A liver lesion is a general medical term for any area of the liver that appears abnormal or distinct from the surrounding healthy tissue on an imaging scan. This finding is relatively common and often occurs incidentally when scans are performed for unrelated reasons. The term “lesion” itself is a descriptive label, not a diagnosis, that flags a focal abnormality for further investigation. While lesions can include masses or tumors, the majority of these findings are harmless and non-cancerous.
Defining Liver Lesions
Liver lesions are broadly categorized into two main structural types based on their composition: solid and cystic. This initial classification is the first step a radiologist takes in characterizing the abnormality seen on a scan. Knowing the internal structure helps narrow down the potential causes of the lesion.
Solid lesions are composed primarily of tissue, such as abnormal cells, blood vessels, or fibrous material. Examples include hemangiomas, focal nodular hyperplasia, hepatocellular adenomas, and malignant tumors like hepatocellular carcinoma or metastases.
Cystic lesions are sacs or cavities filled with fluid, which can be simple water, blood, or proteinaceous material. Simple hepatic cysts are the most common type and are benign, presenting as thin-walled structures. Complex cystic lesions may have internal walls called septations, solid components, or debris, which can raise suspicion and require more detailed analysis.
How Lesions Are Identified
Radiologists use several specialized imaging techniques to detect and characterize liver lesions, each measuring different physical properties of the tissue.
Ultrasound (US) is often the initial screening tool because it is non-invasive and uses high-frequency sound waves to create images. On an ultrasound, lesions are characterized by their echogenicity—how bright or dark they appear relative to the surrounding liver parenchyma.
Computed Tomography (CT) scans utilize X-rays to create cross-sectional images of the liver. Lesions are characterized by their density and are described as hypodense (darker) or hyperdense (brighter). CT is frequently performed with intravenous contrast material to observe how the lesion’s internal structures enhance, providing a dynamic map of its blood supply.
Magnetic Resonance Imaging (MRI) is considered the most powerful tool for soft tissue evaluation and does not use ionizing radiation. MRI characterizes lesions by their signal intensity using different pulse sequences. T2-weighted images are particularly useful as fluid-filled structures, like simple cysts, appear intensely bright, while most solid lesions appear less bright.
Visual Characteristics of Common Lesions
The visual analysis of a liver lesion relies on standardized terminology covering margins, internal signal, and enhancement pattern. Benign lesions often have sharply defined, smooth edges, while malignant lesions frequently show irregular or ill-defined margins with the adjacent liver tissue. The internal signal intensity on MRI or density on CT reflects the lesion’s composition, determining if it is predominantly fluid, fat, or solid tissue.
Simple hepatic cysts are the easiest to characterize, appearing anechoic (black) on ultrasound, having a uniform, low-density interior on CT, and showing extreme brightness on T2-weighted MRI due to their clear fluid content. Crucially, simple cysts show no enhancement after contrast injection because they lack a vascular supply.
Hepatic hemangiomas, the most common benign liver tumor, appear typically hyperechoic (bright) on ultrasound and are characterized by a unique enhancement pattern. After contrast injection, they show peripheral nodular enhancement in the early arterial phase, which gradually fills in toward the center of the lesion in the later phases, matching the surrounding blood vessels.
Focal Nodular Hyperplasia (FNH) is another common benign solid lesion, often presenting with a characteristic central stellate scar. On contrast-enhanced scans, the lesion demonstrates intense, rapid, and uniform enhancement during the arterial phase. It then becomes isointense (blends in) with the surrounding liver tissue in the later phases, though the central scar may show delayed enhancement.
Distinguishing Benign vs. Malignant Findings
Differentiating a harmless lesion from a cancerous one is the ultimate goal of the imaging workup. Certain visual characteristics, often called “red flags,” strongly suggest malignancy and require immediate attention. These include irregular, invasive margins, and signs of rapid growth over a short period.
A common malignant finding is the “washout” pattern, where a lesion enhances brightly in the early arterial phase but then quickly loses the contrast agent in the venous or delayed phases, making it appear dark relative to the liver. Malignant lesions may also present with a hypoattenuating or hypointense halo, a dark ring surrounding the enhanced portion of the mass.
When imaging findings are not definitive, the lesion is often termed “indeterminate.” For lesions with a low suspicion for malignancy, a period of surveillance involving repeat imaging, usually in three to six months, is recommended to monitor for growth or change. If the imaging features remain ambiguous or highly suspicious, a percutaneous biopsy is performed to obtain a tissue sample. Tissue sampling is considered the definitive method to establish a diagnosis by examining the cells under a microscope.