A cryogen-free 0.5 TESLA MRI magnet for head imaging

A compact, cryogen-free 0.5 T superconducting Magnetic Resonance Imager (MRI) magnet for heads and limbs has been developed using conductively-cooled NbTi coils in conjunction with a 4.2 K Gifford-McMahon (GM) refrigerator. The bell-shaped magnet configuration fits over the shoulders of the patient and provides a 20 cm spherical volume to image the head. The magnet is also suitable to image human limbs. The lightweight magnet cartridge is designed for rapid cool-down, quench recovery, and field ramping, as required for clinical applications in small offices, intensive care units, and mobile vans. The NbTi coils are wound on a copper coil form, surrounded by a single aluminum thermal shield and a stainless steel vacuum envelope. The GM cold-head stages are in direct contact with the coil form and thermal shield through flexible connections to provide maximum vibration isolation. Descriptions of the design, fabrication, and test results of the magnet are presented.