How Good Is IDUS in Patients with Isolated Biliary Strictures?

Determining the nature of the isolated bile duct stricture continues to be a challenging problem. Making the proper diagnosis is important for both patients and physicians. Of course, in patients with no prior history of biliary surgery, the majority of the isolated extrahepatic strictures are considered malignant until proven otherwise. Currently, most patients with suspected biliary obstruction will undergo ERCP. ERCP is invaluable in determining the level of obstruction in the biliary tree. Some features, including the length of the stricture or shelf appearance, may suggest malignancy. In addition, the presence of a corresponding stricture in the pancreatic duct: i.e., a “double duct sign” is also suggestive of cancer. However, morphologic changes on cholangiogram are frequently of limited value in determining the nature of the stricture: i.e., benign versus malignant. It has therefore become standard to use additional sampling methods during ERCP. Sampling methods include brush cytology, fine needle aspiration, and transpapillary biopsy. All of these methods are associated with a 90–100% specificity. However, sensitivity and negative predictive value are low. Current data suggest that the use of combined sampling methods is associated with a significant increase in sensitivity (62%) (1). Negative sampling results continue to present management difficulties in many patients. Intraductal ultrasound (IDUS) is a relatively new “accessory” for use during ERCP. The device consists of a probe inserted via the working channel of the ERCP scope and advanced, frequently over a wire, to the area of stricture under fluoroscopic control. Most IDUS probes use a 20 MHz frequency allowing adequate evaluation of the bile duct wall and structures in the immediate vicinity. Limited data by expert ultrasonographers suggest that certain features such as a hypoechoic infiltrating lesion on IDUS can differentiate benign from malignant strictures (2). IDUS appears to be superior to ERCP in defining the nature of the biliary stricture. The exact role of IDUS in the evaluation and local staging of patients with biliary strictures has not been defined. The limited life span of IDUS probes and the need for an ultrasound generator can add to the cost of the procedure. Magnetic resonance (MR) cholangiopancreaticography (MRCP) has emerged as an important diagnostic procedure in the evaluation of the pancreaticobiliary system. Through advances in hardware and software, MRCP techniques have evolved to become fast, robust, and patient friendly, allowing imaging of the entire pancreaticobiliary system within minutes. It is noninvasive, with no oral or intravenous contrast required. It has multiplanar capabilities. Rapid imaging allows even noncooperative patients to be imaged with images of diagnostic quality. The sensitivity of MRCP in the detection of the presence and level of obstruction is high, approaching 100% (3, 4). However, the accuracy rates in differentiation of benign versus malignant disease is more variable, with specificities ranging from 81% to 86%, and sensitivities ranging from 92% to 100% (5). A recent metaanalysis (6) has suggested that MRCP is less reliable in differentiating malignant from benign causes of biliary obstruction. MRCP, like ERCP evaluates the pancreatic and bile duct lumen alone. Both are unable to assess the surrounding soft tissues. Therefore, all patients with malignant strictures require cross-sectional imaging, which includes computed tomography, conventional MR imaging, or transabdominal ultrasound. All of these modalities are able to stage tumors, including the location of the primary lesion, the presence of metastases, and local parenchymal or vascular extension. MR imaging using conventional imaging can increase the sensitivity for malignant tumors, and can easily be performed at the same sitting as the MRCP examination. Addition of gadolinium increases the confidence of diagnosis. Most patients with pancreaticobiliary carcinoma present in an advanced stage, and delineation of nonresectability can be made without more advanced imaging. In those patients who are fortunate enough to present without an obvious mass, but do have biliary obstruction, the role of MRCP is not to replace ERCP, as the limits of spatial resolution with MRCP demand that assessment with ERCP be performed. Instead, MRCP acts as to guide further management. A patient with biliary obstruction of unknown etiology must have evaluation with ERCP with tissue sampling, and a diagnostic MRCP, regardless of high quality, does not obviate this need. It is expected that ERCP, particularly with the addition of IDUS will be better able to characterize strictures in this setting. This is not the case when “all comers” with biliary obstruction are represented. In this setting, MRCP with conventional MR including imaging with gadolinium can provide a comprehensive assessment of etiology of biliary obstruction, stage malignancy, and with the MRCP map the biliary tree for further intervention. This can be performed within a 45 min time frame, making this a simple noninvasive diagnostic test. Techniques for MRCP are evolving and vary between institutions. Though relatively operator independent, the tech-