Development of signal to noise ratio of HTS-rf-SQUID for ultra-low field NMR / MRI by 77K LC resonant circuit

We constructed an ultra-low field (ULF) nuclear magnetic resonance (NMR) / magnetic resonance imaging (MRI) system with a high temperature superconductor (HTS) radio-frequency (rf) superconducting quantum interference device (SQUID) coupled with a LC circuit inside a cryostat filled with liquid nitrogen. Several LC resonant circuits with 139 to 1500 turns coils made from copper wires of 0.2, 0.4, and 0.6 mm in diameter were tested to obtain the optimum configuration for the circuit. For each coil, a capacitance was selected to fix the resonance frequency at around 10 kHz. The LC resonant circuit using a 500 turns coil made from 0.2 mm diameter wire offered the largest signal-to-noise ratio (S/N) of 11.9 with a quality factor of 53.3 at 10.2 kHz and 77 K. With this circuit, the S/N of free induction decay (FID) signals was tested using the ULF SQUID-NMR/MRI system.