| phase contrast microscopy | <investigation> A simple nonquantitative form of interference micoscopy of great utility in visualising live cells. Small differences in optical path length due to differences in refractive index and thickness of structures are visualised as differences in light intensity. (18 Nov 1997) |
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| phase, crystal | <microscopy> A specific crystal structure, usually given a name. (05 Aug 1998) |
| phase encoding | In magnetic resonance imaging, the technique of inducing a gradient in the magnetic field in the Y-axis to induce phase differences with location. Synonym: gradient encoding. (05 Mar 2000) |
| phase I block | Inhibition of nerve impulse transmission across the myoneural junction associated with depolarisation of the motor endplate, as in the muscle paralysis produced by succinylcholine. (05 Mar 2000) |
| phase I clinical trial | <pharmacology> The earliest stage clinical trial for studying an experimental drug in humans. Phase I trials are generally comparatively small and are used to determine toxicity and maximum dose. They provide an initial evaluation of a drug's safety and pharmacokinetics-how the drug is absorbed, what tissues it reaches and how long it takes to leave the body. Such studies also usually test various doses of the drug (dose-ranging) to obtain an indication of the appropriate dose to use in later studies. The patients in these trials usually have advanced disease and have already received best available chemotherapy, therefore, seeing a repose is significant partially because this means there is a lack of cross-resistance between two anti-cancer drugs. (31 Dec 1997) |
| phase II block | Inhibition of nerve impulse transmission across the myoneural junction unaccompanied by depolarisation of the motor endplate, as in the muscle paralysis produced by tubocurarine. (05 Mar 2000) |
| phase II clinical trial | <pharmacology> Usually focus on the activity of the new product as a single agent in a noncomparative, open study. (31 Dec 1997) |
| phase III clinical trial | <pharmacology> An advanced stage clinical trial that should conclusively show how well a drug works as compared to other treatments. Phase III trials are large, frequently multi-institution tests. They generally compare the relative value of the new drug compared with the current standard treatment and measure whether a new drug extends survival or otherwise improves the health of patients on treatment (clinical improvement) rather than just provide surrogate marker data. These studies generally last longer and are larger than phase II trials. (31 Dec 1997) |
| phase image | A magnetic resonance image showing only phase shift information, to detect motion. (05 Mar 2000) |
| phase plate | <microscopy> The plate used near the back focal plane of a microscope objective lens (in conjunction with an annulus at the front focal plane of the condenser lens) to achieve phase contrast. The phase plate selectively shifts the phase of the waves diffracted by the specimen by a quarter wave and reduces the amplitude of the undeviated, direct beam. (05 Aug 1998) |
| phase, resting | More appropriately called interphase. The interval in the cell cycle between two cell divisions when the individual chromosomes cannot be distinguished, interphase was once thought to be the resting phase but it is far from a time of rest for the cell. It is the time when DNA is replicated in the cell nucleus. (12 Dec 1998) |
| phase rule | An expression of the relationships existing between systems in equilibrium: P + V = C + 2, where P is the number of phases, V the variance or degrees of freedom, and C the number of components; it also follows that the variance is, V = C + 2 -P. For H2O at its triple point, V = 1 + 2 -3 = 0, i.e., both temperature and pressure are fixed. Synonym: Gibb's phase rule. (05 Mar 2000) |
| phase separation | <chemistry> The separation of fluid phases that contain different concentrations of common components. Occurs with partially miscible solvents used in many biochemical separation methods. Also temperature dependent phase separation occurs with some detergent solutions. With reference to membranes means the segregation of lipid components into domains that have different chemical composition. (31 Dec 1997) |
| phase shift | <microscopy> A change in the phase relationship between two alternating quantities of the same frequency. (05 Aug 1998) |
| phase variation | <microbiology> Alteration in the expression of surface antigens by bacteria. For example: Salmonella can express either of two forms of flagellin, H1 and H2, that are coded by different genes. Control of which form is expressed is brought about by inversion of the promoter for the H2 gene, which if functional (noninverted) is associated with the expression of H2 and the production of a repressor of the H1 gene. Inversion occurs about every 1000 bacterial divisions and is under the control of another gene, hin, that is within the invertable sequence. (31 Dec 1997) |