The invention relates to a material consisting of an aqueous dispersion of PEG-coated nanoparticles of maghemite. The process for preparing said material consists in polymerizing ethylene oxide in the presence of nanoparticles of maghemite in an aqueous dispersion. The polymerization is initiated using reactive groups which are capable of initiating the polymerization of ethylene oxide without the addition of a catalyst, and which are attached to the nanoparticles of maghemite by means of a coupling agent comprising an alkoxysilyl group and one or more of said reactive groups. Applications: contrast agent for MRI.
