To overcome the negative contrast limitations of iron oxide-based contrast agents and to improve the biocompatibility of Gd-chelate contrast agents, PEGylated Gd-doped iron oxide (PEG-GdIO) NPs as a T-1-T-2 dual-modal contrast agent were synthesized by the polyol method. The transverse relaxivity (r(2)) and longitudinal relaxivity (r(1)) of PEG-GdIO were determined to be 66.9 and 65.9 mM(-1) s(-1), respectively. The high r(1) value and low r(2)/r(1) ratio make PEG-GdIO NPs suitable as a T-1-T-2 dual-modal contrast agent. The in vivo MRI demonstrated a brighter contrast enhancement in T-1-weighted image and a simultaneous darken effect in T-2-weighted MR image compared to the pre-contrast image in the region of glioma. Furthermore, the biocompatibility of PEG-GdIO NPs was confirmed by the in vitro MIT cytotoxicity and in vivo histological analyses (H&E). Therefore, PEG-GdIO NPs hold great potential in T-1-T-2 dual-modal imaging for the diagnosis of brain glioma. (C) 2013 Elsevier Inc. All rights reserved.