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 (2-D) and three-dimensional (3-D) finite-element analyses (FEA) are used to optimize magnet design and to determine appropriate shimming to obtain the desired magnetic field homogeneity. Several innovations of both 2-D FEA and 3-D FEA are described. Using ceramic 10 magnet material, a central magnetic field of approximately 0. 15 T (1500 G) can be obtained with an airgap of 50 cm and a pole piece of 100 cm multiplied by 180 cm multiplied by 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. The 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.