Diffusional mechanisms augment the fluorine MR relaxation in paramagnetic perfluorocarbon nanoparticles that provides a "relaxation switch" for detecting cellular endosomal activation

Abstract

Purpose: To develop a physical model for the 19F relaxation enhancement in paramagnetic perfluorocarbon nanoparticles (PFC NP) and demonstrate its application in monitoring cellular endosomal functionality through a " 19F relaxation switch" phenomenon. Materials and Methods: An explicit expression for 19F longitudinal relaxation enhancement was derived analytically. Monte-Carlo simulation was performed to confirm the gadolinium-induced magnetic field inhomogeneity inside the PFC NP. Field-dependent T 1 measurements for three types of paramagnetic PFC NPs were carried out to validate the theoretical prediction. Based on the physical model, 19F and 1H relaxation properties of macrophage internalized paramagnetic PFC NPs were measured to evaluate the intracellular process of NPs by macrophages in vitro. Results: The theoretical description was confirmed experimentally by field-dependent T 1 measurements. The shortening of 19F T 1 was found to be attributed to the Brownian motion of PFC molecules inside the NP in conjunction with their ability to permeate into the lipid surfactant coating. A dramatic change of 19F T 1 was observed upon endocytosis, revealing the transition from intact bound PFC NP to processed constituents. Conclusion: The proposed first-principle analysis of 19F spins in paramagnetic PFC NP relates their structural parameters to the special MR relaxation features. The demonstrated " 19F relaxation switch" phenomenon is potentially useful for monitoring cellular endosomal functionality. © 2011 Wiley-Liss, Inc.