Magnetic resonance imaging (MRI) contrast agents are used widely in clinical use.The advantage of macromolecular contrast agents was explained by relaxation theory.The chemical structures and development of the contrast agents were also introduced in this review. The paramagnetic Gadolinium (Ⅲ) chelates were modified by synthetic polymers
naturally occurring polymers
targeted agents and fluorescent labeled agents.It is indicated that the specific effects of modified contrast agents such as targeting and florescent labeled would raise the relaxation rate and reduce the toxicity.