US LI-RADS v2024: Observation Categories and VIS A/B/C
Purpose of US LI-RADS v2024
ACR LI-RADS Ultrasound Surveillance v2024 Core is applied for standardized ultrasound surveillance of patients at risk for hepatocellular carcinoma. It is not a diagnostic system for definitive lesion characterization, not a staging system, and not a replacement for multiphase CT, MRI, or CEUS in case of a positive result.
The protocol must always have two independent parts: the US observation category and the VIS assessment. The US category answers whether there is a finding requiring a shortened interval or diagnostic investigation. VIS answers how capable the ultrasound exam is in excluding small lesions.
Observation Categories: US-1, US-2, US-3
The US category is assigned based on the most suspicious finding. For practical reporting, it is important not to overload the text with descriptions of all benign changes but to clearly indicate the size, segment, and feature that determined the category.
| Category | Criterion | Recommended Action |
|---|---|---|
| US-1 Negative | No observations or only definitely benign observations | Continue routine ultrasound surveillance in 6 months |
| US-2 Subthreshold | Observation <10 mm, not definitely benign | Repeat ultrasound in 3–6 months |
| US-3 Positive | Observation ≥10 mm or new thrombus in a vein | Diagnostic contrast imaging per LI-RADS: multiphase CT, MRI, or CEUS based on clinical situation |
US-1 Negative: What to Include
US-1 is used when there are no lesions or the identified changes appear definitely benign. Typical examples in practice are a simple cyst, typical hemangioma, or focal fatty change with a confident ultrasound appearance. Such findings should not elevate the category solely due to the presence of a lesion.
The wording can be brief: “US-1 Negative. No focal lesions suspicious for HCC detected.” If visualization is limited, the US-1 category does not override VIS-B or VIS-C: a negative result with a poor window is less reliable.
US-2 Subthreshold: Small Lesion Less Than 10 mm
US-2 is an intermediate category for a lesion <10 mm that does not appear definitely benign. The logic of the category is not to send every patient with a subcentimeter nodule immediately for diagnostic CT or MRI but to check reproducibility and dynamics over a short interval.
The report should indicate the size in millimeters, location, and comparison with previous studies. ACR recommendation: repeat ultrasound in 3–6 months. If the lesion reaches the threshold of ≥10 mm, the category changes to US-3, and diagnostic contrast imaging is required.
US-3 Positive: When Ultrasound is Considered Positive
US-3 is assigned for an observation ≥10 mm that is not definitely benign, as well as for a new thrombus in a vein. This category does not mean HCC is proven; it means that screening ultrasound is no longer sufficient, and diagnostic contrast-enhanced methodology is needed.
The referral should ideally state that the study is required for diagnostic assessment per LI-RADS. Depending on availability and clinical factors, multiphase CT, MRI, or CEUS are used. The published CEUS LI-RADS has a different intent: it is intended for contrast-enhanced characterization of the observation, not for the primary ultrasound observation category.
VIS A/B/C: Separate Assessment of Visualization Quality
VIS is assigned to each ultrasound study independently of the US category. This is crucial: one can have US-1 VIS-C, meaning “no suspicious lesions seen, but the study is seriously limited.” Such wording is clinically more honest than simply “no pathology detected.”
| VIS Assessment | Meaning | Practical Interpretation |
|---|---|---|
| VIS-A | No or minimal limitations | Quality sufficient for standard ultrasound surveillance |
| VIS-B | Moderate limitations | Sensitivity may be reduced, especially for small lesions |
| VIS-C | Significant limitations | Likely significant reduction in sensitivity; consider alternative surveillance strategy |
What Leads to VIS-B and VIS-C
Visualization limitations are usually associated with marked parenchymal heterogeneity, coarse nodular remodeling, steatosis, attenuation of the ultrasound beam, rib shielding, high diaphragm position, meteorism, or insufficient acoustic window. VIS-B is used when there are limitations, but a significant portion of the liver is still assessable.
VIS-C is appropriate when limitations are so pronounced that the sensitivity of the study for detecting lesions is substantially reduced. In such a situation, a negative US-1 category should not be perceived as a reliable exclusion of small HCC. The ultrasound diagnostician should explicitly state that the study is technically limited and include VIS-C in the final line.
How to Combine US and VIS in One Report
The optimal final line contains both parts: “US-1 Negative, VIS-A” or “US-2 Subthreshold, VIS-B.” If there is a lesion <10 mm and simultaneously poor visualization, the category will be US-2, and the quality will be VIS-C. These parameters do not compete and do not replace each other.
The minimal structured protocol includes: US category, VIS assessment, size and location of significant finding, presence or absence of a new venous thrombus, comparison with previous ultrasound, and specific recommendation for interval or diagnostic imaging.
Typical Phrases for the Protocol
- US-1 VIS-A: “No suspicious focal liver lesions detected. Visualization without significant limitations. Recommended to continue ultrasound surveillance in 6 months.”
- US-2 VIS-B: “An 8 mm observation is noted in segment VI, without signs of a definitely benign lesion. Moderate visualization limitations. Recommended repeat ultrasound in 3–6 months.”
- US-3 VIS-A: “A 14 mm observation is noted in segment VIII. Result positive for ultrasound surveillance. Recommended diagnostic multiphase contrast imaging per LI-RADS.”
- US-1 VIS-C: “No lesions suspicious for HCC detected, however, liver visualization is significantly limited. Consider alternative surveillance strategy.”
Common Mistakes
The first mistake is not indicating VIS. Without VIS, the clinician does not understand how reliable the negative result is. The second mistake is categorizing any subcentimeter nodule as a positive result: according to ACR, the US-3 threshold for observation is ≥10 mm. The third mistake is using US LI-RADS as a diagnostic label for “HCC”: US-3 requires further contrast characterization, not establishing a diagnosis.
It is also important not to confuse US Surveillance LI-RADS v2024 with CEUS LI-RADS. The first system is responsible for grayscale ultrasound surveillance, the second for contrast-enhanced assessment of an already identified observation.
Frequently asked questions
Can US-1 be assigned with VIS-C?
Yes. The US category describes identified observations, while VIS describes the quality of visualization. US-1 VIS-C means no suspicious lesion is identified, but the sensitivity of the study is significantly limited.
What threshold converts a subcentimeter lesion to US-3?
The ACR threshold for a positive ultrasound observation is an observation ≥10 mm. An observation <10 mm that is not definitely benign is classified as US-2.
What is recommended for US-2?
Repeat ultrasound in 3–6 months. If the observation increases to ≥10 mm, the result becomes US-3, and diagnostic contrast imaging is required.