| phase | Any one of the varying aspects or stages through which a disease or process may pass. Origin: Gr. Phasis = an appearance (18 Nov 1997) |
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| phase-amplitude contrast | <microscopy> The separation and recombination of direct vs. Diffracted rays in a light microscope adjusted to Kohler illumination. at the lower focal plane of the condenser there is an annular diaphragm with an opaque central stop. Through this diaphragm rays are focused as a hollow cone onto the specimen. In the back focal plane of the objective there is a conjugate annular diaphragm (phase plate). If here the undiffracted rays are retarded (by a transparent film of proper thickness on the annulus of the phase plate), bright contrast results. If, instead, the phase-delay film is on the central spot, dark contrast results. With either a bright or a dark-contrast phase plate, the annulus is usually coated with a partially absorbing (very thin) film of silver (Zernike method) or carbon soot (Wilska method) to reduce the higher amplitude (intensity) of the undiffracted rays. (05 Aug 1998) |
| phase angle | <microscopy> The angular equivalent of the time displacement between corresponding points on two sine waves of the same frequency. (05 Aug 1998) |
| phase contrast | <microscopy> An optical method devised by F. Zernike for converting the focused image of a phase object (one with differences in refractive index or optical path but not in absorbance), which ordinarily is not visible in focus, into an image with good contrast. (05 Aug 1998) |
| phase-contrast microscope | <instrument> A specially constructed microscope that has a special condenser and objective containing a phase-shifting ring whereby small differences in index of refraction are made visible as intensity or contrast differences in the image; particularly useful for examining structural details in transparent specimens such as living or unstained cells and tissues. (05 Mar 2000) |
| 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) |
| 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) |