This is a copy of the slides presented at the meeting but not formally written up for the volume.

Superparamagnetic iron oxide (SPIO) nanoparticles are prevalent as nanoprobes for molecular magnetic resonance imaging (MRI), providing positive or negative contrast by locally affecting the relaxation of water protons. Fe3O4 nanoparticles are commonly used as a negative MRI contrast agent, implementing various surface functionalization techniques to provide molecular targeting to biological macromolecules. The authors recently demonstrated targeting of cancer antigen 125 (CA125) with differentiable MRI contrast in human ovarian cancer cell lines using monoclonal antibodies covalently conjugated to phospholipid micelle encapsulated 10 nm single crystalline SPIO nanoparticles, demonstrating molecular targeting capabilities via surface functionalization [1]. While molecular targeting of SPIO nanoparticles has been thoroughly demonstrated, the effects of surface modifications have not been studied in regard to proton relaxation. The authors will present spin-lattice (T1) and spin-spin (T2) proton relaxometry of SPIO nanoparticles with varying surface chemistries. The effects of surface modification on T1 and T2 relaxation have not been thoroughly investigated, and results recently reported by the authors indicate a correlation of spin-spin relaxation with SPIO nanoparticle hydrodynamic radius [2]. T1 and T2 relaxometry (Varian 300 MHz NMR) of polyethylene glycol modified (PEGylated) phospholipid micelle encapsulated SPIO nanoparticles and covalently PEGylated SPIO nanoparticles for varying hydrodynamic radii will be presented. These results are of particular interest to molecular imaging applications due to the common practice of SPIO nanoparticle PEGylation to improve biocompatibility. The authors will also present results of magnetic anisotropy studies with respect to proton relaxation by SPIO nanoparticles. Recent work by Roch et al emphasizes the role of magnetic anisotropy in the proton relaxation mechanism of SPIO nanoparticles [3]. The authors have synthesized monodisperse Fe3-xCoxO4 nanoparticles with similar properties to SPIO. Cobalt substitution in SPIO nanoparticles increases the magnetic anisotropy of the SPIO nanoparticles, thus affecting the proton relaxation. The authors will present T1 and T2 relaxometry (Varian 300 MHz NMR) of Fe3-xCoxO4 nanoparticles and corresponding SQUID (superconducting quantum interference device) magnetic anisotropy measurements (Quantum Design, MPMS-7). The results of this study elucidate the role of magnetic anisotropy in the proton relaxation mechanism and demonstrates the feasibility of Fe3-xCoxO4 nanoparticles as a T2 contrast agent.1. Larsen BA et al, Proceedings of the 5th Annual Meeting of Molecular Imaging, 20062. Barker AJ, Larsen BA et al, Proceedings of the ASME SBC, 20063. Roch et al, Journal of Mag Res Imaging 14, pp 94-96, 2001.