Ultrasound Grading of Liver Steatosis: Attenuation and UDFF (WFUMB 2024)
Basics of Quantitative Steatosis Assessment
According to the WFUMB recommendations on multiparametric liver ultrasound (Ferraioli et al., 2024, Part 2 — Guidance on Liver Fat Quantification), ultrasound attenuation measurement methods are used for staging liver fat infiltration. The reference method in the cited works is magnetic resonance proton density fat fraction (MRI-PDFF) — see Cassinotto et al., Radiology 2022;305(2):353-61, where the diagnostic efficacy of attenuation was compared across three ultrasound systems.
Quantitative Thresholds: Attenuation Coefficient and UDFF
Summary of published cut-offs for ATI technology (Canon) and validation data for UDFF (ultrasound-derived fat fraction) relative to MRI-PDFF:
| Indicator | Threshold | Comment |
|---|---|---|
| ATI, steatosis ≥S1 | 0.59–0.69 dB/cm/MHz | range of published cut-offs (review 2025, PMC12149873) |
| ATI, steatosis ≥S2 | 0.67–0.78 dB/cm/MHz | — |
| ATI, steatosis ≥S3 | 0.68–0.86 dB/cm/MHz | — |
| UDFF, steatosis detection | >5% | against MRI-PDFF ≥5.5%: AUC 0.90, sensitivity 94.1%, specificity 63.6% (AJR 2022, PMID 35674351) |
| UDFF, gradations | 5.5% / 15.5% / 17.5% | mild / moderate / severe steatosis (PMID 37568072) |
| MRI-PDFF (reference) | steatosis ≥5.5%; S1 6.4–17.4%, S2 17.4–22.1%, S3 ≥22.1% | reference fat fraction gradations |
Key WFUMB caveat: absolute attenuation values depend on the manufacturer and method implementation (ATI, UGAP, ATT, etc.), so there are no universal inter-vendor thresholds — for monitoring trends, measurements should be performed on the same system, and interpretation should use cut-offs validated for the specific technology.
Qualitative B-mode Assessment
According to Diagnostic Ultrasound: Abdomen & Pelvis (Kamaya et al., 2022), qualitative signs of steatosis in B-mode include:
| Degree | Ultrasound Signs |
|---|---|
| Moderate steatosis | Slight blurring of hepatic vein walls; deep structures and diaphragm still well visualized |
| Severe steatosis | Increased parenchymal echogenicity relative to the kidney, poor differentiation of hepatic and portal veins, signal attenuation in deep liver sections; hepatomegaly possible |
Application in Pediatrics
Attenuation imaging has been studied in the pediatric population: Bulakci et al. (Pediatr Radiol 2023;53:1629-39) conducted a prospective quantitative assessment of liver steatosis in children.
Relation to Fibrosis Assessment
In MASLD/NAFLD, steatosis assessment is complemented by shear wave elastography. According to the LITMUS consortium meta-analysis (WFUMB Part 1, Ferraioli et al., 2024), for significant fibrosis, the AUC is 0.83 for VCTE, 0.86 for pSWE, 0.75 for 2D-SWE; for severe fibrosis — 0.85, 0.89, and 0.72 respectively.
Frequently asked questions
What reference method is used for validating ultrasound attenuation?
MRI-PDFF (proton density fat fraction on MRI) — used as a reference in a prospective comparison of three ultrasound systems (Cassinotto et al., Radiology 2022).
What attenuation thresholds correspond to steatosis grades?
For ATI, published cut-offs are: ≥S1 0.59–0.69, ≥S2 0.67–0.78, ≥S3 0.68–0.86 dB/cm/MHz; thresholds are vendor-dependent, apply values validated for your system.
What UDFF threshold indicates steatosis?
UDFF >5% identifies steatosis (MRI-PDFF ≥5.5%) with an AUC of 0.90 (sensitivity 94.1%, specificity 63.6%); gradation cut-offs are 5.5%/15.5%/17.5% for mild/moderate/severe steatosis.
Is attenuation imaging applicable in children?
Yes, a prospective quantitative assessment of liver steatosis has been conducted in the pediatric population (Bulakci et al., Pediatr Radiol 2023).
How accurate is elastography for fibrosis in MASLD?
According to the LITMUS meta-analysis: for significant fibrosis, AUC is 0.83 (VCTE), 0.86 (pSWE), 0.75 (2D-SWE); for cirrhosis — 0.89, 0.90, and 0.88 respectively (WFUMB Part 1, 2024).