Kazuhiro Tanabe

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.

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.

Magnetic Particle Imaging Utilizing Orthogonal Gradient Field and Third-Harmonic Signal Detection

We have developed a 2-D magnetic nanoparticle (MNP) imaging system using an orthogonal gradient held and third-harmonic signal detection. An ac excitation held was applied to generate third-harmonic signals from MNP on the basis of their nonlinear magnetization curves, and a dc gradient held was applied perpendicular to the excitation held. The dependence of the third-harmonic signal on the dc held was very different from that in the conventional parallel case. We obtained a contour map of the signal held from the MNP located at z = 35 mm below the pickup coil using an excitation held of 1 mT and gradient held of 0.4 T/m. We could obtain spatial resolution of 10 mm even in this small gradient held. Then, we analyzed the contour map using singular value decomposition (SVD) to improve the spatial resolution of the position estimation of MNP. Using SVD, we converted the held map to the MNP distribution. The spatial resolution was improved from 10 to 6.7 mm. We could clearly distinguish three MNP samples separated with 10 mm spacing located at z = 35 mm.

Evaluation of Complex Harmonic Signals From Magnetic Nanoparticles for Magnetic Particle Imaging

We evaluated the complex harmonic signals from magnetic nanoparticles (MNPs) for magnetic particle imaging (MPI). We measured the complex harmonic signals from Resovist MNPs, one of the candidates for an MPI tracer, when only an ac excitation field and both the ac and the dc excitation fields were applied. To evaluate the complex harmonic magnetization spectra, we performed a numerical simulation accounting for the distributions of magnetic moments, anisotropy energy barriers, and hydrodynamic sizes of MNPs in the sample. The simulation results agreed well with experimentally obtained amplitude of the harmonic signals. The simulation results of the phase lag, which was caused by the finite relaxation time, agreed with the experiments for low harmonic numbers. The phase lag deviation between them became significant for higher harmonic numbers. For quantitative evaluation of the phase lag for higher harmonic numbers, a more accurate estimation of the Néel relaxation time is required.

Low Wavelength Dependency Design for MMI (Multi-Mode Interference) Mode Converter

Mode-division multiplexing (MDM) is widely researched, and we are researching mode converter by using multi-mode interference (MMI) waveguide. Wavelength division multiplexing (WDM) must be used in addition to the MDM, however, wavelength dependency exits for MMI in general as it works based on the interference of light. In this paper, we analyze the wavelength dependency of the MMI 0th to 1st mode-converter theoretically. As a result, we clarify the design scheme to suppress the wavelength dependency. Moreover, we show the sufficient performance of less than 1.0 dB in entire C-band for highly confined waveguide (Si/SiO2 waveguide) as a design example.

Narrowband Magnetic Particle Imaging Utilizing Electric Scanning of Field Free Point

We developed a narrowband magnetic particle imaging (MPI) system that uses third-harmonic signal detection and electrical scanning of the field free point (FFP). Comparing with mechanical scanning, we can decrease the measurement time significantly and increase the signal-to-noise ratio as well. For electrical scanning, we designed and constructed gradient and shift coils. The gradient coil consisting of four pieces of planar coils generated the gradient field with a field gradient of 0.4 T/m at a height of 25 mm from the coil surface. The FFP can be moved ±8 mm by supplying a current of ±6.6 A to the shift coil. Using the developed system, we detected two magnetic nanoparticle (MNP) samples located at a depth of 35 mm below the pickup coil with a spacing of 10 mm. By applying an excitation field of 1 mT at 22.75 kHz, we measured the third-harmonic signal from the MNP samples and obtained a contour map of the signal field in an area of 16 × 16 mm2. Then, we converted the field map into an MNP distribution using singular value decomposition method. It was shown that the spatial resolution of the reconstructed MNP distribution was improved compared with that of the measured contour map of the signal field. The spatial resolution for MNP detection in MNP distribution was 5 mm and two MNP samples were distinguished clearly. This result indicated that MPI using electrical scanning of the FFP was successfully performed.

Design criteria for wavelength independent MMI mode converter

The design criteria that achieve the excess loss of less than 1.0 dB in entire C-band has been clarified for MMI mode converter. As a result, we have confirmed that MMI width in addition to access waveguide width ratio over MMI width are the important design criteria.

Preliminarily propagation loss evaluation of core-top etched waveguide for step-core LP21 mode converter

Propagation loss of core-top etched Si/SiO2 waveguide was preliminarily evaluated for the first time. As a result, approximately 8 dB/cm increase has been confirmed after core-top etching.