Organic Radical Contrast Agents for Magnetic Resonance Imaging

We report a molecular design that provides an intravenously injectable organic radical contrast agent (ORCA) for which the molecular 1H water relaxivity (r1) is ca. 5 mM–1 s–1. The ORCA is based on spirocyclohexyl nitroxide radicals and poly(ethylene glycol) chains conjugated to a fourth-generation polypropylenimine dendrimer scaffold. The metal-free ORCA has a long shelf life and provides selectively enhanced magnetic resonance imaging in mice for over 1 h.