||Teruyoshi Sasayama, Takashi Yoshida, Keiji Enpuku, Two-dimensional magnetic nanoparticle imaging using multiple magnetic sensors based on amplitude modulation, Journal of Magnetism and Magnetic Materials, 10.1016/j.jmmm.2020.166765, 505, 166765, 2020.07, 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..
||Teruyoshi Sasayama, Yuya Tsujita, Manabu Morishita, Masahiro Muta, Takashi Yoshida, Keiji Enpuku, Three-dimensional magnetic nanoparticle imaging using small field gradient and multiple pickup coils, Journal of Magnetism and Magnetic Materials, 10.1016/j.jmmm.2016.10.107, 427, 144-150, 2017.04, 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..
||Teruyoshi Sasayama, Tomoki Ishida, Masaaki Matsuo, Keiji Enpuku, Thickness Measurement of an Iron Plate Using Low-Frequency Eddy Current Testing with an HTS Coil, IEEE Transactions on Applied Superconductivity, 10.1109/TASC.2016.2535366, 26, 5, 9001305, 2016.08, 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 L and 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..
||Teruyoshi Sasayama, Takashi Yoshida, M. M. Saari, Keiji Enpuku, Comparison of volume distribution of magnetic nanoparticles obtained from M-H curve with a mixture of log-normal distributions, Journal of Applied Physics, 10.1063/1.4919268, 117, 17, 17D155, 2015.05, 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..