Ultra-Low-Field MRI and Spin-Lattice Relaxation Time of  $^{1}\hbox{H}$ in the Presence of $\hbox{Fe}_{3}\hbox{O}_{4}$  Magnetic Nano-Particles Detected With a High-$T_{\rm C}$  DC-SQUID
We have carried out ultra-low-field nuclear magnetic resonance (NMR) and magnetic resonance imaging experiments in microtesla magnetic fields using a high-Tc dc-SQUID sensor. The measurements were carried out in a home-made magnetically shielded room. NMR spectra of 1H from tap water and other substance samples were obtained in the field range from 10-200 μT. By adapting a transformer coupled detection system, the signal-to-noise ratio of NMR peak in a single-shot measurement for 10 ml tap water was increased significantly as compared with our previously used direct detection method. The improvement was attributed to the increased coupling efficiency and reduced SQUID noise. By applying magnetic field gradient in three directions, one- and two-dimensional imaging results were obtained from water phantom samples. Furthermore, the effect of Fe3O4 magnetic nanoparticles on the spin-lattice relaxation time T1 was studied.