The temporal stability of the magnetic field generated by an MRI magnet is of critical importance for higher resolution medical imaging. However, large screening currents are induced in High Temperature Superconducting (HTS) tapes such as REBCO tapes during the excitation, and attenuated after the excitation due to the flux creep of the HTS tapes. The temporal variation of the magnetic field due to the screening current attenuation in the HTS tapes is a critical problem to apply HTS magnets for MRI. We have verified the effectiveness of the pre-excitation, overshooting, and high temperature magnetization method for MRI magnets to suppress the screening current attenuation in the HTS tapes and the variation rate of the coil magnetic field down to 100 ppm/hour by using a small REBCO double pancake coil.
This study developed a 2 T class REBCO magnet for MRI and accurate magnetic field measurement system with an NMR probe to measure the magnetic field variation of less than 1 ppm/hour. We investigated the effectiveness of the overshooting and high temperature magnetization method to realize less than 1 ppm/hour as the temporal stability of the center field of the magnet, and clarified the suitable excitation method for HTS MRI magnets. Experimental results showed that the overshooting with less than +1% current and high temperature magnetization with the temperature change over 5 K greatly increased the temporal stability of the center field of the magnet at the operational temperature 4 - 20 K. We also clarified that the combined excitation method of the overshooting and high temperature magnetization method can realize the temporal stability of the center field less than 1 ppm/hour with a shorter time as well as smaller load factor of the REBCO magnet.
Acknowledgments: This work was supported by the New Energy and Industrial Technology Development Organization (NEDO).