A permanent magnet for whole-body NMR imaging

Permanent magnets appear to provide a viable alternative to resistive and superconducting magnets for whole-body NMR imaging. Less expensive imaging systems, substantially reduced siting costs, and lower operating costs would be realized. Two dimensional (2D) and three dimensional (3D) finite element analysis (FEA) are used to optimize magnet design and to determine appropriate shimming to obtain the desired magnetic field homogeneity. Several innovations of both 2DFEA and 3DFEA are described in this paper. A whole-body permanent magnet will be constructed in 1985. Using ceramic 10 magnet material, a central magnetic field of approximately 0.15T (1500 gauss) can be obtained with an airgap of 50 cm, and a pole piece of 100 cm × 180 cm × 30 cm. A box-like support structure, with U-shaped sides, constrains the majority of magnetic flux to the vicinity of the magnet, increases the magnetic field in the airgap and improves field homogeneity. Weight of the magnet structure will be approximately 10,000 kg (11 tons). A homogeneity of 30 to 50 ppm is anticipated over a centrally-located 25 cm cube by appropriate mechanical shimming.