¹⁹F spin-lattice relaxation of perfluoropolyethers: Dependence on temperature and magnetic field strength (7.0-14.1 T)

Fluorine (¹⁹F) MRI of perfluorocarbon-labeled cells has become a powerful technique to track the migration and accumulation of cells in living organisms. It is common to label cells for ¹⁹F MRI with nanoemulsions of perfluoropolyethers that contain a large number of chemically equivalent fluorine atoms. Understanding the mechanisms of ¹⁹F nuclear relaxation, and in particular the spin-lattice relaxation of these molecules, is critical to improving experimental sensitivity. To date, the temperature and magnetic field strength dependence of spin-lattice relaxation rate constant (R₁) for perfluoropolyethers has not been described in detail. In this study, we evaluated the R₁ of linear perfluoropolyether (PFPE) and cyclic perfluoro-15-crown-5 ether (PCE) at three magnetic field strengths (7.0, 9.4, and 14.1 T) and at temperatures ranging from 256-323 K. Our results show that R₁ of perfluoropolyethers is dominated by dipole-dipole interactions and chemical shift anisotropy. R₁ increased with magnetic field strength for both PCE and PFPE. In the temperature range studied, PCE was in the fast motion regime (ωτ_c < 1) at all field strengths, but for PFPE, R₁ passed through a maximum, from which the rotational correlation time was estimated. The importance of these measurements for the rational design of new ¹⁹F MRI agents and methods is discussed.