Imaging Strategy for Elevated Liver Function Tests

Initial Imaging Modality

• Ultrasound abdomen is the recommended first‑line screening tool for any patient with elevated liver function tests, as endorsed by the American College of Radiology. 1
• The “starry‑sky” ultrasound pattern (bright portal triads against a darker edematous liver) signifies acute hepatic parenchymal inflammation. 2
• A contracted gallbladder with wall thickening (approximately 5 mm) on ultrasound serves as a marker of hepatic dysfunction and predicts severe hepatitis. 2

Unenhanced CT (Without IV Contrast)

• When ultrasound findings are equivocal, an unenhanced CT abdomen can be used to quantify moderate‑to‑severe hepatic steatosis, providing 100 % specificity and ≈ 82 % sensitivity. 1
• Liver attenuation < 40 HU on unenhanced CT indicates ≥ 30 % fat infiltration; a liver‑to‑spleen attenuation difference > 10 HU confirms moderate‑to‑severe steatosis. 1
• Unenhanced CT has limited sensitivity for detecting early or mild fatty infiltration. 3

Contrast‑Enhanced CT (With IV Contrast)

• Contrast‑enhanced CT should be obtained urgently when ischemic liver injury (shock liver), acute hepatitis complications, or vascular pathology are suspected. 1
• Key CT findings that characterize acute hepatitis include heterogeneous arterial enhancement, peri‑portal hypoattenuation, perihepatic lymphadenopathy > 7 mm, and ascites. 2
• Ischemic hepatitis appears as hypo‑enhancement of the liver parenchyma due to hypoperfusion and carries a high mortality if not recognized promptly. 2
• Contrast‑enhanced CT is useful for delineating the etiology and site of biliary obstruction when ultrasound demonstrates ductal dilatation, and for assessing sequelae of portal hypertension. 1

Critical Pitfalls to Avoid

• Do not perform both unenhanced and contrast‑enhanced CT in the same study for elevated LFTs; it adds unnecessary radiation and cost without clinical benefit. 2
• Do not assume that decreased liver attenuation on contrast‑enhanced CT represents benign steatosis; ischemic injury must be excluded urgently. 1
• Never bypass ultrasound and proceed directly to CT; ultrasound remains the most appropriate initial evaluation. 1

Algorithmic Approach by Clinical Pattern

Mild, Asymptomatic LFT Elevation

• Start with ultrasound to screen for steatosis, biliary dilatation, cirrhosis, or focal lesions. 1
• If steatosis is identified, initiate lifestyle modifications and calculate a FIB‑4 score to assess fibrosis risk. 3
• If biliary dilatation is seen, proceed to MRI with MRCP, which has ≈ 90.7 % accuracy for detecting biliary obstruction. 3

Acute Hepatocellular Injury (Markedly Elevated Transaminases)

• Obtain a portable ultrasound immediately as the first‑line modality. 2
• Add duplex Doppler ultrasound to evaluate vascular patency when an ischemic insult is suspected. 2
• If ischemic hepatitis or other acute complications are suspected, obtain urgent contrast‑enhanced CT. 2

Cholestatic Pattern (Elevated Alk‑Phos/GGT)

• Perform ultrasound first to identify biliary obstruction. 1
• When ductal dilatation is confirmed, use either contrast‑enhanced CT or MRI with MRCP to define the obstruction’s etiology and precise location. 1

Role of MRI

• MRI (with or without contrast, plus MRCP when indicated) is recommended when ultrasound and initial work‑up are inconclusive; it detects ≥ 5 % hepatic fat with 76.7‑90 % sensitivity and 87.1‑91 % specificity, outperforming CT for mild steatosis. 1
• Contrast‑enhanced MRI demonstrates parenchymal inflammation as increased T2 signal, decreased T1 signal, heterogeneous perfusion, and peri‑portal edema appearing hyperintense. 2
• MRI with MRCP is the preferred modality for delineating the cause of biliary obstruction, especially in intra‑hepatic cholestasis or persistent alkaline phosphatase elevation despite a negative ultrasound. 1

CT Evaluation of Diffuse Decreased Liver Attenuation

Hepatic Steatosis (Fatty Infiltration)

• The American College of Radiology states that the most common cause of diffuse low liver attenuation on CT is hepatic steatosis; an unenhanced liver attenuation < 40 HU correlates with ≥ 30 % fat content. 4
• Progressive fat accumulation follows specific attenuation thresholds (unenhanced CT):

• A liver‑to‑spleen attenuation difference > 10 HU or a liver‑to‑spleen ratio < 1 confirms moderate‑to‑severe steatosis (≥30 % fat). 4
• On unenhanced CT, detection of moderate steatosis (≥30 % fat) achieves 100 % specificity and 82 % sensitivity. 4
• Unenhanced CT is limited for mild steatosis (<30 % fat) and may miss early fatty infiltration. 4

Acute Hepatic Injury / Inflammation

• The ACR notes that acute viral hepatitis (A, B, C) and drug‑induced liver injury (e.g., acetaminophen) produce diffuse low attenuation on CT due to parenchymal edema and inflammation. 4
• On contrast‑enhanced CT, acute hepatitis typically shows: arterial phase heterogeneity, peri‑portal hypoattenuation, perihepatic lymph nodes > 7 mm, and ascites. 4

Ischemic Hepatitis (“Shock Liver”)

• According to the ACR, ischemic hepatitis caused by systemic hypoxia or hypotension leads to diffuse hepatic hypo‑enhancement on contrast‑enhanced CT and carries a high mortality if not promptly recognized. Evidence is drawn from two ACR sources. [4][5]
• The presence of diffuse decreased enhancement on contrast‑enhanced CT should trigger urgent evaluation for ischemic injury. 4

Cholestatic Conditions

• Peri‑portal edema associated with cholestasis appears as low attenuation on CT. 4

Diagnostic Algorithm

Unenhanced CT

• Measure absolute liver attenuation; values < 40 HU indicate ≥30 % hepatic fat (moderate steatosis). 4
• Calculate liver‑to‑spleen attenuation difference; a difference > 10 HU confirms moderate‑to‑severe steatosis. 4

Contrast‑Enhanced CT

• The ACR advises that HU measurements are unreliable for steatosis quantification on contrast‑enhanced studies because contrast timing, injection site, and hemodynamics introduce variability. 4
• Evaluate enhancement patterns: heterogeneous arterial enhancement with peri‑portal hypoattenuation suggests acute hepatitis. 4
• Identify diffuse hypo‑enhancement, which indicates possible ischemic injury requiring immediate intervention. 4

Pitfalls

• Do not assume that low attenuation on contrast‑enhanced CT always represents benign steatosis; ischemic liver injury must be excluded urgently. 4
• Routine performance of both unenhanced and contrast‑enhanced CT is unnecessary; select the protocol based on clinical suspicion (unenhanced for steatosis assessment, contrast‑enhanced for acute injury or vascular pathology). 4