Ultra-high relaxivity iron oxide nanoparticles confined in polymer nanospheres for tumor MR imaging

Superparamagnetic iron oxide nanoparticles encapsulated in hydrophilic chitosan nanospheres were prepared by nonsolvent-aided counterion complexation completely in an aqueous solution. The T₂ relaxation of these hybrid nanospheres in vitro and in vivo was investigated. It was found that the molar transverse relaxivity rate r₂ of hybrid nanospheres highly depends upon the payload of iron oxide nanoparticles within hybrid nanospheres. Compared to free iron oxide nanoparticles, the molar transverse relaxivity rate, r₂ of hybrid nanospheres shows an approximately 8-fold increase and reaches the maximum of 533 Fe mM^-1 s^-1. Such a high r₂ value is probably associated with the clustering effect of iron oxide nanoparticles, which are confined in the chitosan nanospheres. The in vivo magnetic resonance imaging (MRI) demonstrates that the hybrid nanospheres shorten transverse relaxation time, T₂ and significantly decrease the signal intensity of the tumor area, giving rise to high contrast tumor MR imaging at a relatively low dose.