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A 10K refrigerator cooled superconducting magnet with an open-ended, C-shaped iron core was developed and tested for the special purpose of magnetic resonance imaging. The magnet employs a single Nb3Sn tape superconducting coil and an open-ended iron loop. The superconducting coil is cooled and maintained at the 10K operational temperature by a Gifford-McMahon(G-M) cryocooler. The magnetic field in the iron open gap is used for magnetic resonance imaging. The field homogeneity is achieved by contoured iron pole faces and specially developed passive shimming procedures. We have achieved less than 1ppm/hour field stability and a 29ppm field homogeneity over a 10 cm diameter spherical volume. High quality images have been produced using this magnet.

A 0.5T, 95cm warm bore, cryogen-free superconducting magnet for whole body MRI has been developed and tested. By using Nb3Sn tape superconductor, this magnet is designed to meet the same performance requirement as existing helium cooled MRI magnets. Persistent operation with drift rate less than 0.1ppm/hour and a stable 5.6ppm field homogeneity in a 30cmx40cm cylindrical volume was achieved in October of 1992. The magnet is cooled and maintained at the temperature of about 10K by a Gifford McMahon(G-M) cryocooler. High quality images were produced by using this magnet with the 0.5T GE SIGNA magnetic resonance imaging system.