Liquid-State Nuclear Magnetic Resonance Measurements for Imaging Using HTS-rf-SQUID in Ultra-Low Field

In our study, we constructed a ultra-low field (ULF) nuclear magnetic resonance (NMR)/magnetic resonance imaging (MRI) system employing a high-temperature superconductor (HTS)-rf-SQUID and room-temperature coils. In the system, we employed a pulsed polarizing field B_p perpendicular to a measurement field B_m; a high-speed switching circuit with an optical FET was used to switch off B_p of approximately 30 mT. When a field gradient dBz/dz of approximately 10 nT/cm was applied to a sample, which was designed to contain water in two compartments, a ¹H-NMR spectrum with two peaks was observed. The frequency difference between the two peaks roughly corresponded to the distance between each center of the water in the two parts. We measured the ¹H -NMR signals for water and mineral oil and found a clear difference in their signal amplitudes owing to their different molecular compositions. The longitudinal relaxation times T₁ of water and the oil were also estimated by changing the polarizing time of B_p. From these results, it was confirmed that ¹H- and T₁-weighted contrast imaging could be realized by using this system.