(PET) a technique in nuclear medicine for *cross-sectional imaging that enables a noninvasive assessment and localization of metabolic activity to be made. Originally used to study activity in the brain, PET is now also used for investigating the chest and abdomen. Emission of a *positron by a radioisotope results in annihilation of the positron on collision with an electron, and the creation of two gamma rays of known energy travelling in exactly opposite directions. The PET scanner has detectors on each side of the patient to detect the simultaneous arrival of the gamma rays. Images are created using reconstruction *algorithms similar to CT scanning. *Fluorodeoxyglucose, labelled with fluorine-18, is used to examine glucose metabolism, and ammonia, labelled with nitrogen-13, gives information on perfusion. Carbon-11 and oxygen-15 can also be used as radioisotopes for PET scanning. Some diseases result in decreased uptake of the radio-labelled material due to decreased function; others show increased glucose metabolism and concentrate the isotope avidly. In this way functional activity of the tissues can be compared with anatomical images obtained by CT or MRI scanning (see PET/CT scanning). Localized areas of increased glucose uptake revealed by PET scans can sometimes signify functional activity of malignant cells, which use more glucose than normal cells, although other processes, such as infection, can produce similar scans. PET scans can help to localize metastatic disease (see metastasis) that is not identified by other scanning techniques. See also tomography. Compare computerized tomography.
A discharge. The term is commonly used to describe the orgasmic ?ow of SEMEN from the erect PENIS that occurs during sleep. Described as a nocturnal emission or, colloquially, as a ‘wet dream’, it is a common event in late PUBERTY.... emission
(CT) a form of X-ray examination in which the X-ray source and detector (CT scanner) rotate around the object to be scanned and the information obtained can be used to produce cross-sectional images (see cross-sectional imaging) by computer (a CT scan). A higher radiation dose is received by the patient than with some conventional X-ray techniques, but the diagnostic information obtained is far greater and should outweigh the increased risk. CT scanning can be used for all parts of the body. The data obtained can be used to construct three-dimensional images of structures of interest. See also multidetector computerized tomography; spiral CT scanning.... computerized tomography
(MDCT) a development of *spiral CT scanning that uses more than one array of detectors opposite the X-ray tube, so that more tissue can be included, with thinner cuts, in a single rotation of the machine. This is particularly important for three-dimensional reconstruction of tissues. It also allows volumetric scanning or *isotropic imaging, which are best achieved when the thickness of the slice is similar to the size resolution of the detectors in the other two planes. Modern CT scanners are now usually equipped with between 16 and 640 detector arrays. The technique is particularly valuable for imaging fast-moving structures, such as the heart.... multidetector computerized tomography
(OCT) a class of optical tomographic techniques that allows extremely high-quality micrometre-resolution three-dimensional images to be obtained from within optical scattering media (e.g. biological tissue). OCT is proving valuable in ophthalmology, for noninvasive imaging of the ocular structures, and in cardiology for visualizing the interior of coronary arteries using a specialized *catheter. See also spectral domain optical coherence tomography.... optical coherence tomography
n. an electrically charged particle released in some radioactive decays, notably fluorine-18 or nitrogen-13, that has the same mass as an electron but opposite charge. It has a very short lifetime as it quickly reacts with an electron (annihilation) to produce a pair of *gamma rays, which are emitted in diametrically opposite directions. The energy of each gamma ray is always 511 keV.... positron
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