positron emission tomography

tomography

 [to-mog´rah-fe]

any method that produces images of single tissue planes. In conventional radiology, tomographic images (body section radiographs) are produced by motion of the x-ray tube and film or by motion of the patient that blurs the image except in a single plane. In reconstruction tomography (CT and PET) the image is produced by a computer program.

computed tomography (CT) (computerized axial tomography (CAT)) a radiologic imaging modality that uses computer processing to generate an image (CAT scan) of the tissue density in a “slice” as thin as 1 to 10 mm in thickness through the patient's body. These images are spaced at intervals of 0.5 to 1 cm. Cross-sectional anatomy can be reconstructed in several planes without exposing the patient to additional radiation.



Since its introduction in 1972, the use of this modality has grown rapidly. Because it is noninvasive and has high contrast resolution, it has replaced some radiographic procedures using contrast media. It also has a better spatial resolution than scintillation imaging (about 1 mm for CAT compared to 15 mm for a scintillation camera).

A CAT scan is divided into a square matrix of pixels (picture elements). The newer CAT scanners use a high resolution matrix with 256 × 256 or 512 × 512 pixels. The region of the tissue slice corresponding to a pixel has a cross-sectional area of 1 × 1 mm to 2 × 2 mm; because of the thickness of the slice, it has a finite height and is therefore referred to as a voxel (volume element).

The actual measurements made by the scanner are the x-ray attenuations along thousands of rays traversing the slice at all angles. The attenuation value for a ray is the sum of the values for all of the voxels it passes through. A computer program called a reconstruction algorithm can solve the problem of assigning attenuation values for all the pixels that add up to the measured values along each ray.

The attenuation values are converted to CAT numbers by subtracting the attenuation value of water and multiplying by an arbitrary coefficient to produce values ranging from −1000 for air to +1000 for compact bone with water as 0. CT numbers are sometimes expressed in Hounsfield units, named after Godfrey Hounsfield, the inventor of the CT scanner; Hounsfield and Allan Cormack were co-winners of the Nobel Prize in physiology or medicine in 1979 for the development of computerized axial tomography.

Computed tomography. Relative position of the x-ray tube, patient, and detectors in a fourth generation CT unit.

electron beam computed tomography (EBCT) ultrafast computed tomography done with a scanner in which the patient is surrounded by a large circular anode that emits x-rays as the electron beam is guided around it.

extended narrow tomography tomography involving an increase in amplitude and increase in exposure angle resulting in greater thinness of the cut for examination.

linear tomography tomography in which the tube and film move in the same direction.

narrow angle tomography a type of tomography that results in thicker sections for examination.

pluridirectional tomography tomography in which there is a great deal of movement in a variety of directions.

positron emission tomography (PET) a combination of computed tomography and scintillation scanning. Natural biochemical substances or drugs tagged with a positron-emitting radioisotope are administered to the subject by injection; the tagged substance (tracer) then becomes localized in specific tissues like its natural analogue. When the isotope decays, it emits a positron, which then annihilates with an electron of a nearby atom, producing two 511 keV gamma rays traveling in opposite directions 180 degrees apart. When the gamma rays trigger a ring of detectors around the subject, the line between the detectors on which the decay occurred is stored in the computer. A computer program (reconstruction algorithm), like those used in computed tomography, produces an image of the distribution of the tracer in the plane of the detector ring.



Most of the isotopes used in PET scanning have a half-life of only 2 to 10 minutes. Therefore, they must be produced by an on-site cyclotron and attached chemically to the tracer and used within minutes. Because of the expense of the scanner and cyclotron, PET is used only in research centers. However, PET is important because it provides information that cannot be obtained by other means. By labeling the blood with ¹¹C-carbon monoxide, which binds to hemoglobin, images can be obtained showing the regional perfusion of an organ in multiple planes. By using labeled metabolites, images can be obtained showing metabolic activity of an organ. ¹⁵O-oxygen and ¹¹C-glucose have been used for brain imaging and ¹¹C-palmitate for heart imaging. ⁸¹Rb, which is distributed like potassium, is also used for heart imaging. By using labeled neurotransmitters, hormones, and drugs the distribution of receptors for these substances in the brain and other organs can be mapped.

single-photon emission computed tomography (SPECT) a type of tomography in which gamma photon–emitting radionuclides are administered to patients and then detected by one or more gamma cameras rotated around the patient. From the series of two-dimensional images produced, a three-dimensional image can be created by computer reconstruction. The technique improves resolution of, and decreases interference by, overlapping organs. It is used particularly for assessment of cardiac disease, stroke, and liver disease; for staging of cancer; and to diagnose physical abnormalities through evaluation of function.

ultrasonic tomography the ultrasonographic visualization of a cross-section of a predetermined plane of the body; see B-mode ultrasonography.

pos·i·tron e·mis·sion to·mog·ra·phy (PET),

creation of tomographic images revealing certain biochemical properties of tissue by computer analysis of positrons emitted when radioactively tagged substances are incorporated into the tissue. Radiotracers used in PET are analogues of physiologic or pharmaceutical agents into which positron-emitting isotopes with short half-lives (2-110 minutes) have been incorporated. Radioisotopes are produced artificially by bombarding stable isotopes with a proton beam generated by a cyclotron. The uptake and metabolism of these positron emitters mimic, at least in part, those of the radiostable natural substances to which they are analogous. Concentrated in particular organs or tissues and incorporated into metabolic processes, they can reflect biochemical function or dysfunction. The glucose analogue 2-(fluorine-18)fluoro-2-deoxy-d-glucose (FDG) is widely used to locate zones of heightened energy metabolism. When a positron emitted by a radiotracer collides with an electron, the particles annihilate each other and two gamma rays are discharged in opposite directions (at 180°). After intravenous administration of the radiotracer, the subject is positioned within a scanner consisting of a ring of scintillation crystals that convert gamma rays into flashes of visible light. These flashes are detected and recorded electronically, and a computer program assembles the data into a 3-dimensional image, color-coded to reflect concentration density.

Unlike other imaging procedures, PET assesses metabolic activity and physiologic function rather than anatomic structure. Because the half-lives of the radionuclides are short and the equipment expensive, PET has not thus far been used extensively in clinical settings. But since its development in the mid-1970s, it has proved the most important tool yet devised for the experimental investigation of the living brain, whether healthy, injured, or diseased. Besides providing important diagnostic information in Alzheimer and other dementias, parkinsonism, and Huntington disease, PET can localize epileptic foci in preparation for surgical intervention, assess intracranial neoplasms, and help to direct therapeutic choices in acute stroke. The sensitivity and specificity of PET in determining malignancy render it valuable in oncology in avoiding biopsies for low grade tumors, in noninvasive differentiation of tumors from radiation necrosis, in early modification of ineffective chemotherapy, and in avoiding unnecessary diagnostic and therapeutic surgery. PET has been employed in cardiology to screen for coronary artery disease, to assess flow rates and flow reserve, and to distinguish viable from nonviable myocardium in bypass and transplant candidates.

positron emission tomography

n. Abbr. PET

Tomography in which a computer-generated image of local metabolic and physiological functions in tissues is produced through the detection of gamma rays that are emitted when introduced radionuclides decay and release positrons.

positron emission tomography (PET)

Etymology: L, positivus + Gk, elektron, amber; L, emittere, to send out; Gk, tome, section, graphein, to record

a computerized radiographic technique that uses radioactive substances to examine the metabolic activity of various body structures. The patient either inhales or is injected with a metabolically important substance such as glucose, carrying a radioactive element that emits positively charged particles, or positrons. When the positrons combine with electrons normally found in the cells of the body, gamma rays are emitted. The electronic circuitry and computers of the PET device detect the gamma rays and construct color-coded images that indicate the intensity of metabolic activity throughout the organ involved. The radioactive isotopes used in PET are very short-lived, so that patients undergoing a PET scan are exposed to very small amounts of radiation. Researchers use PET to examine blood flow and the metabolism of the heart and blood vessels, to study and diagnose cancer, and to investigate the biochemical activity of the brain.

Image produced by a PET scan after a seizure

positron emission tomography

See PET scan.

pos·i·tron e·mis·sion tom·og·ra·phy

(PET) (poz'i-tron ĕ-mish'ŭn tŏ-mog'ră-fē)

Tomographic images formed by computer analysis of photons detected from annihilation of positrons emitted by radionuclides incorporated into biochemical substances; the images, often quantitated with a color scale, show the uptake and distribution of the substances in the tissue, permitting analysis and localization of metabolic and physiologic function.

POSITRON EMISSION TOMOGRAPHY: PET SCAN revealing lung cancer

PET SCAN OF BRAIN

positron emission tomography

Abbreviation: PET

Reconstruction of brain sections by using positron-emitting radionuclides. By using several different radionuclides, researchers can measure regional cerebral blood flow, blood volume, oxygen uptake, and glucose transport and metabolism, and can locate neurotransmitter receptors. PET has been used with fludeoxyglucose F 18 to identify and localize regional lymph node metastases and to help assess response to therapy.

The images produced by PET are in colors that indicate the degree of metabolism or blood flow. The highest rates appear red, those lower appear yellow, then green, and the lowest rates appear blue. The images in various disease states may then be compared to those of normal subjects. Three- and four-dimensional reconstructions are often achieved through the use of computed tomography (CT) with the same machine. See: illustration

illustration

See also: tomography

positron emission tomography

See PET SCANNING.

Positron emission tomography (PET)

A radiology test by which images of cross-sectional planes of the body are obtained, utilizing the properties of the positron. The positron is a subatomic particle of equal mass to the electron, but of opposite charge.

Mentioned in: Esophageal Cancer, Movement Disorders

positron emission tomography (PET) (pˑ·zi·tr),

diagnostic exam in which physiologic images are acquired by detecting subatomic particles emitted from a radioactive substance, administered to the patient before the scan. Also known as
PET imaging or
PET scan.

pos·i·tron e·mis·sion tom·og·ra·phy

(PET) (poz'i-tron ĕ-mish'ŭn tŏ-mog'ră-fē)

Tomographic images formed by computer analysis of photons detected from annihilation of positrons emitted by radionuclides incorporated into biochemical substances.

Patient discussion about positron emission tomography

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