Quantitative viscoelasticity mapping of human liver using supersonic shear imaging: preliminary in vivo feasibility study

Quality Assurance of Point and 2D Shear Wave Elastography through the Establishment of Baseline Data Using Phantoms

Ultrasound elastography has been available on most modern systems; however, the implementation of quality processes tends to be ad hoc. It is essential for a medical physicist to benchmark elastography measurements on each system and track them over time, especially after major software upgrades or repairs. This study aims to establish baseline data using phantoms and monitor them for quality assurance in elastography. In this paper, we utilized two phantoms: a set of cylinders, each with a composite material with varying Young's moduli, and an anthropomorphic abdominal phantom containing a liver modeled to represent early-stage fibrosis. These phantoms were imaged using three ultrasound manufacturers' elastography functions with either point or 2D elastography. The abdominal phantom was also imaged using magnetic resonance elastography (MRE) as it is recognized as the non-invasive gold standard for staging liver fibrosis. The scaling factor was determined based on the data acquired using MR and US elastography from the same vendor. The ultrasound elastography measurements showed inconsistency between different manufacturers, but within the same manufacturer, the measurements showed high repeatability. In conclusion, we have established baseline data for quality assurance procedures and specified the criteria for the acceptable range in liver fibrosis phantoms during routine testing.

Mechanical rheological model on the assessment of elasticity and viscosity in tissue inflammation: A systematic review

Understanding the extent of inflammation is crucial for early disease detection, monitoring disease progression, and evaluating treatment responses. Over the past decade, researchers have demonstrated the need to understand the extent of inflammation through qualitative or quantitative characterization of tissue viscoelasticity using different techniques. In this scientific review, an examination of research on the association between elasticity and Viscosity in diseases, particularly as tissue inflammation progresses, is conducted. A review of utilizing mechanical rheological models to characterize quantitative viscoelastic parameters of normal and inflamed tissues is also undertaken. Based on inclusion and exclusion criteria, we identified 14 full-text studies suitable for review out of 290 articles published from January 2000 to January 2024. We used PRISMA guidelines for the systematic review. In the review, three studies demonstrated the criterion used by the researchers in identifying the best rheological model. Eleven studies showed the clinical application of the rheological model in quantifying the viscoelastic properties of normal and pathological tissue. The review quantified viscoelastic parameters for normal and pathological tissue across various soft tissues. It evaluated the effectiveness of each viscoelastic property in distinguishing between normal and pathological tissue stiffness. Furthermore, the review outlined additional viscoelastic-related parameters for researchers to consider in future stiffness classification studies.

Ultrasound Viscoelastography by Acoustic Radiation Force: A State-of-the-Art Review

Ultrasound elastography (USE) is a promising tool for tissue characterization as several diseases result in alterations of tissue structure and composition, which manifest as changes in tissue mechanical properties. By imaging the tissue response to an applied mechanical excitation, USE mimics the manual palpation performed by clinicians to sense the tissue elasticity for diagnostic purposes. Next to elasticity, viscosity has recently been investigated as an additional, relevant, diagnostic biomarker. Moreover, since biological tissues are inherently viscoelastic, accounting for viscosity in the tissue characterization process enhances the accuracy of the elasticity estimation. Recently, methods exploiting different acquisition and processing techniques have been proposed to perform ultrasound viscoelastography. After introducing the physics describing viscoelasticity, a comprehensive overview of the currently available USE acquisition techniques is provided, followed by a structured review of the existing viscoelasticity estimators classified according to the employed processing technique. These estimators are further reviewed from a clinical usage perspective, and current outstanding challenges are discussed.

Diagnostic utility of two-dimensional shear wave elastography in nephropathic dogs and its correlation with renal contrast-enhanced ultrasound in course of acute kidney injury

Aims of our study were to evaluate the feasibility and diagnostic value of two-dimensional shear wave elastography in dogs with acute kidney injury, chronic kidney disease, and acute on chronic kidney disease, its correlation with renal functional (creatinine, urea), and prognostic parameters (serum calcium-phosphorus product, urinary output), and with contrast-enhanced ultrasound (qualitative and quantitative evaluation). The study was prospective. A group of healthy (Group A) and a group of nephropathic dogs (Group B) were included. Shear wave elastography was performed on the left kidney of the subjects of both groups; contrast-enhanced ultrasound was performed only in dogs with acute kidney injury and acute on chronic kidney disease. Sixty-four dogs were included (Group A, n=24; Group B, n=40). The renal stiffness values were significantly higher in Group B than Group A; optimal cut-off stiffness values for detection of renal pathology were: ≥1.51 m/sec (area under the curve, 0.84; 95% confidence interval 0.74-0.94) and ≥6.75 kPa (area under the curve, 0.84; 95% confidence interval 0.73-0.94). For contrast-enhanced ultrasound, a significant positive correlation was found between renal stiffness, area under the curve, and wash-out area under the curve values of cortex quantitative analysis. No correlations were found between renal stiffness and renal functional and prognostic parameters. Shear wave elastography showed diagnostic utility to detect renal abnormalities in dogs with acute kidney injury, chronic kidney disease and acute on chronic kidney disease, however, it could not differentiate between these different nephropathies.

Multi-Frequency 3D Shear Wave Absolute Vibro-Elastography (S-WAVE) System for the Prostate

This article describes a novel system for quantitative and volumetric measurement of tissue elasticity in the prostate using simultaneous multi-frequency tissue excitation. Elasticity is computed by using a local frequency estimator to measure the three-dimensional local wavelengths of steady-state shear waves within the prostate gland. The shear wave is created using a mechanical voice coil shaker which transmits simultaneous multi-frequency vibrations transperineally. Radio frequency data is streamed directly from a BK Medical 8848 transrectal ultrasound transducer to an external computer where tissue displacement due to the excitation is measured using a speckle tracking algorithm. Bandpass sampling is used that eliminates the need for an ultra-fast frame rate to track the tissue motion and allows for accurate reconstruction at a sampling frequency that is below the Nyquist rate. A roll motor with computer control is used to rotate the transducer and obtain 3D data. Two commercially available phantoms were used to validate both the accuracy of the elasticity measurements as well as the functional feasibility of using the system for in vivo prostate imaging. The phantom measurements were compared with 3D Magnetic Resonance Elastography (MRE), where a high correlation of 96% was achieved. In addition, the system has been used in two separate clinical studies as a method for cancer identification. Qualitative and quantitative results of 11 patients from these clinical studies are presented here. Furthermore, an AUC of 0.87±0.12 was achieved for malignant vs. benign classification using a binary support vector machine classifier trained with data from the latest clinical study with leave one patient out cross-validation.

Risk Stratification in Primary Biliary Cholangitis

Primary Biliary Cholangitis (PBC) is a chronic cholestatic liver disease with a heterogeneous presentation, symptomatology, disease progression, and response to therapy. The current risk stratification assessment, aimed at identifying patients with a higher risk of disease progression, encompasses an in-depth analysis of demographic data, clinical and laboratory findings, antibody profiles, and the evaluation of liver fibrosis using both invasive and noninvasive techniques. Treatment response scores after one year of therapy remain to date a major factor influencing the prognosis of PBC patients. While the initial therapeutic approach with ursodeoxycholic acid (UDCA) is universally applied, new second-line treatment options have recently emerged, with many others under investigation. Consequently, the prevailing one-size-fits-all approach is poised to be supplanted by tailored strategies, ensuring high-risk patients receive the most appropriate treatment regimen from diagnosis. This will require the development of a risk prediction model to assess, at the time of diagnosis, the course, outcome, and response to first and additional treatments of PBC patients. This manuscript provides a comprehensive overview of the current and emerging tools used for risk stratification in PBC and speculates on how these developments might shape the disease landscape in the near future.

Evaluation of Tumor Fibrosis in Pancreatic Ductal Adenocarcinoma by 2-D Shear Wave Elastography: A Pilot Study

OBJECTIVE

The aim of the work described here was to investigate the association of the stromal proportion with the elasticity obtained by 2-D shear wave elastography (SWE) and the diagnostic value of elasticity in evaluating tumor stromal fibrosis in pancreatic ductal adenocarcinoma (PDAC).

METHODS

Patients who met inclusion criteria underwent pre-operative 2-D SWE examination and intra-operative determination of hardness by palpation from July 2021 to November 2022, and the post-operative specimens were used to evaluate pathological features including the tumor stromal proportion. A receiver operating characteristic curve was created to evaluate its diagnostic value in differentiating the degree of tumor stromal fibrosis.

RESULTS

The 2-D SWE measurements in pancreatic lesions were successful in 62 of 69 patients (89.9%). A total of 52 eligible participants were enrolled for subsequent correlation analysis. Elasticity correlated well with tumor stromal proportion (r_s = 0.646) and number of tumor cells (r_s = -0.585) in PDAC. Moreover, pancreatic elasticity determined by 2-D SWE, palpation-determined hardness and tumor stromal proportion were well correlated with each other. Two-dimensional SWE could clearly distinguish mild and severe stromal fibrosis, and its diagnostic performance was better than that determined by palpation even though the difference was not statistically significant (p = 0.103).

CONCLUSION

The elasticity of PDAC obtained using 2-D SWE was closely related to stromal proportion and tumor cellularity and could clearly be used to diagnose the degree of stromal fibrosis, which indicates that 2-D SWE can be a non-invasive predictive imaging biomarker in personalization of therapy and monitoring of treatment.

Evaluating Variability of Commercial Liver Fibrosis Elastography Phantoms

OBJECTIVE

Liver fibrosis has been found to increase the mechanical stiffness of the liver. To mimic different stages of liver fibrosis, commercially available phantoms (Model 039, CIRS, Inc.) have been produced for clinical quality assurance and research purposes. The purpose of this study was to investigate the mechanical property variability of the phantoms in two lots of CIRS Model 039 phantoms.

METHODS

Each lot consisted of phantoms of four stiffness types, and there were 8-10 phantoms of each type. Shear wave elastography measurements were conducted on each phantom at 10 different angles. Group velocity measurements and phase velocity curves were calculated for every SWE acquisition. Multilevel functional principal component analysis (MFPCA) was performed on phase velocity data, which decomposes each phase velocity curve into the sum of eigenfunctions of two levels. The variance of the component scores of levels 1 and 2 were used to represent inter-phantom and intra-phantom variability, respectively. The 95% confidence intervals of phase velocity in a phantom type were calculated to reflect curve variability.

DISCUSSION

The standard deviations of the group velocity for phantoms of any type were less than 0.04 and 0.02 m/s for lots 1 and 2, respectively. For both lots, in every type, the phase velocity curves of most individual phantoms fall within the 95% confidence interval.

CONCLUSION

MFPCA is an effective tool for analyzing the inter- and intra-phantom variability of phase velocity curves. Given the known variability of a fully tested lot, estimation of the variability of a new lot can be performed with a reduced number of phantoms tested.

Clinical correlation of 2D shear wave elastography findings in children with type 1 diabetes mellitus without autoimmune thyroiditis

OBJECTIVES

The aim of study was to evaluate the 2D shear wave sonoelastography (SWE) findings of the thyroid gland in children with type 1 diabetes mellitus (T1DM) with normal gray-scale findings and without thyroid autoimmunity (AIT) and obtain data that will be useful for the early detection of glandular involvement.

METHODS

The study included 46 T1DM patients (mean age: 11.28 ± 3.3 years) and 46 healthy children (mean age: 12.01 ± 3.8 years) as the control group. The thyroid gland mean elasticity value was obtained as kPa and compared in groups. A correlation was investigated between elasticity values and age at diabetes, serum free T4, thyroid stimulating hormone (TSH), anti-thyroglobulin, anti-tissue peroxidase, and hemoglobin A1c values.

RESULTS

No difference was found between T1DM patients and the control group in the thyroid 2D SWE evaluation (the median kPa value: 17.1 (10.2) in the study group and 16.8 (7.0) in the control group) (p=0.15). No significant correlation was found between 2D SWE kPa values and age at diagnosis, serum free T4, TSH, anti-thyroglobulin, anti-tissue peroxidase, and hemoglobin A1c levels in T1DM patients.

CONCLUSIONS

Our study showed that the elasticity of the thyroid gland in T1DM patients without AIT was not affected differently from that of the normal population. If 2D SWE is used in routine follow-up in T1DM patients before the development of AIT, we think that it will be useful in the early detection of thyroid gland affections and AIT, and long-term comprehensive studies in this direction will contribute to the literature.

How to Identify Advanced Fibrosis in Adult Patients with Non-Alcoholic Fatty Liver Disease (NAFLD) and Non-Alcoholic Steatohepatitis (NASH) Using Ultrasound Elastography-A Review of the Literature and Proposed Multistep Approach

Non-alcoholic fatty liver disease (NAFLD), and its progressive form, non-alcoholic steatohepatitis (NASH), represent, nowadays, real challenges for the healthcare system. Liver fibrosis is the most important prognostic factor for NAFLD, and advanced fibrosis is associated with higher liver-related mortality rates. Therefore, the key issues in NAFLD are the differentiation of NASH from simple steatosis and identification of advanced hepatic fibrosis. We critically reviewed the ultrasound (US) elastography techniques for the quantitative characterization of fibrosis, steatosis, and inflammation in NAFLD and NASH, with a specific focus on how to differentiate advanced fibrosis in adult patients. Vibration-controlled transient elastography (VCTE) is still the most utilized and validated elastography method for liver fibrosis assessment. The recently developed point shear wave elastography (pSWE) and two-dimensional shear wave elastography (2D-SWE) techniques that use multiparametric approaches could bring essential improvements to diagnosis and risk stratification.

Hepatocellular Carcinoma in Patients with Nonalcoholic Fatty Liver Disease: The Prognostic Role of Liver Stiffness Measurement

Nonalcoholic fatty liver disease (NAFLD), which is nowadays the most common etiology of chronic liver disease, is associated with an increased risk of hepatocellular carcinoma (HCC), with or without cirrhosis. Owing to the high prevalence of NAFLD worldwide, it becomes crucial to develop adequate strategies for surveillance of HCC and new prediction models aiming at stratifying NAFLD population for HCC risk. To this purpose, several noninvasive tests (NITs) have been proposed in the several last years, including clinical parameters, serum biomarkers, and imaging techniques. Most of these tools are focused on the assessment of liver fibrosis. Both ultrasound (US) elastography (especially transient elastography) and magnetic resonance (MR) elastography have been evaluated to estimate HCC risk in NAFLD patients. Recently, the American Association for the Study of Liver Diseases (AASLD) and the European Association for the Study of the Liver (EASL) include these techniques among the recommended NITs for the assessment of liver fibrosis. The aim of this review is to summarize the most recent data on the role of US and MR elastography in HCC risk stratification in patients with NAFLD.

Shear Wave Dispersion Slope Measured with Shear Wave Dispersion Imaging Is Associated with Variceal Hemorrhage in Cirrhotic Patients

Background and Objectives: Portal hypertension (PH), as the main consequence of cirrhosis, leads to the development of gastroesophageal varices (GEVs). Variceal hemorrhage (VH) caused by the rupture of GEVs is a life-threatening emergency. Thus, the prediction of VH risk is considerably important. Our pilot study aimed to identify the risk factors of variceal hemorrhage (VH) in cirrhosis. Materials and Methods: Cirrhotic patients were prospectively included and divided into two groups according to the presence or absence of VH. Conventional ultrasound and shear wave dispersion (SWD) imaging were conducted to detect the portal vein diameter, spleen diameter, ascites, liver stiffness (LS) and shear wave dispersion slope (SWDS). The laboratory tests were recorded, including platelets (PLT), alanine transaminase (ALT), aspartate aminotransferase (AST), total bilirubin (TBIL) and albumin (ALB). The risk factors of VH were screened using univariate analyses and identified using multivariate logistic regression. The ROC curves were used to assess diagnostic accuracy. Comparisons between AUCs were performed using the Delong method. Results: Sixty-five patients with 22 VHs were finally included. The SWDS, spleen diameter and ascites were identified as independent risk factors for VH. The SWDS showed good performance for diagnosing VH (AUC = 0.768, 95% CI: 0.647−0.864), and sensitively identified 95.5% (95% CI: 77.2%−99.9%) of patients with VH. Including the three risk factors in multivariate logistic regression, we obtained a formula for diagnosing VH: −20.749 + 0.804 × SWDS + 0.449 × spleen diameter + 1.803 × ascites (no ascites = 0, ascites = 1). Comparison of AUCs revealed that the formula (AUC = 0.900, 95% CI: 0.800−0.961) performed better than LS, SWDS, and spleen diameter in diagnosing VH (p < 0.001; p < 0.05; p < 0.05). Conclusions: SWDS is a sensitive parameter for assessing the risk of VH. Combining the SWDS, spleen diameter and ascites resulted in good diagnostic accuracy.

A Prognostic Nomogram with High Accuracy Based on 2D-SWE in Patients with Acute-on-chronic Liver Failure

BACKGROUND AND AIMS

Acute-on-chronic liver failure (ACLF) is associated with very high mortality. Accurate prediction of prognosis is critical in navigating optimal treatment decisions to improve patient survival. This study was aimed to develop a new nomogram integrating two-dimensional shear wave elastography (2D-SWE) values with other independent prognostic factors to improve the precision of predicting ACLF patient outcomes.

METHODS

A total of 449 consecutive patients with ACLF were recruited and randomly allocated to a training cohort (n=315) or a test cohort (n=134). 2D-SWE values, conventional ultrasound features, laboratory tests, and other clinical characteristics were included in univariate and multivariate analysis. Factors with prognostic value were then used to construct a novel prognostic nomogram. Receiver operating curves (ROCs) were generated to evaluate and compare the performance of the novel and published models including the Model for End-Stage Liver Disease (MELD), MELD combined with sodium (MELD-Na), and Jin's model. The model was validated in a prospective cohort (n=102).

RESULTS

A ACLF prognostic nomogram was developed with independent prognostic factors, including 2D-SWE, age, total bilirubin (TB), neutrophils (Neu), and the international normalized ratio (INR). The area under the ROC curve (AUC) was 0.849 for the new model in the training cohort and 0.861 in the prospective validation cohort, which were significantly greater than those for MELD (0.758), MELD-Na (0.750), and Jin's model (0.777, all p <0.05). Calibration curve analysis revealed good agreement between the predicted and observed probabilities. The new nomogram had superior overall net benefit and clinical utility.

CONCLUSIONS

We established and validated a 2D-SWE-based noninvasive nomogram to predict the prognosis of ACLF patients that was more accurate than other prognostic models.

Fibroscan® and Shear Wave correlated well in hepatic fibrosis evaluation of patients with chronic liver diseases "in real life situation"

BACKGROUND

A new noninvasive medical device based on ultrasound elastography such as the Shear Wave Elastography (SWE) was designed in order to measure the liver hardness. The purpose of this work was to evaluate the correlation of the results of the liver elasticity measurements obtained by Fibroscan® (FS) and SWE for patients with chronic liver diseases.

METHODS

Between January and October 2017, the patients who were followed during this period of time underwent noninvasive assessments of liver fibrosis by SWE in the intercostal spaces during abdominal ultrasound procedures and/or FS. The correlation between FS and SWE was estimated and tested at a 0.05 significance level.

RESULTS

Four hundred and seventy-six patients were included in this study. The main etiologies of chronic liver disease were non alcoholic fatty disease (NAFLD), chronic viral hepatitis B (HBV) and chronic viral hepatitis C (HCV). All patients underwent a SWE and 167 among them underwent a FS procedure. The patients who were followed revealed a median FS measurement of 5.80 kpa (Q25 = 4.90 kPa; Q75 = 8 kPa) and a median SWE measurement of 7.00 kPa (Q25 = 6.10 kPa; Q75 = 8.10 kPa). We could observe a significant correlation between the FS and SWE measurements (0.49; P = .001) in the total cohort. The average absolute difference between the measurements of these 2 methods was of 2.54 kPa (sd = 3.39). There was no significant correlation for patients with NAFLD no matter whether they presented with signs of suspected non alcoholic steatohepatitis (NASH) or not (R = 0.20; P = .108). All patients intending to perform the examination were able to undergo the SWE, allowing 33.3% of the patients who failed the FS to have a noninvasive evaluation of their fibrosis.

CONCLUSION

The SWE technique proved to be as efficient as the FS one for the evaluation of the liver fibrosis in real life situation.

Comprehensive experimental assessments of rheological models' performance in elastography of soft tissues

Elastography researchers have utilized several rheological models to characterize soft tissue viscoelasticity over the past thirty years. Due to the frequency-dependent behavior of viscoelastic parameters as well as the different techniques and frequencies employed in various studies of soft tissues, rheological models have value in standardizing disparate techniques via explicit mathematical representations. However, the important question remains: which of the several available models should be considered for widespread adoption within a theoretical framework? We address this by evaluating the performance of three well established rheological models to characterize ex vivo bovine liver tissues: the Kelvin-Voigt (KV) model as a 2-parameter model, and the standard linear solid (SLS) and Kelvin-Voigt fractional derivative (KVFD) models as 3-parameter models. The assessments were based on the analysis of time domain behavior (using stress relaxation tests) and frequency domain behavior (by measuring shear wave speed (SWS) dispersion). SWS was measured over a wide range of frequency from 1 Hz to 1 kHz using three different tests: (i) harmonic shear tests using a rheometer, (ii) reverberant shear wave (RSW) ultrasound elastography scans, and (iii) RSW optical coherence elastography scans, with each test targeting a distinct frequency range. Our results demonstrated that the KVFD model produces the only mutually consistent rendering of time and frequency domain data for liver. Furthermore, it reduces to a 2-parameter model for liver (correspondingly to a 2-parameter "spring-pot" or power-law model for SWS dispersion) and provides the most accurate predictions of the material viscoelastic behavior in time (>98% accuracy) and frequency (>96% accuracy) domains. STATEMENT OF SIGNIFICANCE: Rheological models are applied in quantifying tissues viscoelastic properties. This study is unique in presenting comprehensive assessments of rheological models.

Mechanical validation of viscoelastic parameters for different interface pressures using the Kelvin-Voigt fractional derivative model

The knowledge of the biomechanical properties of tissues is useful for different applications such as disease diagnosis and treatment monitoring. Reverberant Shear Wave Elastography (RSWE) is an approach that has reduced the restrictions on wave generation to characterize the shear wave velocity over a range of frequencies. This approach is based on the generation of a reverberant field that is generated by the reflections of waves from inhomogeneities and tissue boundaries that exist in the tissue. The Kelvin-Voigt Fractional Derivative model is commonly used to characterize elasticity and viscosity of soft tissue when using shear wave ultrasound elatography. These viscoelastic characteristics can be then validated using mechanical measurements (MM) such as stress relaxation. During RSWE acquisition, the effect of interface pressure, induced by pushing the probe on the skin through the gel pad, on the viscous and elastic characteristics of tissue can be investigated. However, the effect of interface pressure on the validity of the extracted viscous and elastic characteristics was not investigated before. Therefore, the purpose of this study was to compare the estimation of the viscoelastic parameters at different thickness of gel pad against the viscoelastic characteristics obtained from MM. The experiments were conducted in a tissue-mimicking phantom. The results confirm that the relaxed elastic constant (μ₀) can be depreciated. In addition, a higher congruence was found in the viscous parameter (η_α) estimated at 6 and 7 mm. On the other hand, a difference in the order of fractional derivative (α) was found.

The Revisited Frequency-Shift Method for Shear Wave Attenuation Computation and Imaging

Ultrasound (US) shear wave (SW) elastography has been widely studied and implemented on clinical systems to assess the elasticity of living organs. Imaging of SW attenuation reflecting viscous properties of tissues has received less attention. A revisited frequency shift (R-FS) method is proposed to improve the robustness of SW attenuation imaging. Performances are compared with the FS method that we originally proposed and with the two-point frequency shift (2P-FS) and attenuation measuring US SW elastography (AMUSE) methods. In the proposed R-FS method, the shape parameter of the gamma distribution fitting SW spectra is assumed to vary with distance, in contrast to FS. Second, an adaptive random sample consensus (A-RANSAC) line fitting method is used to prevent outlier attenuation values in the presence of noise. Validation was made on ten simulated phantoms with two viscosities (0.5 and 2 Pa [Formula: see text]) and different noise levels (15 to -5 dB), two experimental homogeneous gel phantoms, and six in vivo liver acquisitions on awake ducks (including three normal and three fatty duck livers). According to the conducted experiments, R-FS revealed mean reductions in coefficients of variation (CV) of 62.6% on simulations, 62.5% with phantoms, and 62.3% in vivo compared with FS. Corresponding reductions compared with 2P-FS were 45.4%, 77.1%, and 62.0%, respectively. Reductions in normalized root-mean-square errors for simulations were 63.9% and 48.7% with respect to FS and 2P-FS, respectively.

Efficacy of shear wave elasticity for predicting clinical outcomes in patients with significant tricuspid regurgitation

This study aimed to evaluate the efficacy of shear wave (SW) elasticity for assessing clinical outcomes in patients with significant tricuspid regurgitation (TR). Significant TR develops right heart failure (HF) with organ congestion, resulting in adverse outcomes, but appropriate therapeutic strategies remain unclear. The assessment of the degree of hepatic congestion using SW elasticity may be effective for determining therapeutic strategies. We prospectively enrolled 77 patients with moderate or severe TR who underwent SW elastography. Patients were divided into three groups according to the value of SW elasticity: low group (SW elasticity < 6.4 kPa, n = 26), medium group (6.4 ≤ SW elasticity < 9.5 kPa, n = 26), and high group (SW elasticity ≥ 9.5 kPa, n = 25). The endpoint was cardiovascular death or hospitalization for HF. During the median follow-up period of 17 months (range 7-39 months), cardiovascular death or hospitalization for HF occurred in seven patients of high group, in three patients of medium group, and in no patients of low group. In high group, three patients died and seven patients were hospitalized for HF. In medium group, two patients died and one patient was hospitalized. Kaplan-Meier analysis showed that the event-free survival rate was worse in high group than in other groups (log-rank test, p = 0.02). High SW elasticity was independently related to cardiac events as well as right ventricular and left ventricular dysfunction. SW elasticity was a predictor of cardiac events in patients with significant TR by assessing hepatic congestion. SW elasticity can be valuable for determining therapeutic strategies for TR.

Quantifying viscosity and elasticity using holographic imaging by Rayleigh wave dispersion

Viscoelasticity is an important diagnostic parameter to investigate physiological dysfunctions in biological tissues. This Letter reports the quantification of viscoelastic parameters by Rayleigh wave tracing on the surface of tissue-mimicking phantoms using holographic imaging. The Rayleigh wave is induced by an electromechanical actuator on the surface of oil-in-gelatin phantoms and a biological tissue sample followed by holographic imaging and reconstruction of the wave. The frequency-dependent velocity dispersion is fitted to a Voigt model for the quantification of viscous and elastic moduli. The viscoelastic parameters calculated by the proposed method are validated by comparing the results from a conventional mechanical rheometer.

Liquid-Liver Phantom: Mimicking the Viscoelastic Dispersion of Human Liver for Ultrasound- and MRI-Based Elastography

OBJECTIVES

Tissue stiffness can guide medical diagnoses and is exploited as an imaging contrast in elastography. However, different elastography devices show different liver stiffness values in the same subject, hindering comparison of values and establishment of system-independent thresholds for disease detection. There is a need for standardized phantoms that specifically address the viscosity-related dispersion of stiffness over frequency. To improve standardization of clinical elastography across devices and platforms including ultrasound and magnetic resonance imaging (MRI), a comprehensively characterized phantom is introduced that mimics the dispersion of stiffness of the human liver and can be generated reproducibly.

MATERIALS AND METHODS

The phantom was made of linear polymerized polyacrylamide (PAAm) calibrated to the viscoelastic properties of healthy human liver in vivo as reported in the literature. Stiffness dispersion was analyzed using the 2-parameter springpot model fitted to the dispersion of shear wave speed of PAAm, which was measured by shear rheometry, ultrasound-based time-harmonic elastography, clinical magnetic resonance elastography (MRE), and tabletop MRE in the frequency range of 5 to 3000 Hz. Imaging parameters for ultrasound and MRI, reproducibility, aging behavior, and temperature dependency were assessed. In addition, the frequency bandwidth of shear wave speed of clinical elastography methods (Aplio i900, Canon; Acuson Sequoia, Siemens; FibroScan, EchoSense) was characterized.

RESULTS

Within the entire frequency range analyzed in this study, the PAAm phantom reproduced well the stiffness dispersion of human liver in vivo despite its fluid properties under static loading (springpot stiffness parameter, 2.14 [95% confidence interval, 2.08-2.19] kPa; springpot powerlaw exponent, 0.367 [95% confidence interval, 0.362-0.373]). Imaging parameters were close to those of liver in vivo with only slight variability in stiffness values of 0.5% (0.4%, 0.6%), 4.1% (3.9%, 4.5%), and -0.63% (-0.67%, -0.58%), respectively, between batches, over a 6-month period, and per °C increase in temperature.

CONCLUSIONS

The liquid-liver phantom has useful properties for standardization and development of liver elastography. First, it can be used across clinical and experimental elastography devices in ultrasound and MRI. Second, being a liquid, it can easily be adapted in size and shape to specific technical requirements, and by adding inclusions and scatterers. Finally, because the phantom is based on noncrosslinked linear PAAm constituents, it is easy to produce, indicating potential widespread use among researchers and vendors to standardize liver stiffness measurements.

Assessment of congestion and clinical outcomes in patients with chronic heart failure using shear wave elasticity

Aims

The relief of congestion is essential for the prevention of worsening heart failure (HF) resulting in hospitalizations. Assessment of the degree of organ congestion in the chronic phase of HF is important for determining therapeutic strategies. The aim of this study was to evaluate the efficacy of shear wave (SW) elasticity for assessing congestion and clinical outcomes in patients with chronic HF.

Methods and results

We prospectively enrolled 345 consecutive patients with chronic HF who underwent SW elastography at outpatient clinic. Patients were divided into two groups according to the median value of SW elasticity: low group (SW elasticity <6.4 kPa, n = 176) and high group (SW elasticity ≥6.4 kPa, n = 169). The endpoint was cardiovascular death or hospitalization for HF. During the median follow‐up period of 19 months (range: 7–36 months), cardiovascular death or hospitalization for HF occurred in 4 patients of low group and 27 patients of high group. In high group, 8 patients died, and 19 patients were hospitalized for HF. In low group, 3 patients died, and 1 patient was hospitalized. Kaplan–Meier analysis showed that the event‐free survival rate was worse in high group than in low group (log‐rank test, P = 0.004). After adjusting for variables, high SW elasticity was independently related to cardiac events. In multivariate regression analysis, SW elasticity was correlated with left atrial volume index, early diastolic mitral inflow velocity to mitral annular velocity ratio, and inferior vena cava diameter.

Conclusions

The SW elasticity reflected haemodynamic congestion in patients with chronic HF, which was related to cardiac events.

Efficacy of shear wave elasticity for evaluating myocardial hypertrophy in hypertensive rats

Shear wave (SW) imaging is a novel ultrasound-based technique for assessing tissue characteristics. SW elasticity may be useful to assess the severity of hypertensive left ventricular (LV) hypertrophy. This study aimed to evaluate the efficacy of SW elasticity for assessing the degree of myocardial hypertrophy using hypertensive rats. Rats were divided into hypertension group and control group. SW elasticity was measured on the excised heart. Myocardial hypertrophy was assessed histologically. LV weight was greater in hypertension group. An increase in interventricular septum and LV free wall thicknesses was observed in hypertension group. SW elasticity was significantly higher in hypertension group than in control group (14.6 ± 4.3 kPa vs. 6.5 ± 1.1 kPa, P < 0.01). The cross-sectional area of cardiomyocytes was larger in hypertension group than in control group (397 ± 50 μm2 vs. 243 ± 14 μm2, P < 0.01), and SW elasticity was positively correlated with the cross-sectional area of cardiomyocytes (R = 0.96, P < 0.01). This study showed that SW elasticity was higher in hypertensive rats and was closely correlated with the degree of myocardial hypertrophy, suggesting the efficacy of SW elasticity for estimating the severity of hypertensive LV hypertrophy.

Ultrasonic technologies in imaging and drug delivery

Ultrasonic technologies show great promise for diagnostic imaging and drug delivery in theranostic applications. The development of functional and molecular ultrasound imaging is based on the technical breakthrough of high frame-rate ultrasound. The evolution of shear wave elastography, high-frequency ultrasound imaging, ultrasound contrast imaging, and super-resolution blood flow imaging are described in this review. Recently, the therapeutic potential of the interaction of ultrasound with microbubble cavitation or droplet vaporization has become recognized. Microbubbles and phase-change droplets not only provide effective contrast media, but also show great therapeutic potential. Interaction with ultrasound induces unique and distinguishable biophysical features in microbubbles and droplets that promote drug loading and delivery. In particular, this approach demonstrates potential for central nervous system applications. Here, we systemically review the technological developments of theranostic ultrasound including novel ultrasound imaging techniques, the synergetic use of ultrasound with microbubbles and droplets, and microbubble/droplet drug-loading strategies for anticancer applications and disease modulation. These advancements have transformed ultrasound from a purely diagnostic utility into a promising theranostic tool.

Influence of fibrosis progression on the viscous properties of in vivo liver tissue elucidated by shear wave dispersion in multifrequency MR elastography

PURPOSE

Many elastography studies have shown that liver stiffness increases with fibrosis and thus can be used as a reliable marker for noninvasively staging fibrosis. However, the sensitivity of viscosity-related mechanical parameters, such as shear wave dispersion, to liver fibrosis is less well understood.

METHODS

In this proof-of-concept study, 15 healthy volunteers and 37 patients with chronic liver disease and biopsy-proven fibrosis were prospectively investigated by MR elastography at six drive frequencies of 35-60 Hz. Maps of shear wave speed (SWS, in m/s) and loss angle (φ, in rad), as a marker of stiffness and viscous properties, respectively, were generated using tomoelastography data processing. The Child-Pugh score was used to assess cirrhosis severity.

RESULTS

While SWS increased with fibrosis (F0: 1.53 ± 0.11 m/s, F1-F3: 1.71 ± 0.17 m/s, F4: 2.50 ± 0.39 m/s; P < 0.001), φ remained unchanged during mild to severe fibrosis (F0: 0.63 ± 0.05 rad, F1-F3: 0.60 ± 0.05 rad, P = 0.21) but increased in cirrhosis (F4: 0.81 ± 0.16 rad; P < 0.001). Correspondingly, the slope of SWS-dispersion within the investigated range of vibration frequencies increased from insignificant (F0-F3: 0.010 ± 0.007 m/s/Hz) to significant (F4: 0.038 ± 0.025 m/s/Hz; P = 0.005). Significant correlation with the Child-Pugh score was found for φ (R = 0.60, P = 0.01) but not for SWS.

CONCLUSION

Although cirrhosis is associated with liver stiffening and, intuitively, transition towards more rigid material properties, the observed increases in φ and slope of SWS-dispersion indicate abnormally high mechanical friction in cirrhotic livers. This biophysical signature might provide a prognostic imaging marker for the detection of pathological processes associated with fibrosis independent of stiffness.

Multifaceted Design and Emerging Applications of Tissue Adhesives

Tissue adhesives can form appreciable adhesion with tissues and have found clinical use in a variety of medical settings such as wound closure, surgical sealants, regenerative medicine, and device attachment. The advantages of tissue adhesives include ease of implementation, rapid application, mitigation of tissue damage, and compatibility with minimally invasive procedures. The field of tissue adhesives is rapidly evolving, leading to tissue adhesives with superior mechanical properties and advanced functionality. Such adhesives enable new applications ranging from mobile health to cancer treatment. To provide guidelines for the rational design of tissue adhesives, here, existing strategies for tissue adhesives are synthesized into a multifaceted design, which comprises three design elements: the tissue, the adhesive surface, and the adhesive matrix. The mechanical, chemical, and biological considerations associated with each design element are reviewed. Throughout the report, the limitations of existing tissue adhesives and immediate opportunities for improvement are discussed. The recent progress of tissue adhesives in topical and implantable applications is highlighted, and then future directions toward next-generation tissue adhesives are outlined. The development of tissue adhesives will fuse disciplines and make broad impacts in engineering and medicine.

From Anatomy to Functional and Molecular Biomarker Imaging and Therapy: Ultrasound Is Safe, Ultrafast, Portable, and Inexpensive

Ultrasound is the most widely used medical imaging modality worldwide. It is abundant, extremely safe, portable, and inexpensive. In this review, we consider some of the current development trends for ultrasound imaging, which build upon its current strength and the popularity it experiences among medical imaging professional users.Ultrasound has rapidly expanded beyond traditional radiology departments and cardiology practices. Computing power and data processing capabilities of commonly available electronics put ultrasound systems in a lab coat pocket or on a user's mobile phone. Taking advantage of new contributions and discoveries in ultrasound physics, signal processing algorithms, and electronics, the performance of ultrasound systems and transducers have progressed in terms of them becoming smaller, with higher imaging performance, and having lower cost. Ultrasound operates in real time, now at ultrafast speeds; kilohertz frame rates are already achieved by many systems.Ultrasound has progressed beyond anatomical imaging and monitoring blood flow in large vessels. With clinical approval of ultrasound contrast agents (gas-filled microbubbles) that are administered in the bloodstream, tissue perfusion studies are now routine. Through the use of modern ultrasound pulse sequences, individual microbubbles, with subpicogram mass, can be detected and observed in real time, many centimeters deep in the body. Ultrasound imaging has broken the wavelength barrier; by tracking positions of microbubbles within the vasculature, superresolution imaging has been made possible. Ultrasound can now trace the smallest vessels and capillaries, and obtain blood velocity data in those vessels.Molecular ultrasound imaging has now moved closer to clinic; the use of microbubbles with a specific affinity to endothelial biomarkers allows selective accumulation and retention of ultrasound contrast in the areas of ischemic injury, inflammation, or neoangiogenesis. This will aid in noninvasive molecular imaging and may provide additional help with real-time guidance of biopsy, surgery, and ablation procedures.The ultrasound field can be tightly focused inside the body, many centimeters deep, with millimeter precision, and ablate lesions by energy deposition, with thermal or mechanical bioeffects. Some of such treatments are already in clinical use, with more indications progressing through the clinical trial stage. In conjunction with intravascular microbubbles, focused ultrasound can be used for tissue-specific drug delivery; localized triggered release of sequestered drugs from particles in the bloodstream may take time to get to clinic. A combination of intravascular microbubbles with circulating drug and low-power ultrasound allows transient opening of vascular endothelial barriers, including blood-brain barrier; this approach has reached clinical trial stage. Therefore, the drugs that normally would not be getting to the target tissue in the brain will now have an opportunity to produce therapeutic efficacy.Overall, medical ultrasound is developing at a brisk rate, even in an environment where other imaging modalities are also advancing rapidly and may be considered more lucrative. With all the current advances that we discuss, and many more to come, ultrasound may help solve many problems that modern medicine is facing.

Efficacy of shear wave elastography for assessment of liver function in patients with heart failure

AIMS

Liver dysfunction is important for prognosis in heart failure (HF). Shear wave elastography (SWE), which is a novel ultrasound technique for charactering tissues, has been used in liver diseases. However, clinical implication of SWE, including dispersion slope, remains unknown in heart diseases. This study aimed to evaluate the efficacy of SWE assessing liver function in the severity of HF.

METHODS AND RESULTS

We enrolled 316 consecutive patients with or suspected heart diseases, who were classified according to the American College of Cardiology Foundation/American Heart Association stage of HF, including 37 with Stage A, 139 with Stage B, 114 with Stage C, and 26 with Stage D, and 45 normal subjects. Elasticity and dispersion slope of shear wave were assessed according to the HF stage. Elasticity and dispersion slope were not elevated in normal subjects and patients with Stage A. Elasticity was slightly increased from Stage A to Stage C and was remarkably elevated in Stage D (normal: 5.2 ± 1.1 kPa, Stage A: 5.4 ± 1.2 kPa, Stage B: 6.4 ± 1.8 kPa, Stage C: 7.8 ± 3.5 kPa, and Stage D: 17.7 ± 12.7 kPa), whereas dispersion slope was gradually increased from Stage A to Stage D (normal: 9.7 ± 1.7m/s/kHz, Stage A: 10.4 ± 1.6m/s/kHz, Stage B: 11.7 ± 2.4m/s/kHz, Stage C: 13.2 ± 3.4m/s/kHz, and Stage D: 17.6 ± 5.6 m/s/kHz). In the early HF stage, dispersion slope was elevated. In the advanced HF stage, both elasticity and dispersion slope were elevated. Liver function test abnormalities were observed only from Stage C or Stage D.

CONCLUSIONS

Dispersion slope could detect early liver damage, and the combination of elasticity and dispersion slope could clarify the progression of liver dysfunction in HF. SWE may be valuable to manage therapeutic strategies in patients with HF.

Hydrophone Spatial Averaging Correction for Acoustic Exposure Measurements From Arrays-Part I: Theory and Impact on Diagnostic Safety Indexes

This article reports underestimation of mechanical index (MI) and nonscanned thermal index for bone near focus (TIB) due to hydrophone spatial averaging effects that occur during acoustic output measurements for clinical linear and phased arrays. TIB is the appropriate version of thermal index (TI) for fetal imaging after ten weeks from the last menstrual period according to the American Institute of Ultrasound in Medicine (AIUM). Spatial averaging is particularly troublesome for highly focused beams and nonlinear, nonscanned modes such as acoustic radiation force impulse (ARFI) and pulsed Doppler. MI and variants of TI (e.g., TIB), which are displayed in real-time during imaging, are often not corrected for hydrophone spatial averaging because a standardized method for doing so does not exist for linear and phased arrays. A novel analytic inverse-filter method to correct for spatial averaging for pressure waves from linear and phased arrays is derived in this article (Part I) and experimentally validated in a companion article (Part II). A simulation was developed to estimate potential spatial-averaging errors for typical clinical ultrasound imaging systems based on the theoretical inverse filter and specifications for 124 scanner/transducer combinations from the U.S. Food and Drug Administration (FDA) 510(k) database from 2015 to 2019. Specifications included center frequency, aperture size, acoustic output parameters, hydrophone geometrical sensitive element diameter, etc. Correction for hydrophone spatial averaging using the inverse filter suggests that maximally achievable values for MI, TIB, thermal dose ( t ₄₃ ), and spatial-peak-temporal-average intensity ( [Formula: see text]) for typical clinical systems are potentially higher than uncorrected values by (means ± standard deviations) 9% ± 4% (ARFI MI), 19% ± 15% (ARFI TIB), 50% ± 41% (ARFI t ₄₃ ), 43% ± 39% (ARFI [Formula: see text]), 7% ± 5% (pulsed Doppler MI), 15% ± 11% (pulsed Doppler TIB), 42% ± 31% (pulsed Doppler t ₄₃ ), and 33% ± 27% (pulsed Doppler [Formula: see text]). These values correspond to frequencies of 3.2 ± 1.3 (ARFI) and 4.1 ± 1.4 MHz (pulsed Doppler), and the model predicts that they would increase with frequency. Inverse filtering for hydrophone spatial averaging significantly improves the accuracy of estimates of MI, TIB, t ₄₃ , and [Formula: see text] for ARFI and pulsed Doppler signals.

Local Phase Velocity Based Imaging of Viscoelastic Phantoms and Tissues

Assessment of soft tissue elasticity and viscosity is of interest in several clinical applications. In this study, we present the feasibility of the local phase velocity based imaging (LPVI) method to create images of phase velocity and viscoelastic parameters in viscoelastic tissue-mimicking materials and soft tissues. In viscoelastic materials, it is necessary to utilize wave-mode isolation using a narrow bandpass filter combined with a directional filter in order to robustly reconstruct phase velocity images with LPVI in viscoelastic media over a wide range of frequencies. A pair of sequential focused acoustic radiation force push beams, focused once on the left-hand side and once on the right-hand side of the probe, was used to produce broadband propagating shear waves. The local shear wave phase velocity is then recovered in the frequency domain for multiple frequencies, for both acquired data sets. Then, a 2-D shear wave velocity map is reconstructed by combining maps from two separate acquisitions. By testing the method on simulated data sets and performing in vitro phantom and in vivo liver tissue experiments, we show the ability of the proposed technique to generate shear wave phase velocity maps at various frequencies in viscoelastic materials. Moreover, a nonlinear least-squares problem is solved in order to locally estimate elasticity and viscosity parameters. The LPVI method with added directional and wavenumber filters can produce phase velocity images, which can be used to characterize the viscoelastic materials.

Liver stiffness as measured by two-dimensional shear wave elastography overestimates the stage of fibrosis in patients with chronic hepatitis B and hepatic steatosis

Two-dimensional shear wave elastography (2D-SWE) is a non-invasive technique for measuring liver stiffness (LS) and is used to assess the degree of hepatic fibrosis in patients with chronic hepatitis B (CHB). Despite its usefulness, several factors, other than hepatic fibrosis, can affect its diagnostic accuracy. Hepatic steatosis (HS) is a common lesion in CHB that has increasingly been getting attention in the field of disease development; however, its influence on the measurement of LS remains unclear. We aimed to determine whether HS affects the diagnostic accuracy of 2D-SWE in patients with CHB. Serum parameters and LS values were obtained from 161 patients with CHB. The degrees of hepatic fibrosis and inflammatory activity were estimated based on the METAVIR Cooperative Study Group criteria, and the extent of HS was defined as the percentage of hepatocytes containing fat droplets using oil red staining. We found that LS values were independently correlated with HS in the early stages of hepatic fibrosis (F0-F2 or F0-3). Furthermore, LS values in patients with significant steatosis (S≥10%) were higher than the counterpart in fibrosis stages F0-2 (6.82±1.57 vs. 7.92±1.99; p=0.010) and F0-3 (7.18±1.84 vs. 8.25±1.91; p=0.007). Therefore, false positive rates (FPRs) in the diagnosis of advanced fibrosis (16.00% vs. 37.04%, p=0.037) and cirrhosis (6.67% vs. 21.62%, p=0.030) were higher in patients with significant steatosis. In conclusion, the use of 2D-SWE in the measurement of LS overestimates the stage of hepatic fibrosis in CHB patients with HS>10%. This should be taken into consideration to combine LS results with other non-invasive parameters to improve its accuracy.

Efficacy of shear wave elastography for evaluating right ventricular myocardial fibrosis in monocrotaline-induced pulmonary hypertension rats

BACKGROUND

Right ventricular (RV) function is important for outcomes in pulmonary hypertension. Evaluation of RV myocardial characteristics is useful to assess the disease severity. Shear wave elastography (SWE) provides information of shear wave (SW) elasticity, which is related to tissue hardness, and SW dispersion slope, which reflects tissue viscosity. This study aimed to test the hypothesis that SW elasticity is increased and SW dispersion slope is decreased in the right ventricle of monocrotaline (MCT)-induced pulmonary hypertension rats.

METHODS

Rats were divided into MCT-induced pulmonary hypertension group (n = 10) and control group (n = 10). SW elasticity and SW dispersion slope were measured on excised hearts. Myocardial fibrosis was evaluated histologically.

RESULTS

RV hypertrophy was observed in the MCT group. SW elasticity of right ventricle was higher in the MCT group than in the control group (3.5 ± 0.9 kPa vs. 2.5 ± 0.4 kPa, p < 0.01). SW dispersion slope of right ventricle was lower in the MCT group than in the control group (5.3 ± 1.7 m/s/kHz vs. 7.7 ± 1.5 m/s/kHz, p < 0.01). The fibrosis area of right ventricle was increased in MCT group compared with control group (18 ± 5% vs. 8 ± 3%, p < 0.01), and was positively related to SW elasticity and negatively related to SW dispersion slope.

CONCLUSIONS

Higher SW elasticity and lower SW dispersion slope were observed in the fibrotic myocardium of right ventricle in MCT-induced pulmonary hypertension rats. SWE may have the potential to evaluate RV function by assessing myocardial characteristics.

T1ρ magnetic resonance imaging value as a potential marker to assess the severity of liver fibrosis: A pilot study

Purpose

Assessment of liver fibrosis is essential for the management of liver disease. Although liver biopsy is the gold-standard modality for the diagnosis of liver fibrosis, it has some limitations. Thus, other methods are required to overcome the disadvantages of a liver biopsy. T1ρ magnetic resonance imaging (MRI) values are potential biomarkers for liver cirrhosis. This study aimed to assess the relationship between T1ρ MRI values and liver fibrosis severity by measuring the correlation between T1ρ values and shear wave elastography (SWE) values, which are routinely used for the diagnosis of liver fibrosis.

Methods

T1ρ imaging and SWE values were obtained from four healthy volunteers and 16 patients with chronic liver disease. The regions of interest on MR images were drawn and matched with those of the right liver lobe on SWE images.

Results

The mean T1ρ values of the right liver lobe correlated positively with the mean SWE values (Pearson’s correlation coefficient: 0.783; p < 0.0001; 95 % confidence interval: 0.623–0.880).

Conclusion

The mean T1ρ values of the right liver lobe may be correlated with the severity of liver fibrosis.

A review of physical and engineering factors potentially affecting shear wave elastography

It has been recognized that tissue stiffness provides useful diagnostic information, as with palpation as a screening for diseases such as cancer. In recent years, shear wave elastography (SWE), a technique for evaluating and imaging tissue elasticity quantitatively and objectively in diagnostic imaging, has been put into practical use, and the amount of clinical knowledge about SWE has increased. In addition, some guidelines and review papers regarding technology and clinical applications have been published, and the status as a diagnostic technology is in the process of being established. However, there are still unclear points about the interpretation of shear wave speed (SWS) and converted elastic modulus in SWE. To clarify these, it is important to investigate the factors that affect the SWS and elastic modulus. Therefore, physical and engineering factors that potentially affect the SWS and elastic modulus are discussed in this review paper, based on the principles of SWE and a literature review. The physical factors include the propagation properties of shear waves, mechanical properties (viscoelasticity, nonlinearity, and anisotropy), and size and shape of target tissues. The engineering factors include the region of interest depth and signal processing. The aim of this review paper is not to provide an answer to the interpretation of SWS. It is to provide information for readers to formulate and verify the hypothesis for the interpretation. Therefore, methods to verify the hypothesis for the interpretation are also reviewed. Finally, studies on the safety of SWE are discussed.

Dispersion curve calculation in viscoelastic tissue-mimicking materials using non-parametric, parametric, and high-resolution methods

Ultrasound shear wave elastography is a modality used for noninvasive, quantitative evaluation of soft tissue mechanical properties. A common way of exploring the tissue viscoelasticity is through analyzing the shear wave velocity dispersion curves. The variation of phase velocity with frequency or wavelength is called the dispersion curve. An increase of the available spectrum to be used for phase velocity estimation is meaningful for a tissue dispersion analysis in vivo. A number of available methods for dispersion relation estimation exist which can give diffuse results due the presence of noise in the measured data. In this work we compare six selected methods used for dispersion curve calculation in viscoelastic materials. Non-parametric, parametric and high-resolution methods were examined and compared. We tested selected methods on digital phantom data created using finite-difference-based method in tissue-mimicking viscoelastic media as well as on the experimental custom tissue-mimicking phantoms. In addition, we evaluated the algorithms with different levels of added white Gaussian noise to the shear wave particle velocity from numerical phantoms. Tests conducted showed that more advanced methods can offer better frequency resolution, and less variance than the fast Fourier transform. In addition, the non-parametric Blackman-Tukey approach exhibits similar performance and can be interchangeably used for shear wave phase velocity dispersion curves calculation.

Viscoelastic model characterization of human cervical tissue by torsional waves

The understanding of changes in the viscoelastic properties of cervical tissue during the gestation process is a challenging problem. In this work, we explore the importance of considering the multilayer nature (epithelial and connective layers) of human cervical tissue for characterizing the viscoelastic parameters from torsional waves. For this purpose, torsional wave propagations are simulated in three multilayer cervical tissue models (pure elastic, Kelvin-Voigt (KV) and Maxwell) using the finite difference time domain method. High-speed camera measurements have been carried out in tissue-mimicking phantoms in order to obtain the boundary conditions of the numerical simulations. Finally, a parametric modeling study through a probabilistic inverse procedure was performed to rank the most plausible rheological model and to reconstruct the viscoelastic parameters. The procedure consist in comparing the experimental signals obtained in human cervical tissues using the Torsional Wave Elastography (TWE) technique with the synthetic signals from the numerical models. It is shown that the rheological model that best describes the nature of cervical tissue is the Kelvin-Voigt model. Once the most plausible model has been selected, the stiffness and viscosity parameters have been reconstructed of the epithelial and connective layers for the measurements of the 18 pregnant women, along with the thickness of the epithelial layer.

MR elastography: Principles, guidelines, and terminology

Magnetic resonance elastography (MRE) is a phase contrast-based MRI technique that can measure displacement due to propagating mechanical waves, from which material properties such as shear modulus can be calculated. Magnetic resonance elastography can be thought of as quantitative, noninvasive palpation. It is increasing in clinical importance, has become widespread in the diagnosis and staging of liver fibrosis, and additional clinical applications are being explored. However, publications have reported MRE results using many different parameters, acquisition techniques, processing methods, and varied nomenclature. The diversity of terminology can lead to confusion (particularly among clinicians) about the meaning of and interpretation of MRE results. This paper was written by the MRE Guidelines Committee, a group formalized at the first meeting of the ISMRM MRE Study Group, to clarify and move toward standardization of MRE nomenclature. The purpose of this paper is to (1) explain MRE terminology and concepts to those not familiar with them, (2) define "good practices" for practitioners of MRE, and (3) identify opportunities to standardize terminology, to avoid confusion.

Comprehensive Viscoelastic Characterization of Tissues and the Inter-relationship of Shear Wave (Group and Phase) Velocity, Attenuation and Dispersion

We report shear wave phase and group velocity, dispersion and attenuation in oil-in-gelatin viscoelastic phantoms and in vivo liver data. Moreover, we measured the power law coefficient from each dispersion curve and used it, together with the shear wave velocity, to calculate an approximate value for attenuation that agrees with independent attenuation measurements. Results in phantoms exhibit good agreement for all parameters with respect to independent mechanical measurements. For in vivo data, the livers of 20 patients were scanned. Results were compared with pathology scores obtained from liver biopsies. Across these cases, increases in shear wave dispersion and attenuation were related to increased steatosis score. It was found that shear wave dispersion and attenuation are experimentally linked, consistent with simple predictions based on the rheology of tissues, and can be used individually or jointly to assess tissue viscosity. Thus, this study indicates the possible utility of using shear wave dispersion and attenuation to non-invasively and quantitatively assess steatosis.

Three-dimensional shear wave elastography on conventional ultrasound scanners with external vibration

Two-dimensional (2D) ultrasound shear wave elastography (SWE) has been widely used for soft tissue properties assessment. Given that shear waves propagate in three dimensions (3D), extending SWE from 2D to 3D is important for comprehensive and accurate stiffness measurement. However, implementation of 3D SWE on a conventional ultrasound scanner is challenging due to the low volume rate (tens of Hertz) associated with limited parallel receive capability of the scanner's hardware beamformer. Therefore, we developed an external mechanical vibration-based 3D SWE technique allowing robust 3D shear wave tracking and speed reconstruction for conventional scanners. The aliased shear wave signal detected with a sub-Nyquist sampling frequency was corrected by leveraging the cyclic nature of the sinusoidal shear wave generated by the external vibrator. Shear wave signals from different sub-volumes were aligned in temporal direction to correct time delays from sequential pulse-echo events, followed by 3D speed reconstruction using a 3D local frequency estimation algorithm. The technique was validated on liver fibrosis phantoms with different stiffness, showing good correlation (r = 0.99, p < 0.001) with values measured from a state-of-the-art SWE system (GE LOGIQ E9). The phantoms with different stiffnesses can be well-differentiated regardless of the external vibrator position, indicating the feasibility of the 3D SWE with regard to different shear wave propagation scenarios. Finally, shear wave speed calculated by the 3D method correlated well with magnetic resonance elastography performed on human liver (r = 0.93, p = 0.02), demonstrating the in vivo feasibility. The proposed technique relies on low volume rate imaging and can be implemented on the widely available clinical ultrasound scanners, facilitating its clinical translation to improve liver fibrosis evaluation.

The Performance of a 2-Dimensional Shear-Wave Elastography Technique for Predicting Different Stages of Liver Fibrosis Using Transient Elastography as the Control Method

ABSTRACT

The aim was to evaluate the performance of 2-dimensional (2D) shear-wave elastography from general electric (2D SWE-GE), implemented on the new LOGIQ S8 system, for the noninvasive assessment of liver fibrosis, and to identify liver stiffness (LS) cutoff values for predicting different stages of fibrosis using transient elastography (TE) as the control method. We included 179 consecutive subjects, with or without chronic hepatopathies, in whom LS was evaluated in the same session using 2 elastographic techniques: TE (FibroScan, EchoSens) and 2D SWE-GE (LOGIQ S8; GE Healthcare, Chalfont St Giles, United Kingdom). Reliable LS measurements were defined for TE: the median value of 10 measurements with a success rate of 60% or greater and an interquartile range/median ratio (IQR/M) < 0.30; for 2D SWE-GE: the median value of 10 measurements acquired in a homogenous area and IQR/M < 0.30. To discriminate between fibrosis stages by TE, we used the following cutoffs: F2-7; F3-9.5 and F4-12 kPa. Reliable LS measurements were obtained in 97.2% subjects by 2D SWE-GE and in 98.3% by TE (P = 0.72), so that 171 subjects were included for the final analysis. A good correlation was found between the LS values obtained by the 2 methods (r = 0.72, P < 0.0001). The best 2D SWE-GE cutoff value for F ≥ 2 was 6.9 kPa (areas under receiver operating characteristic [AUROC], 0.93; sensitivity, 85.8%; specificity, 90.2%), for F of 3 or greater, it was 8.2 kPa (AUROC, 0.93; sensitivity, 87.5%; specificity, 86.8%) and for F value of 4, it was 9.3 kPa (AUROC, 0.91; sensitivity, 85.7%; specificity, 81.2%). In conclusion, the best 2D SWE-GE (S8) cutoff values for predicting F2, F ≥ 3 and F = 4 were 6.9, 8.2, and 9.3 kPa.

Clinical significance of spleen stiffness in patients with acute decompensated heart failure

Aims

Congestive splenomegaly is a classic sign of organ congestion in acute decompensated heart failure (ADHF). Shear wave elastography (SWE) allows the measurement of spleen stiffness (SS). We hypothesized that SS could quantify the severity of splenic congestion and predict adverse events in ADHF.

Methods and Results

This study included two cohorts: a haemodynamic cohort (62 HF patients) and an outcome cohort (115 ADHF patients). SS was measured by two‐dimensional SWE on the same day of right heart catheterization in the haemodynamic cohort. Right atrial pressure (RAP) independently correlated with SS (β = 0.32, P = 0.002). SS was measured in the outcome cohort before discharge. The 115 patients were divided into three groups on the basis of the tertile value of SS. The third tertile SS group had a higher prevalence of severe tricuspid regurgitation, higher N‐terminal B‐type natriuretic peptide (NT pro‐BNP), and larger right ventricular diastolic diameter, than had the first tertile group and the second tertile group. During a median follow‐up period of 105 (77–135) days, adverse events occurred in 25 patients (one death and 24 rehospitalizations for HF). The third tertile SS group had a significantly higher rate of adverse events (P < 0.001). A higher SS was independently associated with adverse events after adjusting for conventional validated risk score, liver function test, liver stiffness, and estimated RAP.

Conclusions

The degree of SS at discharge can be used as a marker of residual splenic congestion, which is predictive of adverse events in patients with ADHF.

2-D Ultrasound Shear Wave Elastography With Multi-Sphere-Source External Mechanical Vibration: Preliminary Phantom Results

Ultrasound shear wave elastography (SWE) imaging is emerging as a quantitative and non-invasive tissue characterization modality. Shear wave generation using external mechanical vibration (EMV) has received extensive research interest over acoustic radiation force impulse (ARFI) because of its low cost and potential for portability. In this paper, we propose an EMV concept with multiple spherical sources that can be easily reconfigured in three configurations to induce unique shear wave propagation patterns. We introduce two design embodiments of this concept bench test design for proof of concept and a clinically deployable design. The latter is designed to incorporate size, ergonomics, portability and power consumption considerations and constraints. Experimental validation on elasticity phantoms using both EMV designs demonstrates shear wave generation and elasticity reconstruction comparable in performance to ElastQ, a commercial ARFI-based shear elastography technology from Philips. In addition, the local displacement amplitude induced by EMV is 10 times greater than that induced by ARFI at the same given depth. Finally, the multiple configurations of the presented EMV design would allow exploration of advanced elastography methods such as tissue anisotropic elasticity.

Advances in ultrasound elastography for nonalcoholic fatty liver disease

The prevalence of nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) has increased rapidly worldwide, making NAFLD/NASH an important global health problem from both a medical and socioeconomic standpoint. NAFLD is also regarded as a liver component of metabolic syndrome and is reported to be associated with the risk factors for metabolic syndrome. It has been suggested that NAFLD/NASH be recognized both as a liver-specific disease and as an early mediator of systemic diseases. Liver biopsy is recommended as the gold standard method for the diagnosis of NASH and for the staging of liver fibrosis in patients with NAFLD. However, because of its high cost, high risk, and high weightage as a healthcare resource, invasive liver biopsy is a poorly suited diagnostic test for such a highly prevalent condition. Therefore, the development of reliable noninvasive methods for the assessment of liver fibrosis has been sought to estimate the risk of progression of NASH to cirrhosis, estimate the risk of cardiovascular events, aid in the surveillance for HCC, and guide therapy in patients with NAFLD/NASH. In this review, we highlight the principles and recent advances in ultrasound elastography techniques (Real-time Tissue Elastography^®, vibration-controlled transient elastography, point shear wave elastography, and two-dimensional shear wave elastography) used to evaluate the liver fibrosis stage and steatosis grade in patients with NAFLD.

Application of Baveno Criteria and Modified Baveno Criteria with Shear-wave Elastography in Compensated Advanced Chronic Liver Disease

BACKGROUND

We aimed to validate Baveno VI and expanded Baveno VI criteria using two dimensional shear-wave elastography (2D-SWE) in compensated advanced chronic liver disease (cACLD) patients with alcohol as the main etiology.

METHODS

Clinical data from 305 patients with cACLD who underwent a liver stiffness measurement (LSM) with 2D-SWE and endoscopy were consecutively collected.

RESULTS

Among 305 patients, high-risk varix (HRV) was identified in 21.3% (n = 65). The main etiology was alcoholic liver disease (51.8%), followed by hepatitis B virus (29.8%) and hepatitis C virus (9.1%). Baveno VI criteria spared endoscopy in 118 of the 305 (38.7%) patients, and 7 (5.9%) were missed with HRV. Expanded Baveno VI criteria spared more endoscopies (60.0%), but missed more HRV (9.8%) compared with Baveno VI criteria. The other classification described as the modified Baveno VI criteria were LSM < 25 kPa and PLT ≥ 150 × 10³/mm³. In total, 131 of the 305 (43.0%) patients were within the modified Baveno VI criteria, of whom seven (5.3%) had missed HRV. After adding spleen diameter < 12 cm to the modified Baveno VI criteria, the number of spared endoscopies increased by 106/305 (34.8%), with three (2.8%) presenting with HRV, indicating a risk of missing HRV.

CONCLUSION

Baveno VI and expanded Baveno VI criteria with 2D-SWE were insufficient with an HRV miss rate of over 5%. The modified Baveno VI criteria with spleen diameters < 12 cm with 2D-SWE spared more endoscopies with a minimal risk of missing HRV in cACLD patients with alcohol as the main etiology.

Clinical assessment for high-risk patients with non-alcoholic fatty liver disease in primary care and diabetology practices

BACKGROUND

Primary care practitioners (PCPs) and diabetologists are at the frontline of potentially encountering patients with NASH. Identification of those at high risk for adverse outcomes is important.

AIM

To provide practical guidance to providers on how to identify these patients and link them to specialty care.

METHODS

US members of the Global Council on NASH evaluated the evidence about NASH and non-invasive tests and developed a simple algorithm to identify high-risk NASH patients for diabetologists and primary care providers. These tools can assist frontline providers in decision-making and referral to gastroenterology/hepatology practices for additional assessments.

RESULTS

The presence of NASH-related advanced fibrosis is an independent predictor of adverse outcomes. These patients with NASH are considered high risk and referral to specialists is warranted. Given that staging of fibrosis requires a liver biopsy, non-invasive tests for fibrosis would be preferred. Consensus recommendation from the group is to risk-stratify patients based on metabolic risk factors using the FIB-4 as the initial non-invasive test due to its simplicity and ease of use. A FIB-4 score ≥1.3 can be used for further assessment and linkage to specialty care where additional technology to assess liver stiffness or serum fibrosis test will be available.

CONCLUSION

Due to the growing burden of NAFLD and NASH, PCPs and diabetologists are faced with increased patient encounters in their clinical practices necessitating referral decisions. To assist in identifying high-risk NASH patients requiring specialty care, we provide a simple and easy to use algorithm.

Global hepatitis C elimination: history, evolution, revolutionary changes and barriers to overcome

The fundamental discovery of the hepatitis C virus (HCV) in 1989 has led to winning this year's Nobel Prize in Medicine. This achievement guided all the steps in identifying the elements of the virus, in order to develop the treatment and to increase the screening solutions, which have slowed the exposure to the virus. The management of infection started with interferon-alpha (IFN-α), which has later enhanced by adding Ribavirin. Nowadays, HCV treatment is based on direct-acting antiviral agents (DAAs). Currently, HCV infection benefits of curative treatment, with which most patients can be cured. When speaking about hepatitis C future, we can say it is looking bright, considering all the progress that has been made in recent years and all the options that we have for curing all genotypes of HCV infection. The aim of this review is to sum up the historical characteristics of HCV discovery, the evolution of treatment and screening actions, gaps, and stages for achieving the international elimination target of the World Health Organization.

Liver fibrosis with two-dimensional shear-wave elastography in patients with autoimmune hepatitis

BACKGROUND

Two-dimensional shear-wave elastography (2D-SWE) is an ultrasound-based real-time 2D-SWE imaging technique for noninvasive assessment of liver fibrosis. The diagnostic performance of 2D-SWE for assessing liver fibrosis in patients with autoimmune hepatitis (AIH) has not been reported.

AIMS

To evaluate the diagnostic performance of 2D-SWE and serologic tests including the aspartate amino transferase-to-platelet ratio index (APRI) and fibrosis-4 index (FIB-4) for liver fibrosis staging in patients with AIH.

METHODS

103 patients were retrospectively collected. Liver fibrosis was staged according to the Scheuer scoring system. The areas under the receiver operating characteristic curves (AUROCs) were used to assess the accuracy of 2D-SWE and serum fibrosis models for staging liver fibrosis.

RESULTS

Liver stiffness measured by 2D-SWE had a strong correlation with histological fibrosis stage (r = 0.71, P < 0.0001). The AUROCs of liver stiffness in detecting significant fibrosis, severe fibrosis, and cirrhosis were 0.84, 0.84, and 0.94, respectively. The AUROCs of liver stiffness for detecting significant fibrosis and severe fibrosis were higher than those of APRI (0.57, 0.56) and FIB-4 (0.63, 0.66), respectively. The cut-off liver stiffness values for predicting significant fibrosis, severe fibrosis, and cirrhosis were 10.0, 15.8, and 19.3 kPa, respectively.

CONCLUSIONS

2D-SWE shows promising diagnostic performance for staging liver fibrosis in patients with AIH.

Two-Dimensional Shear Wave Elastography versus Transient Elastography: A Non-Invasive Comparison for the Assessment of Liver Fibrosis in Patients with Chronic Hepatitis C

In recent years, several non-invasive methods have been developed for staging liver fibrosis in patients with chronic hepatitis C. A 2D-Shear wave elastography (SWE) technique has been recently introduced on the EPIQ 7 US system (ElastQ), but its accuracy has not been validated in patients with chronic hepatitis C virus (HCV) infection. We enrolled 178 HCV patients to assess their liver fibrosis stage with ElastQ software using transient elastography as a reference standard. The best cut-off values to diagnose ³ F2, ³ F3, and F4 were 8.15, 10.31, and 12.65 KPa, respectively. Liver stiffness values had a positive correlation with transient elastography (r = 0.57; p < 0.001). The area under the receiver operating characteristics (AUROC) was 0.899 for ³ F2 (moderate fibrosis), 0.900 for ³ F3 (severe fibrosis), and 0.899 for cirrhosis. 2D-SWE has excellent accuracy in assessing liver fibrosis in patients with chronic hepatitis C and an excellent correlation with transient elastography.

Increased Brachial Artery Compliance After Arteriovenous Fistula Creation as Measured by Shear Wave Elastography: Results From a Feasibility Study

OBJECTIVES

Arterial stiffness has been proposed as a marker of arteriovenous fistula failure and can be measured locally by using ultrasound shear wave elastography (SWE). This preliminary study aimed to assess whether SWE measurements of the brachial artery were associated with arteriovenous fistula failure.

METHODS

Data were collected on patients who were indicated for fistula creation. Preoperative and postoperative vessel diameters from B-mode ultrasound, brachial artery SWE maps, and demographic data were collected. Logistic and linear regression analyses were used to determine whether any of these variables were related to the outcome of the fistula 3 months after creation.

RESULTS

Data were acquired for 33 patients. Shear wave velocity values decreased after fistula creation (mean ± SD, -1.2 ± 1 m/s; P < .05). No parameters were associated with failure of the fistula in the logistic regression analysis.

CONCLUSIONS

No markers were related to fistula failure, but a decrease in the shear wave velocity was observed in the brachial arteries after fistula creation, indicating increased compliance.