Representative chemical structures and compositions for three different types of MRI contrast agents. Left panel, the chemical structure of a T1 agent, Omniscan™, and T1 relaxation mechanism; middle panel, TEM image of a T2 agent, 9 nm Fe3O4 nanoparticles, and T2 relaxation; right panel, the chemical structure of EuDOTAM3+, a PARACEST agent. A 2-pool exchange model for CEST mechanism and a representative Z-spectrum are illustrated.