The liver is one of the most commonly imaged abdominal organs in CT practice, and the identification of hepatic lesions is among the most challenging and consequential tasks in abdominal radiology. Liver lesions span a wide spectrum from benign, inconsequential findings (simple cysts, haemangiomas, focal nodular hyperplasia) to highly aggressive malignancies (hepatocellular carcinoma, cholangiocarcinoma, metastatic disease). Accurate characterisation critically depends on: understanding normal liver CT anatomy, applying a dedicated multiphasic contrast protocol, and interpreting lesion enhancement patterns in the context of the patient's clinical background — particularly the presence of cirrhosis, known primary malignancy, or metabolic liver disease.

The Multiphasic CT Protocol: Four Phases of Enhancement

Liver lesion characterisation on CT requires a multiphasic (dynamic) contrast-enhanced CT protocol that captures the liver at different time points after IV contrast injection, exploiting the dual blood supply of the liver (portal vein 75%, hepatic artery 25%) and the differential enhancement timing of various lesion types:

  • Non-contrast phase: Detects hyperdense lesions (acute haemorrhage, calcifications, melanoma metastases), establishes baseline attenuation for enhancement calculation
  • Arterial phase (20–35 seconds): Captures hypervascular lesion enhancement — hepatocellular carcinoma (HCC), focal nodular hyperplasia (FNH), haemangioma peripheral nodules, hypervascular metastases (renal cell carcinoma, neuroendocrine tumors)
  • Portal venous phase (60–70 seconds): The primary diagnostic phase — the normal liver parenchyma reaches peak enhancement via the portal vein, making hypovascular lesions (most metastases, early HCC) maximally conspicuous as relatively dark filling defects
  • Delayed/Equilibrium phase (3–5 minutes): Critical for characterising haemangiomas (progressive fill-in), cholangiocarcinoma (progressive delayed enhancement from fibrous stroma), and washout appearance of HCC

LI-RADS: Standardised HCC Categorisation

The Liver Imaging Reporting and Data System (LI-RADS), developed by the American College of Radiology, provides a standardised framework for categorising hepatic observations in patients at high risk for HCC — specifically those with cirrhosis or chronic hepatitis B infection. LI-RADS CT categories range from LR-1 (definitely benign) to LR-5 (definitely HCC), with intermediate categories LR-3 and LR-4 for indeterminate or probably malignant observations. LR-M indicates features suggesting malignancy but not HCC-specific. The major LI-RADS imaging features for HCC assessment include:

  • Arterial phase hyperenhancement (APHE): Non-rim, diffuse, or peripheral enhancement in the arterial phase greater than background liver — the hallmark of HCC vascular biology
  • Washout appearance: Relative hypoattenuation of the lesion compared to liver parenchyma in the portal venous or delayed phase — an LR-5 defining feature when combined with APHE
  • Enhancing capsule: A smooth, rim-like peripheral enhancement in portal venous or delayed phase — representing fibrous pseudocapsule, characteristic of HCC
  • Threshold growth: Increase in lesion diameter of 50% or more within 6 months — indicating rapid growth kinetics consistent with malignancy

Benign Liver Lesions: Key Differentiating Features

Simple Hepatic Cyst: Sharply marginated, water-density (0–10 HU), thin imperceptible wall, no internal septations or enhancement whatsoever on any phase — the most straightforward benign diagnosis. Haemangioma: The most common benign hepatic tumour. Characteristic CT pattern is peripheral nodular enhancement in the arterial phase with progressive centripetal fill-in on delayed images, eventually becoming isoattenuating to the blood pool. Small haemangiomas may show flash-filling (complete enhancement in the arterial phase) mimicking hypervascular malignancy. Focal Nodular Hyperplasia (FNH): Intensely hyperenhancing in the arterial phase with a pathognomonic central scar that appears hypoattenuating in the arterial phase and hyperattenuating on delayed images — the reverse of the central scar of fibrolamellar HCC. FNH is one of the few benign lesions that shows a spoke-wheel arterial pattern on CT angiography phases.

Liver Metastases: The Most Common Hepatic Malignancy

Liver metastases represent the most common hepatic malignancy overall, owing to the rich hepatic blood supply and frequent haematogenous spread from colorectal, lung, breast, gastric, and pancreatic primary tumors. Most liver metastases appear as hypovascular (low-density) lesions relative to enhancing liver parenchyma in the portal venous phase — the best phase for detection. Colorectal metastases classically show a peripheral ring of enhancement ("target" or "halo" sign) with central hypoattenuation representing necrosis. Hypervascular metastases — from renal cell carcinoma, neuroendocrine tumors, thyroid cancer, breast lobular carcinoma, and choriocarcinoma — are best detected in the arterial phase. Treatment response assessment (RECIST criteria) after chemotherapy requires careful measurement of target lesions on the same phase across serial CT examinations.