Alternatives to gadolinium-based metal chelates for Magnetic Resonance Imaging (MRI) have been reported. Magnetic resonance imaging (MRI) has been immensely valuable in diagnostic clinical imaging over the last few decades owing to its exceptional spatial and anatomical resolution. A conceptually different approach to contrast enhancement is based on chemical exchange saturation transfer (CEST). The contact relaxation enhancement is a through-bond effect and can be quite significant for directly coordinated atoms, but its strength rapidly decreases as the number of bonds increases. Chemical exchange in such cases can involve the conventional magnetization transfer (MT) techniques, which entails exchange of magnetization between a semisolid macromolecular phase and bulk water or the more recent CEST techniques that involve exchange between protons of solutes and bulk water. The discovery of paramagnetic lanthanide DOTA-tetra-amide complexes with extremely slow water exchange kinetics has further stimulated new ideas about CEST-based contrast agents.
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