Quantitative Ultrasound Assessment of Liver Steatosis in MASLD/SLD: WFUMB 2024

Why Transition from Visual Echogenicity to Quantitative Ultrasound

Visual assessment of steatosis in B-mode remains subjective: the result depends on the device settings, transducer frequency, body habitus, scanning depth, fibrosis, and the physician's experience. In SLD/MASLD, this is critical because steatosis is an entry sign of the disease, and liver fat dynamics are used to monitor treatment and lifestyle changes.

WFUMB Guidelines/Guidance on Liver Multiparametric Ultrasound. Part 2: Guidance on Liver Fat Quantification, 2024 formalizes the quantitative approach: measuring the physical properties of tissue related to fat and specifying the method, device, units, and measurement conditions. The document does not replace clinical diagnosis of MASLD but sets an ultrasound protocol to objectify steatosis.

What Constitutes Steatosis: Basic Classification

The morphological boundary of steatosis is the accumulation of fat in at least 5% of hepatocytes. This scale is needed by the ultrasound physician as a clinical guide: quantitative ultrasound should detect the presence of steatosis and, if possible, stratify its severity, but not claim to accurately reproduce histology in every patient.

Main Quantitative Ultrasound Parameters

WFUMB highlights three groups of quantitative liver fat indicators. The first is the ultrasound attenuation coefficient, usually expressed in dB/cm/MHz. The more fat, the more the signal attenuates as it passes through the parenchyma.

The second group is the backscatter coefficient, which quantitatively assesses backscattering. On some platforms, it is converted into derived indices, including the ultrasound-derived fat fraction. The third is the speed of sound: the speed of sound in tissue changes with an increase in the fat component, so the indicator can be used as an additional marker of steatosis.

WFUMB's practical conclusion: these parameters cannot be mixed between platforms as interchangeable. The report should specify the method and units, not just 'steatosis by quantitative ultrasound.'

Attenuation Coefficient: How to Understand It

The attenuation coefficient is the most straightforward parameter for implementation because it is directly related to the familiar visual phenomenon of distal signal attenuation. But unlike subjective description, the device calculates a numerical value in a specified area of interest.

WFUMB emphasizes: there is no single universal threshold for the attenuation coefficient that can be safely transferred to all devices. The reasons are different frequencies, gain compensation algorithms, window depth, transducer design, and manufacturers' proprietary algorithms. Therefore, the cut-off should be validated for a specific system or taken from documentation/research specifically for that platform.

Backscatter and Ultrasound-Derived Fat Fraction

The backscatter coefficient assesses not the attenuation of the beam but the nature of ultrasound scattering in the parenchyma. The increase in fat vacuoles changes the acoustic heterogeneity of the tissue, which is reflected in the backscatter magnitude. Some systems provide not the BSC itself but a recalculated indicator, such as the ultrasound-derived fat fraction.

This format is convenient for clinicians because it resembles MRI-PDFF, but WFUMB warns against direct identification: ultrasound-derived fat fractions are the result of a specific model and are not a universal equivalent of MRI measurement. The protocol should specify the name of the indicator as it appears on the device.

Speed of Sound: When Useful

Speed of sound reflects the propagation speed of ultrasound in tissue. When the composition of the liver changes, including an increase in the fat component, the speed may change. The method is potentially useful as an independent parameter, especially in a multiparametric protocol where the result is not based on a single physical characteristic.

The limitation is the same: absolute values depend on the technology implementation. WFUMB considers speed of sound as a promising and clinically applicable parameter with validated equipment, but does not set a general diagnostic threshold for all systems.

Measurement Technique in the Ultrasound Room

Measurements are performed in the right liver lobe through an intercostal approach, selecting a homogeneous area of parenchyma. The ROI should not include large vessels, bile ducts, capsule, focal lesions, rib shadows, and areas of pronounced artifacts. It is important to minimize transducer pressure because compression changes the acoustic window and measurement depth.

To ensure comparability of studies, the same conditions should be maintained: the same device or platform, the same mode, similar ROI depth, standard patient position, and breath-hold without forced inspiration. If the patient is monitored dynamically, it is better to use the same device and the same quantitative module.

What to Write in the Report

The conclusion should be reproducible. The minimum set: device model, name of the quantitative mode, measured parameter, units, number of valid measurements or final value according to the system algorithm, ROI localization, factors that could reduce reliability. The formulation 'attenuation coefficient increased' without a number and without the technology name is insufficient.

Correct clinical conclusion: 'quantitative ultrasound signs of liver steatosis' with an indication that interpretation was performed according to the thresholds of a specific platform. If only visual B-mode signs are used, this should be separated from the quantitative conclusion.

Limitations and Pitfalls

Fibrosis, inflammation, congestion, pronounced obesity, narrow intercostal space, parenchymal heterogeneity, and poor acoustic window can affect the result. Quantitative steatosis does not replace fibrosis assessment: elastography and clinical risk indices are needed for this. Also, MASLD cannot be diagnosed solely by the amount of fat: the new SLD/MASLD nomenclature requires clinical context and cardiometabolic factors.

The main methodological error is using a published cut-off from one platform for another. WFUMB 2024 effectively shifts the task from 'finding a single number' to 'working according to a validated protocol of a specific method.'

Practical Algorithm

1. Confirm that the device has a quantitative module for liver fat: attenuation, backscatter/UDFF, or speed of sound.
2. Perform B-mode to exclude focal lesions, gross artifacts, and select a window.
3. Place the ROI in the right lobe outside vessels, capsule, and shadows.
4. Obtain valid measurements according to the manufacturer's prompts.
5. Interpret the result only according to the thresholds of this platform.
6. In the conclusion, indicate the number, units, method, and limitations.

This approach aligns with WFUMB's position: quantitative ultrasound should become a standard tool for objective assessment of steatosis in SLD/MASLD, but its result must be technically transparent and reproducible.