[en] Magnetically responsive maghemite (γ-Fe_2O_3) nanoparticles (NPs) are the subject of current interest due to their potential use as contrast agents for in vivo magnetic resonance imaging (MRI). In addition, PET (positron emission tomography) is another non-invasive molecular imaging modality based on nuclear medicine and short-lived positron radionuclides incorporated in various bioprobes. In previous work (Haviv et al 2010 J. Am. Chem. Soc. 132 12519–12521), Massart magnetite (Fe_3O_4) oxidation using ceric ammonium nitrate (CAN) was explored. The resulting hydrophilic maghemite (γ-Fe_2O_3) NPs formed a stable aqueous colloid/ferrofluid that disclosed a low level of NP surface doping by Ce"3"/"4"+ atoms/cations. In this work, magnetically responsive gallium(III) or various other lanthanide(III) cation-doped maghemite NPs were prepared in different ways which influenced their r_2* MRI relaxivities, which were usually strengthened vs those of known FDA approved ferrofluids. In addition, this resulting NP surface chemistry enabled their direct labeling with PET-enabled "6"8Ga(III) cations (scheme 1) without using any attached chelating organic species/polymer, providing PET-enabled radioactive magnetic NPs towards a novel dual PET/MRI imaging nanoscale agent. (paper)