Design Principles of Responsive Relaxometric 19F Contrast Agents: Evaluation from the Point of View of Relaxation Theory and Experimental Data

19F magnetic resonance imaging (MRI) is a promising tool in medical diagnostics. An important class of 19F MRI contrast agents is based on paramagnetic resonance enhancement. This effect allows an improvement in sensitivity by increasing the number of scans per unit of time or facilitates the development of responsive contrast agents that are based on changes in relaxation rates as a detection principle. In this work, Bloch- Redfield-Wangsness relaxation theory was used to predict the relaxation properties of existing lanthanoid and transition metal complexes of fluoroorganic ligands and to evaluate several design strategies for responsive contrast agents. Electron-nucleus dipole-dipole, Curie relaxation, and contact interactions were included in the model. Potential significance of chemical shift anisotropy-anisotropic dipolar shielding cross-correlation was discussed. The calculated and experimental results were well aligned. The presented model, along with the optimized field-dependent values of electronic relaxation times, could be used for the preliminary selection of the optimal metal ion for applications in 19F MRI. The results indicate potential advantages of other metal ions in addition to Gd3+ particularly Cu2+, Mn2+, Ni2+, Fe3+, and other lanthanoids as a part of 19F contrast agents.