Imaging and Radiologic Intervention in the Biliary Tract

Ultrasound

US often serves as an initial imaging modality to evaluate the biliary system. In the hands of a trained operator, US can provide physicians with valuable information to help to manage patients with suspected hepatobiliary disease. It may be used in the evaluation of bile ducts for obstruction ( Fig. 107.1A ), masses, or stones. US is noninvasive, inexpensive, and does not involve ionizing radiation, which is particularly important in the setting of pediatric and pregnant patients.

The normal gallbladder is an ovoid, anechoic, fluid-filled structure adjacent to the interlobar fissure, which separates the right and left hepatic lobes. The gallbladder wall should be smooth, and the thickness should not exceed 3 mm. When the gallbladder is contracted, as is often seen in nonfasting patients, the wall may appear falsely thickened. Primary causes of wall thickening include cholecystitis ( Fig. 107.2 ), adenomyomatosis, and cancer. Secondary causes of wall thickening include acquired immune deficiency syndrome (AIDS) cholangiopathy, sclerosing cholangitis, hepatitis, pancreatitis, heart failure, hypoalbuminemia, cirrhosis, portal hypertension, and lymphatic obstruction.

The extrahepatic bile ducts are divided into the common bile duct and common hepatic duct. On US the common hepatic duct is the most easily visualized portion of the extrahepatic biliary system. The cystic duct is normally located posterior to and may join with the common hepatic duct at variable distances, forming the common bile duct. In 10% of the population the cystic duct runs parallel to the common hepatic duct for a long segment, where it is joined by a fibrous sheath, which may cause the common hepatic duct to be misinterpreted as the cystic duct. In the majority of patients the common hepatic duct is anterior and lateral to the portal vein and to the right of the hepatic artery. The hepatic artery separates the common hepatic duct from the portal vein within the hepatoduodenal ligament. However, in 10% to 15% of the patients, the hepatic artery is located anterior to the common hepatic duct. The common bile duct travels toward the second portion of the duodenum as it adopts a more posterior position. The normal diameter of the extrahepatic bile ducts can range from 4 to 8 mm. The size of the common bile duct may increase with age, after cholecystectomy, and after endoscopic manipulation of the duct. The maximum upper limit of normal in the extrahepatic biliary tree after cholecystectomy is 10 mm. However, it is generally accepted that a duct that measures 6 mm or greater in symptomatic patients warrants further investigation.

The intrahepatic bile ducts normally measure less than 2 mm in diameter or less than 40% of the diameter of the accompanying portal vein. Intrahepatic biliary dilation is detected by the parallel channel sign, which is formed by dilated ducts running parallel to the portal vein tributaries. Large intrahepatic biliary ducts located centrally near the porta hepatis should not be confused with pathologic dilatation. Color Doppler US can be used to differentiate dilated intrahepatic ducts from hepatic vessels (see Fig. 107.1B ).

Causes of biliary obstruction include benign etiologies (e.g., stones, infectious, congenital), neoplastic etiologies, and extrinsic compression (e.g., Mirizzi syndrome, pancreatitis, adenopathy). US is able to accurately predict the level of obstruction in 92% of cases and identifies the correct cause in 71% of cases. Further evaluation by CT or MRCP is often needed to help to identify the cause of obstruction or to identify the extent of disease in the case of malignancy prior to therapeutic intervention.

Computed Tomography

Multidetector CT has allowed for improved imaging of the biliary system. Rapid acquisition of volumetric data and the ability to perform multiplanar reconstructions (MPRs) provides more detailed evaluation of both intrahepatic and extrahepatic ducts. Unlike US, CT allows for visualization of the entire common bile duct and better detects etiologies of biliary obstruction. CT is reported to have a sensitivity of 72% to 88% in detecting choledocholithiasis. Disadvantages of CT include exposure to ionizing radiation and use of intravenous (IV) contrast, which may be contraindicated in patients with renal impairment or contrast allergy.

The CT technique for evaluation of the biliary system includes obtaining an unenhanced scan through the liver, gallbladder, common bile duct, and pancreas. Unenhanced scans provide a baseline to determine lesion enhancement and to better detect stones, which can be obscured by contrast material. A portal venous scan is obtained 70 to 80 seconds after IV contrast administration. When there is concern for malignancy, a late arterial phase may be obtained at 45 to 50 seconds. In addition to assessing vascularity within a tumor, a late arterial phase allows for detection of vascular involvement surrounding the tumor, which may alter or preclude surgical management (e.g., pancreatic head lesion with nearby vascular encasement). A 10-minute delayed scan should be added when there is suspicion for cholangiocarcinoma because these tumors often demonstrate delayed enhancement relative to the remainder of the hepatic parenchyma. A thin section technique (1 mm or less) allows for higher quality multiplanar reformats (e.g., coronal, sagittal), which are helpful in the assessment of bile ducts.

Normal intrahepatic bile ducts are faintly visualized on an enhanced CT and are often readily differentiated from dilated ducts, which measure greater than 2 mm ( Fig. 107.3 ). The common hepatic duct and common bile ducts appear as tubular water density structures with near imperceptible walls. The distal common bile duct takes on a round or oval configuration at the level of the pancreatic head.

Procedural guidance with CT-fluoroscopy serves as an effective tool for percutaneous interventional procedures of the chest, spine, abdomen, and pelvis. CT-fluoroscopy provides near real-time image acquisition similar to that of US, allowing for increased procedural efficiency. In the setting of biliary system pathology, CT-fluoroscopy is particularly helpful when undergoing a challenging biopsy (e.g., hepatic lesion adjacent to bile ducts) or a difficult catheter drainage (e.g., biloma near the aorta).

Magnetic Resonance Imaging

US and CT have some advantages over MRI, including cost, availability, speed, and real-time imaging. However, MRI has increasingly played a vital role in biliary imaging. It is considered a highly sensitive and specific noninvasive imaging modality in the evaluation of biliary tract pathology. Indeed, it has become favored over ERCP and percutaneous cholangiogram in most institutions for diagnostic purposes. It is also useful in patients who cannot undergo or have failed ERCP. MRCP may visualize stones as small as 2 mm with sensitivity greatly increasing as stone size increases. Furthermore, it is equally useful in visualizing strictures, biliary leaks ( Fig. 107.4 ), and other pathologies.

MRCP uses T2-weighted imaging to visualize the biliary system. These images are then reformatted into multiple planes using MPR and maximal intensity projections (MIPs), allowing for visualization of much of the biliary tract at once. Three-dimensional imaging or two-dimensional imaging in any other plane may also be obtained. Advances in technique, along with growing experience of MRI technicians, allow high-resolution imaging and decreased imaging time. This allows high-quality diagnostic imaging in severely ill patients who may not be able to be positioned properly for extended periods of time or comply with breathing commands. These advances, along with inherent benefits of MRI, make MRCP a valuable tool both in diagnostic imaging and presurgical planning.

Nuclear Medicine

Hepatobiliary scintigraphy is a noninvasive technique that uses derivatives of iminodiacetic acid tagged with radiotracer (technetium) for the evaluation of the biliary system. It is commonly referred to as hepatobiliary iminodiacetic acid (HIDA) scan, although paraisopropyl iminodiacetic acid (PIPIDA) is now the more prevalent tracer. Chole­scintigraphy is particularly useful in the evaluation for biliary leak, where it is highly sensitive ( Fig. 107.5 ). However, a negative scan in a patient with high clinical suspicion should be followed with ERCP. Cholescintigraphy is also an extremely sensitive examination for other biliary pathology, including obstruction and cholecystitis.