electric Field in deep brain regions by different tms coils
    Physics in Medicine and Biology, 2020
    Co-Authors: Jose Gomeztames, Ilkka Laakso, Akimasa Hirata, Atsushi Hamasaka, S Ueno
    Abstract:

    Deep transcranial magnetic stimulation (dTMS) is a non-invasive technique used for the treatment of depression and obsessive compulsive disorder. In this study, we computationally evaluated group-level dosage for dTMS to characterize the targeted deep brain regions to overcome the limitations of using individualized head models to characterize coil performance in a population. We used an inter-subject registration method adapted to the deep brain regions that enable projection of computed Electric Fields (EFs) from individual realistic head models (n  =  18) to the average space of deep brain regions. The computational results showed consistent group-level hotspots of the EF in the deep brain regions. The halo circular assembly coils Induced the highest EFs in deep brain regions (up to 50% of the maximum EF in the cortex) for optimized positioning. In terms of the trade-off between Field spread and penetration, the performance of the H7 coil was the best. The computational model allowed the optimization of generalized dTMS-Induced EF on deep region targets despite inter-individual differences while informing and possibly minimizing unintended stimulation of superficial regions and possible mixed stimulation effects from deep and cortical areas. These results will facilitate the decision process during dTMS interventions in clinical practice.

  • group level analysis of Induced Electric Field in deep brain regions by different tms coils
    bioRxiv, 2019
    Co-Authors: Jose Gomeztames, Ilkka Laakso, Akimasa Hirata, Atsushi Hamasaka, S Ueno
    Abstract:

    Abstract Deep transcranial magnetic stimulation (dTMS) is a non-invasive technique used in treating depression. In this study, we computationally evaluate group-level dosage during dTMS with the aim of characterizing targeted deep brain regions to overcome the limitation of using individualized head models to characterize coil performance in a population. We use an inter-subject registration method adapted to deep brain regions that enable projection of computed Electric Fields (EFs) from individual realistic head models (n = 18) to the average space of deep brain regions. The computational results showed consistent group-level hotspots of the EF in deep brain region with intensities between 20%-50% of the maximum EF in the cortex. Large co-activation in other brain regions was confirmed while half-value penetration depth from the cortical surface was smaller than 2 cm. The halo figure-8 assembly and halo circular assembly coils Induced the highest EFs for caudate, putamen, and hippocampus. Generalized Induced EF maps of deep regions show target regions despite inter-individual difference. This is the first study that visualizes generalized target regions during dTMS and provides a method for making informed decisions during dTMS interventions in clinical practice.

  • evaluation of the Induced Electric Field and compliance procedure for a wireless power transfer system in an Electrical vehicle
    Physics in Medicine and Biology, 2013
    Co-Authors: Ilkka Laakso, Akimasa Hirata
    Abstract:

    In this study, an Induced Electric Field in a human body is evaluated for the magnetic Field leaked from a wireless power transfer system for charging an Electrical vehicle. The magnetic Field from the wireless power transfer system is modelled computationally, and its effectiveness is confirmed by comparison with the Field measured in a previous study. The Induced Electric Field in a human standing around the vehicle is smaller than the allowable limit prescribed in international guidelines, although the magnetic Field strength in the human body is locally higher than the allowable external Field strength. Correlation between the external magnetic Field and the Induced Electric Field is confirmed to be reasonable at least in the standing posture, which is the case discussed in the international standard. Based on this finding, we discussed and confirmed the applicability of a three-point magnetic Field measurement at heights of 0.5, 1.0, and 1.5 m for safety compliance.

  • fast multigrid based computation of the Induced Electric Field for transcranial magnetic stimulation
    Physics in Medicine and Biology, 2012
    Co-Authors: Ilkka Laakso, Akimasa Hirata
    Abstract:

    In transcranial magnetic stimulation (TMS), the distribution of the Induced Electric Field, and the affected brain areas, depends on the position of the stimulation coil and the individual geometry of the head and brain. The distribution of the Induced Electric Field in realistic anatomies can be modelled using computational methods. However, existing computational methods for accurately determining the Induced Electric Field in realistic anatomical models have suffered from long computation times, typically in the range of tens of minutes or longer. This paper presents a matrix-free implementation of the finite-element method with a geometric multigrid method that can potentially reduce the computation time to several seconds or less even when using an ordinary computer. The performance of the method is studied by computing the Induced Electric Field in two anatomically realistic models. An idealized two-loop coil is used as the stimulating coil. Multiple computational grid resolutions ranging from 2 to 0.25 mm are used. The results show that, for macroscopic modelling of the Electric Field in an anatomically realistic model, computational grid resolutions of 1 mm or 2 mm appear to provide good numerical accuracy compared to higher resolutions. The multigrid iteration typically converges in less than ten iterations independent of the grid resolution. Even without parallelization, each iteration takes about 1.0 s or 0.1 s for the 1 and 2 mm resolutions, respectively. This suggests that calculating the Electric Field with sufficient accuracy in real time is feasible.

  • Reducing the staircasing error in computational dosimetry of low-frequency electromagnetic Fields.
    Physics in Medicine and Biology, 2012
    Co-Authors: Ilkka Laakso, Akimasa Hirata
    Abstract:

    From extremely low frequencies to intermediate frequencies, the magnitude of Induced Electric Field inside the human body is used as the metric for human protection. The Induced Electric Field inside the body can be computed using anatomically realistic voxel models and numerical methods such as the finite-difference or finite-element methods. The computed Electric Field is affected by numerical errors that occur when curved boundaries with large contrasts in Electrical conductivity are approximated using a staircase grid. In order to lessen the effect of the staircase approximation error, the use of the 99th percentile Electric Field, i.e. ignoring the highest 1% of Electric Field values, is recommended in the ICNIRP guidelines. However, the 99th percentile approach is not applicable to localized exposure scenarios where the majority of significant Induced Electric Field values may be concentrated in a small volume. In this note, a method for removing the staircasing error is proposed. Unlike the 99th percentile, the proposed method is also applicable to localized exposure scenarios. The performance of the method is first verified by comparison with the analytical solution in a layered sphere. The method is then applied for six different exposure scenarios in two anatomically realistic human head models. The results show that the proposed method can provide conservative estimates for the 99th percentile Electric Field in both localized and uniform exposure scenarios.

Ilkka Laakso - One of the best experts on this subject based on the ideXlab platform.

  • group level analysis of Induced Electric Field in deep brain regions by different tms coils
    Physics in Medicine and Biology, 2020
    Co-Authors: Jose Gomeztames, Ilkka Laakso, Akimasa Hirata, Atsushi Hamasaka, S Ueno
    Abstract:

    Deep transcranial magnetic stimulation (dTMS) is a non-invasive technique used for the treatment of depression and obsessive compulsive disorder. In this study, we computationally evaluated group-level dosage for dTMS to characterize the targeted deep brain regions to overcome the limitations of using individualized head models to characterize coil performance in a population. We used an inter-subject registration method adapted to the deep brain regions that enable projection of computed Electric Fields (EFs) from individual realistic head models (n  =  18) to the average space of deep brain regions. The computational results showed consistent group-level hotspots of the EF in the deep brain regions. The halo circular assembly coils Induced the highest EFs in deep brain regions (up to 50% of the maximum EF in the cortex) for optimized positioning. In terms of the trade-off between Field spread and penetration, the performance of the H7 coil was the best. The computational model allowed the optimization of generalized dTMS-Induced EF on deep region targets despite inter-individual differences while informing and possibly minimizing unintended stimulation of superficial regions and possible mixed stimulation effects from deep and cortical areas. These results will facilitate the decision process during dTMS interventions in clinical practice.

  • group level analysis of Induced Electric Field in deep brain regions by different tms coils
    bioRxiv, 2019
    Co-Authors: Jose Gomeztames, Ilkka Laakso, Akimasa Hirata, Atsushi Hamasaka, S Ueno
    Abstract:

    Abstract Deep transcranial magnetic stimulation (dTMS) is a non-invasive technique used in treating depression. In this study, we computationally evaluate group-level dosage during dTMS with the aim of characterizing targeted deep brain regions to overcome the limitation of using individualized head models to characterize coil performance in a population. We use an inter-subject registration method adapted to deep brain regions that enable projection of computed Electric Fields (EFs) from individual realistic head models (n = 18) to the average space of deep brain regions. The computational results showed consistent group-level hotspots of the EF in deep brain region with intensities between 20%-50% of the maximum EF in the cortex. Large co-activation in other brain regions was confirmed while half-value penetration depth from the cortical surface was smaller than 2 cm. The halo figure-8 assembly and halo circular assembly coils Induced the highest EFs for caudate, putamen, and hippocampus. Generalized Induced EF maps of deep regions show target regions despite inter-individual difference. This is the first study that visualizes generalized target regions during dTMS and provides a method for making informed decisions during dTMS interventions in clinical practice.

  • evaluation of the Induced Electric Field and compliance procedure for a wireless power transfer system in an Electrical vehicle
    Physics in Medicine and Biology, 2013
    Co-Authors: Ilkka Laakso, Akimasa Hirata
    Abstract:

    In this study, an Induced Electric Field in a human body is evaluated for the magnetic Field leaked from a wireless power transfer system for charging an Electrical vehicle. The magnetic Field from the wireless power transfer system is modelled computationally, and its effectiveness is confirmed by comparison with the Field measured in a previous study. The Induced Electric Field in a human standing around the vehicle is smaller than the allowable limit prescribed in international guidelines, although the magnetic Field strength in the human body is locally higher than the allowable external Field strength. Correlation between the external magnetic Field and the Induced Electric Field is confirmed to be reasonable at least in the standing posture, which is the case discussed in the international standard. Based on this finding, we discussed and confirmed the applicability of a three-point magnetic Field measurement at heights of 0.5, 1.0, and 1.5 m for safety compliance.

  • fast multigrid based computation of the Induced Electric Field for transcranial magnetic stimulation
    Physics in Medicine and Biology, 2012
    Co-Authors: Ilkka Laakso, Akimasa Hirata
    Abstract:

    In transcranial magnetic stimulation (TMS), the distribution of the Induced Electric Field, and the affected brain areas, depends on the position of the stimulation coil and the individual geometry of the head and brain. The distribution of the Induced Electric Field in realistic anatomies can be modelled using computational methods. However, existing computational methods for accurately determining the Induced Electric Field in realistic anatomical models have suffered from long computation times, typically in the range of tens of minutes or longer. This paper presents a matrix-free implementation of the finite-element method with a geometric multigrid method that can potentially reduce the computation time to several seconds or less even when using an ordinary computer. The performance of the method is studied by computing the Induced Electric Field in two anatomically realistic models. An idealized two-loop coil is used as the stimulating coil. Multiple computational grid resolutions ranging from 2 to 0.25 mm are used. The results show that, for macroscopic modelling of the Electric Field in an anatomically realistic model, computational grid resolutions of 1 mm or 2 mm appear to provide good numerical accuracy compared to higher resolutions. The multigrid iteration typically converges in less than ten iterations independent of the grid resolution. Even without parallelization, each iteration takes about 1.0 s or 0.1 s for the 1 and 2 mm resolutions, respectively. This suggests that calculating the Electric Field with sufficient accuracy in real time is feasible.

  • Reducing the staircasing error in computational dosimetry of low-frequency electromagnetic Fields.
    Physics in Medicine and Biology, 2012
    Co-Authors: Ilkka Laakso, Akimasa Hirata
    Abstract:

    From extremely low frequencies to intermediate frequencies, the magnitude of Induced Electric Field inside the human body is used as the metric for human protection. The Induced Electric Field inside the body can be computed using anatomically realistic voxel models and numerical methods such as the finite-difference or finite-element methods. The computed Electric Field is affected by numerical errors that occur when curved boundaries with large contrasts in Electrical conductivity are approximated using a staircase grid. In order to lessen the effect of the staircase approximation error, the use of the 99th percentile Electric Field, i.e. ignoring the highest 1% of Electric Field values, is recommended in the ICNIRP guidelines. However, the 99th percentile approach is not applicable to localized exposure scenarios where the majority of significant Induced Electric Field values may be concentrated in a small volume. In this note, a method for removing the staircasing error is proposed. Unlike the 99th percentile, the proposed method is also applicable to localized exposure scenarios. The performance of the method is first verified by comparison with the analytical solution in a layered sphere. The method is then applied for six different exposure scenarios in two anatomically realistic human head models. The results show that the proposed method can provide conservative estimates for the 99th percentile Electric Field in both localized and uniform exposure scenarios.

S Ueno - One of the best experts on this subject based on the ideXlab platform.

  • group level analysis of Induced Electric Field in deep brain regions by different tms coils
    Physics in Medicine and Biology, 2020
    Co-Authors: Jose Gomeztames, Ilkka Laakso, Akimasa Hirata, Atsushi Hamasaka, S Ueno
    Abstract:

    Deep transcranial magnetic stimulation (dTMS) is a non-invasive technique used for the treatment of depression and obsessive compulsive disorder. In this study, we computationally evaluated group-level dosage for dTMS to characterize the targeted deep brain regions to overcome the limitations of using individualized head models to characterize coil performance in a population. We used an inter-subject registration method adapted to the deep brain regions that enable projection of computed Electric Fields (EFs) from individual realistic head models (n  =  18) to the average space of deep brain regions. The computational results showed consistent group-level hotspots of the EF in the deep brain regions. The halo circular assembly coils Induced the highest EFs in deep brain regions (up to 50% of the maximum EF in the cortex) for optimized positioning. In terms of the trade-off between Field spread and penetration, the performance of the H7 coil was the best. The computational model allowed the optimization of generalized dTMS-Induced EF on deep region targets despite inter-individual differences while informing and possibly minimizing unintended stimulation of superficial regions and possible mixed stimulation effects from deep and cortical areas. These results will facilitate the decision process during dTMS interventions in clinical practice.

  • group level analysis of Induced Electric Field in deep brain regions by different tms coils
    bioRxiv, 2019
    Co-Authors: Jose Gomeztames, Ilkka Laakso, Akimasa Hirata, Atsushi Hamasaka, S Ueno
    Abstract:

    Abstract Deep transcranial magnetic stimulation (dTMS) is a non-invasive technique used in treating depression. In this study, we computationally evaluate group-level dosage during dTMS with the aim of characterizing targeted deep brain regions to overcome the limitation of using individualized head models to characterize coil performance in a population. We use an inter-subject registration method adapted to deep brain regions that enable projection of computed Electric Fields (EFs) from individual realistic head models (n = 18) to the average space of deep brain regions. The computational results showed consistent group-level hotspots of the EF in deep brain region with intensities between 20%-50% of the maximum EF in the cortex. Large co-activation in other brain regions was confirmed while half-value penetration depth from the cortical surface was smaller than 2 cm. The halo figure-8 assembly and halo circular assembly coils Induced the highest EFs for caudate, putamen, and hippocampus. Generalized Induced EF maps of deep regions show target regions despite inter-individual difference. This is the first study that visualizes generalized target regions during dTMS and provides a method for making informed decisions during dTMS interventions in clinical practice.

Stuart Crozier - One of the best experts on this subject based on the ideXlab platform.

  • on the Induced Electric Field gradients in the human body for magnetic stimulation by gradient coils in mri
    IEEE Transactions on Biomedical Engineering, 2003
    Co-Authors: Huawei Zhao, Stuart Crozier
    Abstract:

    Prior theoretical studies indicate that the negative spatial derivative of the Electric Field Induced by magnetic stimulation may be one of the main factors contributing to depolarization of the nerve fiber. This paper studies this parameter for peripheral nerve stimulation (PNS) Induced by time-varying gradient Fields during MRI scans. The numerical calculations are based on an efficient, quasi-static, finite-difference scheme and an anatomically realistic human, full-body model. Whole-body cylindrical and planar gradient sets in MRI systems and various input signals have been explored. The spatial distributions of the Induced Electric Field and their gradients are calculated and attempts are made to correlate these areas with reported experimental stimulation data. The Induced Electrical Field pattern is similar for both the planar coils and cylindrical coils. This study provides some insight into the spatial characteristics of the Induced Field gradients for PNS in MRI, which may be used to further evaluate the sites where magnetic stimulation is likely to occur and to optimize gradient coil design.

Nir Tessler - One of the best experts on this subject based on the ideXlab platform.

  • Molecular control of quantum-dot internal Electric Field and its application to CdSe-based solar cells
    Nature Materials, 2011
    Co-Authors: Nir Yaacobi-gross, Marina Zimin, Shifi Kababya, Asher Schmidt, Michal Soreni-harari, Nir Tessler
    Abstract:

    Inorganic nanocrystals are attractive materials for solar-cell applications. However, the performance of such devices is often limited by an insufficient alignment of energy levels in the nanocrystals. Here, we report that by attaching two different molecules to a single quantum dot or nanocrystal one can induce Electric Fields large enough to significantly alter the electronic and optoelectronic properties of the quantum dot. This Electric Field is created within the nanocrystals owing to a mixture of amine- and thiol-anchor-group ligands. Examining the steady state as well as temporal evolution of the optical properties and the nuclear magnetic resonances of the nanocrystals we found that the first excitonic peak shifts as a function of the capping-layer composition. We also demonstrate that the use of a mixed-ligand-Induced Electric Field markedly enhances the charge generation efficiency in layer-by-layer CdSe-nanocrystal-based solar cells, thus improving the overall cell efficiency.