Home » What is PET? » How does it work

How does PET/CT work

PET/CT combines or merges a PET scan and a CT scan into one set of images.

How does CT work?

CT stands for Computerized Tomography (commonly known as a CAT scan). During the CT scan, the scanner emits X-rays, which go through the patient to detectors. The computer uses this information to generate cross-sectional images of anatomical structures. Your body will not come in contact with the scanner itself. You will be lying on a narrow table, which will move through the scanner or dectors. Each cross-sectional picture or slice gives detailed anatomic location and changes in the anatomy. The use of oral and IV contrast agents can enhance the details by highlighting the gastrointestinal tract (filled by oral contrast) and other organs and blood vessels (filled with IV contrast).

How does PET work?

PET stands for Positron Emission Tomography. PET scans measure metabolic activity and molecular function by using a radioactive glucose injection. The F-18 FDG is injected into the patient. The PET scanner detects the radiation emitted from the patient, and the computer generates three-dimensional images of tissue function or cell activity in the tissues of your body. These functional images can detect disease earlier than the anatomic information gained from CT alone. Like the CT scanner, your body will never come in contact with scanner itself. There are no side effects from this injection and procedure.

All cells use glucose as an energy source. However, cancer cells grow faster than normal healthy cells and they use glucose at much higher rate than normal cells. This is the basis of imaging with F-18 FDG for cancer detection in PET scan.

  • For most clinical PET scans, a small amount of radioactive sugar ("tracer") is introduced into the body through an I.V.. The tracer then travels through your body. After the tracer has been processed (usually 60-90 minutes), a PET scanner detects radioactive tracers' distribution and displays a computerized image of that distribution on a computer monitor. These images are then review by an experienced PET-trained physician and/or radiologist to diagnose, stage, evaluate treatment, or rule out recurrence of disease.
  • Radioactive Sugar
  • Most clinical PET studies in oncology utilize [18F] 2-Fluoro-2-Deoxy-D-Glucose - more commonly known as "FDG". This molecule is shown above. The green atom is the radioactive (positron-emitting) Fluorine-18 atom whose emissions the PET scanner detects.
  • The foundation of PET imaging with FDG rests with the observation as long ago as the 1920s that malignant tissue has a higher metabolic rate for glucose than the normal tissues from which those tumors arise.
  • FDG, an analog of glucose, becomes trapped in the cells that try to metabolize it. Its concentration is tissue builds up in proportion to the rate of glucose metabolism. Because tumors have a high rate of glucose metabolism, they concentrate FDG, and appear as "hot spots" in PET images.
  • Tracer
  • PET is a "tracer" imaging methodology. This means we only use TINY (non-pharmacological) quantities of radioactive tracer in a PET study. For example, when we use FDG, a glucose analog, for our PET studies, we inject less than a millionth of a single grain of sugar!!