Contrast variation, definition

Although ancients and moderns alike have called attention to variability and individuality as factors particularly related to disease susceptibility and moderns have recognized that variability is indispensable to evolution, comparatively little research time and effort have been devoted to definitive study in physiology and biochemistry as to precisely how so-called normal individuals differ from each other. Such study necessarily involves repeated observations on the same individuals, in contrast to a series of single observations on representative populations. No attempt to bring together the available biochemical material on normal variation has been previously made so far as I know. 

There are wide variations in the reported incidence of contrast nephrotoxicity because of differences in patient selection, the type of radiological procedure, and the definition of renal impairment. Contrast nephrotoxicity is relatively uncommon in people with normal renal function, in whom it is 0-10%. Pre-existing renal impairment increases the frequency, with a reported incidence of 12-27% in several prospective controlled studies. In some studies the incidence was as high as 50%, in spite of the use of low-osmolar contrast agents and adequate hydration. Dialysis may be required in some of these patients (SEDA-22, 502). 

The natural abundances of the stable isotopes given in Table 6.3 are global average values, whereas the actual exact values are subjected to small local and temporal variations, due to the slightly different behaviour of isotopologue molecules (by lUPAC definition a molecular entity that differs only in isotope composition, means number of isotopic substitutions in contrast, isotopomer [ isotopic isomer ] molecules are isomers, having the same number of each isotopic atom but in different positions) in the course of chemical reactions or physical processes ([8], kinetic and thermodynamic isotope effects, respectively, see below). The corresponding shifts are so small that they caimot be indicated in the atom-% scale therefore they are expressed in 5-values, differences of the isotope ratio R, e.g., ([ C]/[ C]) of the sample and an international standard relative to this international standard ... 

In 1973, Tatro et al (11)examined the qualitative and quantitative changes in essential oil composition of leaf oils obtained from Juniperus occidentalis, J. osteosperma and J, californica, and found that neither growth medium or seasonal variation had a significant effect on oil composition. By contrast, they observed a definite diurnal cycling which they attributed to the fluctuation in air temperature over a twenty-four hour period. Six years later, Hopfinger et al (16) examined the diurnal variation in the leaf oil of Citrus sinensis (L.) Osbeck. The authors found that there was a two-fold diurnal change in oil... 

Abstract Neutron reflection has proved to be a very successful technique for investigating the structure of layers of surfactants adsorbed at the air/water interface, especially when isotopic substitution may be used to resolve the structure within the layers. Here the extension of the technique to surfactant layers adsorbed at the solid/liquid interface is discussed. The experimental conditions necessary for probing surface structure accurately are mainly determined by the possibilities available for varying the contrast through isotopic substitution. The simplest variation is of the H/D ratio in water, but definitive evidence for bilayer structures at the surface can usually only be obtained by varying... 

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