What is hepatic echogenicity?

Hepatic Echogenicity: Definition and Clinical Significance

Hepatic echogenicity refers to the brightness or reflectivity of liver tissue on ultrasound examination, which primarily reflects the degree of fatty infiltration (steatosis) in the liver parenchyma but does not reliably indicate fibrosis or other structural changes.

Understanding Hepatic Echogenicity

Hepatic echogenicity is a sonographic characteristic observed during ultrasound examination of the liver. It has several key features:

• Definition: The brightness or reflectivity of liver tissue compared to other organs (typically the renal cortex) on ultrasound imaging 1
• Physical basis: Increased echogenicity occurs when lipid droplets within hepatocytes disturb sound wave propagation, causing scatter and attenuation 2
• Normal appearance: The liver should have similar echogenicity to the renal cortex and appear homogeneous 1

Grading System for Hepatic Echogenicity

The American College of Radiology recommends the following grading system for steatosis based on echogenicity 1:

• Normal: Liver echogenicity equal to or slightly less than renal cortex
• Mild steatosis: Mild diffuse increase in liver echogenicity, clear visualization of diaphragm and intrahepatic vessel walls, minimal posterior beam attenuation
• Moderate steatosis: Moderate diffuse increase in liver echogenicity, obscuration of diaphragm and intrahepatic vessel walls, moderate posterior beam attenuation
• Severe steatosis: Marked increase in liver echogenicity, non-visualization of diaphragm and intrahepatic vessel walls, significant posterior beam attenuation

Clinical Significance and Diagnostic Accuracy

Increased hepatic echogenicity has important clinical implications:

• High predictive value for steatosis: Increased echogenicity has a sensitivity of 90% and specificity of 82% for moderate to severe hepatic steatosis (≥30% fat on histology) 3
• Limited value for fibrosis: Echogenicity cannot reliably diagnose fibrosis or cirrhosis, as these conditions may present with normal echogenicity 3
• Associated findings: When increased echogenicity is accompanied by high attenuation and reduced portal vessel wall distinction, the positive predictive value for steatosis increases to 93-94% 3

Differential Diagnosis of Increased Hepatic Echogenicity

While fatty liver is the most common cause of increased hepatic echogenicity, other conditions should be considered 4:

• Hepatic steatosis: Most common cause (86.7% of cases with increased echogenicity) 3
• Cirrhosis: May present with increased echogenicity, but typically has additional features like surface nodularity and signs of portal hypertension 1
• Viral hepatitis: Can cause increased echogenicity in acute or chronic phases
• Glycogen storage disease: Causes diffuse increased echogenicity
• Hemochromatosis: May present with increased echogenicity in early stages

Technical Considerations

Several factors affect the assessment of hepatic echogenicity 1:

• Patient preparation: 6-hour fast before examination is recommended
• Positioning: Supine or slight left lateral decubitus position
• Standard views: Should include subcostal views of liver and portal structures, intercostal views of right lobe, left lobe views through epigastrium, and comparison views of right kidney
• Limitations: Obesity, bowel gas, and machine settings can affect the appearance of echogenicity
• Inter-observer variability: Significant variability exists, with experience playing a major role in accurate interpretation

Clinical Application

When increased hepatic echogenicity is detected:

• Initial assessment: Complete liver chemistry panel, international normalized ratio, creatinine, and complete blood count should be performed 1
• Risk stratification: Calculate FIB-4 score to determine fibrosis risk 1
• Additional imaging: Consider Controlled Attenuation Parameter (CAP) or Transient Elastography (TE) to quantify steatosis and fibrosis, respectively 1
• Evaluation for metabolic risk factors: Assess for obesity, type 2 diabetes, dyslipidemia, and hypertension 1

Pitfalls and Caveats

• Focal fatty infiltration: May appear as masslike areas with characteristic angulated, geometric margins that can be distinguished from true masses 5
• False negatives: About 9% of patients with moderate to severe steatosis may have normal echogenicity 3
• False positives: About 13% of patients with increased echogenicity may have no or only mild steatosis 3
• Fibrosis detection: Increased echogenicity alone cannot diagnose fibrosis; even patients with cirrhosis may have normal echogenicity 3

In conclusion, hepatic echogenicity is a valuable ultrasound parameter primarily reflecting the degree of fatty infiltration in the liver, but it has limitations in detecting fibrosis and should be interpreted in the context of clinical and laboratory findings.