Manganese-Enhanced MRI Contrast Agents: From Small Chelates to Nanosized Hybrids

During the last decades, a wide range of molecular and cellular imaging techniques have been developed and optimized, resulting in important progress in the understanding of diagnostic and biological processes. Among these techniques, magnetic resonance imaging (MRI) is certainly one of the most popular, because of its high spatial resolution and the fact that it does not require the use of radioisotopes. Although gadolinium has long been the most commonly used paramagnetic metal to design efficient MRI contrast agents, the discovery of its potential toxicity has driven researchers to give priority to other paramagnetic metals, as, for example, manganese. With this microreview we would like to highlight the state of the art of manganese-based MRI contrast agents with a particular focus on the chemical structures of these agents and on their chronological evolution from simple chelates to complex nanohybrid systems.