Paramagnetic Ultrasmall Gadolinium Oxide Nanoparticles as Advanced T1 MR1 Contrast Agent: Account for Large Longitudinal Relaxivity, Optimal Particle Diameter, and In Vivo T1 MR Images

Paramagnetic ultrasmall gadolinium oxide (Gd2O3) nanoparticles with particle diameters (d) of similar to 1 nm were synthesized by using three kinds of Gd(III) ion precursors and by refluxing each of them in tripropylene glycol under an 02 flow. A large longitudinal relaxivity (r(1)) of water proton of 9.9 s(-1) mM(-1) was estimated, As a result, high contrast in vivo T-1 MR images of the brain tumor of a rat were observed. This large r(1) 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 r(1) that the optimal range of dfor the maximal r(1), which may be used as an advanced T-1 MRI contrast agent, is 1-2.5 nm.