Longmaid HE III, Adams DF, Neirinckx RD, Harrison CG, Brunner P, Seltzer SE, Davis MA, Neuringer L, Geyer RP. In vivo 19FNMR imaging of liver, tumor, and abscess in rats: preliminary results. Invest Radiol 1985;20:141-145. In vivo magnetic resonance imaging (MRI) has employed almost exclusively the proton because of its high gyromagnetic ratio and natural abundance relative to other nuclei. Recent research has focused on imaging using nuclei other than 1H, but has been limited by the decreased sensitivity and/or low biologic concentrations of the nuclei. Fluorine (19F), with a gyromagnetic ratio second only to that of hydrogen, is a theoretically attractive nucleus for MRI, but fluorine is present in only minute amounts in most tissues. Perfluorochemical emulsions (PFC), developed as blood replacement agents, appear to be safe vehicles for fluorine administration. We report our initial results of in vivo 19F magnetic resonance imaging of liver, tumor, and abscess in rats given exogenous fluorine.