Paramagnetic ultrasmall gadolinium oxide nanoparticles as advanced T ₁ MRI contrast agent: Account for large longitudinal relaxivity, optimal particle diameter, and in Vivo T₁ MR images

Abstract Paramagnetic ultrasmall gadolinium oxide (Gd₂O ₃) nanoparticles with particle diameters (d)of ∼1 nm were synthesized by using three kinds of Gd(III) ion precursors and by refluxing each of them in tripropylene glycol under an O₂ flow. A large longitudinal relaxivity (r₁) of water proton of 9.9 s^-1 mM^-1 was estimated. As a result, high contrast in vivo T₁ MR images of the brain tumor of a rat were observed. This large r₁ is discussed in terms of the huge surface to volume ratio (S/V) of the ultrasmall gadolinium oxide nanoparticles coupled with the cooperative induction of surface Gd(III) ions for the longitudinal relaxation of a water proton. It is found from the d dependence of r₁ that the optimal range of d for the maximal r₁, which may be used as an advanced T₁ MRI contrast agent, is 1-2.5 nm.