Unlike techniques that provide anatomical images, such as X-ray, CT and MRI, PET 
scans show chemical and physiological changes related to metabolism. This is important because these functional changes often occur before structural changes in tissues. PET images may therefore show abnormalities long before they would be revealed by X-ray, CT, or MRI.
Preparation
Before a PET scan, a patient will receive an injection of a radiopharmaceutical, which is a drug labeled with a basic element of biological substances, called an isotope. These isotopes distribute in the organs and tissues of the body and mimic natural substances such as sugars, water, proteins, and oxygen. This radioactive substance is then taken up by cells, thereby allowing the radiologist to visualize areas of increased activity.
Uses of PET
PET is useful for diagnosing many cancers, and for monitoring response to therapy. Effective therapy leads to rapid reductions in the amount of glucose that is taken up by tumors. PET imaging can easily reveal this drop in metabolic activity and show whether a patient is responding positively to a particular course of treatment. PET has been shown effective for predicting outcomes, detecting spread of cancer, and/or monitoring therapeutic response in a wide range of cancers, including breast, colon, lung, ovarian, head, neck, as well as melanoma and lymphoma.
