Positive and negative contrast agents in CT evaluation of the abdomen and pelvis.

The oral administration of a dilute positive iodinated contrast agent such as 2% Gastrografin is usually necessary to achieve optimal delineation of abdominal and pelvic organs. The amount of contrast agent administered and the timing of its administration depends on the site of suspected disease within the body. The simultaneous administration of antiperistaltic agents intravenously is generally unnecessary when utilizing the modern fast CT scanners. However, contrast agent-induced artifact may at times be troublesome, and we therefore suggest that lesions in the left lobe of the liver be scanned without positive contrast in the stomach. Negative contrast may also be useful in delineating disease: gas (carbon dioxide) has been shown to be useful in evaluating bladder tumors. We have also used air insufflation to facilitate the evaluation of small rectal tumors. Intravenous administration of contrast agent aids characterization of various disease entities and also facilitates the recognition of vascular structures (such as veins and arteries in the peripancreatic region). In our opinion both the infusion and bolus modes of contrast material administration have their place in the evaluation of abdominal and pelvic lesions by computed tomography. Newer techniques such as dynamic CT scanning will probably prove to be valuable both in delineating and characterizing disease.