| PET | <radiology> A highly specialised research imaging technique using short lived radioactive substances - usually those made with a cyclotron. This technique is very sensitive in picking up active tumour tissue but does not measure the size of it. Tomographic images are formed by computer analysis of photons detected from annihilation of positrons emitted by radionuclides incorporated into biochemical substances; the images, often quantitated with a colour scale, show the uptake and distribution of the substances in the tissue, permitting analysis and localization of metabolic and physiological function. Because the half-lives of the radionuclides are so short (20 minutes to 2 hours), and the equipment expensive, PET is rarely used in a clinical setting. But since its development in the mid-1970s, it has proved the most important tool yet devised for experimental investigation of the living brain, whether healthy, traumatised, or diseased. With CT and MRI, it represents a new generation of computer imaging techniques that have revolutionised medicine and physiology. Acronym: PET (20 Jun 2000) |
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| PET scan | Positron Emission Tomography. A scanning device which uses low-dose radioactive sugar to measure brain activity. This is a limited-use diagnostic tool. (16 Dec 1997) |
| PET56 methyltransferase | <enzyme> Catalyses the site-specific formation of 2'-o-methylguanosine on in vitro transcripts of both mitochondrial 21s rrna and e. Coli 23s rrna Registry number: EC 2.1.1.- Synonym: pet56 protein, pet56 methylase, pet56 nuclear gene product (26 Jun 1999) |
| peta- | <prefix> Prefix used in the SI and metric systems to signify one quadrillion (10^15). (05 Mar 2000) |
| petal | 1. <plant biology> A member of the inner whorl of non-fertile parts surrounding the fertile organs of a flower, usually soft and coloured conspicuously. 2. <suffix> Seeking; movement toward the part indicated by the main portion of the word. Origin: L. Peto, to seek, strive for (05 Mar 2000) |
| petaled | <botany> Having petals; as, a petaled flower; opposed to apetalous, and much used in compounds; as, one-petaled, three-petaled, etc. Source: Websters Dictionary (01 Mar 1998) |
| petaliform | <botany> Having the form of a petal; petaloid; petal-shaped. Source: Websters Dictionary (01 Mar 1998) |
| petaline | <botany> Pertaining to a petal; attached to, or resembling, a petal. Origin: Cf. F. Petalin. Source: Websters Dictionary (01 Mar 1998) |
| petalism | A form of sentence among the ancient Syracusans by which they banished for five years a citizen suspected of having dangerous influence or ambition. It was similar to the ostracism in Athens; but olive leaves were used instead of shells for ballots. Origin: Gr, fr. A leaf: cf. F. Petalisme. Source: Websters Dictionary (01 Mar 1998) |
| petalite | <chemical> A rare mineral, occurring crystallized and in cleavable masses, usually white, or nearly so, in colour. It is a silicate of aluminia and lithia. Origin: Cf. F. Petalite. Source: Websters Dictionary (01 Mar 1998) |
| petalody | <botany> The metamorphosis of various floral organs, usually stamens, into petals. Origin: Petal + Gr. Form. Source: Websters Dictionary (01 Mar 1998) |
| petaloid | Like a petal, soft in texture and coloured conspicuously. (09 Oct 1997) |
| petaloideous | <botany> Having the whole or part of the perianth petaline. Petaloideous division, that division of endogenous plants in which the perianth is wholly or partly petaline, embracing the Liliaceae, Orchidaceae, Amaryllideae, etc. Source: Websters Dictionary (01 Mar 1998) |
| petalosticha | <zoology> An order of Echini, including the irregular sea urchins, as the spatangoids. See Spatangoid. Origin: NL, from Gr. A leaf + a row. Source: Websters Dictionary (01 Mar 1998) |
| petasus | The winged cap of Mercury; also, a broad-brimmed, low-crowned hat worn by Greeks and Romans. Origin: L, from Gr. Source: Websters Dictionary (01 Mar 1998) |
| bonding, human-pet | The emotional attachment of individuals to pets. (12 Dec 1998) |
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