Abdominal CT scans are inferior to ultrasonography in diagnosing gallstones. The major application of CT scans is to define the course and status of the extrahepatic biliary tree and adjacent structures. It is the test of choice in evaluating the patient with suspected malignancy of the gallbladder, the extrahepatic biliary system, or nearby organs, in particular, the head of the pancreas. Use of CT scan is an integral part of the differential diagnosis of obstructive jaundice . Spiral CT scanning provides additional staging information, including vascular involvement in patients with periampullary tumors.
In many institutions, the initial noninvasive imaging modality of choice for patients with suspected biliary obstruction is CT, specifically helical CT scanning (Fig. 100-2). The latter provides rapid imaging. The disadvantages include
(1) the requirement for intravenous contrast administration (not always possible in patients with renal dysfunction)
(2) the use of ionizing radiation
(3) additional cost as compared to US
CT scanning is less “operator dependent” than US. Thus, it is reproducible and studies are easily compared. It is therefore important for follow-up after biliary surgical or interventional procedures. When correctly performed, CT provides valuable information not only of the intrahepatic and extrahepatic bile ducts but also of structures outside the biliary system (e.g., liver parenchyma, adjacent lymph nodes, the presence of choledocholithiasis, neoplasms).
To optimize biliary tract imaging with helical CT, a 3- to 5-mm collimated scan (pitch 1:1) should be performed from the porta hepatis through the pancreatic head during the portal venous phase of contrast enhancement. Overlapping axial reconstructions of the helical acquisition can aid interpretation. The extrahepatic bile duct is typically visualized throughout its entire course in the hepatoduodenal ligament as a water-density tubular structure. At the level of the pancreatic head, the distal common bile duct has a round or oval configuration. Although normal intrahepatic bile ducts can occasionally be seen with current CT technology, it usually is not difficult to differentiate normal intrahepatic bile ducts from true dilated ducts. Normal intrahepatic ducts should be less than 2 mm and not confluent. Multidetector CT technology shortens acquisition time. This has the advantage of allowing single breath-hold imaging, which is important when scanning pediatric or critically ill patients. Multidetector CT also allows the ability to obtain a three-dimensional data set, which enables image reconstruction for CT angiography and cholangiography
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