What are the indications for a triphasic computed tomography (CT) scan in patients with suspected liver lesions, particularly those with a history of liver disease, such as cirrhosis or hepatitis?

Indications for Triphasic CT Scan

Triphasic CT scan is indicated for the evaluation of suspected hepatocellular carcinoma (HCC) in patients with cirrhosis or chronic liver disease, for characterization of liver nodules detected on screening ultrasound, and for distinguishing hypervascular liver lesions (both benign and malignant) based on their enhancement patterns. 1, 2, 3

Primary Indications

Suspected Hepatocellular Carcinoma

• Order triphasic CT when a liver nodule is identified on ultrasound screening in cirrhotic patients or when serum AFP is rising. 1, 2
• For nodules 1-2 cm in size, use two dynamic imaging modalities (triphasic CT and/or MRI) to establish diagnosis. 1, 3
• For nodules >2 cm, a single triphasic CT showing arterial hyperenhancement with portal/delayed phase washout is diagnostic of HCC without requiring biopsy. 1, 2, 3
• For nodules <1 cm, perform follow-up imaging every 3-4 months; if enlarging, evaluate according to size-based algorithm above. 1, 3

High-Risk Patient Populations Requiring Evaluation

• Cirrhotic patients from any cause (HBV, HCV, alcohol, hemochromatosis, primary biliary cirrhosis) with a detected liver lesion. 1, 2
• Patients with chronic viral hepatitis (even without established cirrhosis) who develop focal liver lesions. 2
• Patients with elevated AFP >200 ng/mL in conjunction with a liver mass >2 cm, which has high positive predictive value for HCC. 1
• Patients with rising AFP levels even without visible mass on ultrasound warrant triphasic CT to detect occult lesions. 1

Characterization of Hypervascular Lesions

Distinguishing Benign from Malignant Lesions

• Triphasic CT differentiates malignant from benign lesions in 74-95% of cases based on enhancement patterns. 4
• HCC demonstrates arterial hypervascularity with washout in portal venous or delayed phases (the classic pattern). 1, 2, 3
• Focal nodular hyperplasia shows arterial hypervascularity without portal washout and may have a central scar. 2, 5
• Hemangiomas typically show peripheral nodular enhancement with progressive centripetal fill-in. 6, 7

Hypervascular Metastases

• Use triphasic CT for suspected hypervascular metastases (particularly from neuroendocrine tumors, renal cell carcinoma, melanoma, thyroid cancer). 8, 6
• The arterial phase detects small hypervascular metastatic lesions not visible on portal venous phase alone in 21% of patients. 8
• For hypovascular metastases (colon, lung, breast), the portal venous phase alone is usually sufficient; triphasic protocol adds limited value. 8, 6

Technical Considerations and Diagnostic Performance

The Three Phases Explained

• Arterial phase (20-27 seconds delay): Detects arterial hypervascularity characteristic of HCC, hypervascular metastases, and FNH. 1, 8, 7
• Portal venous phase (45-80 seconds delay): Optimal for detecting hypovascular lesions and assessing background liver. 1, 8, 7
• Delayed/equilibrium phase (5-10 minutes): Critical for identifying washout pattern that distinguishes HCC from benign lesions. 1, 7, 9

Diagnostic Performance by Lesion Size

• Sensitivity is size-dependent: 61-73% for lesions >2 cm, 44-65% for lesions 1-2 cm, and only 10-43% for lesions <1 cm. 3
• Washout in the equilibrium phase increases positive predictive value to 92.9% for diagnosing HCC, while hypervascular lesions without washout have 97.1% negative predictive value for malignancy. 9

Common Pitfalls and Caveats

Avoid These Mistakes

• Never order single-phase CT for liver lesion characterization—it is inadequate and may miss critical diagnostic information. 4
• Do not order CT without contrast for evaluating liver lesions; it has very limited diagnostic utility. 4
• Be aware that arterial phase imaging can detect benign lesions (FNH, transient hepatic perfusion differences) that mimic malignancy, creating diagnostic dilemmas. 8, 5
• CT underestimates tumor burden by 25-30%, particularly for lesions <2 cm, even with optimal technique. 3

When Biopsy Is Still Needed

• Biopsy is required when imaging is atypical or non-diagnostic, particularly for 1-2 cm nodules showing classic enhancement on only one imaging modality. 1, 2
• Avoid biopsy of potentially resectable lesions when possible due to 1-3% risk of needle tract seeding. 1
• In non-cirrhotic livers, biopsy may be necessary as non-invasive criteria are less reliable. 2

When to Consider MRI Instead

• MRI offers superior sensitivity and specificity in nodular cirrhotic livers compared to triphasic CT. 3
• For atypical cysts or lesions, MRI with contrast establishes definitive diagnosis in 95% of cases versus 90% with CT. 4
• Consider MRI when triphasic CT findings are equivocal or when additional characterization is needed. 1, 3