Endoscopic Ultrasonography (EUS): Definition and Applications
Endoscopic ultrasonography (EUS) is a specialized diagnostic and therapeutic procedure that combines endoscopy with high-frequency ultrasound imaging, incorporating an ultrasonic transducer into the tip of an endoscope to provide detailed visualization of the gastrointestinal tract wall layers and adjacent structures.
What is EUS?
EUS is a hybrid technology that integrates:
- An endoscope for direct visualization of the GI tract lumen
- A high-frequency ultrasound probe (mechanical or electronic) at the tip of the endoscope
- The ability to visualize structures beyond the mucosal surface
Equipment Types
- Radial echoendoscopes: Provide 360° cross-sectional imaging
- Linear echoendoscopes: Allow for interventional procedures with needle guidance
- Miniprobes: Small catheter-based ultrasound probes that can be passed through standard endoscopes
Imaging Capabilities
- Visualizes 5 distinct layers of the GI tract wall corresponding to:
- Interface echo between superficial mucosa and acoustic coupling medium
- Deep mucosa
- Submucosa plus acoustic interface
- Muscularis propria
- Serosa and subserosal fat 1
Diagnostic Applications
Subepithelial Lesions
- Superior to standard endoscopy: Can differentiate intramural lesions from extrinsic compression with high sensitivity (89-98%) 1
- Layer identification: Determines exact layer of origin, size, and morphologic features of subepithelial masses 1
- Characterization: Distinguishes between different types of lesions based on echogenicity (hypoechoic, hyperechoic, anechoic) 1
Pancreaticobiliary Disorders
- Unexplained pancreatitis: First-line diagnostic test for unexplained acute and recurrent pancreatitis 1
- Biliary evaluation: Can exclude common bile duct stones before laparoscopic cholecystectomy 2
- Pancreatic masses: Superior diagnostic accuracy (95.9%) for determining causes of suspected obstruction 3
Cancer Staging
- Esophageal cancer: Essential for T and N staging, particularly useful for assessing T4b status with invasion toward airways, pericardium or aorta 1
- Lung cancer: Combined with endobronchial ultrasound (EBUS) for mediastinal nodal staging with high sensitivity (93-100%) 1
- Gastric and pancreatic tumors: Accurate preoperative staging 2
Therapeutic Applications
EUS has evolved from purely diagnostic to interventional and therapeutic:
- Tissue acquisition: EUS-guided fine-needle aspiration (FNA) or biopsy for pathologic diagnosis 2, 4
- Drainage procedures: Pancreatic pseudocyst drainage 2
- Pain management: Celiac plexus neurolysis 2
- Biliary access: Alternative route when ERCP fails 3
- Anti-tumor treatments: Fine-needle injection of anti-tumoral agents, tumor ablation, and fiducial placement 4
Advantages Over Other Imaging Modalities
- Superior resolution: Images obtained from inside the body, near or touching the examined surface 5
- Layer visualization: Shows 5 layers of digestive tract wall, comparable to microscopic examination 5
- Complementary to CT/MRI: Detects small lesions (<4mm) missed by other modalities 3
- Diagnostic accuracy: Higher sensitivity for pancreatic tumors (90.8%) compared to CT (50%) 3, 5
Recent Advancements
- EUS-elastography: Evaluates tissue elasticity to differentiate various lesions 6
- Contrast-enhanced EUS: Distinguishes benign from malignant lesions using intravenous contrast agents 6
- Forward-viewing instruments: Newer development allowing better access to certain areas 7
Limitations
- Operator dependence: Requires significant expertise and experience to achieve high diagnostic accuracy 5
- Limited field of view: May not visualize all areas equally well 3
- Invasive procedure: Carries small risk of complications (up to 6.3% of patients) 3
- Access limitations: May be difficult to perform in patients with stenotic lesions 1
EUS represents a significant advancement in both diagnostic and therapeutic gastroenterology, providing detailed imaging and interventional capabilities that complement conventional imaging techniques.