Monodisperse Gd2O3-embedded iron oxide (GdIO) nanoparticles can simultaneously enhance the local magnetic field intensities of each other under an external magnetic field and result in synergistic enhancement of T-1 and T-2 effects. GdIO nanoparticles have the unique property to be both T-1 and T-2 contrast agents and can potentially lead to higher accuracy in cancer diagnosis, particularly liver tumors.