The present invention relates to labeled compounds suitable for positron emission tomography (PET) imaging and/or fluorescence imaging such as near-infrared fluorescence imaging (NIRF). The invention further relates to the use of these compounds for carrying out PET scans. imaging calcium sensing receptor (CSR)-positive organs. excising parathyroid tissue. protecting parathyroid tissue during thyroid surgery, and treating disorder(s) of a CSR-positive tissue in a subject.