Contrast media limitations

The first CT system was shown to be approximately 100 times more sensitive than the conventional X-ray system (837). Contrast enhancement in CT is measured in terms of a density unit. In the original article, water was arbitrarily set at a value of 0 unit air, 500 unit fat, 50 unit, and so forth. On this scale, a number of tissues in the body fell within 20 units above water or "4% window" in scanning. This unit was later named the Hounsfield Unit (HU) in honor of the author (840). Using HUs, air has a value of -1000, water a value of 0, and dense bone a value of +1000. The white tone on the picture represents dense material with enhanced contrast. Materials such as tumor will be less enhanced by the contrast medium because of limited blood supply and will appear hypodense, gray, or less white. Organs or tissues not contrast enhanced will appear black on the picture. A similar unit, known as the CT number, for measuring the image contrast or conspicuity, discussed by Dawson (836), is based on a com-... 

The authors of the Emopean multicenter study pointed out some limitations in their report. These included lack of provocation testing, the need for more control subjects exposed to a contrast medium but without clinical signs of a hypersensitivity reaction, the culprit drug was not... 

A fasting period is only required for sedated children (not for contrast medium injection). It should be limited to 3 h in young children and to 4 h in older ones. Dehydration should always be prevented. 

The ideal soil is defined as a loose, granular medium that is devoid of cohesion but possesses internal friction. In contrast, an ideal cohesive medium is one that is devoid of internal friction. Real soils generally fall between the foregoing two limiting definitions. 

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