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Leung K.


Molecular Imaging and Contrast Agent Database (MICAD) [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2004-2013.
2004 Nov 01 [updated 2007 Dec 12].


Magnetic resonance imaging (MRI) maps information about tissues spatially and functionally. Protons (hydrogen nuclei) are widely used to create images because of their abundance in water molecules. Water comprises about 80% of most soft tissues. The contrast of proton MRI depends mainly on the density of nuclear (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 development of contrast agents. Most of the contrast agents affect the T1 and T2 relaxation of the surrounding nuclei, mainly the protons of water. T-2* is the spin-spin relaxation time composed of variations from molecular interactions and intrinsic magnetic heterogeneities of tissues in the magnetic field [1]. Superparamagnetic iron oxide (SPIO) structure is composed of ferric iron (Fe3+) and ferrous iron (Fe2+) in the general formula of Fe23+OFe2+O. The iron oxides particles are coated with a layer of dextran or other polysaccharide. These particles have a 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 reticuloendothelial system is by endocytosis or phagocytosis. SPIO particles are 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 (about 20 nm to 3,500 nm in diameters) as large SPIO agents (Ferumoxsil or AMI-121, Ferucarbotran, OMP), standard SPIO (SSPIO) agents (Ferumoxides or AMI-25, SHU 555A), ultrasmall SPIO (USPIO) agents (Ferumoxtran or AMI-227, NC100150) and monocrystalline iron oxide nanoparticles (MION) agents [1]. Ferumoxsil is composed of iron particles of about 10 nm and the hydrodynamic diameter is about 300 nm. The iron particles are coated with a non-biodegradable and insoluble matrix (siloxane) and suspended in viscosity-increasing agents such as starch and cellulose [2]. They are used in oral large SPIO preparations. Ferumoxsil has been tested in clinical trials as negative contrast agent that decreases signal on T2 images. Ferumoxsil is used in bowel MR imaging.

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