IV contrast is frequently utilized for emergency CT studies, because it creates a more detailed image of many structures, particularly abdominal, pelvic, and retroperitoneal organs. The contrast material causes beam attenuation that is directly related to the iodine concentration achieved by the vascular supply to that tissue. Abnormal tissue, whether due to a malignant, inflammatory, or infectious etiology, has different contrast enhancement patterns than normal tissue and may appear avascular, hypovascular, isodense, or hypervascular. Images can be obtained at various phases following contrast administration. Depending on the diagnosis in question, the study should be tailored to acquire data at the appropriate time ( Fig.„„296-6).
FIG. 296-6. Scans of a 16-year-old boy after a skiing accident. A. Scan before intravenous (IV) contrast bolus enhancement is displayed at standard abdominal soft tissue settings; the intrahepatic clot and laceration are difficult to see. B. After bolus IV contrast administration during dynamic scanning, a scan at the same level as in (A) shows the hepatic laceration well. The laceration extends immediately behind but does not apparently involve the right hepatic vein. Note that the clot appeared hyperdense relative to the liver before IV contrast administration and appears hypodense relative to the enhanced liver after bolus contrast. The clot did not change density. The liver increased in density because of the circulating metal in the IV contrast. C. The upper abdominal component of this patient's relatively small hemoparitoneum is seen as blood in the Morison pouch (the right posterior subhepatic space between the liver and kidney).
In certain circumstances, it may be necessary to perform both a noncontrast CT scan and a contrast CT scan. CT scans to evaluate patients with potential urinary tract calculi must initially be performed without IV contrast, because the presence of contrast can easily obscure any renal stones present. In addition, an obstructing ureteral calculus may often be detected on an unenhanced CT study. However, IV contrast can be useful to determine the exact level of ureteral obstruction or to determine whether a pelvic calcification lies within the ureter. It may also provide information on the function of the kidneys. In general, the best way to optimize any study is to discuss the purpose of the CT adequately with the radiologist and/or technologist performing the study.
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