A chemical compound has the general formula: Ex4-linker-Sar(64Cu)-Fl in which Ex4 is an extendin-4 analog; linker is a polyethylene glycol (PEG) chain, for example formed with four ethylene glycol residues; Fl is a photoluminescent moiety, and Sar(64Cu) is an atom of copper-64 chelated in a sarcophagine moiety is useful as a multimodality imaging agent for detection and localization of insulinoma cells and &bgr;-cell masses.