What are the key anatomical features of the liver as visualized through ultrasound (USG)?

Liver Ultrasound Anatomy: Key Features and Clinical Significance

Normal Anatomical Structures

The liver can be comprehensively visualized on ultrasound through systematic scanning, with key anatomical landmarks including the hepatic veins, portal veins, bile ducts, and segmental architecture that guide both diagnostic interpretation and interventional procedures. 1

Parenchymal Characteristics

• Normal liver parenchyma appears homogeneous with medium-level echogenicity, typically equal to or slightly greater than the renal cortex 2
• The hepatic echotexture should be uniform without coarsening or heterogeneity, which would suggest underlying pathology 3
• Liver echogenicity compared to kidney (hepatorenal index) provides an objective assessment standard 2

Vascular Anatomy

• Portal veins appear as echogenic structures with bright walls due to surrounding connective tissue, distinguishing them from hepatic veins 1
• Hepatic veins have thin or imperceptible walls and increase in caliber as they approach the inferior vena cava 1
• Normal right hepatic vein measures ≥5 mm in diameter; measurements <5 mm suggest cirrhosis 3
• Portal vein flow should be hepatopetal (toward the liver) with normal velocity on Doppler examination 3

Segmental Localization

• The liver can be visualized through transgastric and transduodenal scanning approaches 1
• Segmental identification uses anatomical landmarks including the portal vein branches, hepatic veins, and gallbladder fossa 1
• Right posterior segments (VI and VII) are technically difficult to visualize as they are located far from the stomach and duodenum 1

Pathological Features Indicating Cirrhosis

Surface and Contour Changes

• Nodular liver surface is the most specific imaging finding for cirrhosis, with 98% positive predictive value when detected 4
• Surface nodularity has 86% sensitivity on the undersurface versus 53% on the superior surface 4
• Nodularity of the anterior left lobe is particularly characteristic of cirrhotic change 3

Morphologic Alterations

• Right lobe atrophy with caudate lobe and lateral segment left lobe hypertrophy indicates advanced cirrhosis 3, 4
• Caudate-to-right lobe ratio >0.90 (modified ratio) suggests cirrhosis 3, 4
• Right hepatic posterior "notch" represents characteristic contour deformity 3, 4
• Expanded gallbladder fossa (>10 mm thickness of hilar periportal space) 3, 4
• Atrophied medial segment of left lobe 3

Parenchymal Texture Changes

• Coarsened or heterogeneous hepatic echotexture associates with cirrhosis, though this finding is subjective and machine-dependent 3
• The sonographic appearance of steatosis and cirrhosis often overlap with a "fatty-fibrotic" pattern 3, 2
• These morphologic features are present only in later stages of fibrosis, not early disease 3, 4

Portal Hypertension Features

• Splenomegaly is readily detected and indicates portal hypertension 3, 4
• Slow velocity or hepatofugal (reversed) portal vein flow on color Doppler indicates advanced cirrhosis 3, 4
• Decreased phasicity of hepatic venous waveforms correlates with hepatic fibrosis 3
• Portosystemic collaterals and varices can be identified 5
• Ascites detection is straightforward with ultrasound 5

Focal Lesion Characteristics

Benign Lesions

• Simple cysts appear as anechoic structures with posterior acoustic enhancement and no internal vascularity 3
• Hemangiomas typically show hyperechoic appearance on grayscale imaging 6
• Focal nodular hyperplasia demonstrates spoke-wheel vascular pattern with centrifugal filling on contrast-enhanced ultrasound 3
• Hepatic adenomas show centripetal enhancement pattern (periphery to center) on contrast studies 3

Malignant Features

• Hepatocellular carcinoma appears as hypoechoic nodules >10 mm in cirrhotic liver, requiring careful surveillance 5
• Metastases show variable echogenicity patterns 6
• Abscesses demonstrate variable echogenicity from anechoic to hyperechoic with arterial rim enhancement 3

Steatosis Assessment

• Increased liver echogenicity compared to renal cortex indicates fatty infiltration (steatosis) 2
• Grading includes mild, moderate (slightly impaired vessel visualization), and severe (marked increase with poor vessel/diaphragm visualization) 2
• Ultrasound has 84.8% sensitivity and 93.6% specificity for moderate-to-severe hepatic fat deposition 2
• Ultrasound has limited sensitivity (53-65%) for mild steatosis 2
• Depth-dependent attenuation causes posterior signal loss 2

Critical Limitations and Pitfalls

• Evaluation is limited in obese patients due to poor ultrasound beam penetration 3
• Gain settings can artificially alter apparent echogenicity, potentially causing misdiagnosis 2
• Fatty liver can mask underlying focal lesions due to increased background echogenicity 2
• Cirrhosis and steatosis cannot be reliably differentiated based on echogenicity alone 2
• Morphologic features of cirrhosis are subjective and operator-dependent 3

Clinical Applications

• Ultrasound guides percutaneous liver biopsy for tissue diagnosis 3, 7
• Real-time guidance improves accuracy for focal lesion sampling and reduces complications in patients with anatomical variations 3
• Screening for hepatocellular carcinoma in cirrhotic patients requires systematic surveillance 3, 4
• Assessment of treatment response to interventional procedures 8
• Portal pressure evaluation and monitoring 1