Quantitative (1)H MRI, (19)F MRI, and (19)F MRS of cell-internalized perfluorocarbon paramagnetic nanoparticles.

In vivo molecular imaging with targeted MRI contrast agents will require sensitive methods to quantify local concentrations of contrast agent, enabling not only imaging-based recognition of pathological biomarkers but also detection of changes in expression levels as a consequence of disease development, therapeutic interventions or recurrence of disease. In recent years, targeted paramagnetic perfluorocarbon emulsions have been frequently applied in this context, permitting high-resolution (1)H MRI combined with quantitative (19)F MR imaging or spectroscopy, under the assumption that the fluorine signal is not altered by the local tissue and cellular environment. In this in vitro study we have investigated the (19)F MR-based quantification potential of a paramagnetic perfluorocarbon emulsion conjugated with RGD-peptide to target the cell-internalizing α(ν)β(3)-integrin expressed on endothelial cells, using a combination of (1)H MRI, (19)F MRI and (19)F MRS. The cells took up the targeted emulsion to a greater extent than nontargeted emulsion. The targeted emulsion was internalized into large 1-7 µm diameter vesicles in the perinuclear region, whereas nontargeted emulsion ended up in 1-4 µm diameter vesicles, which were more evenly distributed in the cytoplasm. Association of the targeted emulsion with the cells resulted in different proton longitudinal relaxivity values, r(1), for targeted and control nanoparticles, prohibiting unambiguous quantification of local contrast agent concentration. Upon cellular association, the fluorine R(1) was constant with concentration, while the fluorine R(2) increased nonlinearly with concentration. Even though the fluorine relaxation rate was not constant, the (19)F MRI and (19)F MRS signals for both targeted nanoparticles and controls were linear and quantifiable as function of nanoparticle concentration.