Relaxometry and Contrast Agents
The perturbation on the relaxation of solvent water protons induced by the presence of paramagnetic metal-containing systems has been exploited for the design of contrast agents in magnetic resonance imaging applications. Complexes of Gd3+ ions were identified early as the candidates of choice and several, highly stable, Gd-containing contrast agents entered into clinical practice two to three decades ago. Overall, the relationships between the structure and dynamics of paramagnetic metal complexes and the determinants of the proton relaxation enhancement have been well defined. On this basis a number of Gd-containing complexes have been synthetized and tested. Besides the currently used procedures devoted to visualizing tumor perfusion or abnormalities in organ function, Gd-containing contrast agents have found application in quantitative assessments such as DCE-MRI (dynamic contrast enhanced-MRI). At preclinical level, a number of paramagnetic systems have already been proposed as targeting agents of overregulated receptors/transporters or as responsive agents to tissue microenvironment parameters. These studies pave the way to molecular imaging studies with MRI modality.