Examining the Current Landscape of Liver Assessment by means of Viscosity and Shear Wave Elastography: A State-of-the-Art Review

Abstract

Ultrasound plays a central role in the diagnosis, monitoring, and management of liver diseases. Assessing hepatic fibrosis is essential in chronic liver disease, and several diagnostic modalities are available. The gold standard remains percutaneous liver biopsy, an invasive method using a 16–18-gauge needle. A breakthrough came in 2003 with transient elastography (TE), a non-invasive technique that measures liver stiffness (kPa) via elastic wave propagation. Later, shear wave elastography (SWE), integrated into modern ultrasound systems, was developed to assess tissue elasticity. SWE generates shear waves (SWs) through acoustic radiation force, assuming tissues to be linearly elastic and homogeneous, and provides quantitative stiffness data. Recent evidence shows hepatic tissue is viscoelastic, with wave propagation varying by frequency. Quantifying viscosity remains a challenge. Fibrosis affects viscoelastic properties and shear wave speed (SWS), while necroinflammation predominantly alters the viscous component, influencing the shear wave dispersion slope (SWDS). This review provides an overview of ultrasound elastography methods, including stiffness and viscosity assessment, their physical principles, and clinical applications in hepatology.

Zusammenfassung

Ultraschall ist zentral für die Diagnose, Verlaufskontrolle und Therapie von Leber-Erkrankungen. Die Einschätzung des Fibrosegrads ist entscheidend im Management chronischer Leber-Erkrankungen. Goldstandard bleibt die invasive Leberbiopsie mit einer 16–18-Gauge-Nadel. Seit 2003 bietet die transiente Elastografie (TE) eine nicht invasive Alternative, die die Lebersteifigkeit in kPa durch Messung elastischer Wellen bestimmt. Später wurde die Scherwellen-Elastografie (SWE) entwickelt, heute in moderne Ultraschallgeräte integriert. SWE erzeugt laterale Scherwellen (SWs) mittels akustischem Strahlungskraft-Impuls und basiert auf der Annahme, dass Gewebe homogen und linear elastisch ist. Dies ermöglicht eine quantitative Bestimmung der Gewebesteifigkeit. Neuere Studien zeigen jedoch, dass das Lebergewebe viskoelastisch ist – Wellen unterschiedlicher Frequenz breiten sich mit variabler Phasengeschwindigkeit aus. Die exakte Messung der Viskosität bleibt methodisch anspruchsvoll. Eine Fibrose verändert die viskoelastischen Eigenschaften und die Scherwellen-Geschwindigkeit (SWS), während nekroinflammatorische Prozesse vor allem die viskose Komponente beeinflussen und damit den Scherwellen-Dispersionsslope (SWDS). Diese Übersicht beleuchtet die Methoden der Elastografie zur Beurteilung der Steifigkeit und Viskosität, deren physikalische Grundlagen sowie klinische Anwendungen in der Hepatologie.

Keywords

liver - ultrasound - viscosity - dispersion - noninvasive assessment

Introduction

Ultrasound plays a central role in the diagnosis, monitoring, and management of liver diseases. It is used to assess the liver parenchyma in acute and chronic conditions, to detect and characterize nodules, and to screen for hepatocellular carcinoma. Ultrasound is also essential in all phases of liver transplantation, from preoperative evaluation to post-operative monitoring of complications.

Liver fibrosis can be assessed by several methods, with percutaneous liver biopsy remaining the gold standard [1]. However, its invasiveness, risk of complications, and sampling variability limit its use [2] [3]. Elastography has emerged as a non-invasive alternative for assessing fibrosis [4] [5], with guidelines already being established [6] [7].

A major advance was achieved in 2003 with the introduction of transient elastography (TE) via Fibroscan (Echosens, Paris), which measures liver stiffness in kilopascals (kPa) [8] [9]. TE is effective for diagnosing cirrhosis and detecting significant fibrosis [10]. Shear wave elastography (SWE) is a newer ultrasound technique that evaluates tissue elasticity by means of shear waves (SWs) generated by acoustic radiation force [11] [12]. SWE has a higher accuracy than TE for severe fibrosis. However, the differentiation of intermediate stages remains challenging [13] [14] [15]. These limitations may be the result of the neglecting of viscoelastic properties, which could mask early fibrosis since elasticity may remain normal [16] [17].

Recent evidence highlights the fact that soft tissues exhibit complex, non-linear, viscoelastic, and poroelastic behavior. Linear models may oversimplify tissue characterization, thereby limiting diagnostic accuracy. Novel biomarkers such as viscosity and non-linearity are being explored for improved disease evaluation [18].

Viscosity provides biochemical insight distinct from elasticity [19]. It relates to shear wave (SW) speed dispersion and attenuation, both of which increase with frequency in viscous tissues [20]. Thus, dispersion analysis offers an indirect method to assess viscosity [21]. Measuring viscosity and non-linearity may allow more precise and operator-independent evaluation, thus reducing variability linked to probe pressure [18] [22].

This review provides a comprehensive overview of ultrasound elastography methods, including both stiffness and viscosity assessment, their physical principles, and their clinical relevance across different liver diseases.

Principles of Viscoelasticity and Non-linearity

The biological microscale consists mainly of the extracellular matrix (ECM) and active cells, particularly fibroblasts and smooth muscle cells [23]. The ECM preserves tissue integrity via a fibrous scaffold of collagen and elastin within a proteoglycan-rich matrix. It supports mechanical load transmission and fluid flow [22]. Collagen content significantly affects tissue shear modulus [24], and its composition varies by organ. In the liver, it represents about 5–10% of tissue content [25].

Elastin fibers, loosely linked to collagen, are randomly distributed and help prevent plastic deformation under repeated loads [26]. The gelatinous matrix, composed of water and proteoglycans (PGs), enhances stiffness and resists compression with fluid support [22]. PGs, with glycosaminoglycans (GAGs), attract water, generating osmotic pressure and acting as dampeners [27] [28]. ECM composition induces anisotropy due to fiber orientation and crosslinking. Shear wave propagation reflects shear modulus, sensitive to microstructure. However, variability across tissues and systems limits standardization.

Soft tissue biomechanics rely on 2 key assumptions: incompressibility due to high water content, and isotropy for simplified modelling [22]. Stress and strain are decomposed into volumetric and deviatoric components [29] [30]. These assumptions are mainly under low strain.

Tissues exhibit time-dependent mechanics influenced by viscosity, often quantified together with poroelasticity. Fluid movement under load delays strain, making deformation velocity key [31] [32]. Dispersion reflects ECM structure, affected by viscosity and frequency [33, 22].

Disregarding viscosity can bias elasticity assessments. Viscosity-sensitive imaging techniques may detect microstructural changes earlier than stiffness-based methods, thereby potentially reducing unnecessary biopsies [34].

Technical Considerations and Potential Pitfalls

In the absence of specific guidelines for shear wave dispersion (SWD), standard 2D-SWE protocols are recommended, as accurate SW speed measurement is essential for dispersion slope calculation [35]. The patient should lie in a supine position or in a slight left lateral decubitus position, with the right arm elevated to optimize the intercostal acoustic window. The transducer is placed intercostal, perpendicular to the liver capsule, avoiding refraction. B-mode imaging is adjusted for optimal acoustic and color gain settings.

Measurements are taken during breath-hold to minimize motion artifacts. Deep breathing or Valsalva maneuvers should be avoided, as they affect stiffness values [36]. The region of interest (ROI) is positioned 1.0–1.5 cm below the Glisson capsule to reduce artifacts and avoid subcapsular stiffness. A 3 x 3 cm ROI is typically used, with a circular 1.0 cm measurement ROI being placed to exclude vessels, hot spots, and drop-out areas.

While 10 SWE measurements are generally advised [37], some studies have used fewer readings with similar accuracy [38]. No specific quality criteria exist for SWD, but SW speed IQR can be used to assess data quality. An IQR/median <0.30 (kPa) or <0.15 (m/s) indicates acceptable data, suitable for dispersion analysis [37] ([Fig. 1]).

Fig. 1 Multiparametric ultrasound assessment of a liver graft. Shear wave elastography and shear wave dispersion (viscosity) imaging are shown.

Applications

Pre-clinical Studies

Sugimoto et al. evaluated shear wave speed (SWS) and shear wave dispersion slope (SWDS) in animal models with varying necroinflammation and fibrosis. SWS was significantly higher in the fibrosis model, while dispersion slope was higher in necroinflammation. Fibrosis correlated with SWS; necrosis correlated with dispersion slope [21].

Furuichi et al. also found higher SWDS in rat models with predominant necroinflammation versus fibrotic models [39].

Normal Values in Healthy Subjects

A prospective study including 131 healthy individuals reported a mean liver viscosity of 1.59 Pa.s. The measurement, based on the propagation and frequency variation of SWS, reflects tissue shear-wave dispersion. Viscosity values were influenced by liver stiffness, age, and BMI [40].

A recent study using the Aplio i800 evaluated the attenuation imaging coefficient (ATI), SWE, and SWD in children. ATI correlated with age; SWD depended on BMI SDS; SWE was influenced by abdominal wall thickness and sex. SWE and SWD were correlated, but ATI showed no relation to either. The study provided norm values and reference charts for ATI, SWE, and SWD in pediatrics, considering covariates like age, sex, and BMI [41]. SWDS in children appears slightly higher than in adults, likely due to age-related viscoelastic differences.

Assessment of Liver Fibrosis

SWE allows quantitative evaluation of liver stiffness, which closely correlates with fibrosis severity. Beyond stiffness, hepatic viscosity, estimated through shear wave dispersion slope, reflects viscoelastic properties influenced by necroinflammation and microstructural complexity. Although viscosity alone is less predictive than elasticity for fibrosis staging, combining both may enhance diagnostic accuracy, especially for assessing disease activity and early structural changes [20, 17, 42].

In one of the first studies on viscosity in liver disease, Chen et al. evaluated elasticity (kPa) and viscosity (Pa.s) using shear wave dispersion ultrasound vibrometry (SDUV) on an iU22 system (Philips Healthcare). Viscosity had a lower predictive value for fibrosis staging compared to liver stiffness (AUC 0.86 vs. 0.98 for F0–F1 vs. F2–F4 fibrosis). However, the study did not assess viscosity’s role in evaluating disease activity or steatosis [20].

Deffieux et al. measured stiffness, viscosity, and dispersion slope in patients with viral hepatitis-induced fibrosis using the Aixplorer system. Viscosity was less predictive of fibrosis than stiffness (AUC 0.76 vs. 0.89 for significant fibrosis; AUC 0.87 vs. 0.87 for cirrhosis) and was a poor predictor of activity and steatosis [17].

Ferraioli et al. prospectively studied 367 patients using TE as a reference. They found a significant correlation between 2D-SWE stiffness values and SWDS (Aplio i800, Canon), with SWDS differentiating between F0–1, F2, and F3–4 (9.8, 13.6, 17.5 [m/s]/kHz, respectively) [42].

Zhang et al. analyzed SWDS in 159 patients with chronic liver disease, including hepatitis B and NAFLD. Dispersion slope (DS) values varied with necroinflammation (P = .02) and fibrosis stage (P < .001). DS was linked to fibrosis only after subgroup analysis considering necroinflammation (P < .001). AUCs for DS detecting ≥F2 fibrosis, cirrhosis, and moderate-severe necroinflammation were 0.72, 0.71, and 0.64, respectively. Combining DS with stiffness did not improve performance after excluding DS [43].

Wang et al. examined 210 patients undergoing hepatectomy for hepatocellular carcinoma. They found 2D-SWE to be more accurate for advanced fibrosis (F ≥ 3 and F = 4) than SWDS (AUC 0.88 and 0.85 vs. 0.82 and 0.79). No significant differences were observed for mild to moderate fibrosis prediction. SWDS showed mild correlation with necroinflammatory activity [44]. A summary of key studies assessing liver stiffness, viscosity, and dispersion slope in liver fibrosis is shown in [Table 1]. An overview of the ultrasound systems and technologies available for assessing viscosity and elasticity is provided in Supplementary Tab. 1.

Table 1 Assessment of Liver Fibrosis.

Study	Technique	Study Participants	Main Findings	Performance
Chen et al. [20]	Shear wave dispersion ultrasound vibrometry iU22 Philips healthcare	Chronic liver disease	Viscosity < stiffness for fibrosis staging	AUC: 0.86 (viscosity), 0.98 (stiffness)
Deffieux et al. [17]	Supersonic shear imaging vs. FibroScan	120 patients (70 cases of viral hepatitis)	Stiffness: AUC 0.85–0.90 (similar to FibroScan) Viscosity: AUC 0.76 significant fibrosis; 0.87 cirrhosis; not correlated to steatosis or disease activity.	AUC: 0.76 vs. 0.89 (≥ F2); both 0.87 (F4) Supersonic shear imaging is effective for fibrosis staging; viscosity limited for disease activity and steatosis.
Ferraioli et al. [41]	2D-SWE – Aplio i800 ultrasound system Canon medical systems Transient elastography as reference standard	367 patients	Strong correlation between 2D-SWE and TE (r = 0.87, P < 0.0001) Diagnostic accuracy for fibrosis staging: AUC 0.97 for ≥ F2 and 0.97 for ≥ F3 Best SWE cutoffs: >7 kPa (≥ F2), >9 kPa (≥ F3) SWDS highly correlated with fibrosis (r = 0.85, P < 0.0001), but weakly with steatosis.	2D-SWE and SWDS accurate tools for fibrosis staging; SWDS does not correlate with steatosis.
Zhang et al. [42]	2D-SWE Aplio i900 Canon	159 patients (CHB-NAFLD)	Dispersion slope (DS) correlated with necroinflammation and fibrosis. DS provided little additional value compared to SWE alone.	AUC: 0.72 (≥ F2), 0.71 (F4), 0.64 (≥ A2)
Wang et al. [43]	2D-SWE & SWDS Canon Aplio i900	210 patients scheduled to undergo hepatectomy for HCC	SWD correlated with fibrosis and inflammation; with lower accuracy than SWE for severe fibrosis and cirrhosis.	AUCs: SWE (0.88 for ≥ F3, 0.85 for F4) vs. SWD (0.82 for ≥ F3, 0.79 for F4). SWE > SWDS for advanced fibrosis

Viral Hepatitis

In chronic viral hepatitis, the diagnostic value of SWDS and viscosity is limited. SWE reliably assesses liver stiffness, but SWDS and viscosity are less effective for fibrosis staging or necroinflammation assessment. Several studies report inconsistent correlations with histology, suggesting a restricted role for these parameters in viral hepatitis [17] [44].

Deffieux et al. found AUCs for viscosity of 0.60, 0.83, and 0.65 for inflammation grades A ≥ 1, A ≥ 2, and A = 3, respectively, while SWDS showed consistently lower AUCs [17]. Similarly, Zhang et al. concluded that DS did not improve diagnostic performance for fibrosis, necroinflammation, or steatosis in viral hepatitis [43].

Metabolic Liver Disease

SWE, SWDS, and attenuation coefficient (AC) form a multiparametric ultrasound approach for the non-invasive evaluation of metabolic-associated steatotic liver disease (MASLD). While SWE correlates with fibrosis severity, SWDS reflects necroinflammation-related viscoelastic changes, and AC quantifies steatosis. Combined, these parameters improve diagnostic accuracy with regard to identifying high-risk non-alcoholic steatohepatitis (NASH), providing better results than traditional imaging for staging and monitoring [21] [45] [46] [47].

Sugimoto et al. assessed SWE and SWDS in biopsy-proven NAFLD. Fibrosis stage correlated with SWE (P = 0.037), and inflammation grade with SWDS (P = 0.022). SWE better predicted fibrosis, while SWDS was more accurate for necroinflammation [21].

In 2021, Sugimoto et al. developed the “LAD-NASH score”, which integrates multiparametric ultrasound (including SWD) to detect high-risk NASH (NAS ≥ 4, fibrosis ≥ F2). They achieved AUCs of 0.86 (derivation, 111 patients) and 0.88 (validation, 102 patients) [45].

Jang et al. conducted a multicenter study on 132 patients with suspected NASH. SWDS showed AUCs of 0.86, 0.86, and 0.79 for inflammation grades ≥1, ≥2, and =3, respectively. They developed a NASH risk score combining AC, SWDS, and SWE, yielding an AUC of 0.94, sensitivity of 81%, specificity of 96%, PPV of 93%, and NPV of 88%. Multivariate analysis revealed SWDS was linked to inflammation, AC to steatosis, and SWE to both inflammation and fibrosis [46].

In 2024, Sugimoto et al. evaluated DS, AC, and SWS in MASLD. DS correlated with lobular inflammation (regression coefficient 0.06, P = .02; AUC 0.72, 95% CI: 0.63–0.82). AC showed excellent performance for steatosis (AUC 0.92, 95% CI: 0.87–0.97, P < .001). SWS was highly accurate for significant fibrosis (≥F2, AUC 0.91, 95% CI: 0.86–0.96, P < .001). DS was most associated with inflammation but was limited in advanced fibrosis or obesity. AC and SWS were robust markers for steatosis and fibrosis, respectively [47].

These findings support the conclusion that SWDS and SWE enhance diagnosis, staging, and monitoring in NASH beyond traditional imaging. Recently Jung et al. provided a report of first experiences with a multiparametric ultrasound platform (M-Ref) integrating viscosity, SWE, and B-mode, and showed feasibility with respect to assessing tissue alterations, although multicenter validation will be required to establish reference values across systems [48]. The key diagnostic findings and performance of SWE, SWDS, and AC in metabolic liver disease are summarized in [Table 2].

Table 2 Metabolic Liver Disease.

Study	Technique	Key Findings
Sugimoto et al. 2018 [21]	Shear wave elastography (SWE) – Shear wave dispersion slope (SWDS)	SWE correlated with fibrosis (P = 0.037) – SWDS with inflammation (P=0.022)
Sugimoto et al. 2021 [44]	Multiparametric US	LAD-NASH score: AUC 0.86 0.88 (validation) for high-risk NASH
Jang et al. 2022 [45]	SWE-SWDS – Attenuation coefficient (AC)	NASH risk score (SWDS, AC, SWE): AUC 0.94; SWDS = inflammation; AC = steatosis; SWE = fibrosis and inflammation
Sugimoto et al. 2024 [46]	Dispersion slope (DS) – AC – Shear wave speed (SWS)	DS AUC 0.72 (inflammation); AC AUC 0.92 (steatosis); SWS AUC 0.91 (fibrosis); DS has limited use in advanced fibrosis and obesity

Liver Transplantation

Nenadić et al. assessed viscosity to differentiate transplanted livers with severe rejection from those without. Using shear wave elastography attenuation (Amuse), they characterized viscoelastic parameters without rheological models, showing high concordance with biopsy findings [49].

Lee et al. evaluated 104 liver transplant recipients undergoing biopsies. SWDS (Aplio i900, Canon) was significantly higher in allograft damage vs. no damage (14.4 vs. 10.4 (m/s)/kHz, p ≤ 0.01). SWDS had higher diagnostic accuracy for allograft damage than liver stiffness (AUC 0.86 vs. 0.75). Fibrosis alone influenced stiffness, while both fibrosis and necroinflammation affected SWDS. Thus, SWDS may better assess allograft damage and inflammatory involvement [50].

Future Directions and Conclusions

Viscosity is increasingly recognized as a relevant non-invasive imaging biomarker, offering additional insight into diffuse liver pathology. However, suboptimal in vivo shear wave signal quality may contribute to the lower performance of viscosity compared to elasticity, as shown in some studies. Discrepancies across published data may result from various factors, including differences in imaging technologies, absence of standard guidelines, diverse study designs, and heterogeneous patient populations. Quantifying viscosity precisely remains challenging, with model selection directly impacting accuracy.

Fibrosis alters viscoelastic properties and primarily affects SWS, while necroinflammation, which modifies the viscous component, influences SWDS. Understanding the relationship between viscosity and elasticity across liver diseases is critically important. Large-scale prospective studies using standardized methodologies are essential to validate these emerging techniques in varied clinical settings. In this regard, CEUS perfusion imaging has also been highlighted by the EFSUMB guidelines [51] and further explored in clinical studies [52], thereby providing complementary functional information. MRI-based elastography remains a valuable reference standard, but broader use is constrained by methodological complexity, limited availability, and feasibility [53]. These considerations may be even more relevant in fragile patients, such as those in the early post-transplant period.

Contributorsʼ Statement

SOFIA Maria BAKKEN: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing - original draft, Writing - review & editing. Matteo Serenari: Formal analysis, Investigation, Methodology, Supervision, Validation, Visualization, Writing - review & editing. Giulia Fiorini: Data curation, Investigation, Methodology, Resources, Supervision, Visualization, Writing - review & editing. Jorge Ruiz-Rodríguez: Data curation, Visualization, Writing - review & editing. Andrea Boccatonda: Conceptualization, Formal analysis, Investigation, Methodology, Visualization, Writing - original draft. Carla Serra: Conceptualization, Data curation, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization.

Conflict of Interest

The authors declare that they have no conflict of interest.

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Publication History

Received: 08 May 2025

Accepted after revision: 24 September 2025

Accepted Manuscript online:
24 September 2025

Article published online:
16 January 2026