Positron Emission Tomography
Positron emission tomography (PET) is a highly specialized imaging technique using short-lived radiolabeled substances to produce powerful images of the body's biological function.
Besides being used to investigate the metabolism of normal organs, PET has also become the technique of choice to investigate various neurological diseases and disorders, including stroke, epilepsy, Alzheimer's disease, Parkinson's disease, and Huntington's disease. Various psychiatric disorders, such as schizophrenia, depression, obsessive-compulsive disorder, attention-deficit/hyperactivity disorder, and Tourette syndrome, are also imaged by PET.
PET is especially useful in the context of cancer because it can detect metastatic tumors that may not be visualized by other imaging techniques. It is also being increasingly used not only as a cancer diagnostic tool, but also to help physicians design the most beneficial therapies. For example, it may be used to assess response to chemotherapy. PET imaging is very accurate in differentiating malignant from benign cell growths, and in assessing the spread of malignant tumors. PET is also used to detect recurrent brain tumors and cancers of the lung, colon, breast, lymph nodes, skin, and other organs.
In some cases, patients may be allergic to the radioactive agents used for PET. A patient with known allergies should discuss this with their specialist before undergoing the PET scan.
PET is used in conjunction with compounds that closely resemble a natural substance used by the body, such as a simple sugar (e.g. glucose), labeled with a radioactive atom and injected into the patient. These compounds (radionuclides or radiopharmaceuticals) emit particles called positrons. As positrons emitted from the radionuclides encounter electrons in the body, they produce high-energy photons (gamma rays) that can be recorded as a signal by detectors surrounding the body. The radionuclides move through the body and accumulate in the organs targeted for examination. A computer collects the distribution of radioactivity and reassembles them into actual images.
By further defining a lesion seen on other imaging modalities, PET may enhance assessment of tumors exceedingly well. This is because of its operating principle. The radiolabeled sugars injected into the patient will be used by all body cells, but more sugar will be used by cells that have an increased metabolism. Cancer cells are highly metabolic, meaning that they use more sugar than healthy nearby cells, and they are easily seen on the PET scan. PET images thus show the chemical functioning of an organ or tissue, unlike x ray, computed tomography, or magnetic resonance imaging, which show only body structure.
The radiopharmaceutical is given by intravenous injection or inhaled as a gas a few minutes before the PET procedure. How it is administered depends on the radiopharmaceutical used and which one is selected depends on what organ or body part is being scanned. During the scan, the patient lies comfortably; the only discomfort involved may be the pinprick of a needle used to inject the radiopharmaceutical.
No special aftercare measures are indicated for PET.
Some of radioactive compounds used for PET scanning can persist for a long time in the body. Even though only a small amount is injected each time, the long half-lives of these compounds can limit the number of times a patient can be scanned. However, PET is a relatively safe procedure. PET scans using radioactive fluorine result in patients receiving exposures comparable to (or less than) those from other medical procedures, such as the taking of x rays. Other scanning radiopharmaceuticals—for instance, 6-F-dopa or radioactive water—normally cause even less exposure.
The PET scan of a healthy organ or body part will yield images without contrasting regions, because the radiolabeled sugar will have been metabolised at the same rate.
The PET scan of a diseased organ or body part however, will yield images showing contrasting regions, because the radiolabeled sugar will not have been metabolized at the same rate by the healthy and diseased cells.
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von Schulthess, G.K., ed. Clinical Positron Emission Tomogra phy. Philadelphia: Lippincott, Williams & Wilkins, 1999.
Anderson, H., and P. Price. "What Does Positron Emission Tomography Offer Oncology?" European Journal of Can cer 36 (October 2000):2028-35
Arulampalam, T. H., D.C. Costa, M. Loizidou, D. Visvikis, P.J. Ell, and I.Taylor. "Positron Emission Tomography and
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Monique Laberge, Ph.D.
—A noncancerous cell growth that does not metastasize and does not recur after treatment or removal.
—A procedure designed to detect cancer even though a person has no symptoms, usually performed using an imaging technique.
—An imaging technique that uses a computer to combine multiple x-ray images into a two-dimensional cross-sectional image.
—One of the small particles that make up an atom. An electron has the same mass and amount of charge as a positron, but the electron has a negative charge.
—A high-energy photon, emitted by radioactive substances.
—The time required for half of the atoms in a radioactive substance to disintegrate.
—The sum of all physical and chemical processes occurring in the body to maintain its integrity and also the transformations by which energy is made available for its uses.
—A special imaging technique used to image internal parts of the body, especially soft tissues.
—A light particle.
—One of the small particles that make up an atom. A positron has the same mass and amount of charge as an electron, but the positron has a positive charge.
QUESTIONS TO ASK THE DOCTOR
- How many PET scans will I have to undergo?
- Are there any risks associated with the radiopharmaceuticals that will be injected?
- How reliable are PET scans for my type of cancer?