Ultrasmall Water-Soluble and Biocompatible Magnetic Iron Oxide Nanoparticles as Positive and Negative Dual Contrast Agents

Monodispersed water‐soluble and biocompatible ultrasmall magnetic iron oxide nanoparticles (UMIONs, D = 3.3 ± 0.5 nm) generated from a high‐temperature coprecipitation route are successfully used as efficient positive and negative dual contrast agents of magnetic resonance imaging (MRI). Their longitudinal relaxivity at 4.7 T (r1 = 8.3 mM−1 s−1) is larger than that of clinically used T1‐positive agent Gd‐DTPA (r1 = 4.8 mM−1 s−1), and three times that of commercial contrast agent SHU‐555C (r1 = 2.9 mM−1 s−1). The transversal relaxivity (r2 = 35.1 mM−1 s−1) is six times that of Gd‐DTPA (r2 = 5.3 mM−1 s−1), half of SHU‐555C (r2 = 69 mM−1 s−1). The in vivo results show that the liver signal from T1‐weighted MRI is positively enhanced 26%, and then negatively decreased 20% after injection of the iron oxide nanoparticles, which is stronger than those obtained from Gd‐DTPA (