A nuclear magnetic resonance and imaging system was constructed to study spin-lattice relaxation time T1, spin-spin relaxation time T2, and effective relaxation time T2* of gadolinium (Gd) chelates using a high-Tc superconducting quantum interference device in microtesla magnetic fields. In the presence of the magnetic contrast T2* is related to T2 by the relation: 1/T2*=1/T2+γΔB+ΓGd-chelates, where γ=42.58 kHz/mT and γΔB is the relaxation rate due to the inhomogeneity field ΔB in measuring coil at the sample position and ΓGd-chelates is the intrinsic relaxation rate of Gd chelates. It is found that T1, T2, and 1/ΓGd-chelates decay exponentially as the concentration (or magnetic susceptibility) of Gd chelates increases. The Gd chelates cause a diffusive motion of nuclear spins and hence enhance the relaxation rates. Enhanced image contrast has been demonstrated in a water phantom with Gd chelates in microtesla magnetic fields.