# Hepatic Mass Characterization: LI-RADS and Beyond

## LI-RADS Overview

### Purpose and Applicability

The Liver Imaging Reporting and Data System (LI-RADS) is a standardized system for interpreting and reporting CT and MRI observations in patients at risk for hepatocellular carcinoma (HCC). It applies only to patients at risk for HCC, including those with cirrhosis, chronic hepatitis B, or prior HCC, as well as liver transplant candidates and recipients. LI-RADS does not apply to patients under 18 years, patients without HCC risk factors, or those with cirrhosis from vascular causes such as Budd-Chiari syndrome or cardiac cirrhosis.

### At-Risk Population

The at-risk population includes patients with cirrhosis of any etiology except vascular causes, patients with chronic hepatitis B even without cirrhosis, and patients with current or prior HCC.

## LI-RADS Categories

### LR-1: Definitely Benign

This category encompasses definite cysts, definite hemangiomas, definite hepatic fat deposition or sparing, and definite vascular anomalies. No further workup is needed.

### LR-2: Probably Benign

This category includes probable cysts, probable hemangiomas, and distinctive nodules without HCC features. Follow-up is guided by clinical judgment.

### LR-3: Intermediate Probability of Malignancy

Observations that do not meet criteria for LR-4 or LR-5 and are not definitely or probably benign are classified as LR-3. Follow-up in 3-6 months is recommended.

### LR-4: Probably HCC

This category applies to observations with imaging features suggestive but not diagnostic of HCC. Multidisciplinary discussion should be undertaken, with consideration of biopsy or close follow-up.

### LR-5: Definitely HCC

Observations with imaging features diagnostic of HCC are classified as LR-5 and can be treated without tissue confirmation. This designation requires all major features at specific size thresholds.

### LR-M: Probably or Definitely Malignant, Not Specific for HCC

This category captures features suggesting non-HCC malignancy, such as intrahepatic cholangiocarcinoma, combined HCC-CCA, or metastasis. A **targetoid appearance** with rim arterial phase hyperenhancement, peripheral washout, and delayed central enhancement is characteristic. Biopsy is typically recommended.

### LR-TIV: Tumor in Vein

This category denotes definite enhancing soft tissue in the portal vein, hepatic vein, or IVC and is diagnostic of macrovascular invasion, usually from HCC.

## Major LI-RADS Features

### Arterial Phase Hyperenhancement (APHE)

APHE is defined as enhancement in the arterial phase that is unequivocally greater than the surrounding liver. **Non-rim APHE**, which may be diffuse, heterogeneous, or homogeneous, is characteristic of HCC. **Rim APHE**, or peripheral enhancement, favors non-HCC malignancy and leads to an LR-M categorization.

### Non-Peripheral Washout

Non-peripheral washout is a visually assessed reduction in enhancement relative to the liver in the portal venous or delayed phase. It is one of the strongest predictors of HCC.

### Enhancing Capsule

An enhancing capsule appears as a smooth, uniform, peripheral rim of enhancement in the portal venous or delayed phase, representing a true fibrous capsule around HCC.

### Threshold Growth

Threshold growth is defined as an increase in size of a mass by 50% or more within 6 months or less. It indicates malignancy regardless of other features.

### Size

Size is used in the diagnostic algorithm alongside the major features, with thresholds of 10 mm or 20 mm depending on the feature combination.

## LI-RADS Diagnostic Table (Simplified)

| Size | Non-rim APHE Only | APHE + 1 Additional Feature (washout OR capsule) | APHE + 2 Additional Features (washout AND capsule) | APHE + Threshold Growth |
|------|-------------------|--------------------------------------------------|---------------------------------------------------|------------------------|
| <10 mm | LR-3 | LR-4 | LR-4 | LR-4 |
| 10-19 mm | LR-4 | LR-4 | LR-5 | LR-5 |
| >=20 mm | LR-4 | LR-5 | LR-5 | LR-5 |

For an observation with non-rim APHE: a lesion 10 mm or larger with washout plus capsule qualifies as LR-5; a lesion 10 mm or larger with washout or capsule plus threshold growth also qualifies as LR-5; a lesion 20 mm or larger with washout or capsule (without threshold growth) qualifies as LR-5; a lesion 10 mm or larger with only one additional feature is LR-4; a lesion 10-19 mm with APHE only is LR-4; and a lesion smaller than 10 mm with APHE only is LR-3.

## Ancillary Features

### Favoring Malignancy

Ancillary features favoring malignancy include mild-moderate T2 hyperintensity, restricted diffusion, corona enhancement, mosaic architecture, nodule-in-nodule appearance, fat in the mass greater than the surrounding liver, and blood products within the mass.

### Favoring Benignity

Features favoring benignity include marked T2 hyperintensity (as seen in hemangioma), homogeneous marked T2 hyperintensity, undistorted vessels, signal paralleling blood pool, size stability for 2 years or more, and size reduction.

## Common Hepatic Lesions in Non-At-Risk Patients

### Hemangioma

Hemangioma is the most common benign hepatic tumor. On CT it appears as a well-defined hypodense lesion with **peripheral nodular discontinuous enhancement** in the arterial phase and progressive centripetal fill-in on portal venous and delayed phases. On MRI it is markedly T2 hyperintense (the "light bulb" bright sign) with the same enhancement pattern. A **flash-filling hemangioma** is a small hemangioma that enhances homogeneously in the arterial phase and can mimic a hypervascular metastasis; confirmation relies on marked T2 hyperintensity and stability over time.

### Focal Nodular Hyperplasia (FNH)

FNH is the second most common benign hepatic tumor and occurs predominantly in young women. On CT and MRI it shows homogeneous arterial enhancement with near-isointensity on the portal venous phase. The hallmark **central scar** is T2 hyperintense with delayed enhancement on MRI, which distinguishes it from the scar of fibrolamellar HCC, which is T2 hypointense. On the hepatobiliary phase with gadoxetate (Eovist), FNH is iso- to hyperintense because it retains the hepatobiliary contrast agent, and this is the key feature distinguishing it from adenoma.

### Hepatic Adenoma

Hepatic adenoma is a benign tumor associated with oral contraceptive use, anabolic steroids, and glycogen storage disease. It carries a risk of hemorrhage and malignant transformation, especially the beta-catenin mutated subtype. On CT and MRI it shows arterial enhancement with washout and may contain fat or hemorrhage. On the hepatobiliary phase it is typically hypointense because it does not retain gadoxetate in most subtypes. The recognized subtypes include HNF1-alpha mutated (steatotic with low malignant risk), inflammatory (with rim enhancement and telangiectatic features), and beta-catenin mutated (highest malignant risk).

### Common Hepatic Lesion Comparison

| Lesion | Key Enhancement Pattern | T2 Signal | Hepatobiliary Phase (Gadoxetate) | Distinguishing Feature |
|--------|------------------------|-----------|----------------------------------|----------------------|
| Hemangioma | Peripheral nodular, centripetal fill-in | Markedly hyperintense ("light bulb") | Hypointense | Marked T2 brightness, progressive fill-in |
| FNH | Homogeneous arterial, near-isointense PVP | Mild hyperintense scar | Iso- to hyperintense (retains contrast) | Central scar with delayed enhancement |
| Adenoma | Arterial enhancement, possible washout | Variable | Hypointense (does not retain) | Fat/hemorrhage; OCP association |
| HCC | Non-rim APHE, washout, capsule | Mildly hyperintense | Hypointense | Cirrhotic liver; portal vein invasion |

### Hepatocellular Carcinoma (HCC)

HCC is the most common primary hepatic malignancy and arises in the setting of cirrhosis. The classic imaging pattern consists of arterial phase hyperenhancement followed by portal venous or delayed washout with an enhancing capsule. Mosaic architecture, intralesional fat, and restricted diffusion support the diagnosis. HCC may invade the portal vein, producing tumor thrombus that appears as enhancing soft tissue within the portal vein (LR-TIV).

<image>A four-phase CT/MRI diagram showing the classic enhancement pattern of HCC. Four sequential images are arranged horizontally representing: (1) pre-contrast with a subtle lesion, (2) arterial phase showing diffuse hyperenhancement of the lesion (APHE, non-rim), (3) portal venous phase showing washout with the lesion now hypointense relative to the liver, and (4) delayed phase showing an enhancing capsule as a bright rim around the lesion. Each phase is labeled, and the major LI-RADS features (APHE, washout, capsule) are annotated with arrows. A text box indicates that this combination in a lesion >= 10 mm qualifies as LR-5 (definitely HCC).</image>

<image>A comparison panel showing enhancement patterns of four common liver lesions on dynamic contrast-enhanced MRI. Row 1 (Hemangioma): peripheral nodular enhancement in arterial phase, progressive centripetal fill-in on portal venous and delayed phases, marked T2 hyperintensity. Row 2 (FNH): homogeneous arterial enhancement, near-isointensity on portal venous phase, T2 hyperintense central scar with delayed enhancement, iso-hyperintense on hepatobiliary phase. Row 3 (Adenoma): heterogeneous arterial enhancement with intracellular fat (signal drop on opposed-phase), hypointense on hepatobiliary phase. Row 4 (HCC): non-rim APHE, washout, enhancing capsule, mild T2 hyperintensity. Key distinguishing features are highlighted in a sidebar.</image>

<image>A flowchart depicting the LI-RADS algorithm for observations with non-rim arterial phase hyperenhancement. The chart starts with a node labeled "Non-rim APHE present" and branches by size (less than 10 mm, 10-19 mm, >= 20 mm). Each size branch further subdivides based on the presence of additional major features (washout, capsule, threshold growth). Terminal nodes are color-coded to the corresponding LI-RADS category (LR-3 in yellow, LR-4 in orange, LR-5 in red). A separate branch shows that rim APHE or targetoid features lead to LR-M regardless of other features.</image>

## Clinical Pearls

LI-RADS applies only to patients at risk for HCC (those with cirrhosis or chronic hepatitis B), and its categories should not be applied to patients without these risk factors. LR-5 is diagnostic of HCC and can be treated without biopsy, making correct application of LI-RADS criteria critically important -- this is unique in oncologic imaging. Rim APHE (targetoid appearance) should be categorized as LR-M, not LR-5, because it favors non-HCC malignancy such as cholangiocarcinoma. On MRI with hepatobiliary agents (gadoxetate), FNH retains contrast and appears iso- to hyperintense in the hepatobiliary phase, while adenoma and most malignancies appear hypointense, making this the single most useful feature for differentiating FNH from adenoma. A "light bulb bright" lesion on T2-weighted MRI with peripheral nodular enhancement is virtually diagnostic of hemangioma in any clinical context. When a lesion shows arterial enhancement and washout in a non-cirrhotic patient, adenoma, hypervascular metastasis, or FNH should be considered rather than HCC.

## References

- ACR LI-RADS v2018 Core Document (American College of Radiology)
- Chernyak V, et al. "Liver Imaging Reporting and Data System (LI-RADS) Version 2018: Imaging of Hepatocellular Carcinoma in At-Risk Patients." *Radiology*, 2018
- Elsayes KM, et al. "LI-RADS: A Conceptual and Historical Review." *RadioGraphics*, 2019
- Grazioli L, et al. "Hepatic Adenomas: Imaging and Pathologic Findings." *RadioGraphics*, 2001
- Brancatelli G, et al. "Focal Nodular Hyperplasia: CT Findings with Emphasis on Multiphasic Helical CT." *Radiology*, 2001