Iron-oxide nanoparticles can act as contrast agents in magnetic resonance imaging (MRI), while radiolabeling the same platform with nuclear medicine isotopes allows imaging with positron emission tomography (PET) or single-photon emission computed tomography (SPECT), modalities that offer better quantification. For successful translation of these multifunctional imaging platforms to clinical use, it is imperative to evaluate the degree to which the association between radioactive label and iron oxide core remains intact in vivo. We prepared iron oxide nanoparticles stabilized by oleic acid and phospholipids which were further radiolabeled with (59)Fe, (14)C-oleic acid, and (111)In. Mouse biodistributions showed (111)In preferentially localized in reticuloendothelial organs, liver, spleen and bone. However, there were greater levels of (59)Fe than (111)In in liver and spleen, but lower levels of (14)C. While there is some degree of dissociation between the (111)In labeled component of the nanoparticle and the iron oxide core, there is extensive dissociation of the oleic acid component.
