What is a PET scan?

No, it's not for our four-legged friends, although many animals have had PET scans in research studies. PET is short for Positron Emission Tomography, and it's one of the newer imaging studies that is helping to change the detection and treatment of cancer and Alzheimer's disease. 

PET was first used in research over 50 years ago. The development of today's PET scanners and the availability of new radioactive drugs have brought this exciting technology out of the research labs and into local imaging facilities. Over the past five years, PET scans have become the fastest area of growth in medical images. The advent of much faster computers and better PET machines has markedly increased image quality, shortened scan time and reduced cost. Now the technology of PET has been married to the technology of X-ray Computed Tomography (CT) to make the new PET/CT scanner.

How is a PET scan done? PET scans use radioactive tracers made from radioisotopes. The radiotracer used most is 2-Fluorine-18 fluoro-2-deoxyglucose, or FDG. Individuals having the scan are given an intravenous injection of FDG after they have fasted for 4 hours. Fasting helps decrease the level of glucose (sugar) in the blood, which in turn increases the amount of FDG the cells in the body will use. Once the FDG is in the cells, it is changed by the cells and becomes trapped in the cells. One hour after being given FDG, the patient is ready for the scan. The scan takes 15 to 30 minutes on the new PET/CT scanners. The scan is then processed and sent to the radiologist or nuclear medicine physician for interpretation who sends a report to the referring physician.

How is the PET scan used? The PET/CT scan information is used by physicians to detect, stage, evaluate treatment and follow cancers. Most cancers will use more sugar than normal tissue due to increased metabolism. This increased uptake of FDG allows the PET scan to identify locations of cancers and possible spread or metastasis. The metabolic information helps to detect smaller cancers and metastases that are often not seen on other imaging studies. The PET can also show if the cancer metabolism is decreasing after cancer therapy. This will allow the treating physician to know if the therapy is working much earlier than other imaging studies which usually look only at the size of the cancer. PET scans can also detect the reoccurrence of cancer earlier, especially in tissue that is scarred or deformed from prior treatments.

PET scans are now used in neurology to scan for dementias such as Alzheimer's disease. Brain cells are very active and use large amounts of glucose. Brain tissue that has been damaged by disease or stroke will have decreased metabolism and thus have a decreased uptake of FDG. The patterns of decreased uptake of FDG can be used to make the diagnosis of Alzheimer's disease and some other neurological disorders at earlier stages than CT or Magnetic Resonance Imaging (MRI) scans. Although there is no cure for Alzheimer's disease at this time, early treatment helps to delay the progression of the disease and increases the functional status of the patient.

What is new with PET? Most PET scans are done on a PET/CT scanner which does a PET scan and a low-dose CT scan on the same machine. This allows the merging of the metabolic PET image with the anatomic CT image to give better detection and location of abnormal accumulation of FDG. The CT data is usually used to make the PET image look better by a process called attenuation correction. This same improved computer software allows the PET images to be merged with other imaging studies such as MRI.

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Brett L. Austin, M.D., "What is a PET scan?" Woman's Way, 2005.