positron emission tomography
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tomography
[to-mog´rah-fe]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.

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.
pos·i·tron e·mis·sion to·mog·ra·phy (PET),
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
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)positron emission tomography
Abbreviation: PETThe 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
positron emission tomography
See PET SCANNING.Positron emission tomography (PET)
pos·i·tron e·mis·sion tom·og·ra·phy
(PET) (poz'i-tron ĕ-mish'ŭn tŏ-mog'ră-fē)Patient discussion about positron emission tomography
Q. Can a pet allergy cause your nose to plug, give you a sore throat and a bad cough? I recently adopted a puppy. I felt fine for about a week an a half, but 2 days ago I began to sneeze a lot. My nose plugged and I had to blow almost constantly. I also came down with a sore throat and a bad cough. Is this a result of a pet allergy or did I just come down with a cold? I've been around dogs all my life and I don't ever recall reacting like this before.
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