In recent years, endoscopic ultrasonography (EUS) has emerged as an important tool for the diagnosis and management of pancreaticobiliary disease. The close proximity of the echoendoscope to the biliary system allows detailed imaging of the gallbladder and adjacent structures. EUS is useful for the detection of occult cholelithiasis and biliary sludge and in the evaluation of suspected choledocholithiasis. It can be used to classify and predict neoplasia in polypoid lesions of the gallbladder and also to diagnose and stage gallbladder carcinoma. This article reviews the use of EUS in these diseases of the gallbladder.
Endoscopic ultrasonography (EUS) was introduced in 1980 and has developed considerably in the past 30 years. Primarily useful for the detection and staging of gastrointestinal cancers, EUS is now established as an important diagnostic modality that is necessary for the optimal management of gastrointestinal disease. EUS and magnetic resonance cholangiopancreatography (MRCP) have been largely responsible for the diminishing role of diagnostic endoscopic retrograde cholangiopancreatography (ERCP). EUS has become an essential tool for the complete pancreaticobiliary endoscopist.
EUS is an accurate modality for imaging gallbladder structures because of the close proximity of the duodenum to the gallbladder and extrahepatic biliary tree ( Fig. 1 ). EUS is considered superior to transabdominal ultrasonography (US) for imaging the biliary system, using higher ultrasound frequencies (5–12 MHz vs 2–5 MHz). EUS can differentiate the double-layered structure of the gallbladder wall and provide higher resolution for imaging small polypoid lesions. Both types of echoendoscopes, radial and linear (transverse and longitudinal imaging, respectively), can be used to image the biliary tree. In addition to imaging, EUS-guided needle puncture using the linear instrument enables transluminal aspiration of tissue for diagnosis and provides direct access to the biliary tree for therapeutic interventions. The development of intraductal ultrasonography (IDUS) miniprobes has further advanced the study of pancreaticobiliary tree disorders.
Clinical situations in which EUS can be used for evaluation of gallbladder disease include investigation of suspected cholelithiasis or biliary sludge, evaluation of suspected choledocholithiasis, imaging of polypoid lesions of the gallbladder, and diagnosis and staging of gallbladder cancer. This article reviews the use of EUS in these settings.
EUS instruments
The currently available instruments for biliary imaging with EUS include radial and linear echoendoscopes and catheter-based IDUS probes. There are 2 types of echoendoscopes, denoted radial or linear based on the piezoelectric crystals that generate the EUS image. In EUS, ultrasound pulses are generated by a transducer containing a piezoelectric crystal that converts an electronic pulse into an acoustic wave that propagates into the tissue. The same transducer then detects returning acoustic waves that contain information about the tissue through which the waves have propagated. In a radial echoendoscope, the crystals are arranged in a band around the shaft of the endoscope, perpendicular to the long axis of the instrument, generating a cross-sectional image ( Fig. 2 ). Radial scanning instruments provide detailed circumferential images, making them useful for orientation, and electronic radial instruments with Doppler capabilities help distinguish small vessels from ducts and improve vascular staging. In a linear echoendoscope, the crystals are arranged along one side of the endoscope’s tip, generating a longitudinal image parallel to the long axis of the instrument. Only the linear echoendoscope can be used to guide fine-needle puncture. The ability of the linear echoendoscope to provide scanning in the same plane as the instrument’s shaft allows the endoscopist to trace the path of a needle as it is inserted out of the working channel of the echoendoscope ( Fig. 3 ). Hence, a linear echoendoscope can be used for diagnostic evaluation and to facilitate interventional EUS, such as EUS-guided fine-needle aspiration, EUS-guided injection therapies, and EUS-guided drainage procedures. The availability of high-frequency catheter ultrasound probes allows imaging from within the biliary tree. IDUS probes are placed during ERCP, most often over a guide wire, and can be advanced into the common bile duct, hilar region, intrahepatic ducts, gallbladder, and across biliary strictures.
