A 60 cm bore 2.0 tesla high homogeneity magnet for magnetic resonance imaging

A 60 cm warm bore imaging and spectroscopy magnet has been constructed and placed in operation at the Francis Bitter National Magnet Laboratory (FBNML). The magnet achieved its design central field of 2.0 T but is currently being operated at 1.5 T. It operates in the persistent mode with a measured decay rate of less than 0.03 ppm/hr. Employment of both 10 superconducting shims and small ferromagnetic shims located close to the warm bore has resulted in a homogencity of better than 3 ppm throughout the 25 cm diameter spherical volume (DSV). Room temperature shim coils have not been incorporated into the system. A novel form of compact shielded pulsed gradient coil system has been designed, constructed and tested. In such a system, appropriate configuration of an external shield coil results in cancellation of external flux without the introduction of impurity harmonics that degrade the linearity of the gradients. Six sets (X, Y, Z coils, and X, Y, Z shields) have been incorporated into a unit of 6 cm build. The all aluminum cryostat employs a 77 K nitrogen recondenser and a shield cooler operating at less than 20 K. Steady state helium consumption is about 50 ml/hour. The system is currently being used for both high resolution, in-vivo³¹P-NMR spectroscopy and a variety of MRI experiments including²³Na imaging of eyes.