Initial experiences with dynamic,quality indicator-based multimodal tissue analysis (M-Ref) with parallel assessment of viscosity and shear wave elastography in liver parenchyma alterations 1

Abstract

BACKGROUND AND OBJECTIVE:

Modern ultrasound technology enables detailed tissue morphology analysis. A novel approach involves measuring viscoelasticity or viscosity. This pilot study investigates the potential of a novel high-end ultrasound system with dynamic quality indicators and the M-Ref tool.

METHODS:

Using a novel premium high-end ultrasound system (Resona A20/Mindray), comparative investigations were conducted on 52 patients, evaluating B-mode morphology, shear wave tissue elastography (STE), and viscosity (STVi) of the liver parenchyma. The study utilized a cohort of 25 healthy volunteers as a control group. The examinations were performed intercostally using a multifrequency convex probe SC7-1U (1–7 MHz) and breath-hold technique, ensuring that at least the highest or second-highest score in the dynamic quality control (5 stars) was achieved. Measurements were made in a color-coded region with a maximum 2 cm diameter and a depth of no more than 2 cm, avoiding bile ducts or blood vessels, at a depth up 2 cm from the liver capsule.

RESULTS:

A minimum of 10 measurements were taken for each parameter: liver steatosis (based on acoustic attenuation coefficient, USAT), viscosity (STVi), and shear wave elastography (STE) with correlation to fibrosis grade. Reference values for the control group were <1.4 m/s and <5 kPa for STE, with cirrhosis criteria defined as values >2.6 m/s and >15 kPa. For steatosis, values up to 0.5 dB/cm/MHz were considered normal, while values >0.8 dB/cm/MHz indicated fatty liver. Viscosity values <1.7 Pa.s were deemed normal, with >3.6 Pa.s indicating significant abnormality. Major causes of increased viscosity included severe steatosis, active hepatitis, hepatic tumors, or post-ablative states. In all cases, a high-quality indicator score (>93%) was achieved with at least 4/5 top reference markers in green.

CONCLUSIONS:

This pilot study confirms the comprehensive capabilities of multimodal imaging for tissue characterization using B-mode, elastography, and new techniques for assessing viscoelasticity. However, extensive multicenter evaluations will be needed to definitively establish reference values specific to the type of transducer and equipment used.

Keywords

Liver ultrasound quality indicators shear wave elastography viscosity multiparametric (M-Ref)

1 Background

Modern ultrasound technology enables detailed tissue morphology analysis, with high-resolution multifrequency probes allowing increasingly accurate B-mode assessments, detecting even small lesions a few millimeters in size [1, 2]. Additionally, the assessment of tissue stiffness has become feasible, with manual compression strain elastography and automated shear wave elastography (STE) established as key techniques [3]. These methods enable more precise tissue analyses, identifying homogeneity or irregular stiffness patterns, particularly in tumor-suspected areas, using color-coded imaging. Recent guidelines have integrated these techniques into liver fibrosis assessments, correlating shear wave elastography values with fibrosis stages up to cirrhosis [4].

Current advancements in ultrasound technology have enabled more accurate regional measurements of shear wave propagation, providing insights into tissue identification. Additionally, new dynamic 2D techniques, such as strain and shear wave elastography, can now be displayed dynamically and computed in greater detail. Automated quality indicators are also increasingly utilized to optimize examination results and ensure representative data [5].

A novel approach involves measuring viscoelasticity or viscosity, with early studies suggesting that these parameters may reflect inflammatory components or enhanced tumor neovascularization, contributing to changes in tissue properties [5, 6]. This opens new diagnostic possibilities previously reserved for specialized MRI sequences or histopathology.

While these newly developed ultrasound tissue analysis techniques are promising, it is essential to first assess their applicability and correlate findings with clinically relevant outcomes. In the clinical routine liver ultrasound evaluations, assessing the degree of liver steatosis is of critical importance. Comparative investigations such as the hepatorenal index (HRI) and dispersion measurements, initially limited to techniques like Fibroscan, have now been integrated into high-end ultrasound systems.

Worldwide non-alcoholic fatty liver disease (NAFLD), is the most prevalent chronic liver disease with a prevalence of about 25% of the population [7, 8]. Increasingly, efforts are being made to incorporate all available elastography techniques into a comprehensive and detailed analysis, encompassing not only fibrosis and cirrhosis but also steatosis, non-steatotic fibrosis (NASH), and inflammatory liver changes to non-invasively detect various forms of hepatitis using modern ultrasound technology.

This pilot study investigates the potential of a novel high-end ultrasound system with dynamic quality indicators and the M-Ref tool for simultaneous assessment of B-mode imaging, shear wave elastography, and viscosity concerning liver parenchymal alterations.

2 Material and methods

All examinations were performed by an experienced examiner (>3000 examinations/year, >20 years of experience) with an SC7-1 convex transducer (1–7 MHz) using the Acoustic Intelligence Technology platform (AIT) available on the new premium high-end ultrasound system (Resona A20, Mindray). The study included patients referred by the liver outpatient clinic or following tumor board decisions. Written informed consent was obtained for all contrast-enhanced ultrasound (CEUS) examinations, which were performed according to EFSUMB guidelines. A positive vote by the local Ethics Committee has been received.

Potential liver lesions were measured in B-mode, and macrovessel structures were assessed using color-coded Doppler sonography (CCDS) in high-resolution (HR) mode with the option of using glazing flow. The additional flow adapted microflow was documented using short cine loops and single images. All new flow options were used in the preset UMA (Ultra-Micro Angiography; it is an innovative Doppler technique able to improve the detection of very low flow.). In focal nodular hyperplasia (FNH), a spoke-wheel pattern was typically observed.

Liver parenchymal echogenicity was assessed using the hepatorenal index (HRI, ideally HRI = 1) and compared to healthy renal parenchyma. Using color-coded elastography, the shear wave velocity was measured within circular, individually adjustable regions of interest (ROI) in kPa or m/s. A target value of <1.4 m/s was established for healthy tissue, with focused measurements 2–4 cm deep from the capsule using point shear wave technology quantitatively (STQ). Ten measurements were taken whenever possible, avoiding vessels and bile ducts. A quality indicator with a maximum of five green stars was used to ensure optimal imaging. STQ values between 1.4 m/s and 2.6 m/s suggested progressive fibrosis (F1–F4), with higher values indicating cirrhosis.

Two additional tools were used to assess the degree of a potential steatosis, documenting potential artifacts and evaluating inflammatory changes via the liver tissue index (LTI). Measurements were taken at depths of 1–5 cm, avoiding vascular structures and motion artifacts. Overall, a comprehensive evaluation was possible using the acoustic attenuation coefficient of liver tissue (USAT). For fat measurements, up to 10 individual measurements were targeted. Ideal values were considered measurements up to 0.5 dB/cm/MHz. Measurement values indicating significant steatosis were possible with values >0.8 dB/cm/MHz.

The new M-Ref tool allowed simultaneous shear wave (STE), steatosis (USAT), and viscosity (STVi) assessments. Viscosity was measured in color-coded regions of interest (ROI), ranging from a homogeneous red to irregular orange. Reference values for normal viscosity coded as green (<1.7 Pa.s). As the values for viscoelasticity increased, the color coding was applied using yellow or red on a scale corresponding to a traffic light. Ten individual measurements were taken within a 5 mm ROI, in areas up to 3 x 4 cm, at depths of 2–5 cm, avoiding vessels and bile ducts. For tumor cases, viscosity measurements were also performed within tumor lesions for comparison with surrounding liver tissue [8].

In a final protocol, alongside the scales and a comprehensive tabular breakdown of measurements for fibrosis, fat changes, and viscosity, a graphical representation similar to a target was created. Ideal values were depicted in the green central area, slightly elevated values were shown as yellow rings, and highly pathological values were represented by red rings on the outer edge, with arrows originating from the green center. Whenever possible, the reference used was other imaging modalities, follow-up examinations, and in some cases, histology obtained through biopsy.

Contrast-enhanced ultrasound (CEUS) was employed to clarify non-cystic focal liver lesions, if needed. A bolus of 1 to 2.4 ml of sulfur hexafluoride microbubbles (SonoVue®, Bracco, Milan, Italy) with 10 ml sodium chloride was administered intravenously via the cubital vein. The primary lesion to be assessed was digitally recorded as cine loops in DICOM format for evaluation of microvascularization. Using low MI (mechanical index <0.15) technology, the penetration depth was adapted to the frequency via image quality (RES, GEN or PEN) functions. The up to 60-second-long inflow, with possible washout starting in the early portal venous phase, was analyzed by a perfusion analysis using false color and time-intensity curve (TIC) analysis.

For parametric CEUS, 10 seconds of arterial inflow were color-coded in false colors. Red and orange represented highly hypervascularized areas, yellow showed moderate perfusion, and green and blue indicated low perfusion. A regular pattern typically appeared in benign lesions, such as a nodular pattern from the edges in hemangiomas, homogenous patterns in fat redistribution disorders, a spoke-wheel appearance from the center in FNH, and a centripetal pattern in adenomas. The main criteria for malignant lesions included irregular arterial microvascularization in CEUS, often nearly chaotic in HCC, and an increasing washout from 3 to 5 minutes in the late phase. This washout kinetics was captured with short cine loops of 10 seconds, recorded every 60 seconds across the entire liver. Benign lesions typically showed a more continuous contrast agent accumulation in the lesion until it matched the surrounding tissue. Malignant lesions exhibited microshunts with irregular hypervascularization, a rapid rise in the curve, followed by a washout and flattening of the curve. Post-ablation defects or scars after tumor resection should appear avascular.

The statistical analysis was carried out as follows: All samples are described with mean and standard deviation. Two group comparisons were performed with student’s t-test. The differences between the four patient groups with regard to the frequency of liver parenchymal damage were determined using a one-way classification Chi-squared test. p-values lesser than 0.05 were considered significant.

3 Results

A total of 52 patients (38 male, 14 female) with liver parenchymal changes, aged 59.4±15.9 years, were compared to 25 apparently healthy volunteers (10 male, 15 female), aged 48.3±7.4 years (p < 0.0001). The echogenicity of the liver parenchyma could be assessed with the highest quality in all patients and volunteers (see Figs. 1–4). The RLB index was 100% for all healthy subjects. Among the 52 patients, the RLB index was also 100% in 50 patients, and 97% in one patient. In one patient, the distance to the liver was too large due to ascites, affecting the assessment.

Fig. 1

Case of normal liver stiffness, normal levels of fat tissue (0.41 dB/cm/MHz) and normal level of viscosity (0.84 Pa.s) and no fibrosis (1.4 m/s). M-Ref with color evaluation of the normal (green), higher (yellow) and pathological levels (red) of liver stiffness, fat evaluation and viscosity.

Fig. 2

Case of high levels of liver stiffness, normal levels of fat tissue (0.41 dB/cm/MHz) and higher levels of viscosity (3.01 Pa.s) in a case of Hepatitis C and severe liver fibrosis (2.59 m/s). M-Ref with color evaluation of the normal (green), higher (yellow) and pathological levels (red) of liver stiffness, fat evaluation and viscosity.

Fig. 3

Case of hepatocellular carcinoma (HCC) of the right liver lobe, normal levels of fat tissue (0.45 dB/cm/MHz), higher levels of viscosity (2.69 Pa.s) and particular liver fibrosis (1.7 m/s) in a case of Hepatitis C. B-Mode: irregular tumor, SR CEUS with irregular orange neovascularization, parametric CEUS capillary tumor changes red and yellow in false colors. M-Ref with color evaluation of the normal (green), higher (yellow) and pathological levels (red) of liver stiffness, fat evaluation and viscosity.

Fig. 4

Case of a stomach cancer metastasis of the right liver lobe, normal levels of fat tissue (0.35 dB/cm/MHz), higher levels of viscosity (2.98 Pa.s) and particular liver fibrosis (1.67 m/s). B-Mode: irregular tumor (arrows), UMA and SR CEUS with irregular orange neovascularization, CEUS capillary tumor changes with washout. M-Ref with color evaluation of the normal (green), higher (yellow) and pathological levels (red) of liver stiffness, fat evaluation and viscosity.

Table 1

Different categories for liver viscosity. Green being a healthy liver and red as a fibrosis

	Patients	Control group
Red	4	0
Orange	10	0
Yellow	21	1
Green	17	24

Interventions were performed in 5 patients. The average liver elasticity in patients was 1.72±0.43, with 23 values above 1.7 m/s, while no such cases were observed in the healthy volunteers, whose average elasticity was 1.39±0.11 m/s (p = 0.0003). The mean fat content in patients was 0.56±0.12 dB/cm/MHz, compared to 0.48±0.11 dB/cm/MHz in the healthy subjects (p = 0.0028). The liver tissue viscosity in patients was 2.08±0.93 Pa.s, while it was 1.16±0.31 Pa.s in the healthy group (p < 0.00001).

There is a statistically significant relationship of the parenchyma state between the group of healthy subjects and the patients (p < 0.0001).

Even within the patient group, the frequencies of the four classes differ significantly (p = 0.0042). The differences in the frequencies of liver parenchymal damage among the four patient groups were determined using a one-way classification chi-squared test.

4 Discussion

This pilot study confirms the comprehensive capabilities of multimodal imaging for tissue characterization using B-mode, elastography, and new techniques for assessing viscoelasticity. The M-Ref documentation system also allows for the integration of dynamic quality indicators, ensuring high image quality, which facilitates the comparability and monitoring of measurement values over time. Although liver biopsy histopathology remains the gold standard at present, increasingly precise techniques such as shear wave elastography, dispersion measurements, and the latest methods for analyzing fat deposits and viscosity changes using ultrasound enable ever more detailed analyses of tissue changes. Normal reference values are confirmed for tissue structures, with fibrosis measurements below 1.4 m/s or <5 kPa, liver fat changes <0.48 dB/cm/MHz, and viscosity <1.7 Pa.s [8].

Non-alcoholic fatty liver disease (NAFLD) is the most widespread chronic liver condition globally. There is an increasing presence of hepatological tissue changes with high viscoelasticity values in cases of severe steatosis, cirrhosis, or active inflammations, such as various forms of hepatitis, which can be better differentiated through the combination of evaluating the degree of fibrosis, fat alterations, and viscosity.

The fibrosis of the liver is a significant prognostic factor for chronic liver diseases. So far, a two-dimensional shear-wave elastography is widely recommended for assessing the tissue stiffness. However, inflammation caused by fibrosis might influence the elastography measurements. Hence, new imaging technologies are being developed for measuring the stiffness without any bias. For new techniques it is always vital to first collect reference values among healthy subjects.

The impact of medications and harmful substances, such as in NASH or non-steatotic fibrosis, can be better assessed and monitored over time with comprehensive ultrasound tissue analyses like M-Ref. Even in cases of tumors, parenchymal changes, particularly increased viscosity, can be better observed over time [7].

However, challenges arise in the comparability of different methods, such as liver fibrosis measurement, assessment of liver steatosis, or viscoelasticity, because currently, very different techniques with varying units are still under evaluation. Therefore, efforts are being made with guidelines, such as those from EFSUMB, to establish standards for high-quality ultrasound tissue examinations of the liver using new techniques. It is important to emphasize that modern multimodal liver diagnostics with ultrasound should also include contrast-enhanced ultrasound (CEUS) with supplementary perfusion analysis, such as time intensity curve (TIC) analysis or parametric approaches. CEUS becomes essential when the detection and characterization of liver tumors is required. Furthermore, assessing transplant changes, for example, after liver transplantation (LTX), necessitates not only a vascular analysis with CCDS in cases of serious complications but also perfusion analysis with CEUS, especially in cases of rejection reactions [10 –13].

So far only multiparametric quantitative MR-imaging is seen as an alternative to the gold standard liver biopsy in all non-alcoholic liver diseases. However, compared to ultrasound MRI is more expensive, has higher risk factors and contraindications and takes a longer time. Thus, it was recommended by the World Federation for Ultrasound and Medicine and Biology (WFUMB) as a standardized and reproducible point-of-care method for the detection of HS [14 –19]. There are also indications in the literature of differing viscosity values between benign and malignant liver tumors as well as other liver diseases [5, 20].

It should be noted critically that high-end techniques for tissue analysis with ultrasound are not yet universally available. In addition, multimodal tissue analysis with ultrasound requires experienced examiners, and reference values still need to be comprehensively defined regarding units and examination procedures. Nevertheless, stable measurement values are emerging for cirrhosis at >2.4 m/s or >50 kPa, for severe steatosis at >0.78 dB/cm/MHz, and for severe inflammatory reactions at >3.5 Pa.s. However, extensive multicenter evaluations will be needed to definitively establish reference values specific to the type of transducer and equipment used. It is also always important to better analyze borderline situations such as organ failure, rejection reactions, and multi-infarct syndrome. The further development of these techniques remains to be seen with excitement.

5 Limitations

Only a small group was surveyed. Further studies in the sense of multicenter studies are essential.

Conflicts of interest

The authors declare no existing conflicts of interest.

Author contributions

EMJ: Writing the manuscript, Developing the methodology and design, Review of the manuscript.

IW: Writing the manuscript, Review of the manuscript.

UK: Review and Editing of the manuscript, Carrying out the data collection.

FJ: Writing the manuscript, Carrying out the statistical analysis, Review of the manuscript.

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