Surface impact on nanoparticle-based magnetic resonance imaging contrast agents

Magnetic resonance imaging (MRI) is one of the most widely used diagnostic tools in the clinic. To improve imaging quality, MRI contrast agents, which can modulate local T₁ and T₂ relaxation times, are often injected prior to or during MRI scans. However, clinically used contrast agents, including Gd³⁺-based chelates and iron oxide nanoparticles (IONPs), afford mediocre contrast abilities. To address this issue, there has been extensive research on developing alternative MRI contrast agents with superior r₁ and r₂ relaxivities. These efforts are facilitated by the fast progress in nanotechnology, which allows for preparation of magnetic nanoparticles (NPs) with varied size, shape, crystallinity, and composition. Studies suggest that surface coatings can also largely affect T₁ and T₂ relaxations and can be tailored in favor of a high r₁ or r₂. However, the surface impact of NPs has been less emphasized. Herein, we review recent progress on developing NP-based T₁ and T₂ contrast agents, with a focus on the surface impact.