Drug tracer imaging of small molecule therapeutic compounds employs isotopologues or radiolabeled analogues. For example, I-124 PU-H71 (top right) is an isotopologue of the epichaperome inhibitor therapeutic compound, PU-H71 (top left), identical in molecular structure, differing only in the isotopic form of a constituent atom (stable iodine-127 versus the positron-emitting iodine-124) [7,8]. F-18 SKI-249380 (bottom right) is a radiolabeled analogue of the tyrosine kinase inhibitor therapeutic compound, dasatinib (bottom left). The positron-emitting fluorine-18 atom, in F-18 SKI-249380, replaces a hydroxyl group found in dasatinib; this substitution has minimal pharmacologic effect, enabling F-18 SKI-249380 to be an effective drug tracer of dasatinib [9,10].