Magnetic Resonance Imaging Contrast Agents Based on Iron Oxide Superparamagnetic Ferrofluids

Abstract

Novel systems based on suspensions of colloidal magnetic nanoparticles have been investigated as perspective superparamagnetic contrast agents (CA) for magnetic resonance imaging (MRI). The nanostructures that we have studied contain surfactant-capped magnetite (Fe₃O₄) inorganic cores with different controlled sizes, ranging from 5.5 to 12 nm. The as-synthesized nanostructures are passivated by hydrophobic surfactants and thus are fully dispersible in nonpolar media. The magnetic nanocrystals have been transferred into aqueous media by a procedure based on the surface intercalation and coating with an amphiphilic polymer shell. The MRI efficiency in contrasting images, i.e., the NMR relaxivities r₁ and r₂, have been compared with Endorem and Sinerem, commercial superparamagnetic MRI contrast agents. We found that our nanostructures exhibit r₁ and r₂ relaxivities comparable to those of commercial CA over the whole frequency range. The MRI efficiency of our samples was related to their microstructural and magnetic properties. The transverse relaxivity r₂, leading the contrast in “negative” superparamagnetic agents, was found to improve as the diameter of the inorganic core is increased. The NMR relaxometry profile confirmed the nature of the physical mechanisms inducing the increase of the nuclear relaxation rates at low (magnetic anisotropy) and high (Curie relaxation) fields.