Advances in biliary imaging have improved making accurate diagnoses of the presence and causes of biliary obstruction. Abdominal ultrasound is a useful screening tool because it is highly specific for choledocholithiasis. New developments in CT and MRI have also been useful in the diagnosis of biliary disease. Although diagnosis of biliary disease can be achieved in a noninvasive manner, there are limitations to modern MRI and CT cholangiographic techniques; their use may not be necessary or cost effective. MRI and CT imaging of the biliary tract provides opportunities for less-invasive diagnostic techniques but should be used judiciously before interventional endoscopy.
Key points
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Initial evaluation of suspected biliary obstruction should include liver biochemical tests and a transabdominal ultrasound (US).
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Patients with suspicion of choledocholithiasis should be risk stratified; those who are high risk should undergo endoscopic retrograde cholangiopancreatography (ERCP) without further radiologic imaging. Patients with intermediate risk of choledocholithiasis should undergo endoscopic ultrasonography (EUS), magnetic resonance cholangiopancreatography (MRCP), or intraoperative cholangiography, depending on the availability.
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Patients with suspected malignancy should undergo a CT scan or an MRI/MRCP to evaluate extent of disease, followed by further endoscopic diagnosis and/or therapy, if technically feasible.
Introduction
Biliary diseases and conditions associated with biliary obstruction affect a significant portion of the worldwide population, and accurate diagnosis of the presence and cause of biliary obstruction is central to providing cost-effective work-up and treatment. Since its advent in 1968, ERCP has become the gold standard in the setting of biliary obstruction. It is one of many invasive direct cholangiography techniques, with a reported complication rate of 1% to 9% and a mortality rate from 0.2% to 0.5%.
Advances in biliary radiology with US, CT, and MRI technology in recent years, however, have allowed accurate, noninvasive imaging of the biliary tree and pancreas. In the setting of obstructive jaundice, imaging helps assess the severity and cause of the obstruction. In some cases, a multimodality imaging approach may be necessary. This article focuses on the general approach to patients with suspected biliary obstruction and discusses the applications of various imaging modalities that may be useful before performing ERCP.
Introduction
Biliary diseases and conditions associated with biliary obstruction affect a significant portion of the worldwide population, and accurate diagnosis of the presence and cause of biliary obstruction is central to providing cost-effective work-up and treatment. Since its advent in 1968, ERCP has become the gold standard in the setting of biliary obstruction. It is one of many invasive direct cholangiography techniques, with a reported complication rate of 1% to 9% and a mortality rate from 0.2% to 0.5%.
Advances in biliary radiology with US, CT, and MRI technology in recent years, however, have allowed accurate, noninvasive imaging of the biliary tree and pancreas. In the setting of obstructive jaundice, imaging helps assess the severity and cause of the obstruction. In some cases, a multimodality imaging approach may be necessary. This article focuses on the general approach to patients with suspected biliary obstruction and discusses the applications of various imaging modalities that may be useful before performing ERCP.
Abdominal ultrasound
Abdominal US is widely recommended as the initial imaging test in evaluating patients presenting with jaundice because it is noninvasive, safe, inexpensive, and widely available. In addition, US is portable and allows for bedside imaging.
US for Gallstones
Gallstones appear as echogenic foci that cast acoustic shadows and seek gravitational dependence ( Fig. 1 ). The accuracy of US in the diagnosis of cholelithiasis has been well established, with sensitivities reported at approximately 96%, although the size and number of gallstones cannot be accurately determined on US. The sensitivity of US in diagnosing gallbladder stones is comparable to that of MRCP (97.7%), although it is less sensitive (27% compared with 50% for MRCP) for detecting microlithiasis or biliary sludge. US has also been shown accurate in diagnosing complications of gallstones, in particular cholecystitis, and provided a definitive diagnosis in nearly 80% of patients with suspected cholecystitis in one study.
Even though US has a high specificity (100% in multiple studies ) for choledocholithiasis, it is insensitive: sensitivity of US ranges from 22% to 33% for detecting common bile duct stones. US, however, is able to detect dilatation of the common bile duct (sensitivity, 77% ), a finding often seen with choledocholithiasis. Biliary dilatation greater than 6 mm often indicates biliary obstruction in patients with an intact gallbladder. With a normal-caliber duct (<6 mm), gallstones were found in only 6% of cases at time of surgery, although 37.5% of patients with duct diameters greater than 5 mm had stones at time of surgery. In patients who are postcholecystectomy, the predictive value of duct dilatation for duct stones was 71% and the predictive value of nondilated ducts in excluding stones was 83%. Therefore, US should not be considered an adequate screening test for choledocholithiasis.
US is limited in its ability to detect common bile duct stones for many reasons. Intestinal gas often obscures the distal common duct, preventing detection of ampullary stones. Also, calculi require surrounding bile for sonographic contrast, so it can be difficult to visualize stones in normal-caliber bile ducts. Operator experience also plays a major role in sensitivity: when the examination is performed by an expert operator, sensitivity is nearly double (77%–90%) that of an operator with little experience (37%–47%).
Although US has limitations, it can be useful in establishing the best ERCP strategy for endoscopists to use on patients undergoing laparoscopic cholecystectomy. Unnecessary diagnostic ERCPs can be avoided with rigid criteria. The recent algorithm proposed by the American Society for Gastrointestinal Endoscopy states that a normal abdominal US, with normal levels of serum alkaline phosphatase and bilirubin on initial patient presentation, has a negative predictive value of 95% to 96% for common bile duct stones.
US for Malignancy
Evaluating patients who have suspected malignant biliary obstruction is more complex, because the cause, level of obstruction, and extent of the malignancy all need to be addressed. The presence of metastases is particularly important, because it dictates clinical management. In this scenario, the drawbacks of US remain, including the dependency on the operator’s interpretive skill and suboptimal imaging because of obesity or intestinal gas.
Abdominal US has a sensitivity of 71% at defining the etiology of biliary obstruction and is particularly sensitive for detecting the level of biliary obstruction in jaundice (sensitivity 88%–90%). The sensitivity of US has increased to 90% for the diagnosis of primary gallbladder cancer, with the infiltrating type still the most difficult to find.
US, however, is less sensitive for diagnosing other tumors, including pancreatic carcinoma, ampullary carcinoma, and cholangiocarcinoma, with a rate less than 50% in defining cause. The most common finding is dilatation of the common bile duct, often as an indirect sign of tumor, and small carcinomas of the distal duct are often not well visualized. Malignancy is reported as the most common cause of obstruction at the suprapancreatic level or proximal to the level of porta hepatis. Although the sensitivity of US for detecting bile duct cancer has been reported as high (over 90% in one study ), further imaging is needed if a malignancy is suspected, particularly for staging purposes.
Using US to assess the spread of bile duct cancers has only been reported with Klatskin tumors. Although US with Doppler has been fairly reliable in detecting the invasion of Klatskin tumors into the liver, portal vein, and bile ducts, it has not been helpful in detecting lymph node and peritoneal metastases, and further endoscopic or radiologic imaging is required. The sensitivity of US for diagnosing pancreatic tumors is also variable, reported between 48% and 89%, with low specificity and accuracy, particularly if the tumor is less than 2 cm in diameter.
CT
CT, although usually not the first imaging modality used to evaluate patients with biliary obstruction, can be helpful in diagnosing biliary disease. CT allows for detailed evaluation of the biliary tract, particularly with the advent of modern CT cholangiographic techniques that include the use of biliary excreted contrast material.
CT for Gallstones
Again, CT is usually not the first modality of choice for detecting cholelithiasis, although it can be depicted on CT scan. The appearance of gallstones on CT imaging varies with the chemical composition of the stone, because those stones that are heavily calcified show up radiopaque, but soft tissue attenuation stones can be difficult to visualize. Approximately 10% to 20% of gallstones are composed of pure cholesterol, which are low in density, and, because they are isoattenuating with bile, are hard to see on CT images. This can account for many false-negative CT scans.
Compared with US, CT imaging is better able to accurately demonstrate the location (97%) and cause (94%) of biliary obstruction; however, US is still more sensitive, specific, and accurate for diagnosis of cholelithiasis and has been shown to have a much higher positive predictive value (75% vs 50% for CT) and negative predictive value (97% vs 89% for CT) for diagnosing acute biliary disease. US, therefore, may be more useful than CT for initial screening of acute biliary disease.
Optimization of CT technique, including using thin sections and unenhanced helical CT, can improve the detection of filling defects and has been shown to have greater sensitivity, specificity, and accuracy for the diagnosis of choledocholithiasis. Overall, the sensitivity of CT for choledocholithiasis is reported as between 65% and 88%, with conventional CT imaging having a specificity reported as high as 73% to 97%.
CT cholangiography combines the use of helical CT imaging with the use of intravenous biliary contrast material to generate cholangiographic images of opacified bile ducts. These intravenous contrast materials, such as iopanoic acid, are excreted into bile, but often require the preimaging administration of antihistaminic drugs. In the United States, intravenous contrast-enhanced CT cholangiography has been used mostly to define second-order or third-order bile duct anatomy before living liver transplant donation.
CT cholangiography is helpful for patients with suspected choledocholithiasis (sensitivity 86%–93%, specificity 100%). It has also been shown to have sensitivity in diagnosing bile duct stones comparable with magnetic resonance cholangiography (>90%) and higher than unenhanced helical CT.
CT cholangiography has its limitations, however. In one study comparing CT cholangiography with fiberoptic cholangioscopy, common bile duct stones less than 5 mm in diameter were missed by CT cholangiography in several cases and subsequently detected by invasive cholangioscopy. Furthermore, the excretion of these noninvasive contrast materials, such as iopodic acid, can have variable effects. In particular, patients with liver insufficiency and high serum bilirubin levels (levels >3 mg/dL) often have CT cholangiographic images with insufficient opacification of bile ducts. Concerns regarding the higher doses of radiation, compared with conventional helical CT, and the potential toxicity of the contrast agents have limited the use of CT cholangiography.
CT for Malignancy
Although CT is not the best imaging technique for choledocholithiasis, it is frequently performed during the work-up of jaundice. The sensitivity of CT in detecting gallbladder carcinoma has been reported as greater than 90%. The finding of a mass with variable enhancement is associated with an ill-defined contour of the gallbladder wall or possibly even a fungate mass within the gallbladder itself. Helical CT has been accurate in diagnosing and staging of T2 tumors, but the accuracy of CT for local staging is higher for tumors presenting as a mass lesion as opposed to tumors that present, on imaging, as thickened wall. Overall, dual-phase helical CT has been useful for determining the resectability of gallbladder carcinoma.
Cholangiocarcinoma is often depicted on CT scan as an abrupt termination of bile duct dilatation. The multidetector technique that enables reformations with quality images has been helpful in showing the site and cause of obstruction (97% accuracy for determining the level; 94% accuracy for cause of obstruction ), but sensitivity can still be low (40%). CT is more sensitive in detecting hilar carcinoma, but it may not detect small masses. Tumors can have nonspecific appearances, with variable enhancement patterns, although they usually present as patchy peripheral enhancement that is most prominent during the portal venous phase. Usually, cholangiocarinomas are of lesser attenuation than liver parenchyma on unenhanced CT, but appearances can still be nonspecific. Thin sections on imaging with newer helical multidetector CT are more sensitive for detecting subtle, small cholangiocarcinomas as the cause of biliary obstruction.
Magnetic resonance cholangiopancreatography
MRI offers inherent advantages over CT imaging: there is no risk of radiation or additional contrast, and the appearance of gallstone does not seem affected by their internal composition. MRCP, which uses heavy T2 weighting and rapid image acquisition, provides accurate imaging of the biliary tree and pancreatic duct and has been available as a noninvasive alternative to diagnostic ERCP for 20 years. The T2 weighting allows for bile and pancreatic juice to appear bright, with a high-signal intensity that contrasts with nearby hepatic and pancreatic tissue that appears dark with a low-signal intensity. Because MRCP is noninvasive and does not carry any of the morbidity and mortality associated with ERCP, it has been widely adopted for use in the work-up of biliary obstruction. It also does not require sedation, as endoscopic US and ERCP do, so MRCP avoids risks associated with sedation. Some patients undergoing MRCP, however, may ultimately require diagnostic (tissue/cytology sampling) or therapeutic (sphincterotomy, removal of gallstones, or stent placement) ERCP. Decision-analysis models on the value of MRCP have not demonstrated a reduction in the number of ERCPs in patients with choledocholithiasis or other biliary diseases.
MRCP for Gallstones
Gallstones are well depicted on MRCP, regardless of their location and appear, on T2 images, as foci of low-signal intensity surrounded by bright bile ( Fig. 2 ). Several studies have demonstrated that the sensitivity and specificity for MRCP in diagnosing choledocholithiasis is high (85%–92% and 93%–97%). In one study, however, sensitivity decreased to 64% with stones less than 5 mm in diameter, even though MRCP can demonstrate calculi as small as 2 mm. Multiple studies show that, even with thin-section 3-D imaging techniques, the sensitivity for stones that are 3 mm or smaller may be less than 50%. Additional limitations have also been reported, such as mistaking multiple impacted stones (with minimal surrounding bile) for a stricture or cholangitis or misidentifying pneumobilia for stones because pneumobilia also manifests as a signal void.
