A radioconjugate containing a monoclonal antibody directed against guanylyl cyclase C (GCC) labeled with the radioisotope gallium Ga 68, with positron emission tomography (PET) imaging activity. The monoclonal antibody moiety of MLN6907 selectively binds to GCC, a transmembrane receptor normally found on intestinal cells and dopaminergic neurons in the brain, that is also overexpressed on the surface of a variety of cancer cells. Upon internalization of the agent and PET, tumors expressing GCC can be visualized.