Introduction Imaging Technique

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The imaging techniques employed to evaluate the biliary system are usually selected on the basis of the diagnostic information required, the clinical presentation, and the body habitus. Although ultrasound (US) and computed tomography (CT) are the primary non-invasive imaging modalities for screening patients with biliary tree pathology, MR cholangiopancreatography (MRC) is rapidly assuming a more important role in non-invasive evaluation of the intra- and extrahepatic bile ducts. Percutaneous transhepatic cholangiography (PTC) and endoscopic retrograde cholangiopancreatography (ERCP) are able to provide detailed information on ductal anatomy and pathology through direct opacification of the bile ducts [72]. These techniques are also very useful as non-surgical therapeutic methods for biliary drainage, stent placement, stone removal and stricture dilatation.

US is generally used to screen patients with suspected biliary ductal disease. It is usually performed using the highest-frequency transducer at 3.5 MHz in obese patients and 5 MHz in thin patients. However, whereas the choledochal and common hepatic ducts can usually be seen, the intra-hepatic ducts are rarely seen unless they are dilated. Usually the common hepatic duct and the common biliary duct are evaluated using parasagittal scans and appear as tubular hypoechoic structures. The common hepatic duct is identified anteriorly and laterally to the proximal main portal vein or the undivided right portal vein. The right hepatic artery passes between the posteriorly located portal vein and the anteriorly located common hepatic duct which can usually be measured at this level. The common bile duct is difficult to evaluate in the distal portion because of overlying gas in the duodenum and hepatic flexure. However, visualization can be improved by scanning the patient in a semi-erect position [67]. Color Doppler US is very helpful in distinguishing bile ducts from small hepatic vessels, especially in the left lobe where parallel branching hepatic anterior and portal veins may mimic dilated left intrahep-atic ducts.

Frequently, US is able to visualize not only the dilated ducts in patients with biliary obstruction, but also the lesion associated with the duct dilatation. Biliary dilatation may also be observed in the absence of obstruction in patients who have undergone prior biliary surgery and cholecystecto-my, or in subjects with resolved obstruction [54].

Whereas most abdominal US examinations are performed after prolonged fasting, the administration of a fatty meal prior to examination of the bil iary tree is an adjunctive maneuver that provides functional information in addition to increasing the accuracy of obstruction detection. Whereas a normal, non-obstructed duct decreases in size or does not change in caliber, an increase in caliber of 2 mm or more suggests some degree of ductal obstruction and the need for further evaluation [58].

On CT, bile ducts appear as water-dense tubular branching structures converging at the porta he-patis. The common hepatic duct and the common biliary ducts have a similar shape, and are generally visible within the hepatoduodenal ligament. The distal common biliary duct appears on cross-section as a circular, low-density structure in the pancreatic head. The normal hepatic duct on CT scans is 3-6 mm in diameter, and the common bile duct is 6-7 mm in diameter. Most CT examinations are performed in three phases (pre-contrast, arterial and portal-venous), frequently after the ingestion of 500-800 ml of water to distend the gastrointestinal tract.

Multi-detector computed tomography (MDCT) has greatly enhanced the capabilities of CT to assess the upper abdomen, and is increasingly proving useful for the evaluation of biliary duct disease. In addition, because the total length of the common bile duct can be delineated more completely, MDCT may be a more useful means of precisely defining the site and cause of biliary obstruction. This imaging modality permits precise evaluation of biliary tract abnormalities such as choledocholithiasis, hepatolithiasis, cholangiocar-cinoma, extrinsic lesions obstructing the biliary tract, and congenital biliary tract anomalies [26].

MR imaging (MRI) has become more useful for biliary system imaging since the introduction of fast imaging strategies, gradient echo sequences, fast spin-echo sequences, and half-Fourier acquisition single-shot sequences (HASTE). MRC in association with morphologic imaging using T1- and T2-weighted sequences, has emerged as an accurate, non-invasive alternative to diagnostic endo-scopic retrograde cholangiography (ERC) for the evaluation of diseases of the biliary tract [75].

MRC is performed using heavily T2-weighted sequences that depict the hyperintense fluid contained within the bile ducts with high signal intensity, whereas suppression of the signal of surrounding, non-fluid containing structures is achieved due to the long echo-time. MR images are acquired in the coronal and axial planes, typically with the use of a phase-array imaging coil to increase overall accuracy [48].

MRC is usually performed using a multisection technique involving the acquisition of multiple 1-3 mm thick source images of the pancreaticobiliary tract. Because of the orientation of the normal bile duct, the biliary tract is usually partially visualized on each of several images rather than visualized in its entirety on a single image. Although diagnostic decisions are usually made on the basis of the source images, three-dimensional (3D) images can be obtained with maximum intensity projection (MIP) and multiplanar reconstruction (MPR) techniques. Moreover, 3D images are able to provide a road map of the obstructed ductal system, delineate complex strictures, and assist in the planning of percutanous, surgical, and endoscopic procedures [19].Another approach, which obviates the need for MIP post-processing, is to employ a single-shot projection technique to obtain a 30-70 mm thick image, during a three second acquisition.

The availability of MR contrast agents with hepatobiliary properties such as Gd-BOPTA, Mn-DPDP and Gd-EOB-DTPA, which are in part eliminated through the hepatobiliary system, permit the biliary tree to be visualized on Tl-weighted images. This approach may prove useful for evaluating leakage from the biliary tree after hepatic resection or liver transplantation [21, 23].

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