Combined PET/CT uses two imaging methods, CT and positron emission tomography (PET), in one procedure. CT is done first to create anatomic pictures of the organs and structures in the body, and then PET is done to create pictures that provide functional data about the metabolic pathways (chemical reactions that take place in a cell to create and use energy) that are active in tissues or cells. Cancer cells often use different metabolic pathways than normal cells.
Patients undergoing a PET/CT procedure are administered a positron-emitting (radioactive) substance, or radiopharmaceutical, that is designed to target cancer cells specifically. Numerous radiopharmaceuticals have been developed. However, the most common PET procedure uses an imaging agent called FDG (a radioactive form of the sugar glucose). Because cancerous tumors usually metabolize glucose more rapidly than normal tissues, they take up more FDG and appear different from other tissues on a PET scan.
Other PET imaging agents can provide information about the level of oxygen in a particular tissue, the formation of new blood vessels, the presence of bone growth, whether tumor cells are actively dividing and growing, and whether cancer may have spread.
Combining CT and PET may provide a more complete picture of a tumor’s location and growth or spread than either test alone. The combined procedure may improve the ability to diagnose cancer, to determine how far a tumor has spread, to plan treatment, and to monitor response to treatment. Combined PET/CT may also reduce the number of additional imaging tests and other procedures a patient needs.