岡庭 信司

Shinji OKANIWA

飯田市立病院消化器内科

Department of Gastroenterology, Iida Municipal Hospital

キーワード : bile duct, differential diagnosis, ultrasound, endoscopic ultrasound, intraductal ultrasound

超音波検査（ultrasound : US）は特異度と感度がともに高く，胆道癌が疑われる患者では第一に選択されるべき検査法である．しかし，肝外胆管（extrahepatic bile duct：EHBD）は複雑な解剖とガス像の影響により全体像を描出しにくい．EHBDを明瞭に描出する鍵は，左側臥位での「J字」操作と高周波プローブによる拡大画像の利用である．さらに，胆嚢の腫大，肝外胆管の拡張，胆嚢・肝外胆管内のデブリエコーといった間接所見も，特にファーター乳頭部の潜在病変を検出するうえで重要となる．肝外胆管の壁肥厚の鑑別診断では，まず長軸および短軸方向の進展パターンを評価し，そのうえで最内側高エコー層（innermost hyperechoic layer：IHL）と最外側高エコー層（outermost hyperechoic layer：OHL）の特徴を評価する．非対称性の壁肥厚，IHLの欠如，OHLの不整・不連続は胆管癌（cholangiocarcinoma：CCA）に特徴的な所見である．CCAは胆管の隆起性病変の中で最も頻度が高く，腫瘍の進展範囲や深達度の評価が重要である．結節型CCAは通常低エコーで，垂直方向への進展を認めやすい．一方，乳頭型CCAは高エコーのことが多く，側方進展を認める．これらの鑑別診断に造影USが有用なことがある．ただしCCAの可能性を否定できない場合や確定診断が必要な場合には，経乳頭的生検や超音波内視鏡下組織採取を検討する．

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. Nodulartype 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.