Design of injectable calcium phosphate cements suitable for spectral photon counting computed tomography imaging

Abstract

Rare-earth trifluoride nanoparticles (REF₃ NPs with REF = Gd, Ho and Y) were combined with an injectable calcium phosphate cement (CPC) used in clinics, in order to develop formulations suitable for CT-imaging and monitoring of the in vivo CPC behavior during the bone regeneration process. Preliminary in vitro experiments were performed by spectral photon-counting computed tomography (SPCCT) using a new generation detector that allowed selection of an energy window centred around the K-edge value of the considered rare-earth trifluorides (i.e. Gd or Ho). A loading of circa 10 mg of GdF₃ or HoF₃ NPs per mL of cement paste was sufficient to allow the easy and specific visualization of the resulting cement blocks when embedded in a contrast agent free CPC. Quantification of the rare-earth trifluoride content in the samples was consistent with the expected value. Since the REF₃ NPs were found to aggregate in the CPC liquid phase of the cement, solid state NMR experiments were carried out using YF₃ NPs, thus showing that phosphate ions present in the liquid bind to the NP surface, leading to their rapid precipitation. Different surface modifications were investigated for the stabilization that was successfully achieved when coating the NPs with polymers, in particular, pectin.