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 .