Engineering the Colloidal Properties of Iron Oxide Nanoparticles for High T ₁ MRI Contrast at 64 mT

Low-field magnetic resonance imaging (LF-MRI), an emerging form of portable and accessible MRI, has tremendous potential for point-of-care diagnostics and democratization of medical imaging. As LF-MRI evolves, there is a need to develop workflows, materials, and technologies designed specifically for the low-field regime. Here, monodisperse iron oxide nanoparticles (IONs) are evaluated for applications as positive contrast T ₁ agents using 64 mT LF-MRI. The nanoparticles were synthesized with relatively large diameters (16 and 22 nm) to promote high relaxivity at 64 mT. The particles were also stabilized with poly-(ethylene glycol) (PEG), a biocompatible ligand. The 16 nm particles showed an especially high longitudinal relaxivity, 90 L mmol^-1 s^-1, representing more than a 10× increase compared to a common Gd-based agent. The effects of colloidal stability were investigated by functionalizing the 22 nm particles with PEG ligands of varying molecular weights. The IONs displayed aggregation that depended on the length of the PEG ligands. The clustering reduced the longitudinal relaxivity of the ION-filled solutions.