What are the causes of diffuse low attenuation of the liver on CT?

Causes of Diffuse Decreased Attenuation in the Liver on CT

The most common cause of diffuse decreased liver attenuation on CT is hepatic steatosis (fatty infiltration), which produces liver attenuation values <40 HU on unenhanced CT and correlates with ≥30% fat content. 1

Primary Causes by Attenuation Pattern

Hepatic Steatosis (Most Common)

• Progressive fat accumulation produces specific attenuation thresholds: 64.4 HU (0% fat), 59.1 HU (1-25% fat), 41.9 HU (26-50% fat), and 25.0 HU (>50% fat). 1
• A liver-to-spleen attenuation difference >10 HU and attenuation ratio <1 confirms moderate-to-severe steatosis. 1
• Unenhanced CT achieves 100% specificity and 82% sensitivity for detecting moderate steatosis (≥30%). 1
• Associated metabolic risk factors include obesity, type 2 diabetes, hypertension, and dyslipidemia. 2

Acute Hepatic Injury/Inflammation

• Acute viral hepatitis (hepatitis A, B, C) and drug-induced liver injury (acetaminophen) cause diffuse low attenuation due to parenchymal edema and inflammation. 1
• On contrast-enhanced CT, acute hepatitis demonstrates arterial heterogeneity, periportal hypoattenuation, perihepatic lymphadenopathy (>7 mm), and ascites. 1
• Diffuse transient hepatic attenuation differences (THAD) with a patchy pattern indicate acute hepatic injury in patients with markedly elevated ALT (>400 IU/L). 3

Ischemic Liver Injury (Shock Liver)

• Ischemic hepatitis from systemic hypoxia or hypotension produces diffuse hypoenhancement of liver parenchyma and carries high mortality if unrecognized. 1
• Results from hepatic hypoperfusion due to heart failure, constrictive pericarditis, or systemic shock. 4
• Contrast-enhanced CT is essential to identify this life-threatening condition. 5

Hepatic Congestion/Vascular Disorders

• Budd-Chiari syndrome produces mosaic enhancement with a patchy pattern of diffuse low attenuation. 4
• Portal vein thrombosis causes central-peripheral enhancement phenomenon with relative hypodensity of central parenchyma. 4
• Heart failure-related hepatic stasis creates diffuse mosaic enhancement pattern. 4

Cholestatic Disorders

• Biliary obstruction with dilatation of the entire biliary tree produces peribiliary pattern of low attenuation around dilated bile ducts. 4
• Periportal edema appears as low attenuation on CT in cholestatic conditions. 1

Other Pathologic Causes

• Massive hepatic necrosis produces heterogeneous low-attenuation changes (readings <50 HU) and suggests severe acute injury. 6
• Acute cellular rejection in transplant recipients can manifest as homogeneous low attenuation. 6
• Cholestasis, liver cell ballooning, and centrilobular congestion may all produce low-attenuation changes. 6

Critical Diagnostic Algorithm

On Unenhanced CT:

• Measure absolute liver attenuation in HU: <40 HU indicates ≥30% steatosis. 1, 7
• Calculate liver-to-spleen ratio: difference >10 HU confirms moderate-to-severe steatosis. 1
• Attenuation >75 HU suggests iron overload rather than decreased attenuation. 7

On Contrast-Enhanced CT:

• Do not rely on HU values for steatosis quantification because contrast timing, injection site, and hemodynamics create variable measurements. 1
• Assess enhancement pattern: heterogeneous arterial enhancement with periportal hypoattenuation suggests acute hepatitis. 1
• Look for hypoenhancement: diffuse decreased enhancement indicates ischemic injury requiring urgent intervention. 1, 5

Pattern Recognition:

• Homogeneous low attenuation: typically steatosis, cholestasis, or diffuse edema. 6
• Heterogeneous low attenuation: suggests massive necrosis, acute inflammation, or vascular compromise. 6
• Sectoral wedge-shaped THAD with clear borders: portal branch thrombosis or arterioportal shunting. 4
• Mosaic/patchy pattern: Budd-Chiari syndrome or heart failure. 4
• Central-peripheral pattern: portal trunk thrombosis or cirrhosis. 4

Critical Pitfalls to Avoid

• Never assume decreased attenuation on contrast-enhanced CT represents benign steatosis—ischemic liver injury must be excluded urgently as it is life-threatening. 5
• Do not perform both unenhanced and contrast-enhanced CT routinely—choose based on clinical suspicion (unenhanced for steatosis quantification, contrast for acute injury/vascular pathology). 5
• Unenhanced CT has limited sensitivity for mild steatosis (<30% fat) and cannot detect early fatty infiltration. 1
• Sampling errors on biopsy may miss diffuse disease that CT can identify. 8

Clinical Context Integration

• For asymptomatic mild LFT elevation: unenhanced CT quantifies steatosis severity and guides metabolic risk stratification. 9
• For acute hepatocellular injury with markedly elevated transaminases: contrast-enhanced CT urgently evaluates for ischemic injury, acute hepatitis complications, or vascular thrombosis. 5, 9
• For cholestatic pattern: contrast-enhanced CT or MRI with MRCP defines biliary obstruction etiology after ultrasound confirms ductal dilatation. 5, 9