Published papers

ABSTRACT Background Physical exercise is recommended for sarcopenia prevention and treatment. Patients with frailty often show poor tolerance. To achieve universal availability, electrical stimulation of skeletal muscles has been devised, but concerns remain regarding efficacy and safety. Alternatively, magnetic stimulation could theoretically induce skeletal muscle contractions by remotely inducing electric fields, which may be a promising noninvasive approach. Our objective is to examine the efficacy and underlying mechanisms of magnetic stimulation against disuse muscle atrophy in a rat model of wire hindlimb immobilization. Methods Rats were divided into the following three groups: control ( n  = 5), those with spiral wire immobilization (SWI)‐induced hindlimb muscle atrophy ( n  = 6) and those with SWI undergoing magnetic stimulation ( n  = 5). Magnetic stimulation was set at 30 Hz, intensity 25%, 800 trains and 12 000 pulses/day for 5 days. Rats were evaluated for muscle strength and harvested soleus and gastrocnemius muscles for transcriptome and metabolome analysis. Results The body weight (144.8 ± 6.5 g vs. 175.2 ± 5.6 g) and the weight of the soleus (0.037 ± 0.01 g vs. 0.065 ± 0.01 g) and gastrocnemius muscles (0.599 ± 0.02 g vs. 0.966 ± 0.05 g) were lower in the SWI than the controls ( p  < 0.05). Intriguingly, the weight of gastrocnemius muscle (0.654 ± 0.024 g vs. 0.599 ± 0.01 g, p  < 0.05) and maximal knee extension twitch torque (110.49 ± 8.57 mNm vs. 84.57 ± 6.61 mNm, p  < 0.05) were greater in rats with SWI and magnetic stimulation than in rats with SWI alone. RNA‐seq identified 469 genes that were downregulated by SWI, 190 genes that were upregulated by magnetic stimulation. Metabolome profiling indicated altered glutathione metabolism in SWI muscles (−Log10(P‐value) = 3.2, −Log10(false discovery rate, FDR) = 1.6), whereas magnetic stimulation influenced purine metabolism (−Log10(P‐value) = 2.8, −Log10(FDR) = 0.9), partially supported by RNA‐seq differentially expressed gene list. In engineered human muscle tissues, treatment with 1 μM L‐alanosine, a purine metabolism inhibitor, negatively affected the maintenance of the electric pulse stimulation‐induced contractile function; twitch forces were 0.12 ± 0.09 mN with 0.69 ± 0.14 mN as control ( p  < 0.05) and tetanic forces were 0.28 ± 0.23 mN with 1.55 ± 0.33 mN as control ( p  < 0.05). Conclusions Magnetic stimulation counteracted the loss of weight and function of murine gastrocnemius muscle induced by immobilization and may have the potential to halt the progression of sarcopenia via alteration of the metabolic pathway in the muscle.

Abstract While magnetocardiography (MCG) is a non-invasive tool for assessing cardiac electrophysiology, its conventional planar sensor array often struggles to accurately estimate sources distant from the sensor. This limitation hinders the precise localization of deep cardiac activity. This study aims to overcome this challenge by investigating a novel cylindrical sensor array designed to improve source current estimation accuracy. We combined numerical simulations, phantom measurements, and in vivo rat experiments to compare a cylindrical sensor array with a conventional planar array. Single dipole localization was used in the simulation and phantom experiments under controlled focal source conditions, whereas both single-dipole fitting and minimum norm estimation (MNE)-based distributed source analyses were applied to the animal MCG data. The simulations and phantom measurements showed that the cylindrical array achieved more accurate single-dipole localization than the planar array under comparable sensor–source distance conditions. In the animal experiments, this advantage was not clearly observed using single-dipole fitting. The MNE-based distributed source analysis showed geometry-dependent reconstruction patterns, with the cylindrical array producing more posteriorly distributed activity and higher source map repeatability. The cylindrical sensor array provided complementary spatial information to anterior planar sampling, particularly for deeper or posterior cardiac activity. These findings suggest that surrounding sensor coverage may provide complementary information for MCG-based reconstruction, although further validation with realistic activation models remains necessary.

In this study, the theoretical feasibility of utilizing optically pumped magnetometers for on-board magnetoencephalography measurements was explored. Simulations were conducted to generate steady-state visually evoked response (SSVER) signals that incorporate vehicle noise, and a noise reduction strategy specifically designed for on-board applications is proposed. Upon engine activation, the magnetic field vibration of a conventional gasoline-powered vehicle measured in an urban environment was found to be approximately seven times greater in the vertical direction than in the horizontal direction. The maximum signal-to-noise ratio of the SSVER in an automotive environment was simulated to be −110 dB. A 350-mm side-length, 20-turn active compensation coil can achieve an attenuation rate of approximately 28 dB at a target frequency of 24 Hz for measurements inside the vehicle cabin. Therefore, an increase in the number of coil turns would result in a higher attenuation rate. Further noise attenuation to the level inside a magnetically shielded room requires approximately 80 dB.

We propose a novel method based on an inverse problem to design a single channel radio frequency (RF) receive coil for magnetic resonance imaging (MRI) with an optimized signal-to-noise ratio (SNR) for imaging the rat kidney. We identified a dedicated curved-surface coil design for use on the rat body surface utilizing inverse problem analysis using direct current calculation and quantitatively evaluated the indicators of coil performance using alternating current calculation. The proposed coil achieved increased SNR and signal intensity responses that were respectively 1.05- and 2-fold higher than those of conventional surface coils. In the future, we will fabricate a prototype coil and perform the MRI of the rat kidney to diagnose kidney diseases.

To perform transcranial magnetic stimulation therapy at home, a stimulation coil that is robust against misalignment must be employed. A dome-shaped coil is one such coil; however, it has the disadvantage that the induced electric field is smaller than that of the conventional figure-8 coil. In this study, we developed a double D-shaped coil with a horizontal return path to suppress the influence of the return current, while reducing the inductance of the coil. After optimizing geometric parameters of the double D-shaped coil through numerical simulations, we fabricated the coil and evaluated its performance on healthy subjects. It was confirmed that the developed coil was found to tolerate a wider range of positioning errors than the figure-8 coil when stimulating the motor cortex.

We present a novel method for estimating the magnetic field that can generate the desired magnetic force to move magnetic nanoparticles in a region of interest. The magnetic field is expanded using vector spherical harmonics, and the optimal multipole moments are estimated. The multipole moments are included in quadratic form because the magnetic force is defined as the product of the magnetic field and its gradient. We transform the system of quadratic equations into a rank-one-matrix estimation problem to construct a convex numerical algorithm based on a well-established proximal operation. The proposed method was validated using numerical simulations and can be used to design magnetic force patterns that cannot be generated using a simple coil pair.

We employ a dry-type phantom to evaluate the performance of a diamond quantum magnetometer with a high sensitivity of about 6 pT/Hz from the viewpoint of practical measurement in biomagnetic sensing. The dry phantom is supposed to represent an equivalent current dipole (ECD) generated by brain activity, emulating an encephalomagnetic field. The spatial resolution of the magnetometer is evaluated to be sufficiently higher than the length of the variation in the encephalomagnetic field distribution. The minimum detectable ECD moment is evaluated to be 0.2 nA m by averaging about 8000 measurements for a standoff distance of 2.4 mm from the ECD. We also discuss the feasibility of detecting an ECD in the measurement of an encephalomagnetic field in humans. We conclude that it is feasible to detect an encephalomagnetic field from a shallow cortex area such as the primary somatosensory cortex.

The electrical properties (EPs) of biological tissue, consisting of conductivity and permittivity, provide useful information for the diagnosis of malignant tissues and the evaluation of heat absorption rates. Recently, magnetic resonance electrical properties tomography (MREPT), by which EPs are reconstructed from internal magnetic field data measured by using magnetic resonance imaging (MRI), has been actively studied. We previously proposed an explicit pointwise reconstruction method for MREPT based on a complex partial differential equation (PDE), known as the D-bar equation, of the electric field and its explicit solution given by an integral formula. In this method, as in some other conventional methods, EP values on the boundary of the region of interest must be given as a Dirichlet boundary condition of the PDE. However, it is difficult to know these values precisely in practical situations. Therefore, in this paper, we propose a novel method for reconstructing EPs in a circular region without any knowledge of boundary EP values. Starting from the integral solution to solve the D-bar equation in a circular region with the Neumann boundary condition, we show that the contour integral term of the integral formula is eliminated by using Faraday's law and solve the PDE based only on magnetic field data measured by using MRI. Numerical simulations show that the proposed method yields a good reconstruction results without any knowledge of boundary EP values.

Introduction Magnetoencephalography (MEG) can measure weak magnetic fields produced by electrical brain activity. Transcranial direct current stimulation (tDCS) can affect such brain activities. The concurrent application of both, however, is challenging because tDCS presents artifacts on the MEG signal. If brain activity during tDCS can be elucidated by MEG, mechanisms of plasticity-inducing and other effects of tDCS would be more comprehensively understood. We tested the technical feasibility of MEG during tDCS using a phantom that produces an artificial current dipole simulating focal brain activity. An earlier study investigated estimation of a single oscillating phantom dipole during tDCS, and we systematically tested multiple dipole locations with a different MEG device. Methods A phantom provided by the manufacturer was used to produce current dipoles from 32 locations. For the 32 dipoles, MEG was recorded with and without tDCS. Temporally extended signal space separation (tSSS) was applied for artifact rejection. Current dipole sources were estimated as equivalent current dipoles (ECDs). The ECD modeling quality was assessed using localization error, amplitude error, and goodness of fit (GOF). The ECD modeling performance with and without tDCS, and with and without tSSS was assessed. Results Mean localization errors of the 32 dipoles were 1.70 ± 0.72 mm (tDCS off, tSSS off, mean ± standard deviation), 6.13 ± 3.32 mm (tDCS on, tSSS off), 1.78 ± 0.83 mm (tDCS off, tSSS on), and 5.73 ± 1.60 mm (tDCS on, tSSS on). Mean GOF findings were, respectively, 92.3, 87.4, 97.5, and 96.7%. Modeling was affected by tDCS and restored by tSSS, but improvement of the localization error was marginal, even with tSSS. Also, the quality was dependent on the dipole location. Discussion Concurrent tDCS-MEG recording is feasible, especially when tSSS is applied for artifact rejection and when the assumed location of the source of activity is favorable for modeling. More technical studies must be conducted to confirm its feasibility with different source modeling methods and stimulation protocols. Recovery of single-trial activity under tDCS warrants further research.