Shigeru Kakugawa

A study on optimal coil configurations in a split-type superconducting MRI magnet

Optimal coil configurations in a split-type superconducting MRI magnet have been investigated, and the feasibility of the compact and high performance magnet is presented. A highly homogeneous magnetic field of about 10 ppm/50 cm diameter spherical volume (DSV) is attainable by eliminating error fields of higher orders and, moreover, the radius of the outermost coil can be reduced to about 70 percent of that of the magnet with positive current simply by alternately arranging coils with positive and negative currents.

Three-dimensional optimization of correction iron pieces for open high field MRI system

We have developed a new magnet for open high field (0.7 T) MRI systems. In order to homogenize magnetic fields in the field of view, an optimization method to determine initial configurations of large correction iron pieces was developed. In our method, we combined linear programming (LP) and 3D magnetic FEA and, using analytic formulae for LP, we computed field harmonics due to the volume of magnetized iron.

Shielding stray magnetic fields of open high field MRI magnets

We have developed a new magnet for open high field (0.7 T) MRI systems. Since MRI systems are installed in hospital sites, it is important to reduce stray magnetic fields of the magnets. We studied passive, active and hybrid shielding methods and compared them. In order to study feasibility of active shielding, we considered higher order multipole stray fields. We present details of the calculation method and the results.

Electrical properties of superconducting joint between composite conductors

To investigate the electrical properties of the joint between superconducting multifilamentary wires, the authors used a radio-frequency superconducting quantum interference device (RF-SQUID) voltmeter to measure the extremely small voltage induced across the joint. They measured the time variation of both the current induced in a loop of superconducting wire and the voltage induced across the joint. A voltage sensitivity in the picovolt range was obtained with this apparatus. Comparisons of current decay between superconducting loops with soldered, spot-welded and cold-welded joints were made. A flux-creep-like phenomenon was seen in the current decay for the cold-welded and spot-welded joints.

An optimal design of coaxial coils with constraints on inner and outer multipole magnetic fields

An optimal design method for magnets with double-layered coaxial coils to generate a uniform central magnetic field and a small stray magnetic field has been proposed. To find the optimal current density distribution, we used linear programming with constraints on the multipole field strengths, computed with analytical formulae, of the central and stray fields. The final coil positions were optimized with a quasi-Newton method, again taking into account the inner and outer multipole field strengths.