What does EUS (Endoscopic Ultrasonography) mean?

Endoscopic Ultrasonography (EUS): Definition and Applications

EUS (Endoscopic Ultrasonography) is a diagnostic and therapeutic technique that combines endoscopy with high-frequency ultrasound by incorporating an ultrasonic transducer into the tip of an endoscope, allowing for detailed imaging of the gastrointestinal tract wall layers and adjacent structures. 1, 2

Technical Aspects of EUS

• EUS utilizes specialized endoscopes with ultrasound transducers that generate images either mechanically or electronically, with electronic techniques allowing for additional features like Doppler ultrasound 2
• The procedure involves advancing an endoscope (typically oblique-viewing for upper GI tract or forward-viewing for colorectal examination) to the area of interest and obtaining ultrasound images from within the lumen 2
• EUS provides superior resolution compared to external imaging techniques like CT, MRI, or transabdominal ultrasound because the ultrasound probe is positioned directly adjacent to the target tissue 3

Diagnostic Capabilities

• EUS can visualize the 5 distinct layers of the gastrointestinal tract wall with microscopic precision, allowing for detailed examination of intramural lesions 1
• The technique reliably differentiates between intramural lesions and extrinsic compression with sensitivity between 89% and 98% 1
• EUS is particularly valuable for evaluating subepithelial lesions (SELs), determining their layer of origin, size, echogenicity, and margins, which helps narrow the differential diagnosis 1
• For subepithelial lesions, EUS can determine whether they originate from the mucosa (layer 2), submucosa (layer 3), or muscularis propria (layer 4), which has diagnostic and therapeutic implications 1

Clinical Applications

• EUS is the preferred initial diagnostic test for unexplained acute and recurrent pancreatitis, with diagnostic yield of 29-88% in identifying potential etiologies 1
• For pancreatic masses, EUS offers high-resolution imaging of the pancreas and can detect lesions missed by other imaging modalities 1
• In biliary strictures, EUS provides detailed examination of the extrahepatic biliary tree with sensitivity of 78% and specificity of 84% for diagnosing malignant strictures 1
• EUS is considered complementary to pancreas protocol CT in pancreatic cancer staging but is not recommended as a routine staging tool for vascular involvement 1

Advanced EUS Techniques

• EUS-guided fine needle aspiration (EUS-FNA) and fine needle biopsy (EUS-FNB) allow tissue acquisition from lesions with diagnostic accuracies of 46-93% 1
• Contrast-enhanced harmonic EUS uses ultrasound contrast agents to visualize blood flow in fine vessels, aiding in differentiating benign from malignant lesions 4
• EUS elastography measures tissue elasticity, which can be useful in diagnosing chronic pancreatitis and pancreatic cancer 4
• Interventional EUS procedures include tissue sampling, drainage, anastomosis creation, and therapeutic substance delivery 5

Limitations and Considerations

• EUS is highly operator-dependent, requiring significant expertise and experience to achieve high diagnostic accuracy 3
• The accuracy of results varies based on the endosonographer's experience, with at least 80% accuracy in diagnosis recommended for optimal treatment planning 3
• For some conditions, EUS should be performed after a delay (e.g., 2-6 weeks after resolution of acute pancreatitis) to avoid interference from inflammatory changes 1
• While EUS is excellent for local staging and tissue acquisition, it has a limited field of view and cannot detect pathology beyond the region adjacent to the probe 1

Comparison with Other Imaging Modalities

• EUS is superior to CT and MRI for examining the intestinal wall layers and detecting small lesions due to its higher resolution 3
• For biliary obstruction evaluation, EUS is complementary to ERCP, with ERCP having a more therapeutic role while EUS provides detailed imaging and tissue sampling capabilities 1
• MRI/MRCP can be complementary to EUS, particularly if the EUS examination is unrevealing, and remains a reasonable alternative when EUS expertise is not available 1