A 2-tesla active shield magnet for whole body imaging and spectroscopy

Abstract

A 2-T superconducting active shield magnet with a 0.99-m-diameter warm bore for whole-body magnetic resonance imaging (MRI) and spectroscopy has been developed and tested. The magnet and cryostat were designed to meet the same performance standards as existing MRI magnets, but with the volume of the stray field region reduced to less than 4% of that for an unshielded magnet. The 0.5-mT stray field contour is within 5 m axially and 3 m radially of the magnet center. The system weight is only 14 t. The magnet operates at a current of 530 Amps with a stored energy of 9.5 MJ using low cost, high-copper-to-superconductor-ratio conductors on split-construction aluminum formers. The design involved extensive analysis of the stresses in the coils and formers and special consideration of the behavior during quench to avoid burnout or high-voltage breakdown. A low-loss cryostat was also developed with a titanium suspension system and two radiation shields but no liquid nitrogen, the shields being cooled by a single standard Gifford-McMahon refrigerator.