The invention provides a use of a compound of the formula wherein V is a folate, wherein the folate is a substrate for receptor-mediated transmembrane transport in vivo, or a folate receptor binding derivative or analog thereof L is a divalent linker R is a side chain of an amino acid M is a cation of a radionuclide n is 1 or 0 and k is 1 or 0 for imaging a population of cells in an animal pre-injected with unlabeled folate. The combination of the compound with unlabeled folate is also provided.
