Polyethylenimine (PEI) is an organic polymer with a high density of amino groups that can be protonated. At physiological pH, the positively charged PEI binds to DNA as a gene carrier (1). PEI binds to negatively charged heparin sulfate proteoglycans on the cell surface, thus facilitating the transfection of eukaryotic cells (2). Currently, [18F]fluoro-2-deoxy-d-glucose ([18F]FDG) (3) and 64Cu-pyruvaldehyde-bis(N4-methylthiosemicarbazone) (64Cu-PTSM) (4) have been studied as positron emission tomography (PET) cell-trafficking agents. However, studies of FDG-labeled cells were limited to ~6 h because of the short physical half-life of 18F (110 min). Li et al. (5) labeled PEI with 64Cu without using a metal chelator. 64Cu-PEI was evaluated as a tumor-imaging probe and as a cell-labeling agent for cell trafficking in comparison to 64Cu-PTSM.