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lüll Polyethylene glycol-gold nanoparticles Leung KMolecular Imaging and Contrast Agent Database (MICAD)-/-ä 2004[]; ä (ä): äX-Ray imaging, or computed tomography (CT), visualizes tissue density differences that provide the image contrast produced by X-ray attenuation between soft tissues and electron-dense bone (1). Radiopaque X-ray contrast agents are needed to enhance the degree of contrast between diseased tissues and normal tissues. Water-soluble X-ray contrast agents are generally based on small tri-iodobenzene compounds such as monomers or dimers (2), which can be ionic (high osmolality) or nonionic (low osmolality). When injected intravenously, commonly via intra-arterial catheterization, these agents exhibit highly nonspecific vascular permeation and rapid renal excretion, which limits their targeting performance. Gold has not been used as an X-ray contrast agent in vivo. Gold has a higher atomic number and a higher absorption coefficient than iodine, providing 2.7-fold greater contrast/weight than iodine (3). Furthermore, imaging gold at 80-100 keV reduces interference from bone absorption and provides lower soft tissue absorption, which would reduce radiation to patients. Hainfeld et al. (3) used gold nanoparticles (AuNPs; 1.9 nm in diameter, ~50 kDa) as a CT contrast agent in mice; these experiments showed enhanced CT contrast of the vasculature, kidneys, and tumor in mice. However, plasma proteins in blood adsorb onto the surface of bare AuNPs, which produces large aggregates (4) that may result in altered pharmacokinetics and biodistribution of AuNPs (5). Polyethylene glycol (PEG) is found to minimize nonspecific adsorption of proteins onto NPs and to reduce their uptake by the liver (5). PEG-AuNPs are being studied as cancer CT imaging and photothermal agents (6).ä |