What is positron emission tomography (PET) scanning?
Positron emission tomography (PET) scanning permits the noninvasive determination of biological function, metabolism, and pathology following the administration of short-lived positron-emitting radiopharmaceuticals (radioactive pharmaceuticals). Positron-emitting radiopharmaceuticals are drugs that contain a radioactive atom that is transformed into a more stable atom by emitting a positron. A positron is a subatomic particle that has the same mass and charge as an electron, but the charge is positive rather than negative. When an energetic positron is emitted by a radioactive atom, it quickly loses its energy in the surrounding medium and comes to rest. The positron then combines with a free electron in the medium, the two particles are annihilated, and their energy is converted into two 511,000-volt-potential (511 keV) photons that travel in exactly opposite directions. The detection of these two photons in coincidence (simultaneously) by placing a ring of small radiation detectors around the patient is the basic principle used in PET scanning. The radiation detectors convert these light photons into electrical signals that are fed to a computer, which reconstructs the distribution of radioactivity in the desired organ and presents the information as an image on a video screen.
PET techniques are generally used to measure metabolic rates quantitatively in normal and abnormal tissues. The positron-emitting radionuclides oxygen-15, nitrogen-13, and carbon-11 are well suited for studying tissue metabolism because of their short physical half-lives and their ubiquitous presence in biomolecules. Oxygen-15 gas is useful for studying oxygen metabolism, whereas oxygen-15 water and oxygen-15 carbon monoxide are used to study blood flow and blood volume, respectively, in any organ. Fluorodeoxyglucose (FDG) labeled with fluorine-18 is another positron-emitting radiopharmaceutical that is useful in PET for the quantitative measurement of glucose metabolism. Glucose metabolism is high in brain tumors when compared to normal tissue. Hence, PET with fluorodeoxyglucose-18 is widely used in the detection of brain tumors and assessment of the degree of malignancy, since low-grade tumors are less metabolically active than high-grade tumors. Fluorodeoxyglucose-18 PET is also used to identify persistent tumors after surgery.
The radiopharmaceuticals carbon-11 aminoisobutyric acid, rubidium-82 chloride, and gallium-68 citrate are useful for studying the blood-brain barrier permeability. This modality is also being used increasingly in patients with seizure disorders, dementia, and movement disorders. The application of PET in these cases may provide previously unavailable information about these diseases to help the practicing neurologist. PET imaging is also being used to understand the mechanisms behind normal brain processes. Among the mechanisms studied are language processing, speech, vision, and brain development. Recent improvements in whole-body PET scanning technology are useful in clinical oncology. This technique is being employed widely in the qualitative imaging of primary or recurrent tumors, lymph nodes, and distant metastases.
PET is also being used with increasing frequency to measure myocardial perfusion (heart function). A number of different perfusion agents labeled with rubidium-82, gallium-62, oxygen-15, and nitrogen-13 are being investigated. Some of these agents are being used to measure not only relative perfusion but also the absolute blood flow in selected regions of the myocardium. Carbon-11 palmitate is employed to determine cardiac fatty acid metabolism. These radiopharmaceuticals are generally safe because they involve low doses of radiation with minimal risks.
The most desirable radionuclides used in PET procedures—carbon-11, nitrogen-13, oxygen-15, and fluorine-18—can be produced only with a cyclotron. Because of their very short physical half-lives, a dedicated cyclotron and fully automated equipment for radionuclide separation are usually necessary. Radionuclides that emit positrons but do not require a nearby cyclotron for their production—such as copper-62, gallium-68, and rubidium-82—are receiving attention because of their reduced cost. Although PET scanning provides valuable diagnostic information that in many cases cannot be obtained using other modalities, the expensive nature of the highly technical equipment necessary may limit the availability of these procedures to specialized medical centers. The changing health care environment and the public demand for cost containment may hinder further advances in this area of diagnostic radiology. Technological advances in PET scanning instrumentation, however, may result in a markedly reduced cost for the procedure in the future.
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