Contrast Enhancement in Computed Tomography
The development of computed tomography made available a radio diagnostic tool of extremely high sensitivity for the detection of relatively-low differences in tissue density. Users of CT observed early in its history that an intravenous infusion of iodinated contrast material could be of great assistance in the diagnosis and characterization of numerous lesions. Both Riding and Gado showed that an iodine concentration of 1 mgm/ml raised the average attenuation of the solution by 24 to 30 Houndsfield units on a scanner operating at 120 kV (1,2). Gado et al demonstrated that the high values of the tissue-blood ratio of enhancement in pathologic conditions cannot be explained by iodinated blood volume per unit volume of tissue (1). These authors reasoned that such very high values can be accounted for only by an extravascular component of contrast material crossing a pathologically defective blood-brain barrier. Subsequent studies by the same author determined the tissue-blood ratios of a red blood cell tracer (51 CR-RBC) and a plasma tracer (125 IHSA) and confirmed their thesis that pathological CT contrast enhancement is produced by an extravascular component of jodinated contrast material as well as intravascular iodinated blood (3).
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