| absorptiometry, photon | A noninvasive method for quantitating bone mineral content. It is used especially in the diagnosis of osteoporosis and also in measuring bone mineralization in infants. (12 Dec 1998) |
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| photon | <physics> A quantum unit of light energy. (09 Oct 1997) |
| photon density | The number of counted events recorded in scintigraphy per square centimeter or per square inch of imaged area. Synonym: count density. (05 Mar 2000) |
| single photon emission computed tomography | <radiology> Tomographic imaging of metabolic and physiological functions in tissues, the image being formed by computer synthesis of photons of a single energy emitted by radionuclides administered in suitable form to the patient. The method uses radionuclides which emit a single photon of a given energy. The camera is then rotated 180 or 360 degrees around the patient to capture images at multiple positions along the arc. The computer then reconstructs the transaxial, sagittal, and coronal images from the 3-dimensional distribution of radionuclides in the target area scanned. The advantages of SPECT are that it can be used to observe biochemical and physiological processes as well as the size and volume of the organ. The disadvantage is that, unlike positron emission tomography where the positron-electron annihilation results in the emission of 2 photons at 180 degrees from each other, SPECT requires physical collimation to line up the photons, which results in the loss of available photons and hence degrades the image. Acronym: SPECT (20 Jun 2000) |
| absorption | The process of absorbing, specifically: 1. <physiology> The movement and uptake of substances (liquids and solutes) into cells or across tissues such as skin, intestine and kiidney tubules, by way of diffusion or osmosis. 2. <chemistry> The drawing of a gas or liquid into the pores of a permeable solid. 3. <psychology> The devotion of thought to one object or activity, with inattention to others. 4. <radiology> The taking up of energy by matter with which the radiation interacts. 5. <physics> The loss of (electromagnetic) energy to a medium. For instance, an electromagnetic wave which propagates through a plasma will set the electrons into motion. If the electrons make collisions with other particles, they will absorb net energy from the wave. 6. <immunology> A process in which an antigen or antibody is used to pull an analogous antigen or antibody out of a solution. Compare: adsorption. Origin: L. Absorptio (12 Nov 1997) |
| absorption band | The range of wavelengths or frequencies in the electromagnetic spectrum where radiant energy is absorbed by passage through a gaseous, liquid, or dissolved substance; it is exploited for analytical purposes in colourimetry or spectrophotometry, and is usually described in terms of the wavelength where maximum absorbance occurs (i.e., lambdamax). (05 Mar 2000) |
| absorption cell | A small glass chamber with parallel sides, in which absorption spectra of solutions can be obtained. (05 Mar 2000) |
| absorption chromatography | <investigation> Techniques for separating molecules based on differential absorption and elution. Term for separation methods involving flow of a fluid carrier over a nonmobile absorbing phase. (18 Nov 1997) |
| absorption coefficient | <physics> Measures the degree of wave absorption defined as the fraction of wave energy lost as the wave travels a unit distance. See: absorption. (15 Jan 1998) |
| absorption collapse | Pulmonary collapse due to rapid complete obstruction of a large bronchus. (05 Mar 2000) |
| absorption fever | An elevation of temperature often occurring, without other untoward symptoms, shortly after childbirth, assumed to be due to absorption of uterine discharges through abrasions of the vaginal wall. (05 Mar 2000) |
| absorption lines | The dark line's in the solar spectrum due to absorption by the solar and the earth's atmosphere; the phenomenon occurs because rays passing from an incandescent body through a colder medium are absorbed by elements in that medium. (05 Mar 2000) |
| absorption spectroscopy | <investigation> This is the use of a spectrophotometer to measure the ability of particles (solutes) in a solution to absorb light through a range of specific wavelengths. Every compound absorbs light differently, so absorption spectra can be used to identify compounds, measure concentrations, and determine reaction rates. (15 Jan 1998) |
| absorption spectrum | <chemistry> A graph of the amount of light a substance absorbs, plotted as a fuction of energy, frequency or wavelength. (15 Jan 1998) |
| atomic absorption spectrophotometry | Determination of concentration by the ability of atoms to absorb radiant energy of specific wavelengths. (05 Mar 2000) |