Biliary System: Imaging Approach and Differential Diagnosis

Imaging Indications and Protocols

Oral and IV cholangiography have been supplanted by newer cross-sectional imaging and cholescintigraphy. MR hepatobiliary IV contrast agents may be used to supplement CT or MR cholangiography.

Cholescintigraphy is a nuclear medicine study used to evaluate the morphology and function of the biliary tree. In a " HIDA scan ," the patient receives an IV administration of Tc-99m iminodiacetic acid compound, an agent that has rapid uptake by the liver and excretion in the bile. A normal study has a hepatic parenchymal phase followed by identification of the radiotracer within the gallbladder (GB), indicating a patent cystic duct, and subsequent passage into the duodenum, indicating a patent common duct. While the anatomic detail of a HIDA scan is limited, the functional information is valuable in diagnosing cystic duct or common duct occlusion or a biliary leak.

Ultrasonography (US) is the primary imaging modality for most GB and biliary abnormalities. US detects gallstones within the GB with almost 100% accuracy. The diagnosis of acute cholecystitis is established with only slightly less accuracy based on the findings of gallstones, GB wall thickening, and focal tenderness over the GB (the sonographic Murphy sign). US may also allow diagnosis of complications of cholecystitis, such as gangrene or perforation, although CT is often better suited for the evaluation of disease beyond the GB wall. GB wall mass lesions, such as polyps and carcinoma, are also well depicted by US.

CT is less sensitive than US in diagnosis of gallstones, because the attenuation of gallstones may vary from less than water to densely calcified. Furthermore, CT detects sludge (the viscous, echogenic layer of material within the GB that is often present in fasting patients and those with GB dysfunction) much less frequently than US. CT is accurate in diagnosing complications of acute cholecystitis and in revealing the mass (biliary, hepatic, or pancreatic), which is the usual cause of painless jaundice. Newer thin-section and multiplanar CT detects > 70% of stones in choledocholithiasis and shows indirect signs, such as abrupt narrowing of the common bile duct (CBD), in a higher percentage of cases. CT or MR is the primary modality for diagnosis and staging of pancreaticobiliary neoplasms. Multiplanar displays, especially along the course of the biliary and pancreatic ducts and vessels, are especially effective in providing findings that have an impact on diagnosis and management of pancreaticobiliary diseases.

CT cholangiography is a noninvasive alternative to direct or MR cholangiography and is useful for the preoperative evaluation of the biliary anatomy in a potential living liver donor, as some common ductal anomalies may preclude or complicate this procedure. A standard without-and-with IV contrast CT scan is initially performed using the conventional nonionic contrast media. Next, a slow IV drip infusion of iodipamide is performed with delayed CT imaging, and multiplanar reformations of the opacified biliary tree are obtained. Spatial resolution usually exceeds that of MRCP but is less than that of direct cholangiography.

MR cholangiopancreatography (MRCP) has largely supplanted endoscopic retrograde cholangiopancreatography (ERCP) in the diagnosis of lesions affecting the biliary or pancreatic ducts. MRCP is a primary tool in the evaluation of biliary obstruction (from calculi or intrinsic and extrinsic masses), while ERCP is usually reserved for interventions, such as the placement of a biliary stent to bypass an obstructed bile duct.

MRCP utilizes a variety of heavily T2-weighted sequences to show the bile (and pancreatic duct) as bright signal fluid. While spatial detail is limited, it is usually sufficient to establish the diagnosis and guide management. MRCP can also be combined with other sequences in multiple planes to yield comprehensive evaluation of the liver, biliary tree, and pancreas.

The IV administration of gadoxetate (Eovist or Primovist) allows high-quality MR evaluation of the hepatic parenchyma during the arterial and venous phases of imaging, and delayed imaging can provide unique advantages over the usual gadolinium-based contrast media for some specific indications. Gadoxetate (Eovist) has the unique property of having 50% hepatobiliary excretion. Imaging after a 20-minute delay shows dense enhancement of normal hepatic parenchyma as well as enhancement of the biliary tree. This allows for better quality MR cholangiography for such indications as preoperative evaluation of potential hepatic donation, biliary leaks following trauma or surgery, and biliary obstruction. The presence of hepatic dysfunction (e.g., with elevated serum bilirubin) may impair the quality of the cholangiographic phase of a gadoxetate-enhanced MR scan.

Direct cholangiography retains an important role in the diagnosis and treatment of biliary disease. Percutaneous transhepatic cholangiography is the optimal modality for patients with known or suspected biliary obstruction when ERCP is unavailable (e.g., following prior surgical biliary diversion) or to diagnose, stage, and treat intrahepatic or proximal extrahepatic biliary obstruction (e.g., Klatskin tumor).

ERCP is performed for known or suspected biliary obstruction that may require endoscopic placement of a biliary stent, retrieval of stones, or the acquisition of a biopsy specimen or brush cytology/histology confirmation of malignancy.

ERCP is also the modality of choice for diagnosis and treatment of traumatic or postsurgical bile leaks, which will usually resolve following placement of a biliary stent.

Postoperative (T-tube) cholangiography is a valuable and easy means of evaluating a biliary tree that has been altered by surgery (e.g., liver transplantation, choledochoenterostomy) when the surgeon has left a tube in place within the CBD with an external limb that can be accessed for injection of contrast medium. A T-tube cholangiogram allows for convenient and safe diagnosis of retained stones, leaks, or strictures.

Imaging Evaluation of Jaundiced Patient

A patient who has jaundice or significant elevation of liver function tests, especially alkaline phosphatase or bilirubin, either has biliary obstruction or severe diffuse hepatic disease. The role of imaging in this setting is to determine the presence, level, and cause of biliary obstruction.

Criteria for diagnosing biliary dilation vary somewhat among investigators and according to the age of the patient. As a general rule, the presence of visible continuous arborization (branching) of the intrahepatic ducts indicates dilation. The bile ducts course along the portal triads and should not be > 40% of the diameter of the adjacent portal vein. The common hepatic duct should measure < 6 mm at the porta hepatis and the CBD < 8 mm, although it commonly measures up to 10 mm in elderly patients who have had a prior cholecystectomy. Dilation of the extrahepatic ducts should always be correlated with any clinical or biochemical evidence of obstruction before recommending extensive evaluation.

The character of the transition from a dilated to narrow duct is an important criterion. Abrupt narrowing is usually due to tumor, stone, or iatrogenic injury, while tapered narrowing is more commonly due to inflammation, such as from pancreatitis or cholangitis. Malignant tumors also cause eccentric narrowing of the duct and a mass in or around the duct and may be associated with other signs of "invasiveness," such as vessel encasement.

The level of the obstruction is determined by the point of transition from dilated to narrowed ducts.

Intrahepatic causes of obstruction include primary sclerosing cholangitis and liver tumors, usually malignant.

Porta hepatis obstruction is most commonly due to cholangiocarcinoma (Klatskin tumor). Primary sclerosing cholangitis, GB carcinoma, metastases, or iatrogenic injury (usually from a laparoscopic cholecystectomy) are other etiologies that may result in obstruction at the porta hepatis.

Intrapancreatic causes of obstruction include pancreatic carcinoma, chronic pancreatitis, CBD stones, cholangiocarcinoma, and ampullary lesions (dysfunction or tumor).

Differential Diagnosis