Advanced ultrasound diagnosis of extrahepatic bile duct lesions.

Ultrasound (US) has high specificity and sensitivity, and it should be performed first for patients with suspicion of biliary tract cancer. However, the complicated anatomy in addition to the gas images makes it difficult to delineate the entire extrahepatic bile duct (EHBD). The keys to depiction of EHBD are the "J" shape manipulation in the left lateral decubitus position and the use of magnified images with high-frequency transducers. Furthermore, indirect findings such as gallbladder (GB) distension, BD dilatation, and debris echo in the GB and BD are also important for detecting occult lesions, particularly in the ampullary region of Vater. For the differential diagnosis of BD wall thickening, the spreading pattern in the long and short axial directions should be assessed first. Then, the characteristics of the innermost hyperechoic layer (IHL) and outermost hyperechoic layer (OHL) should be evaluated. Asymmetrical wall thickening, absence of IHL, and presence of irregularity or discontinuity in OHL are characteristic patterns of cholangiocarcinoma (CCA). Because CCA is the most common BD polypoid lesion, it is important to diagnose tumor extension and depth invasion in addition to differential diagnosis. Nodular-type CCA is usually hypoechoic and more likely to invade vertically. In contrast, papillary-type CCA is often hyperechoic and extends laterally. Contrast‑enhanced US may be useful for evaluating these findings. However, if the possibility of CCA cannot be ruled out or a definitive diagnosis is needed, a transpapillary biopsy or endoscopic US-guided tissue acquisition should be considered.