Nanoscale Metal-Organic Frameworks: Magnetic Resonance Imaging Contrast Agents and Beyond

Magnetic resonance imaging (MRI) is an imaging technique based on the detection of nuclear spin reorientations in a magnetic field. MRI provides high spatial resolution and soft-tissue contrast in anatomical images, and a large penetration depth; however, it is relatively insensitive and typically relies on large doses of administered contrast agents to give adequate contrast between normal and diseased tissues. This review covers the development of nanoscale metal-organic frameworks (NMOFs) as novel MRI contrast agents. NMOFs not only provide an effective vehicle for delivering large amounts of paramagnetic metal centers but also exhibit enhanced per-metal-based relaxivity relative to clinically used small-molecule contrast agents. The in vitro and in vivo effectiveness of these agents has also been demonstrated. The potential of NMOFs as contrast agents for other imaging modalities, such as optical imaging (OI) and X-ray computed tomography (CT), and as chemical and biological sensors is also discussed.