ESSENTIALS OF DIAGNOSIS
Endoscopic ultrasound (EUS) is a critical tool in cancer staging and is superior to computed tomography (CT) and positron emission tomography (PET), in locoregional esophageal cancer staging.
EUS-guided fine-needle aspiration (EUS-FNA) is the most sensitive method for diagnosing pancreatic cancer; overall accuracy of EUS is similar to CT for assessing vascular invasion.
EUS helps evaluate subepithelial lesions, although the yield of EUS-FNA is lower for subepithelial than for extraluminal lesions.
EUS is as an important tool in diagnosing chronic pancreatitis.
Diagnostic yield for EUS is similar to magnetic resonance cholangiopancreatography (MRCP) in detecting choledocholithiasis with the choice of modality guided by local availability.
EUS is the premier tool for staging gastrointestinal cancers and has revolutionized the role of gastrointestinal endoscopy in diagnostic imaging inside and outside the gastrointestinal tract. EUS merges endoscopy, which is limited to visualizing the lumen of the gastrointestinal tract, with ultrasonography, which allows imaging of the layers of the gastrointestinal wall and surrounding structures. Tissue diagnosis can be obtained with EUS-FNA, which uses a catheter with a retractable needle that can be advanced into visualized tissue. Exciting advances in the therapeutic application of EUS have begun, including the well-established endoscopic pseudocyst drainage and more recent biliary and pancreatic duct access techniques as well as possibilities of EUS-guided fine-needle injection (EUS-FNI) therapy.
Radial scanning echoendoscopes were the first instruments used with the advent of EUS in the 1980s (Figure 37–1). The ultrasound transducer is mounted on the tip of the endoscope and rotates 360 degrees to provide cross-sectional images perpendicular to the long axis of the endoscope (Figure 37–2). The endoscopic viewing optic is mounted proximal to the ultrasound transducer and provides an oblique view of the lumen. A new forward-viewing echoendoscope has been developed. To overcome the problem of ultrasound waves traveling through an air-filled lumen, a balloon that can be filled with water surrounds the ultrasound transducer. Various ultrasonic frequencies between 5 and 20 MHz are used.
A. Radial echoendoscope. B. Radial echoendoscope with balloon inflated.
Imaging planes for radial and linear echoendoscopes. The radial echoendoscope provides images in a plane perpendicular to the long axis. The linear echoendoscope provides images along the long axis in an 80–105-degree arc with fine-needle aspiration capability.
Linear array echoendoscopes provide ultrasound images along the long axis of the endoscope usually in a 120- to 180-degree arc (Figures 37–2 and 37–3). This is critical to allow simultaneous imaging of the needle and target lesion during EUS-FNA. A variety of needles, including 19-, 22-, and 25-gauge, can be passed through the endoscopic working channel.