Hirotaka Takeshima

Superconducting magnet apparatus using superconducting multilayer composite member, method of magnetizing the same and magnetic resonance imaging system employing the same

A superconducting magnet apparatus which includes a cryostat vessel for containing therein a coolant for realizing superconductivity, a magnetic flux generating unit disposed within the cryostat vessel and including a superconducting multilayer composite member constituting a sustaining medium for a persistent current which generates a magnetic flux along a center axis of a predetermined magnetic field space, and members holding the magnetic field generating unit within the cryostat vessel.

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.

Accurate and convenient evaluation method of magnetic field

An evaluation method for the magnetic field employed in an MRI (magnetic resonance imaging) system is described. In the past, the magnetic-field distribution was approximated by a linear combination of spherical harmonics, and the coefficients were used for the evaluation of the field homogeneity. The accuracy of the evaluation method presented is demonstrated by comparisons between field distributions computed from the coefficients and measurements. The results give an estimate for the number of coefficients needed for a good approximation. The contour map computed in this way conveniently given the field distribution from measurements at only a few points. The image distortion estimated from the field distribution is demonstrated to be a valuable index for evaluating the specification of the magnetic circuit in an MRI system. >

Magnetic resonance imaging apparatus with noise suppressing structure

A magnetic resonance imaging apparatus capable of suppressing noise caused by vibration of a gradient magnetic field coil and improving image quality includes a large number of ferromagnetic shims disposed in a large number of holes formed in a shim tray, and vibration dampers disposed in holes formed in the shim tray to reduce noise generated by vibration of a gradient magnetic field generating coil.