Basics of Endoscopic Ultrasonography (EUS) Anatomy
Endoscopic ultrasonography (EUS) is the modality of choice for evaluating gastrointestinal tract anatomy, providing superior visualization of the gut wall layers and adjacent structures with high diagnostic accuracy. 1
EUS Probe Orientation and Image Display
- EUS follows specific conventions for probe orientation that differ from traditional ultrasound:
- The probe marker is typically directed toward the patient's right (9 o'clock position) for subcostal views 2
- Unlike cardiology, emergency physicians typically do not reverse screen orientation during EUS exams, which simplifies the procedure in urgent settings 2
- For kidney views, the probe indicator is conventionally directed toward the head or vertebral end of the rib on both sides 2
Gastrointestinal Wall Layer Visualization
EUS uniquely visualizes the GI tract as a 5-layer structure from lumen outward:
- Layer 1: Superficial mucosa (hyperechoic)
- Layer 2: Deep mucosa (hypoechoic)
- Layer 3: Submucosa (hyperechoic)
- Layer 4: Muscularis propria (hypoechoic)
- Layer 5: Serosa/adventitia (hyperechoic)
This layered visualization is critical for diagnosis as different pathologies arise from specific layers:
- Layer 2 lesions: Typically carcinoids, pancreatic rests
- Layer 3 lesions: Often lipomas or vascular lesions
- Layer 4 lesions: Commonly GISTs or leiomyomas
- Layer 5 lesions: External compression or serosal-based lesions 1
Key Anatomical Windows
Cardiac Views
Subcostal/Subxiphoid Four-Chamber View:
- Probe placed under rib cage/xiphoid process
- Directed toward patient's left shoulder
- Visualizes right atrium, tricuspid valve, right ventricle, left atrium, left ventricle, and pericardial spaces 2
- Liver serves as sonographic window
Parasternal Long Axis View:
- Obtained using 3rd-5th intercostal spaces left of sternum
- Probe marker directed to patient's left hip (4 o'clock position)
- Visualizes right ventricle, septum, left atrium, left ventricle, aortic valve, and aortic root 2
Abdominal/Renal Views
- Kidneys are best identified in longitudinal axis
- Orientation defined with respect to organ axes rather than standardized anatomic planes
- Transverse views often require navigating rib shadows 2
Technical Considerations
- EUS interpretation is highly operator-dependent
- Scope positioning can be challenging, particularly for fundal lesions 1
- Different needle sizes are recommended based on location:
Advanced EUS Techniques
EUS-Guided Fine Needle Aspiration/Biopsy (FNA/FNB):
Contrast-Enhanced EUS:
EUS Elastography:
- Evaluates tissue elasticity to differentiate various lesions
- Particularly useful for pancreatic masses and lymph nodes 3
Clinical Applications
EUS demonstrates superior accuracy compared to CT for:
- Wall penetration assessment (85% vs. 15% for esophageal-GE junction tumors) 4
- Lymph node evaluation (60-100% vs. 25-67% accuracy depending on location) 4
- Subepithelial lesion characterization based on layer of origin 1
Common Pitfalls and Limitations
- EUS has limited ability to reliably distinguish between neoplastic and inflammatory tissue 5
- Small lesion sampling may have insufficient diagnostic yield 1
- Technical challenges in certain anatomical locations (e.g., fundus) 1
- Pre-procedural imaging (MRCP or CECT) is recommended before interventional EUS procedures 2
Training Considerations
- Proficiency in EUS requires observation in high-volume centers, followed by hands-on training in porcine models before performing procedures in patients 2
- ERCP skills are beneficial for endoscopists learning EUS-guided procedures 2
- Multidisciplinary support is recommended for interventional EUS procedures 2