A diagnostic agent composition is provided which comprises a nanoparticle composition and a pharmaceutically acceptable carrier or excipient. The nanoparticle composition comprises a nanoparticulate metal oxide and a phosphorylated polyol, wherein the phosphorylated polyol comprises at least two phosphate groups and one or more hydrophilic groups selected from the group consisting of polyethylene ether moieties, polypropylene ether moieties, polybutylene ether moieties, and combinations of two or more of the foregoing hydrophilic moieties. The disclosure provides detailed guidance on methods of making and using such diagnostic agent compositions. The diagnostic agent compositions provided by the present invention are useful as contrast agents for medical diagnostic imaging techniques such as magnetic resonance (MR) imaging and X-ray imaging. The diagnostic agent composition may be administered to a subject via a variety of techniques, among them injection, inhalation, and ingestion.