What shows up on pet scan but not on cat scan?

Glucose metabolism. PET with F18 FDG, glucose metabolite, determines glucose metabolism of normal and abnormal structures.For example, sometimes lymph nodes are visualized on CT and do no know whether benign or malignant. In general malignant lesions are more active and have higher metabolism of glucose.
Cellular activity. A cat scan is a sophisticated computer driven x-ray image creator. It depends upon absorptive differences of static tissue and generates an image of the tissues in question. A pet scan depends upon the cellular activity of things like cancers, to pick up a radiolabeled sugar which is concentrated more in active tissue and is detected by the scan, generating a picture with a blob of activity.
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.
Metabolic info/data. A ct scan is an anatomic , static exam of the anatomy. It is just a picture of your anatomic family. A pet/ct scan shows the same anatomy , but tells you also how active ..The anatomy / anatomic family is. Generally the more metabolically active something is ..The more you may need to worry.