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.
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