Metabolite profile in mouse tissues after intravenous injection of parent radiotracers, FCH and D4-FCH, assessed by radio-HPLC. The % radioactivity remaining at 30 min in the form of parent radiotracer, phosphocholine or betaine analogs, as well as that of unknown metabolites are shown for (a) liver, (b) kidney and (c) HCT116 tumor. In the case of tumor, additional comparisons were done with [11C]choline.