Molecular Imaging & Contrast Agents - Sunrise Course Preclinical & Clinical Applications of Metal Based Paramagnetic/Iron Contrast

Introduction Magnetic resonance imaging is a powerful tool for visualizing cellular/macrophage tracking and migration or inflammatory processes. MRI examinations can be performed longitudinally and non-invasively, at isotropic image resolutions approaching 50 microns within reasonable scan times in live animals. Whereas vascular and interstitial compartments are investigated using Gadolinium-based contrast agents, specific molecular or cellular imaging can be obtained using metal-based contrast agent like Manganese (Mn) or iron oxide particles. To provide enhanced contrast on an MR image, cells are loaded with an MRI contrast agent prior to or during imaging. Superparamagnetic iron oxide (SPIO) nanoparticles can be used to generate contrast based on magnetic susceptibility differences between the labeled cells and the surrounding tissue. Cellular detection is most often accomplished via T2 or T2* imaging (negative contrast) after incorporation of iron oxide either within or attached to the cell. Coupling of the particles with fluorophores allows correlation of in vivo and ex vivo MRI with optical imaging and histology. At sufficient iron concentration and high resolution, single cells can be detected, even in vivo. Methods to provide positive and/or quantifiable contrast from iron containing cells are currently important topics of research. On the other hand, the T1 paramagnetic effect of Mn gives directly a positive contrast effect that can be used to assess the intracellular distribution of this ion.