Identification of primary tumors and metastasis sites is an essential step in cancer diagnostics and the following treatment. Positron emission tomography-computed tomography (PET/CT) is one of the most reliable methods for scanning the whole organism for malignancies. In this work, we synthesized an C-labeled oligonucleotide primer and hybridized it to an anti-cancer DNA aptamer. The C-aptamer was applied for imaging of Ehrlich ascites carcinoma and its metastases in mice using PET/CT. The imaging experiments with the C-aptamer determined very small primary and secondary tumors of 3 mm and less. We also compared C imaging with the standard radiotracer, 2-deoxy-2-[fluorine-18]fluoro-D-glucose (F-FDG), and found better selectivity of the C-aptamer to metastatic lesions in the metabolically active organs than F-FDG. C radionuclide with an ultra-short (20.38 min) half-life is considered safest for PET/CT imaging and does not cause false-positive results in heart imaging. Its combination with aptamers gives us high-specificity and high-contrast imaging of cancer cells and can be applied for PET/CT-guided drug delivery in cancer therapies.