Super-Stable Superconducting MRI Magnet Operating for 25 Years

This paper describes a superconducting magnet performance continuing persistent current operation for 25 years. The central magnetic field is 0.5 T for whole-body magnetic resonance imaging (MRI). The observed stability of the magnetic field was extremely stable compared with the specification. Because of the ultrafine electrical resistances in persistent current mode operation, it was difficult to observe an accurate magnetic field stability at the factory test. By long-term operation, we are able to observe the magnetic field stability and the joint resistance precisely. The magnet was started to energize on January 1988 and has been operated consecutively without re-excitation. At that time, a superconducting whole-body MRI system was appeared in the world among a normal conducting magnet and a permanent magnet system. We made superconducting joints to connect superconducting conductors between superconducting coils. We carried out the measurement of the central magnetic field by the nuclear magnetic resonance method. Because the device is used for the medical examination at the hospital, the magnetic field data acquisition is in a limited situation. The stability of the last magnetic field becomes 0.0006 ppm/h and gets the good performance that is more than 100 times than the specification. The total resistance of the superconducting joints is 4 ×10
^-12
Ω and supports good superconducting connection.