Magnetic resonance imaging (MRI) is a powerful tool for disease diagnosis such as cancer metastasis and inflammation, using different metal chelates. Metal chelates increase the contrast signal between normal and diseased tissues by catalyzing the relaxation of water protons in their proximities. Typical examples are Gd3+ low-molecular-weight chelates, and superparamagnetic iron oxide (SPIO). "In vivo" administration of these agents allows the labeling of tumor cells; or cells can be labeled "in vitro" with contrast agents and then they can be injected and monitored "in vivo" by using MRI techniques.