Teruyoshi Sasayama

Parallelization of Finite Element Analysis of Nonlinear Magnetic Fields Using GPU

The acceleration of a nonlinear magnetic field analysis by parallelizing the finite element method (FEM) is examined using the graphics processing unit (GPU). It is shown that the speedup of the magnetic field analysis is realized by parallelizing the variable preconditioned conjugate gradient (VPCG) method. The Jacobi over-relaxation (JOR) method, conjugate residual (CR) method and conjugate gradient (CG) method are also applied in the variable preconditioning. The results of computations demonstrate that VPCG using the GPU significantly improve the performance. Especially, CG applied by variable preconditioned on GPU is 39 times faster than ICCG on a CPU.

Comparison of volume distribution of magnetic nanoparticles obtained from M-H curve with a mixture of log-normal distributions

We studied the distributions of the magnetic moment and magnetic volume of magnetic nanoparticles (MNPs). These distributions were estimated by applying the singular value decomposition method to the M-H curve measured in the liquid phase. The estimated distributions were compared with a mixture of log-normal distributions, and two results agree well with each other. Using the estimated distribution of the magnetic moment, we also analyzed the M-H curve of immobilized MNPs in order to estimate the average value of the anisotropy energy constant Ku and the characteristic time τON that determines the Neel relaxation of immobilized MNPs. The values Ku and τON are estimated as 4 kJ/m3 and 1×10−9 s, respectively, for Resovist MNPs.

Characterization of the magnetic moment distribution in low-concentration solutions of iron oxide nanoparticles by a high-Tc superconducting quantum interference device magnetometer

We developed a highly sensitive AC/DC magnetometer using a high-temperature superconductor superconducting quantum interference device for the evaluation of magnetic nanoparticles in solutions. Using the developed system, we investigated the distribution of magnetic moments of iron oxide multi-core particles of 100 nm at various iron concentrations that are lower than 96 μg/ml by analyzing the measured magnetization curves. Singular value decomposition and non-regularized non-negative least-squares methods were used during the reconstruction of the distribution. Similar distributions were obtained for all concentrations, and the iron concentration could be determined from the measured magnetization curves. The measured harmonics upon the excitation of AC and DC magnetic fields curves agreed well with the harmonics simulated based on the reconstructed magnetization curves, implying that the magnetization curves of magnetic nanoparticles were successfully obtained as we will show in the article. We compared...

Estimation of magnetic moment and anisotropy energy of magnetic markers for biosensing application

We present a method to evaluate the magnetic moment (m) and the anisotropy energy (E) of magnetic markers, which are the key parameters employed in biosensing applications. The distributions of the m and E values in the marker are evaluated by analyzing the static magnetization (M-H) curve of the suspended markers and the frequency dependence of the AC susceptibility of the immobilized markers, respectively. Then, we obtain the relationship between m and E. In the experiment, four markers made of multicore and single core nanoparticles are examined. We obtain distributions of the m and E values, which show the particular characteristics of each marker. Although the m and E values are widely distributed in the marker, a clear relationship is obtained between the values. Therefore, the obtained m-E curve, as well as the distribution of the m and E values, provides a framework to discuss the dynamic behavior of the immobilized markers. The difference in the estimated m-E curves between the markers is also di...

The effect of Neel relaxation on the properties of the third harmonic signal of magnetic nanoparticles for use in narrow-band magnetic nanoparticle imaging

We study the third harmonic signal of magnetic nanoparticles (MNPs) for use in narrow-band magnetic nanoparticle imaging. We measured the properties of the third harmonic signal, such as frequency and magnetic field dependencies, when the behavior of MNPs was dominated by Neel relaxation. It was shown that the third harmonic signal had both real and imaginary parts, although only the real part is expected from the conventional Langevin function. The real and imaginary parts exhibited different dependences on the frequency and magnetic field. The dynamic behavior of MNPs was analyzed by taking into account the Neel relaxation of MNPs. It was shown that the imaginary part was generated due to Neel relaxation. We obtain an analytical expression for the third harmonic signal, in which distributions of magnetic moment and anisotropic energy of MNPs in the sample were also considered. We show that the analytical results quantitatively explain the experimental results. Our results indicate that the properties of the third harmonic signal of immobilized MNPs are significantly affected by Neel relaxation.

Optimization of 3-D Magnetic Circuit of Linear Oscillatory Actuator for Diaphragm Blower

The efficiency and materials cost of a linear oscillatory actuator (LOA) for a diaphragm blower depends on the design of magnetic circuit. Therefore, it is necessary to optimize the magnetic circuit in order to develop a high efficiency and low price LOA by employing the magnetic field analysis method and an optimization method. In this paper, the 3-D multi-objective optimization of LOA with large magnetic force and small magnet volume is carried out using the combined 3-D finite element method (FEM), evolution strategy (ES), and particle swarm optimization (PSO).

Hysteresis Loss of Fractionated Magnetic Nanoparticles for Hyperthermia Application

Magnetic hyperthermia using magnetic nanoparticles (MNPs) draws significant interest for application in heat therapy for cancerous tumors, wherein it is important to improve the heating efficiency, i.e., to increase the hysteresis loss. In this paper, we examined the hysteresis loss of magnetically fractionated MNPs for hyperthermia application. Original Resovist MNPs were magnetically fractionated into three types, and their hysteresis loops were measured with an excitation field of 2.5 mT (rms) at a frequency of 20 kHz. The hysteresis loss of fractionated MNPs with the larger magnetic moment was approximately two times that of the original Resovist MNPs. A numerical simulation based on the Langevin function was performed to support the experimental results. From the experimental and simulation results, we can conclude that the efficiency of hyperthermia is improved by magnetically separating MNPs.

Narrowband magnetic nanoparticle imaging using cooled pickup coil and gradient field

Magnetic particle imaging (MPI) has been extensively studied for in-vivo biomedical diagnosis. We developed a narrowband MPI system utilizing third harmonic detection. The third harmonic signal from the magnetic nanoparticles (MNPs) was detected with a pickup coil cooled to 77 K, and its output was read out with a resonant circuit. The noise of the detection system was fT/Hz1/2 at a signal frequency of 8.79 kHz. We also introduced the so-called gradient field with a field gradient of 0.3 T/m in order to improve the MPI spatial resolution. We first clarified the properties of MNPs, which provided the basis for MPI using the gradient field. Next, we measured the signal-field map generated from the MNPs when an excitation field with a root mean square value of 1.6 mT and frequency of 2.93 kHz was applied. Using a mathematical technique called singular value decomposition (SVD), we reconstructed an image of the MNP distribution from the measured map. We demonstrated the detection of MNP samples as small as 1 µg at a distance of 50 mm. The spatial resolution of the reconstructed MNP distribution was approximately 10 mm. These results will indicate the feasibility of the system for the application to breast cancer detection.

Application of minimum variance beamformer for estimation of tip position of a nasogastric tube

When a patient is not capable of eating a meal due to difficulty in swallowing, a nasogastric tube can be inserted through the nose to the stomach to provide nourishment in liquid form. However, it is possible to insert the tube into the lungs by mistake. In this paper, we apply a minimum variance beamformer that is commonly used in brain signal source detection to estimate the position of the magnet attached to the tip of a nasogastric tube. To reduce the computational cost, the signal source power is calculated with two different resolutions. Furthermore, the signal source power calculations are performed in parallel. The position may be estimated in less than 1.0 s, making this method suitable for detecting the tip position of a nasogastric tube in real time.

Application of linearly-constrained prewhitening beamformer to reconstruction of highly correlated EEG signal sources

In this study, we compared the performances of minimum variance beamformer (MVBF), linearly-constrained (LC)-MVBF, prewhitening beamformer (PWBF), and LC-PWBF with regard to the estimation of signal location in the brain neocortex. Equivalent current dipoles were located in the primary motor cortex in performed numerical simulations by considering applications in brain-computer interfaces based on modulation of µ and β rhythms. It was confirmed that when the signals were highly correlated, the location bias of LC-PWBF was smaller than that of MVBF, LC-MVBF, and PWBF. Obtained results suggest that LC-PWBF is useful for estimating the location of signal sources that are highly correlated and have low signal-to-noise ratio.