Test Results for a 1.5 T MRI System Utilizing a Cryogen-free YBCO Magnet

A magnetic resonance imaging (MRI) system based on a 1.5 T magnet using a Y-Ba-Cu-O (YBCO) high-temperature superconductor (HTS) is described. The magnet is conduction cooled to 20 K by a pulse tube refrigerator and shielded using an iron yoke. With active thermal stabilization of the yoke the long-term temporal drift of the magnetic field is better than 10 ppm. Hysteretic changes in field strength and homogeneity observed immediately after energizing the magnet are attributed to screening currents. However these changes were relatively small and reproducible, and did not prevent the field homogeneity from being shimmed to 50 ppm peak-to-peak over a 115-mm diameter imaging volume. The magnet was designed to minimize eddy currents induced by pulsed field gradients and was fitted with a set of unshielded gradient coils. A conventional shielded birdcage resonator was used to acquire spin-echo images from test objects. The results indicate that YBCO HTS magnets can be constructed with field homogeneity and temporal stability suitable for clinical quality MRI.