Inducing Defects in 19F-Nanocrystals Provides Paramagnetic-free Relaxation Enhancement for Improved In Vivo Hotspot MRI

Paramagnetic relaxation enhancement (PRE) is the current strategy of choice for enhancing magnetic resonance imaging (MRI) contrast and for accelerating MRI acquisition schemes. Yet, debates regarding lanthanides' biocompatibility and PRE-effect on MRI signal quantification have raised the need for alternative strategies for relaxation enhancement. Herein, we show an approach for shortening the spin-lattice relaxation time (T 1 ) of fluoride-based nanocrystals (NCs) that are used for in vivo 19 F-MRI, by inducing crystal defects in their solid-crystal core. By utilizing a phosphate-based rather than a carboxylate-based capping ligand for the synthesis of CaF 2 NCs, we were able to induce grain boundary defects in the NC lattice. The obtained defects led to a 10-fold shorter T 1 of the NCs' fluorides. Such paramagnetic-free relaxation enhancement of CaF 2 NCs, gained without affecting either their size or their colloidal characteristics, improved 4-fold the obtained 19 F-MRI signal-to-noise ratio, allowing their use, in vivo, with enhanced hotspot MRI sensitivity.