Metabolic activity. Pet/ct scans are used in staging cancers-figuring out whether and how far a tumor has spread. While a ct scan shows anatomy (where things are), a pet scan uses fluorescent labeled sugar molecules to show cells that have an increased activity. Cells with a higher metabolic rate like cancer cells are more likely to show up. Benign conditions such as inflammation and infection will also light up.
Anatomy vs Physiolog. Ct scans look at the structure of the body, the anatomy, and the way diseases change or distort it. Pet looks at the physiology, or how the body uses, or metabolizes something. For pet, the most common metabolism pathway looked at is how glucose is used. A low dose ct is often acquired at the same time as the pet (as in pet/ct scan), to correlate the pet findings with the ct anatomy.