What abdominal organs are more accessible by Endoscopic Ultrasonography (EUS) and less so by Transabdominal Ultrasonography (USG)?

Abdominal Organs More Accessible by EUS Than Transabdominal Ultrasound

EUS provides superior access to the pancreas (especially head and body), distal common bile duct, gastric wall lesions, periampullary region, and left hepatic lobe compared to transabdominal ultrasonography, which has limited visualization of these structures due to bowel gas interference and anatomical positioning. 1

Pancreas

The pancreas is the primary organ where EUS demonstrates clear superiority over transabdominal ultrasound. 1

• EUS achieves sensitivity, specificity, and accuracy of 90.8%, 96.5%, and 91% respectively for solid pancreatic tumors when combined with FNA, particularly for lesions in the pancreatic head and body 1
• The pancreatic head and body are directly visualized through the duodenum and stomach with EUS, allowing detection of small mass lesions, chronic pancreatitis features, and cystic lesions 1
• Transabdominal ultrasound faces significant challenges visualizing the pancreas due to interference from stomach and duodenal gas, requiring multiple technical maneuvers (water filling, position changes, respiratory timing) that still result in incomplete visualization in many patients 2, 3
• The pancreatic tail is particularly difficult to visualize with transabdominal ultrasound but is readily accessible via EUS from the gastric fundus 4, 5

Distal Common Bile Duct and Periampullary Region

EUS provides direct visualization of the distal CBD and ampulla of Vater with 91% accuracy for diagnosing biliary strictures, structures that are poorly visualized by transabdominal ultrasound 1

• The extrahepatic biliary tree is accessed when the EUS probe is positioned in the duodenum, allowing visualization of strictures and guidance for tissue acquisition 1
• EUS can detect small stones, strictures, and periampullary neoplasms in the distal CBD that are missed by transabdominal approaches 1
• The periampullary region is specifically evaluated for separating invasive from noninvasive lesions 6

Gastric Wall Lesions

Small gastric wall lesions (≤30 mm) have only 50-61% visualization rates with transabdominal ultrasound, whereas EUS can visualize nearly all gastric subepithelial masses regardless of size. 6

• Transabdominal ultrasound visualization is inversely proportional to the size of gastric wall masses, with adequate visualization ranging only 69-93% overall 6
• EUS allows layer-by-layer assessment of gastric wall lesions, determining their origin (third vs. fourth layer) and characteristics crucial for diagnosing GISTs and other subepithelial masses 6
• The proximity of the EUS probe to the gastric wall and use of higher-frequency transducers provides superior resolution compared to transabdominal approaches 6

Gallbladder (Selective Advantage)

EUS demonstrates superior characterization of gallbladder polyps with 95% sensitivity, particularly for distinguishing true polyps from tumefactive sludge when transabdominal ultrasound is equivocal 1

• The gallbladder fundus and body are evaluated when adjacent to the duodenum or gastric antrum 1
• EUS better identifies gallbladder polyp types (97% vs 71% for transabdominal ultrasound) through detection of tiny echogenic foci characteristic of cholesterol polyps and microcysts in adenomyomatosis 6
• However, transabdominal ultrasound remains the first-line modality for gallbladder evaluation; EUS is reserved for problem-solving when lesions >10 mm require further characterization 6

Left Hepatic Lobe

The left hepatic lobe is accessible via EUS from the gastric fundus or body with 92% accuracy for diagnosing focal lesions, an area that may be difficult to visualize with transabdominal ultrasound depending on patient body habitus 1

• EUS allows FNA of focal lesions in the left lobe under direct visualization 1
• The right hepatic lobe remains better evaluated by transabdominal ultrasound due to anatomical positioning 1

Peripancreatic Fluid Collections in the Pancreatic Tail

All peripancreatic fluid collections located at the pancreatic tail require EUS-guided drainage rather than conventional transmural drainage, as they are unlikely to cause luminal compression visible on transabdominal imaging 4

• Transabdominal ultrasound and CT fail to provide adequate access for drainage procedures in the pancreatic tail region 4
• EUS-guided drainage was 100% successful for tail collections after conventional approaches failed 4

Critical Limitations to Consider

EUS has a narrow field of view (10-20% limitation) and cannot detect pathology beyond structures physically adjacent to the probe, making it unsuitable as a first-line screening modality 1

• Transabdominal ultrasound provides broader anatomical survey capabilities despite inferior resolution for specific structures 1
• EUS is operator-dependent and requires significant technical expertise, with effectiveness varying based on endoscopist experience 6
• Pre-procedural imaging with MRCP or contrast-enhanced CT is recommended before EUS procedures to understand anatomy and plan the optimal approach 6