Ultrasound Quantification of Liver Steatosis: Attenuation Coefficient by WFUMB 2024
What the WFUMB 2024 Guidelines Changed
WFUMB Guidelines/Guidance on Liver Multiparametric Ultrasound. Part 2: Guidance on Liver Fat Quantification systematizes the quantitative ultrasound assessment of liver fat using built-in scanner tools. The focus has shifted from subjective B-mode features and CAP without full B-mode navigation to methods that operate in the grayscale image field: attenuation coefficient, backscatter indicators, and sound speed.
For the practicing physician, the main takeaway is simple: when suspecting MASLD, it is advisable not to limit the formulation to "diffuse liver changes of the steatosis type," but to specify the quantitative ultrasound parameter, method model, and measurement conditions. WFUMB emphasizes that there is currently no cross-platform interchangeability of thresholds.
Why the Attenuation Coefficient is More Important than Visual Assessment
Fat in the liver enhances the attenuation of the ultrasound beam. In B-mode, this manifests as increased echogenicity of the parenchyma, impaired visualization of the vascular pattern, diaphragm, and deep liver sections. However, such signs depend on the probe frequency, gain, body habitus, scanning depth, and physician experience.
The attenuation coefficient quantitatively describes how quickly the signal amplitude decreases with depth. In practice, this allows transitioning from a qualitative grading of "steatosis present/absent" to a number that can be used for dynamic monitoring, patient selection for further examination, and comparison with laboratory and elastographic data.
ATT, UGAP, ATI: Different Names for the Same Concept
The WFUMB guidelines discuss commercial implementations of the attenuation coefficient. Clinical protocols feature designations such as ATT, UGAP, ATI, and other proprietary names. All are based on attenuation assessment, but algorithms, calibration, depth range, quality control, and final thresholds differ.
Therefore, it is insufficient to simply write "attenuation coefficient increased" in the conclusion. Required are: the technology name, manufacturer or platform model, numerical value with unit of measurement, protocol compliance, measurement quality, and clinical interpretation within the validated thresholds of that specific system.
Steatosis Classification: What Can Be Transferred Between Methods
The morphological scale describes the proportion of hepatocytes with macrovesicular fat. It is not a direct ultrasound threshold for ATT/UGAP/ATI but remains the clinical language through which non-invasive methods are validated.
| Class | Morphological Criterion | How to Use in Ultrasound Quantification |
|---|---|---|
| S0 | <5% hepatocytes with fat | Absence of steatosis on the histological scale; the ultrasound parameter should not be interpreted outside the method's thresholds |
| S1 | 5–33% | Mild steatosis; the zone where subjective B-mode is particularly unreliable |
| S2 | 34–66% | Moderate steatosis; quantitative methods usually have higher clinical utility than visual assessment |
| S3 | >66% | Severe steatosis; it is important to consider beam penetration limitations and measurement quality |
Why CAP Thresholds Should Not Be Transferred to ATT
CAP is measured during transient elastography and is not the same as the attenuation coefficient built into a diagnostic ultrasound scanner. CAP does not use full B-mode navigation of the measurement site, whereas modern ultrasound methods allow placing ROI under visual control and avoiding vessels, capsules, focal changes, and pronounced artifacts.
WFUMB does not suggest replacing one CAP number with an "equivalent" ATT/UGAP/ATI number. These are different instrumental implementations, different algorithms, and different sets of validated data. The correct formulation: quantitative ultrasound assessment of liver fat can be used as an independent image-guided method when protocol compliance and platform-specific interpretation are ensured.
Standardized Measurement Protocol
Measurements are performed in the right liver lobe through an intercostal approach, in a position that provides a stable acoustic window. The area of interest should be in homogeneous parenchyma, away from large vessels, bile ducts, focal lesions, capsules, and reverberation zones. Deep sections with pronounced attenuation and the near field with artifacts are not suitable for reliable assessment.
Not only the number is important but also reproducibility. It is necessary to save the image with ROI, document the number of valid measurements, variability indicator, or other quality criteria provided by the manufacturer. If the system provides a reliability map, the measurement is accepted only in the area that meets the requirements of this map.
Confounders and Typical Errors
WFUMB separately emphasizes the need to consider factors that change the acoustic properties of the liver not only due to fat. These include inflammatory activity, edema, cholestasis, pronounced fibrosis, congestive hepatopathy, technically difficult acoustic window, obesity, narrow intercostal spaces, and parenchymal heterogeneity.
A common mistake is to interpret an increased attenuation coefficient as a diagnosis of MASLD without clinical context. Ultrasound quantification answers the question of the probability and severity of fat infiltration but does not establish etiology. In the conclusion, it is advisable to separate the measured phenotype "liver steatosis" from the clinical diagnosis, which requires assessment of metabolic factors, alcohol, drug causes, and comorbidities.
The Role of Backscatter and Sound Speed
Besides attenuation, WFUMB considers quantitative indicators of backscatter and sound speed. Fat infiltration changes not only beam attenuation but also microstructural scattering and acoustic speed in the tissue. These methods are promising, especially as part of a multiparametric protocol.
However, in terms of accumulated clinical data, the attenuation coefficient remains the most practically implemented tool. Backscatter and sound speed are advisable to use when they are available on the platform, validated for the liver, and accompanied by clear quality criteria.
How to Write a Conclusion
An optimal conclusion should be structured. Example: "Liver: diffuse increase in echogenicity. Quantitative fat assessment performed by ATI/UGAP/ATT method on the ... platform. Attenuation coefficient: ... dB/cm/MHz, measurement technically satisfactory/limited. Data correspond/do not correspond to steatosis according to the validated thresholds of the used system. Clinical-laboratory correlation and fibrosis assessment are recommended if risk factors are present."
If the quality is unsatisfactory, this should be clearly stated: "Quantitative steatosis assessment is technically limited, result not interpreted." Such wording is better than issuing a questionable number without comment.
Practical Algorithm for MASLD
- Evaluate B-mode: echogenicity, vascular pattern, depth attenuation, signs of cirrhosis or focal changes.
- Perform quantitative fat assessment using a built-in method with B-mode navigation.
- Check measurement quality according to system criteria.
- Interpret the result only according to thresholds validated for the specific technology.
- If there are risk factors for progression, supplement the examination with fibrosis assessment, as the amount of fat does not replace fibrosis staging.
The final position of WFUMB 2024: the attenuation coefficient is a practical and most mature ultrasound parameter for quantitative liver fat assessment, but it should not become a "new CAP" with universal thresholds. Reliability is ensured by B-mode navigation, standardized protocol, quality control, and platform-specific interpretation.
Frequently asked questions
Can CAP be replaced by the attenuation coefficient on an ultrasound scanner?
It can be used as an independent image-guided method for quantitative liver fat assessment, but CAP thresholds cannot be transferred to ATT/UGAP/ATI. These are different technologies with different algorithms and validated thresholds.
Is there a single ATT threshold for diagnosing steatosis?
According to WFUMB 2024, there is no universal threshold for all platforms. The value needs to be interpreted in units and according to the criteria of the specific system, considering measurement quality and clinical context.
Is quantitative fat assessment sufficient for managing a patient with MASLD?
No. Quantitative ultrasound assessment describes steatosis but does not replace etiological diagnosis and fibrosis assessment. Clinical-laboratory correlation and fibrosis staging are required if risk factors are present.