Magnetic resonance imaging (MRI) maps information about tissues spatially and functionally. Protons (hydrogen nuclei) are widely used in imaging because of their abundance in water molecules. Water comprises 80% of most soft tissue. The contrast of proton MRI depends mainly on the density of the nucleus (proton spins), the relaxation times of the nuclear magnetization (T1, longitudinal and T2, transverse), the magnetic environment of the tissues, and the blood flow to the tissues. However, insufficient contrast between normal and diseased tissues requires the development of contrast agents. Most contrast agents affect the T1 and T2 relaxation times of the surrounding nuclei, mainly the protons of water. T2* is the spin–spin relaxation time composed of variations from molecular interactions and intrinsic magnetic heterogeneities of tissues in the magnetic field (1). The superparamagnetic iron oxide (SPIO) structure is composed of ferric iron (Fe) and ferrous iron (Fe). The iron oxide particles are coated with a layer of dextran or other polysaccharide. These particles have large combined magnetic moments or spins, which are randomly rotated in the absence of an applied magnetic field. SPIO is used mainly as a T2 contrast agent in MRI, though it can shorten both T1 and T2/T2* relaxation processes. SPIO particle uptake into the reticuloendothelial system (RES) is by endocytosis or phagocytosis. SPIO particles are also taken up by phagocytic cells such as monocytes, macrophages, and oligodendroglial cells. A variety of cells can also be labeled with these particles for cell trafficking and tumor-specific imaging studies. SPIO agents are classified by their sizes with coating material (20–3,500 nm in diameter) as large SPIO (LSPIO) nanoparticles, standard SPIO (SSPIO) nanoparticles, ultrasmall SPIO (USPIO) nanoparticles, and monocrystalline iron oxide nanoparticles (MION) (1). USPIO nanoparticles are composed of iron nanoparticles ~4–6 nm in diameter and the hydrodynamic diameter with dextran coating is ~20–50 nm. USPIO nanoparticles have a long plasma half-life because of their small size. The blood pool half-life of plasma relaxation times is calculated at ~24 h in humans (2) and 2 h in mice (3). Because of its long blood half-life, USPIO can be used as blood pool agent during the early phase of intravenous administration (4). In the late phase, USPIO is suitable for the evaluation of RES in the body, particularly in lymph nodes (5). Integrins are a family of heterodimeric glycoproteins on cell surfaces that mediate diverse biological events involving cell–cell and cell–matrix interactions (6). Integrins consist of an α and a β subunit and are important for cell adhesion and signal transduction. The αβ integrin is the most prominent receptor affecting tumor growth, tumor invasiveness, metastasis, tumor-induced angiogenesis, inflammation, osteoporosis, and rheumatoid arthritis (7-12). Expression of αβ integrin is strong on tumor cells and activated endothelial cells, whereas expression is weak on resting endothelial cells and most normal tissues. The αβ antagonists are being studied as antitumor and antiangiogenic agents, and the agonists are being studied as angiogenic agents for coronary angiogenesis (11, 13, 14). A tripeptide sequence consisting of Arg-Gly-Asp (RGD) has been identified as a recognition motif used by extracellular matrix proteins (vitronectin, fibrinogen, laminin, and collagen) to bind to a variety of integrins, including αβ. Various radiolabeled antagonists have been introduced for imaging of tumors and tumor angiogenesis (15). Most of the cyclic RGD peptides comprise five amino acids. Haubner et al. (16) reported that various cyclic RGD peptides exhibit selective inhibition of binding to αβ integrin (inhibition concentration (IC), 7–40 nM) but not to αβ (IC, 600–4,000 nM) or αβ (IC, 700–5,000 nM) integrins. Various radiolabeled cyclic RGD peptides have been found to have high accumulation in tumors in nude mice (17). The cyclo(Arg-Gly-Asp-D-Try-Glu) (c(RGDyE)) peptide was conjugated to USPIO nanoparticles for non-invasive MRI of αβ expression on activated endothelial cells in tumor (18).