A liver lesion is an abnormal area or growth of tissue found within the liver. These lesions are often discovered incidentally when patients undergo imaging studies for unrelated conditions. While the term “lesion” can sound alarming, the majority of these growths are non-cancerous. Medical imaging provides detailed characteristics that help distinguish between benign and malignant conditions.
How Doctors Visualize Liver Lesions
The three main tools used to visualize liver lesions are ultrasound (US), computed tomography (CT), and magnetic resonance imaging (MRI). Ultrasound is frequently the initial screening method because it is widely available, low-cost, and non-invasive, using sound waves to assess tissue density, or echogenicity.
A CT scan involves a series of X-rays taken from different angles and often uses an iodine-based contrast agent injected into a vein. This contrast material allows doctors to observe how a lesion takes up and releases the agent during different phases of blood flow, which is a powerful clue to its nature.
MRI generally offers the most detailed visual information regarding soft tissue structure and composition. Like CT, MRI employs contrast agents, but it can use hepatocyte-specific agents that are taken up by normal liver cells but not usually by cancerous ones. Because MRI does not use ionizing radiation and provides superior soft tissue contrast, it is often the preferred modality for characterizing lesions found on initial screening.
Visual Characteristics of Non-Cancerous Lesions
The appearance of non-cancerous lesions often exhibits uniform and predictable visual patterns. Simple hepatic cysts are common, fluid-filled sacs that appear anechoic, or purely dark, on ultrasound. On MRI and CT, these cysts have thin, smooth, well-defined margins and show no internal enhancement after contrast injection.
Hemangiomas, which are benign tangles of blood vessels, are the most frequently encountered non-cancerous liver lesion. Their characteristic visual trait on contrast-enhanced scans is a discontinuous, nodular enhancement that begins at the periphery of the lesion. In the later phases of the scan, this contrast slowly and progressively fills in toward the center of the mass.
Focal nodular hyperplasia (FNH) is often characterized by a central scar that can be clearly seen on MRI. The lesion shows bright, homogeneous enhancement during the arterial phase of contrast injection. The fibrous central scar typically remains relatively dark during this early phase, only showing delayed enhancement in later images. The lesion generally becomes iso-intense, or visually similar to the surrounding liver tissue, during the portal venous phase.
Visual Characteristics of Cancerous Lesions
Malignant lesions, such as hepatocellular carcinoma (HCC) or metastases, often display visual traits that suggest aggressive growth. Unlike the smooth, well-defined borders of many benign lesions, cancerous masses frequently present with irregular or ill-defined margins. The presence of multiple lesions scattered throughout the liver is highly suggestive of metastatic disease.
A hallmark feature of HCC on dynamic imaging is its specific pattern of contrast behavior known as “washout.” Due to their unique blood supply, these tumors typically show marked hyper-enhancement during the early arterial phase. This is followed by the contrast rapidly leaving the tumor, causing it to appear darker than the surrounding normal liver tissue in the subsequent portal venous or delayed phases.
Other concerning visual traits include a heterogeneous internal structure, sometimes described as a mosaic pattern, which reflects the disorganized nature of malignant tissue. A smooth, enhancing peripheral rim, known as a capsule appearance, is another feature often seen in progressed HCC on delayed-phase imaging.
When Appearance Requires Further Diagnosis
Although modern imaging provides detailed visual clues, the appearance of a lesion is not always definitive, particularly for masses with atypical or indeterminate features. A physician may rely on a combination of imaging, patient history, and blood tests to reach a probable diagnosis.
For lesions that cannot be confidently characterized as benign based on imaging alone, a tissue sample is often required. This involves a liver biopsy, where a small amount of tissue is extracted using a needle, usually guided by ultrasound or CT imaging. A pathologist then examines this tissue under a microscope to confirm the exact nature of the cells. For small, incidental lesions that do not display concerning features, surveillance involves watching the lesion over time with follow-up imaging to monitor for any growth or change.