Intraabdominal Areas Accessible by EUS
EUS provides high-resolution imaging of the pancreaticobiliary tree, mediastinal structures, and select intra-abdominal organs by advancing an ultrasound-equipped endoscope into the duodenum or esophagus, with access limited to structures immediately adjacent to the gastrointestinal tract. 1
Primary Pancreaticobiliary Access
The pancreatic head and distal common bile duct (CBD) represent the primary targets for EUS imaging, offering superior visualization compared to transabdominal ultrasound due to the proximity of the probe to these structures. 1 EUS achieves sensitivity, specificity, and accuracy of 90.8%, 96.5%, and 91% respectively for solid pancreatic tumors when combined with fine-needle aspiration (FNA). 1
Specific Pancreaticobiliary Structures:
- Pancreatic head and body: Direct visualization for mass lesions, chronic pancreatitis features, and cystic lesions 2, 3
- Distal CBD: Detection of small stones (<4 mm), strictures, and periampullary neoplasms 1
- Ampulla of Vater: Local staging of periampullary tumors 1
Biliary Tree and Gallbladder
EUS accesses the extrahepatic biliary tree when the endoscope is positioned in the duodenum, allowing visualization of biliary strictures and guidance for tissue acquisition. 1 The gallbladder fundus and body can be evaluated when adjacent to the duodenum or gastric antrum, particularly useful for characterizing gallbladder polyps and distinguishing them from tumefactive sludge. 1
Liver Access
The left hepatic lobe is accessible via EUS when the transducer is positioned in the gastric fundus or body, allowing for FNA of focal lesions. 1, 4 The right hepatic lobe remains largely inaccessible due to anatomic constraints. 4
Mediastinal and Thoracic Structures
When performed via the esophageal approach (EUS-B), the following mediastinal lymph node stations are accessible:
- Stations 2L, 4L (left paratracheal nodes) 1
- Station 7 (subcarinal nodes) - accessible from both esophageal and tracheal approaches 1
- Stations 8 and 9 (lower mediastinal/paraesophageal nodes) 1
- Lung tumors located immediately adjacent to the esophagus 1
Stations 2R and 4R (right paratracheal) are difficult to reach because the trachea lies between the transducer and these nodes, though large nodes (>2 cm) may occasionally be sampled. 1
Subdiaphragmatic Structures
EUS can access select structures below the diaphragm when the scope is advanced to the gastric fundus or duodenum:
- Left adrenal gland: Accessible for staging of lung cancer or evaluation of adrenal masses 1
- Celiac trunk region: Retroperitoneal lymph nodes near the aorta and celiac axis 1
- Celiac plexus: For neurolysis in pancreatic cancer pain management 2
Gastrointestinal Wall Layers
EUS visualizes five distinct layers of the GI tract wall, corresponding to acoustic interfaces rather than true histological boundaries. 5 This capability allows:
- Subepithelial lesion characterization: Determining layer of origin (mucosa, submucosa, muscularis propria) 5
- Staging of GI malignancies: Assessing depth of invasion (T-staging) 4
- Lymph node evaluation: Perigastric, periduodenal, and celiac nodes 4
Critical Limitations
EUS has a narrow field of view and cannot detect pathology beyond structures physically adjacent to the probe. 1 This means:
- Hilar lymph nodes (stations 10-12) require EBUS rather than esophageal EUS 1
- Intrahepatic biliary tree beyond the left lobe is not accessible 1
- Pancreatic tail lesions may be difficult to visualize depending on anatomic position 2
- Right-sided mediastinal structures are poorly accessed due to interposed trachea 1
Procedural Considerations
EUS is performed under general anesthesia in an interventional suite or operating room, with complication rates up to 6.3% (most commonly post-procedural pancreatitis). 1 The invasive nature and narrow diagnostic field limit its role as a first-line imaging modality, reserving it for situations where tissue diagnosis is needed or when cross-sectional imaging is equivocal. 1