Principles and advantages of MRI reagents for Fe complexes: Nuclear magnetic resonance imaging (MRI) is a technique that uses the principle of nuclear magnetic resonance (NMR) to detect the electromagnetic waves emitted by an applied gradient magnetic field to map the structure of an object. MRI shows broad application prospects in tumor imaging diagnosis, but it is difficult for conventional MRI to achieve early sensitive diagnosis of tumors as a result, so the development of advanced MRI molecular imaging probes is needed. The Fe complex is a MRI molecular imaging probe based on Fe (III) ions, with the advantages of high thermodynamic stability, low long-term toxicity, and large relaxation rate under high magnetic field. Fe complexes can regulate their solubility, targeting and activation properties according to different ligand structures and functional groups, thus achieving specificity and sensitivity for tumor imaging.