lodinated intravascular contrast agents are water-soluble molecules that contain several radiodense iodine atoms per molecule. Following injection, contrast agents remain in the vascular space for only a short time and are rapidly distributed into the extracellular space. Contrast agents do not enter the intracellular space or cross an intact blood-brain barrier. They are rapidly excreted by the kidney via glomerular filtration without tubular resorption.
Since the radiodense iodine atoms absorb more x-rays than do biological tissues, contrast agents appear white (like the bones) if they are present in high enough concentration during examinations made with x-rays (radiographs). Contrast agents are visible on arteriography during the short time they are in high concentration in the vascular space. They are visible in the urinary tract during intravenous urography (IVU), after excretion by the kidney. Because of its ability to discriminate subtle differences in radiodensity, computed tomography (CT) shows the presence of contrast agents in all tissue compartments even though they may be present in relatively low concentration (compared to arteriography or radiography).
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