Paramagnetic Ultrasmall Gadolinium Oxide Nanoparticles as Advanced <i>T</i>₁ MRI Contrast Agent: Account for Large Longitudinal Relaxivity, Optimal Particle Diameter, and <i>In Vivo</i> <i>T</i>₁ MR Images

Paramagnetic ultrasmall gadolinium oxide (Gd2O3) 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.