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Rectangular wave eddy current testing (RECT), which is performed using a rectangular wave excitation current, can simultaneously obtain multiple datasets. However, the high-frequency harmonic signal detected by the detection coil interferes with the low-frequency signal based on Faraday’s law of induction. The method proposed in this study is a type of electronic bridge, wherein a compensation method is implemented to enhance the low-frequency signal of the RECT using a digital-to-analog converter (DAC). The compensation wave generated by the DAC is determined such that the output signal becomes zero when the probe does not detect any flaws. A 12 mm thick aluminum plate with flat-bottom drill holes on the backside is used as the specimen. The holes have a diameter of 3 mm and depths of 2, 4, and 6 mm, respectively. The results demonstrate that the flaw signal cannot be detected without compensation. However, the flaw signal can be successfully detected around these holes with compensation.

In magnetic nanoparticle tomography (MNT), the reduction of artefacts and the calculation time can be used to estimate the position of magnetic nanoparticles (MNPs). Non-negative least squares (NNLS) inverse problem analysis has been used in MNT systems for this task. However, owing to the presence of measurement noise and the high sensitivity of the NNLS method, it often estimates certain MNPs inaccurately, i.e., it generates artefacts. In addition, its calculation time is very high. In this study, we applied the minimum variance spatial filter (MV-SF) inverse problem analysis to MNT and estimated the position of an MNP sample containing 100 μ g of Fe. Using the MV-SF method, MNP samples placed at a depth of 25–40 mm were observed to have no artefacts. Moreover, the MV-SF method was also observed to be faster than the NNLS method by a factor of approximately 20. These results verify the feasibility of the MV-SF method for estimating the MNP positions in an MNT system.

For the development of electric propulsion aircrafts with light weight, low emission and high efficiency, MW-class fully superconducting synchronous machines operating at liquid nitrogen temperature were conceptually designed with REBa2Cu3O7- δ (REBCO) superconducting tapes in our previous studies. To verify the actualization of the structure and cooling method, a 1 kW-class prototype fully superconducting synchronous motor was designed and constructed in this study. The fixed armature was cooled with subcooled liquid nitrogen at 65 K. The rotor was cooled with helium gas. The pole number was two for the future high speed operation. The applicability of the complicated casing structure with three chambers into fully superconducting motor was also investigated from the viewpoint of thermal insulation. The operations as a motor up to 500 rpm and a generator demonstrated that the designed structure and cooling method were reasonable and effective for cooling the fixed armature and rotating field windings.

In the application of the magnetic nanoparticle (MNP) imaging technique, which uses a large ac magnetic field and detects harmonic signals, it is important to attenuate the fundamental signal generated by the magnetic field used for excitation. In this article, we propose a method to reduce this fundamental signal using a compensation circuit with a digital-to-analog (D/A) converter, which can be interpreted as an electrical bridge. We apply the proposed method to an MNP imaging system. The system contains one excitation coil that generates an ac magnetic field and 16 pickup coils that detect the third-harmonic signals from the MNP samples. The position of an MNP sample containing $100~\mu \text{g}$ of Fe was estimated using this system via an inverse problem analysis. The results demonstrate that the fundamental signal is suppressed successfully and also show that the distance between the estimated and actual positions of the MNP sample is less than 10 mm at a depth of 50 mm.

Magnetic nanoparticles (MNPs) are physically immobilized in some biomedical applications. In this case, dynamic ac magnetization that determines MNPs’ performance has strong dependence on the relative direction between the magnetic easy axes of MNPs and the applied excitation field. To study this dependence, we prepared immobilized MNP samples with different degrees of easy axes’ alignment. The degree of alignment was changed by setting the strength of a dc field that is applied during the immobilization process. The magnetic properties of these samples, such as ac magnetization, coercive field, amplitude of third-harmonic signal, and hysteresis loss, are compared with those of a randomly oriented easy axes’ sample. It was shown that these values are significantly affected by the degree of easy axes’ alignment when the strength of the ac excitation field, $H_{0}$ , is relatively small. The alignment effect gradually becomes smaller when $H_{0}$ is increased. These findings are important for immobilized MNP applications in hyperthermia and magnetic particle imaging.

Accurate, easy, and fast inspection of defects on the backside of thick steel plates is essential for the maintenance of infrastructures. Low frequency eddy current testing (LF-ECT) is a promising method to detect defects of the backside of steel plates, with a thickness of approximately 10 mm. However, it is possible that the signal from the backside defect is smaller than that from the surface magnetic noise, causing difficulty identifying the backside defect. In this study, we propose a method to reduce the surface noise by employing a square wave inverter to generate a harmonic signal (rectangular wave ECT, or RECT), and the result demonstrates that the surface noise is successfully reduced using the harmonic signal.

High power density motors are required for electric propulsion aircrafts. The highest power density of the conventional motor is reported as 5.2 kW/kg up to now. Superconducting rotating machines have a potential to realize higher power density due to its high current density and a large magnetic field property. In this study, the induction motor with air-cored superconducting armature windings was conceptually designed. The output power range was 3 to 6 MW and the output power density aimed to be over 20 kW/kg. The armature voltage should be below kV-class and joule loss below 122 kW. The target efficiency was over 95%. The electromagnetic design and analysis were carried out by JMAG Designer. The number of magnetic poles and operating temperatures were set as two poles and liquid hydrogen temperature of 20 K, respectively. The parameter was power frequency in the range of 100-250 Hz. As a result, all the requirements were satisfied when the power frequency was 130 Hz. The feasibility of the MW-class superconducting induction motor was investigated, and in conclusion, the superconducting induction motor can be applied to the aircraft propulsion system.

Currently, feasibility studies on fully turboelectric propulsion systems for electric aircrafts are being conducted worldwide. A fully superconducting rotating machine can realize fully turboelectric propulsion systems with light weight and high power density. In the simple casing model considered in a previous study, the field winding forms a large percentage of the generator weight. Therefore, to reveal the relationship between the operation temperature of the field winding and output power density, we compared an inner cryostat model having a field winding operation temperature of 20 K with the simple casing model having a field winding operation temperature of 64 K in terms of the output power density, efficiency, and increasing temperature. The stator and rotor rooms of the inner cryostat model were insulated by introducing the vacuum layer. The inner cryostat casing structure can have a lower refrigerant gas temperature, e.g., 20 K. Consequently, the temperature of the simple model increased from 5.4 to 7.1 K and that of the inner cryostat model increased from 3.0 to 6.1 K. The simple and inner cryostat models’ power densities were 18.8 and 21.7 kW/kg, respectively. Both models achieved 99% efficiency. No major difference was observed between both models in terms of efficiency and output power density.

With air traffic predicted to increase in the future, CO2 emission must be decreased to prevent global warming. Our research group previously proposed electric propulsion systems based on superconducting technology, which is promising for reducing emissions. In this study, propulsive 2 MW fully superconducting motors were conceptually designed. At operating temperatures of 20 and 64 K, we observed the influence of the superconducting magnetic shield on the electromagnetic properties, especially the output power density, AC loss, and efficiency. The superconducting magnetic shield can reduce leakage magnetic field to the outside of the motor by inducing a current, and the shield is reproduced with a conductor of extremely high conductivity for simplicity in electromagnetic analysis. As a result, at an operating temperature of 20 K, the superconducting motors with the superconducting magnetic shields showed a high power density greater than 40 kW/kg and high efficiency of 99.7%. Even at an operating temperature of 64 K, the motors with the superconducting shield showed a power density and efficiency of 20 kW/kg and 99.4%, respectively.

We have developed a new type of magnetometer using a high-critical-temperature superconductor (HTS) coil. This magnetometer consists of pickup and input coils made from HTS tape. The two coils are connected with very low joint resistance, and the signal flux that is collected by the pickup coil is transferred to the input coil. The signal at the input coil is then read out using a newly developed inductance-modulation scheme. The inductance of the input coil is modulated over time using a magnetic wire that is inserted into the input coil, and the magnetic flux is converted into a signal voltage using this time-dependent inductance. It is demonstrated that this magnetometer can measure low-frequency signal fields down to 0.03 Hz without any reduction in responsivity. The noise level was measured to be 0.85 pT/Hz 1/2 at frequencies above 25 Hz and reached 11 pT/Hz 1/2 at 1 Hz when using a pickup coil with 50 turns and an average diameter of 25 mm.

Future aircraft are expected to have low noise pollution, low emission, and low fuel consumption. A fully turbo-electric propulsion system was proposed by NASA with these aims. The advantages of fully superconducting rotating machines are their weight and efficiency, owing to a high current density resulting in no iron core and no joule heat. In this paper, 10-MW fully superconducting generators with REBa 2 Cu 3 Oy (RE: Rare Earth, Y, Gd, Eu, and others, REBCO) superconducting tapes were studied in relation to their output power density and efficiency. The target output power density is over 20 kW/kg. Fully superconducting generators of 10 MW were designed, then a numerical simulation of the rated operation was conducted. The ac loss of the superconducting windings, iron loss of the yoke, total efficiency, dry weight, and output power density were evaluated, and their dependence on the operating temperature and the synchronous rotating speed were investigated. Here laser-scribing of the wires into a multifilamentary structure and transposed parallel conductors were introduced for ac loss reduction and current capacity enhancement. Consequently, the efficiency reached 99.2% in the case where the REBCO superconducting tapes were scribed into a 10-filament structure. The best model achieved a power density of 26.7 kW/kg. The iron yoke accounts for approximately half the total weight of all the models. The result suggests that the development of superconducting shields brings significant improvement to the output power density.

Defect detection on the backside of thick steel plates is essential for the safety maintenance of most infrastructures in which they are used. Eddy-current testing (ECT) is a promising method to detect the defects in a steel plate, and inspections at low frequencies are necessary to suppress the skin effect. However, if the frequency is reduced further, the signal from the detection coil also becomes smaller. Therefore, we assessed the optimal frequency for the detection of slit defects on the backside of a 10 mm thick steel plate based on experiments and electromagnetic simulations using the finite-element method. The results suggest that the optimal frequency to detect the defects is approximately 8–10 Hz when the height of the slit exceeds 6 mm. Similarly, in the simulation, the signal was maximized when the frequency was 8–10 Hz, which validates the experimental results. Furthermore, we propose a method to emphasize the deeper defects using multi-frequency data as a fundamental study. The results show that the backside defects are depicted more clearly.

The structure of magnetic nanoparticles affects the signal intensity and resolution of magnetic particle imaging, which is derived from the harmonics caused by the nonlinear response of magnetization. To understand the key effects of particle structures on the magnetization harmonics, the dependence of the harmonics on the size and anisotropy of different structures was investigated. We measured the harmonic signals with respect to different magnetic nanoparticle structures by applying an AC field with a gradient field for magnetic particle imaging, which was compared with the numerically simulated magnetization properties. In addition, the dynamics of the easy axis of magnetic nanoparticles in the liquid state were evaluated. The difference between the harmonics in the solid and liquid states indicates the effective core size and anisotropy due to particle structures such as single-core, chainlike, and multicore particles. In the case of the chainlike structure, the difference between the harmonics in the solid and liquid states was larger than other structures. In the numerical simulations, core diameters and anisotropy constants were considered as the effective values, such as the increase in anisotropy in the chainlike structure due to dipole interaction. The multicore particles showed high harmonics owing to their large effective core diameters. The superparamagnetic regime in the multicore structure despite the large effective core diameter was derived from the small effective anisotropy. The effective core size and the effective anisotropy of each particle structure and their impacts on the harmonic signals were revealed.

Generators used in electric aircraft require a high-power density, and AC loss is also a significant problem. We designed 10 MW REBCO superconducting synchronous generators at 64 K to achieve a power density of 20 kW/kg and low AC loss. In this study, electromagnetic analyses were performed using finite element method software. Consequently, the thickness of the back yoke is 50 mm or less so that the generator can meet the target power density. The method of decreasing the magnetic field of the armature winding and increasing that of the field winding was used to effectively reduce the AC loss. As a result, the generator achieved a high-power density of 21.0 kW/kg, reducing the AC loss from over 600 kW to 415 kW.

The evaluation of opposite-side defects in thick ferromagnetic objects (for example, steel plates) is essential for safety maintenance. Eddy-current testing (ECT) can detect defects in metallic specimens; however, owing to the skin effect, its use is limited to the evaluation of surface or subsurface defects. Therefore, in this paper, we propose a non-contacting inspection system that consists of two excitation coils and a detection coil to detect slit defects on the backside of a steel plate using low-frequency ECT (LF-ECT). Using the proposed system, we examine whether LF-ECT mainly detects the leakage or the flux induced by the eddy current, and also examine which direction of the excitation currents in the two excitation coils is suitable for detecting slit defects. The results suggest that the method to drive the current in the same direction and to detect the imaginary part of the magnetic flux density, which corresponds to the flux induced by the eddy current, is suitable for detecting slit defects. Subsequently, we examine the detection limit of the height of the slit defect when the thickness of the steel plate is 10 mm and the width and length of the slit defect are constant, at 50 and 1 mm, respectively. The results demonstrate that the slit defect can be detected by using the proposed system when the height is greater than or equal to 4 mm. Therefore, the proposed system is a promising tool to detect critical defects on the backside of steel plates.

Naso- or oro-gastric tubes are used for patients who have difficulty swallowing, to deliver nutrients or medicines to the stomach. To observe whether the tube is inserted into the stomach correctly, we have proposed a method that estimates the position of the permanent magnet attached to the tip of the tube using several magnetic sensors placed outside the patient’s body [1], [2]. The mean of the tip position estimation error is less than 10 mm when the distance between the sensor plane and the permanent magnet is 150 mm [2]. However, if the system is used on pregnant or fat patients, the system needs to perform accurately even for distances greater than 150 mm. To achieve the requirement mentioned above, the sensitivity of the magnetic sensors should be enhanced. Although high-sensitivity magnetoimpedance (MI) and magnetoresistance (MR) sensors can detect signals of the order of nT, the performance is only achieved in a magnetically shielded room due to the limitation of the measurement range and the existence of the geomagnetic field; this creates a problem for the system’s practical use in hospitals. Therefore, we propose a method to compensate the geomagnetic field using offset coils.

Magnetic nanoparticles (MNPs) have been widely studied for bio-sensing applications, where suspended and immobilized MNPs can be magnetically distinguished using their different magnetic properties. We study magnetic properties of suspended and immobilized MNPs when the Neel relaxation time is much shorter than the Brownian. We show in both numerical simulation and experiment that they have different magnetic properties such as AC magnetization curves and harmonic spectra even though the dynamic behavior of both MNP types is primarily dominated by Neel relaxation. This difference is caused by the partial alignment of the easy axes in suspended MNPs when an AC magnetic field is applied. We introduce a distribution function for the angle of easy axis alignment. We also show a method to evaluate the distribution function from the measured AC magnetization curve and clarify the relationship between easy axis alignment and the AC field strength. Using the distribution function, we can quantitatively discuss the effect of easy axis alignment on the magnetic properties of suspended MNPs. The obtained results provide a basis for using MNPs in bio-sensing.

To correctly insert a gastric tube into the stomach, the position of an attached permanent magnet is estimated by solving the inverse problem using measurement data obtained from magneto-impedance (MI) sensors placed outside the patient’s body. The ubiquity and power of tablet computers make them promising tools to easily confirm the position of the tip of the gastric tube. However, a single mobile device may be insufficient for actually solving the inverse problem. Therefore, we propose a distributed cooperative local access network system for the estimation of the position of a permanent magnet attached to the tip of a gastric tube. In addition, we evaluate whether ferromagnetic materials near a sensor affect the performance of the system. Experimental results show that the position estimation error is less than 10 mm, even when the tip is 150 mm from a MI sensor, and that the tip position can be displayed on a tablet computer every 0.25 s. These results suggest that the proposed method holds promise for hospital applications.

The magnetic core-size distribution of magnetic nanoparticles (MNPs) was estimated from three independent measurements, i.e., magnetization, ac susceptibility (ACS), and magnetic relaxation (MRX). First, distribution of magnetic moment m in MNP sample was estimated by analyzing the static magnetization ( $M$ – $H)$ curve of the MNP sample in suspension. Next, distribution of anisotropy energy E was estimated by analyzing the ACS of immobilized MNP sample measured from 10 Hz to 1 MHz. The ACS measurement in much lower frequency range was substituted by the MRX measurement. MRX of immobilized MNP sample was measured from 2 to $10^{4}$ s, corresponding to the frequency range from $10^{-4}$ to 0.5 Hz in ACS measurement. The relaxation curve was analyzed using a newly developed analytical method to estimate the E distribution of the MNPs in the range of large E values. Then, the estimated distributions of m and E were transferred to the distributions of magnetic core size. Core-size distributions obtained from m and E distributions reasonably agreed with each other. Namely, we obtained similar core-size distribution from static and dynamic properties of MNP sample, confirming the validity of the present method. Therefore, the present method can be useful to estimate the core-size distribution of MNP sample.

We propose a magnetic particle imaging (MPI) method based on third harmonic signal detection using a small field gradient and multiple pickup coils. First, we developed a system using two pickup coils and performed three-dimensional detection of two magnetic nanoparticle (MNP) samples, which were spaced 15 mm apart. In the experiments, an excitation field strength of 1.6 mT was used at an operating frequency of 3 kHz. A DC gradient field with a typical value of 0.2 T/m was also used to produce the so-called field-free line. A third harmonic signal generated by the MNP samples was detected using the two pickup coils, and the samples were then mechanically scanned to obtain field maps. The field maps were subsequently analyzed using the nonnegative least squares method to obtain three-dimensional position information for the MNP samples. The results show that the positions of the two MNP samples were estimated with good accuracy, despite the small field gradient used. Further improvement in MPI performance will be achieved by increasing the number of pickup coils used.

In some biomedical applications of magnetic nanoparticles (MNPs), the particles are physically immobilized. In this study, we explore the effect of the alignment of the magnetic easy axes on the dynamic magnetization of immobilized MNPs under an AC excitation field. We prepared three immobilized MNP samples: (1) a sample in which easy axes are randomly oriented, (2) a parallel-aligned sample in which easy axes are parallel to the AC field, and (3) an orthogonally aligned sample in which easy axes are perpendicular to the AC field. First, we show that the parallel-aligned sample has the largest hysteresis in the magnetization curve and the largest harmonic magnetization spectra, followed by the randomly oriented and orthogonally aligned samples. For example, 1.6-fold increase was observed in the area of the hysteresis loop of the parallel-aligned sample compared to that of the randomly oriented sample. To quantitatively discuss the experimental results, we perform a numerical simulation based on a Fokker-Planck equation, in which probability distributions for the directions of the easy axes are taken into account in simulating the prepared MNP samples. We obtained quantitative agreement between experiment and simulation. These results indicate that the dynamic magnetization of immobilized MNPs is significantly affected by the alignment of the easy axes.

Iron oxide magnetic nanoparticles (MNPs), whose typical sizes are in the range of few nanometers to tens of nanometers, are widely studied for biological applications because they offer some attractive possibilities. First, their size can be controllable in order that they can get close to a biological target of interest. Being coated with biological molecules, MNPs can bind to a biological target. Second, they can be manipulated at a distance using an external magnetic field gradient. Third, the MNPs respond to an AC magnetic field and exhibit different responses depending on their size, the frequency of the magnetic field, and so on. Owing to these attractive properties of the MNPs, many researchers have explored the biological applications of the MNPs such as the separation of biological targets, drug delivery, immunoassay, hyperthermia, and magnetic particle imaging (MPI) for biomedical imaging [1]. In this presentation, we focus on the applications of the MNPs for immunoassay and MPI. Immunoassay is a detection of biological target, such as disease related proteins and cells, for medical diagnosis. We developed a magnetic immunoassay system utilizing magnetic markers [2]. In this method, the marker is made of MNPs coated with antigen-specific antibodies. The markers coupled to the biological target are detected by measuring the magnetic signal from the bound marker. One of the advantages of the magnetic method is that we can perform immunoassay in the liquid phase, i.e., we can magnetically distinguish bound markers, which are coupled to the biological target, from unbound (free) markers without utilizing a time-consuming washing process called B/F separation. This function can be realized by using the difference in Brownian relaxation time between the bound and free markers [2]. So far we performed a magnetic immunoassay using streptavidin coated magnetic markers, biotins as target which are fixed on the surface of large polymer beads, and anisotropic magneto-resistive sensors. The sensitivity was estimated as high as 10 fM in terms of the molecular-number concentration. MPI, which was introduced by Gleich and Weizenecker [3], is a new modality for imaging the spatial distribution of MNPs, especially for in-vivo diagnostics. Since the direct nonlinear magnetization signals from MNPs are sensitively detected with no background tissue signal, MPI could be applied for clinical applications such as angiography, stem cell tracking, and cancer imaging. The performance of MPI strongly depends on the magnetic properties of the MNPs, such as magnetic moment, relaxation time, and their distributions. In this presentation, we will show the method to estimate these important parameters and their distributions in MNPs sample [4]. We will also discuss how the viscosity of surrounding medium affects the dynamic magnetization properties of MNPs from the view point of sensitivity and spatial resolution of the image in MPI [5]. References: [1] Q. A. Pankhurst, et al., J. Phys.D: Appl. Phys. 42, (2009) 224001. [2] K. Enpuk, et al., Applied Physics Express 7, (2014) 097001. [3] B. Gleich and J. Weizenecker, Nature 435, (2005), 1214. [4] T. Yoshida, et al., J . A ppl. P hys. 114, (2013) 173908. [5] T. Yoshida, et al., J. Magn. Magn. Mat. 340, (2015) 105.

We applied low-frequency eddy current testing (ECT) using a high-temperature superconducting (HTS) coil in order to measure the thickness of an iron plate. Using this method, we measured changes in coil impedance when the iron plate was placed below the coil. Although low-frequency measurements were necessary to avoid the skin effect, changes in coil impedance became very small at low frequencies. For this reason, an HTS coil was used in order to sensitively measure these small changes. First, changes in the inductance L and the resistance R of the coil were measured when the iron plate was positioned 18 mm below the HTS coil, and the thickness of the plate was changed in increments from 6 to 22 mm. The results show that we were able to estimate the thickness of the plate up to 22 mm based on the changes in R when an excitation frequency of 4 Hz was used. Next, the effect of the liftoff between the iron plate and the HTS coil on the changes in Land R was studied while the liftoff was changed from 18 to 28 mm. The results show that the liftoff could be estimated from the changes in L. We were therefore able to determine the liftoff and the thickness of the iron plate by measuring changes in L and R, respectively. Results obtained from numerical simulation using the finite element method were in agreement with the experimental results.

We propose an imaging system for magnetic nanoparticles (MNPs) using multiple magnetic sensors based on amplitude modulation. The system was used to obtain two-dimensional images of two MNP samples spaced 15 mm apart at a depth of 30 mm. An AC magnetic field was used at an operating frequency of 5.4 kHz, and a third-harmonic signal generated by the MNP samples was detected using 16 pickup coils. To increase the position information of the MNP samples, the amplitude of the AC magnetic field was changed from 0.24 to 4.8 mT. The third-harmonic signals were subsequently analyzed using nonnegative least squares to image the MNP samples. The results show that the positions of the two MNP samples were estimated with good accuracy.

We studied the properties of a magnetometer using high-Tc superconducting coils and an inductance modulation scheme. First, we obtained analytical expressions for the responsivity and magnetic field noise of the magnetometer that could be used for the design of a magnetometer. Next, a prototype magnetometer was fabricated and its performance was measured in liquid nitrogen. It was observed that the measured results agreed well with the calculated results, which indicated the validity of the present design method. We obtained a magnetic field noise of 0.8 pT/Hz1/2 above 25 Hz when using a pickup coil with 50 turns and an average diameter of 25 mm. At low frequencies, the field noise increased when decreasing the frequency, and reached 9 pT/Hz1/2 at 1 Hz. We also discuss the improvement of the magnetometer performance when the size of the pickup coil is increased.

We demonstrate the wash-free detection of biological targets based on the Brownian relaxation of magnetic markers. The Brownian relaxation time of bound markers was prolonged by using a cluster (or agglomerate) of bound markers that was formed via the binding reaction between markers and targets. Thus, bound and free markers were differentiated magnetically by using the difference in their relaxation times. Two types of magnetic properties, i.e., third-harmonic signal and magnetic relaxation, were used for measurement. The concentration of target was detected from the decrease in the third-harmonic signal and the increase in the magnetic-relaxation signal. We demonstrated the detection of C-reactive proteins using these two properties. In both cases, a good correlation was obtained between the measured signal and the concentration of target. It was also shown that the two methods had different concentration dependences, which will be caused mainly by the distribution of cluster size of the bound markers.

In this paper, we propose improved methods of liquidphase detection of biological targets utilizing magnetic markers and a high-critical-temperature superconducting quantum interference device (SQUID). For liquid-phase detection, the bound and unbound (free) markers are magnetically distinguished by using Brownian relaxation of free markers. Although a signal from the free markers is zero in an ideal case, it exists in a real sample on account of the aggregation and precipitation of free markers. This signal is called a blank signal, and it degrades the sensitivity of target detection. To solve this problem, we propose improved detection methods. First, we introduce a reaction field, Bre, during the binding reaction between the markers and targets. We additionally introduce a dispersion process after magnetization of the bound markers. Using these methods, we can obtain a strong signal from the bound markers without increasing the aggregation of the free markers. Next, we introduce a field-reversal method in the measurement procedure to differentiate the signal from the markers in suspension from that of the precipitated markers. Using this procedure, we can eliminate the signal from the precipitated markers. Then, we detect biotin molecules by using these methods. In an experiment, the biotins were immobilized on the surfaces of large polymer beads with diameters of 3.3 μm. They were detected with streptavidinconjugated magnetic markers. The minimum detectable molecular number concentration was 1.8 × 10−19 mol/ml, which indicates the high sensitivity of the proposed method. key words: high-critical-temperature superconducting quantum interference device (SQUID), magnetic marker, immunoassays, liquid-phase detection

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 $\times$ 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.

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.

A nasogastric tube is commonly used to pour nutrients into the stomachs of patients who have difficulty swallowing in order to provide them with the required nutrition. However, it is possible to inadvertently insert the tube into the lungs. Thus, a method of obtaining the tip position of the tube is desired. In this paper, we use predata of magnetoimpedance (MI) sensors to estimate the position of the magnet attached to the tube tip using a minimum variance beamformer. Experimental results show that the position estimation error is

We performed three-dimensional magnetic particle imaging using third-harmonic signal detection. The measurement system consists of three coil types, i.e., an ac excitation coil, a dc gradient coil, and five pickup coils cooled to 77 K. In the experiment, two magnetic nanoparticle samples containing 10 µg (Fe) of magnetic nanoparticles were arranged three-dimensionally with a spacing of 16 mm. An ac excitation field with a strength of 1.6 mT at 3 kHz and a dc gradient field with a field gradient of 0.2 T/m were used. A third-harmonic signal generated from the magnetic nanoparticle samples was detected using the five pickup coils. The noise of the detection system was as low as SB^(1/2) = 7 fT/Hz^(1/2) at a signal frequency of 9 kHz. A magnetic field map generated from the two magnetic nanoparticle samples was obtained by scanning the samples two-dimensionally. Field maps obtained with the five pickup coils were analyzed to reconstruct a three-dimensional distribution image of the magnetic nanoparticle sample. A mathematical technique called the nonnegative least squares method was used for this purpose. We successfully demonstrated three-dimensional magnetic particle imaging. Namely, we reconstructed the three-dimensional positions of the two magnetic nanoparticle samples with good accuracy. Quantities of the two magnetic nanoparticle samples were also estimated reasonably well. These results indicate the feasibility of the present system for three-dimensional magnetic particle imaging.