Daiki Tamada

Skeletal Age Assessment in Children Using an Open Compact MRI System

MRI may be a noninvasive and alternative tool for skeletal age assessment in children, although few studies have reported on this topic. In this article, skeletal age was assessed over a wide range of ages using an open, compact MRI optimized for the imaging of a childs hand and wrist, and its validity was evaluated. MR images and their three‐dimensional segmentation visualized detailed skeletal features of each bone in the hand and wrist. Skeletal age was then independently scored from the MR images by two raters, according to the Tanner–Whitehouse Japan system. The skeletal age assessed by MR rating demonstrated a strong positive correlation with chronological age. The intrarater and inter‐rater reproducibilities were significantly high. These results demonstrate the validity and reliability of skeletal age assessment using MRI. Magn Reson Med, 2013.

Development of a superconducting bulk magnet for NMR and MRI.

A superconducting bulk magnet composed of six vertically stacked annular single-domain c-axis-oriented Eu-Ba-Cu-O crystals was energized to 4.74 T using a conventional superconducting magnet for high-resolution NMR spectroscopy. Shim coils, gradient coils, and radio frequency coils for high resolution NMR and MRI were installed in the 23 mm-diameter room-temperature bore of the bulk magnet. A 6.9 ppm peak-to-peak homogeneous region suitable for MRI was achieved in the central cylindrical region (6.2 mm diameter, 9.1 mm length) of the bulk magnet by using a single layer shim coil. A 21 Hz spectral resolution that can be used for high resolution NMR spectroscopy was obtained in the central cylindrical region (1.3 mm diameter, 4 mm length) of the bulk magnet by using a multichannel shim coil. A clear 3D MR image dataset of a chemically fixed mouse fetus with (50 μm)(3) voxel resolution was obtained in 5.5 h. We therefore concluded that the cryogen-free superconducting bulk magnet developed in this study is useful for high-resolution desktop NMR, MRI and mobile NMR device.

Adaptive optics retinal scanner for one-micrometer light source

We developed an adaptive optics (AO) retinal scanner by using a light source with a center wavelength of 1-microm. In a recent study on optical coherence tomography (OCT), it was proved that 1-microm light provided higher image contrast of deep region of the eye than 840-nm light. Further, high lateral resolution retinal images were obtained with AO. In this study, we performed measurements on two normal subjects in the AO-SLO mode and analyzed its performance toward developing the AO-OCT. With AO correction, we found that the residual RMS wavefront error of ocular aberration was less than 0.1 microm. We also found that the AO retinal scanner in the AO-SLO mode enabled enhanced observation of photoreceptor mosaic.

Magnetic field shimming of a permanent magnet using a combination of pieces of permanent magnets and a single-channel shim coil for skeletal age assessment of children.

We adopted a combination of pieces of permanent magnets and a single-channel (SC) shim coil to shim the magnetic field in a magnetic resonance imaging system dedicated for skeletal age assessment of children. The target magnet was a 0.3-T open and compact permanent magnet tailored to the hand imaging of young children. The homogeneity of the magnetic field was first improved by shimming using pieces of permanent magnets. The residual local inhomogeneity was then compensated for by shimming using the SC shim coil. The effectiveness of the shimming was measured by imaging the left hands of human subjects and evaluating the image quality. The magnetic resonance images for the child subject clearly visualized anatomical structures of all bones necessary for skeletal age assessment, demonstrating the usefulness of combined shimming.

Power Optimization of a Planar Single-Channel Shim Coil for a Permanent Magnet Circuit

We propose a new method of designing a power-optimized single-channel shim coil (SCSC), which enables high homogeneity in the magnetic field of a magnetic resonance imaging system. The design method is based on a superposition of multiple circular currents to account for the power dissipated in the current-carrying coils. With a power-optimized SCSC, magnetic field inhomogeneity is largely corrected and there is negligible degradation of the magnetic field in continuous use.

Design and Evaluation of a Planar Single-Channel Shim Coil for a Permanent Magnetic Resonance Imaging Magnet

We propose a straightforward method of designing a planar single-channel shim coil for magnetic resonance imaging (MRI) using a narrow-gap permanent magnet. The design method is based on the superposition of the current densities produced by planar second-order shim coil elements and optimization of the coefficients used for the superposition. The magnetic field homogeneity was improved from 13 to 3.3 ppm (root mean square) in the central spherical area (diameter = 18 mm), revealing that the planar single-channel shim coil is a useful device for narrow-gap permanent MRI magnets.

Two-dimensional Compressed Sensing using the Cross-sampling Approach for Low-field MRI Systems

A compressed sensing method using a cross sampling and self-calibrated off-resonance correction is proposed. Estimation of the magnetic field inhomogeneity based on image registration enables the off-resonance correction with no additional radio-frequency pulses or acquisitions. In addition to this advantage, a fast and straightforward calculation was achieved by using the first-order components of the magnetic field inhomogeneity. Imaging experiments using a phantom and a chemically fixed mouse demonstrated practical benefits in improving blurring and artifacts in magnetic resonance images in low field magnetic resonance imaging systems.

A New Planar Single-Channel Shim Coil Using Multiple Circular Currents for Magnetic Resonance Imaging

We propose a new planar single-channel shim coil for magnetic resonance imaging (MRI) permanent magnets. The coil design is based on the superposition of multiple circular currents and the stream function method. The designed shim coil was implemented for a permanent magnet with 1.0 T and a 90 mm gap. When the shim coil current was optimized, the magnetic field inhomogeneity decreased from 240 to 97 ppm (peak-to-peak) in the central cylindrical area (54.6 mm diameter, 60.0 mm height), demonstrating that the single-channel shim coil proposed here is a useful device for permanent narrow-gap magnets with complicated magnetic field distribution.

Development of a temperature-variable magnetic resonance imaging system using a 1.0 T yokeless permanent magnet

A temperature variable magnetic resonance imaging (MRI) system has been developed using a 1.0 T permanent magnet. A permanent magnet, gradient coils, radiofrequency coil, and shim coil were installed in a temperature variable thermostatic bath. First, the variation in the magnetic field inhomogeneity with temperature was measured. The inhomogeneity has a specific spatial symmetry, which scales linearly with temperature, and a single-channel shim coil was designed to compensate for the inhomogeneity. The inhomogeneity was drastically reduced by shimming over a wide range of temperature from -5°C to 45°C. MR images of an okra pod acquired at different temperatures demonstrated the high potential of the system for visualizing thermally sensitive properties.

A gradient coil design for a high-temperature superconducting bulk magnet using the finite-difference method

The screening current induced in a bulk superconductor presents a critical problem for gradient coil design because it gives rise to undesirable effects on the gradient field: namely, a decrease in the efficiency and a distortion of the field. In this paper, a new coil design method for the bulk magnet is proposed using the finite-difference method (FDM) and the target-field method. The calculation using the FDM was implemented with the boundary conditions of the bulk magnet. Transverse and longitudinal coils were designed and fabricated for a high-temperature superconducting bulk magnet. The calculated results of the efficiency and the nonlinearity distribution of the gradient field agreed well with the measured results.