We have synthesized a novel chelator, 4-DEAP-ATSC, for loading 64Cu into liposomal drugs for positron emission tomography (PET). The 64Cu-loading technique based on the transmembrane ion gradient principle results in > 90% labeling efficiency without post-labeling purification. The 64Cu-liposomes are stable in vitro and in vivo; preclinical PET in tumor xenografts demonstrated that image-based analysis provides an accurate non-invasive quantification of the liposomal drug’s in vivo distribution.