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

摘要

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 $B_{rm p}$ perpendicular to a measurement field $B_{rm m}$; a high-speed switching circuit with an optical FET was used to switch off $B_{rm p}$ of approximately 30 mT. When a field gradient ${rm d}B_{z}/{rm d}z$ of approximately 10 nT/cm was applied to a sample, which was designed to contain water in two compartments, a $^{1}{rm H}$-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 $^{1}{rm H}$ -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 ${rm T}_{1}$ of water and the oil were also estimated by changing the polarizing time of $B_{rm p}$. From these results, it was confirmed that $^{1}{rm H}$- and ${rm T}_{1}$-weighted contrast imaging could be realized by using this system.