Designing High-Accuracy Permanent Magnets for Low-Power Magnetic Resonance Imaging

Abstract

We present a highly scalable and versatile family of dipole magnet designs suitable for high-resolution magnetic resonance imaging (MRI), constructed entirely from cylindrical permanent magnet rods of two distinct radii. The multiplicity, $N$ , of the main set of rods (which dominate the generated field) can vary from 10 to 26. For each such $N$ , we present several designs that produce magnetic fields uniform to better than 100 parts per million (ppm) over spherical regions of radius half that of the bore. This design family includes peak fields ranging from 0.15 to 0.50 T, adjustable by changing $N$ , with the best accuracy obtained for $N=14$ with a peak field of 0.32 T. With optional correction rods that start out in a zero field configuration, these designs can be tuned to compensate for manufacturing errors simply by adjusting the rotations of these rods. Most of the 100 ppm accuracy radius can thereby be recovered, even in realistic imperfect configurations. We specifically present dimensions for full-body MRI with a standard bore (clearance) diameter of 70 cm, but this family of designs can be scaled uniformly to produce high-uniformity magnetic fields for applications at any length scale.