Positron emission tomography

Definition

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.

Purpose

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.

Precautions

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.

Description

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.

Preparation

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.

Aftercare

No special aftercare measures are indicated for PET.

Risks

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.

Normal results

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.

Abnormal results

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.

See Also Imaging studies; Nuclear medicine scans

Resources

BOOKS

Balazs, G., ed. Positron Emission Tomography: A Critical Assessment of Recent Trends. Norwell, MA: Kluwer Aca demic Publishers, 1998.

von Schulthess, G.K., ed. Clinical Positron Emission Tomogra phy. Philadelphia: Lippincott, Williams & Wilkins, 1999.

PERIODICALS

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 Colorectal Cancer." British Journal of Surgery 88 (Febru ary 2001): 176-89

Roelcke, U., and K.L. Leenders. "PET in Neuro-oncology." Journal of Cancer Research and Clinical Oncology 127 (January 2001): 2-8

Lisa Christenson

Monique Laberge, Ph.D.

KEY TERMS

Benign growth

—A noncancerous cell growth that does not metastasize and does not recur after treatment or removal.

Cancer screening

—A procedure designed to detect cancer even though a person has no symptoms, usually performed using an imaging technique.

CT scan

—An imaging technique that uses a computer to combine multiple x-ray images into a two-dimensional cross-sectional image.

Electron

—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.

Gamma ray

—A high-energy photon, emitted by radioactive substances.

Half-life

—The time required for half of the atoms in a radioactive substance to disintegrate.

Malignant growth

— A cell growth or tumor that becomes progressively worse and that can metastasize elsewhere in the body.

Metabolism

—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.

MRI

—A special imaging technique used to image internal parts of the body, especially soft tissues.

Photon

—A light particle.

Positron

—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?

Positron emission tomography

Definition

Positron emission tomography (PET) is a highly specialized imaging technique using short-lived radiolabeled substances to produce extremely high resolution images of the body's biological function.

Purpose

Besides being used to investigate the metabolism of normal organs, PET has also become the technique of choice to investigate various neurological diseases, 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, because these disorders have changes in specific areas of the brain . Additionally, PET scanning is a powerful research tool to detect changes or abnormalities in areas that may be difficult to visualize using other radiological procedures. In the field of mental health, a PET scan may be used when a patient seeks medical help for symptoms that could possibly be caused by a brain tumor. These symptoms may include headaches, emotional abnormalities, or intellectual or memory problems. In these cases, a PET scan may be performed to "rule out" a tumor, so that other tests can be performed in order to establish an accurate diagnosis .

PET is especially utilized in persons affected by 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 researchers 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.

Precautions

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.

Description

PET is used in conjunction with compounds that closely resemble a natural substance used by the body, such as a simple sugar (glucose, for example), 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.

Preparation

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.

Aftercare

No special aftercare measures are indicated for PET.

Risks

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.

Normal results

The PET scan of a healthy organ or body part will yield images without contrasting regions, because the radiolabeled sugar will have been metabolized at the same rate.

Abnormal results

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 (breaking down a large molecule to a smaller molecule that can be used by the body) at the same rate by the healthy and diseased cells.

Resources

BOOKS

Balazs, G., ed. Positron Emission Tomography: A Critical Assessment of Recent Trends. Norwell, MA: Kluwer Academic Publishers, 1998.

von Schulthess, G. K., ed. Clinical Positron Emission Tomography. Philadelphia: Lippincott, Williams and Wilkins, 1999.

PERIODICALS

Anderson, H., and P. Price. "What Does Positron Emission Tomography Offer Oncology?" European Journal of Cancer 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 Colorectal Cancer." British Journal of Surgery 88 (February 2001): 176–89.

Roelcke, U., and K. L. Leenders. "PET in Neuro-oncology." Journal of Cancer Research and Clinical Oncology 127 (January 2001): 2–8.

Lisa Christenson

Monique LaBerge, Ph.D.

pet¹ / pet/ • n. a domestic or tamed animal or bird kept for companionship or pleasure and treated with care and affection: the pony was a family pet. ∎  a person treated with special favor, esp. in a way that others regard as unfair: Liz was teacher's pet. ∎  used as an affectionate form of address: don't cry, pet, it's all right. • adj. (of an animal or bird) kept as a pet: a pet cat. ∎  of or relating to pet animals: a pet shop pet food. ∎  denoting a thing that one devotes special attention to or feels particularly strongly about: another of her pet projects was the arts center my pet hate is bad telephone manners. ∎  denoting a person or establishment that one regards with particular favor or affection: his pet performer was Hollander. • v. (pet·ted , pet·ting ) [tr.] stroke or pat (an animal) affectionately: the cats came to be petted. ∎  treat (someone) with affection or favoritism; pamper: I was cosseted and petted and never shouted at. ∎  [intr.] engage in sexually stimulating caressing and touching: couples necking and petting in the cars. DERIVATIVES: pet·ter n. pet² • n. [in sing.] a fit of sulking or ill humor: Mother's in a pet.

PET scan or positron emission tomography , a medical imaging technique that monitors metabolic, or biochemical, activity in the brain and other organs by tracking the movement and concentration of a radioactive tracer injected into the bloodstream. The technique uses special computerized imaging equipment and rings of detectors surrounding the patient to record gamma radiation produced when positrons (positively charged particles) emitted by the tracer collide with electrons.

PET scans are especially valuable in imaging the brain. They are used in medicine to diagnose brain tumors and strokes, and to locate the origins of epileptic activity; in psychiatry to examine brain function in schizophrenia , bipolar disorder , and other mental illnesses; and in neuropsychology to study such brain functions and capabilities as speech, reading, memory, and dreaming.

positron emission tomography (PET, positron emission computed tomography, PECT) n. a cross-sectional imaging technique of nuclear medicine that enables assessment and localization of metabolic activity in tissues of the brain, chest, and abdomen by measuring their uptake of injected radioisotopes (e.g. fluorodeoxyglucose, using fluorine-18, to examine glucose metabolism). As the radioisotope is metabolized, it emits positrons, which are annihilated with the production of gamma rays; these are detected by the PET scanner and used to construct images.

positron emission tomography (PET) Medical imaging technique (used particularly on the brain) that produces three-dimensional images. Radioisotopes, injected into the bloodstream prior to imaging, are taken up by tissues where they emit positrons that produce detectable photons.

Pet ♀ Short form of Petula, in part influenced by the common affectionate term of address ‘pet’, derived from the vocabulary word for a tame animal kept for companionship.

PET / pet/ • abbr. ∎  polyethylene terephthalate. ∎  positron emission tomography, used esp. for brain scans.

pet1 animal domesticated and treated as a favourite; indulged child XVI; darling, favourite XVIII. orig. Sc. and north. dial.; of Celt. orig. (cf. Ir., Gael. peata).

PET Polyethylene terephthalate; clear plastic used in packaging, especially bottles for drinks.

pet2 offence at being slighted. XVI. of unkn. orig.
Hence pettish (-ISH1) XVI.

positron emission tomography see PET scan .

PET n. see positron emission tomography.

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