Shear wave spectroscopy for in vivo quantification of human soft tissues visco-elasticity

Jingle Cell Rock: Steering Cellular Activity With Low-Intensity Pulsed Ultrasound (LIPUS) to Engineer Functional Tissues in Regenerative Medicine

Acoustic manipulation or perturbation of biological soft matter has emerged as a promising clinical treatment for a number of applications within regenerative medicine, ranging from bone fracture repair to neuromodulation. The potential of ultrasound (US) endures in imparting mechanical stimuli that are able to trigger a cascade of molecular signals within unscathed cells. Particularly, low-intensity pulsed ultrasound (LIPUS) has been associated with bio-effects such as activation of specific cellular pathways and alteration of cell morphology and gene expression, the extent of which can be modulated by fine tuning of LIPUS parameters including intensity, frequency and exposure time. Although the molecular mechanisms underlying LIPUS are not yet fully elucidated, a number of studies clearly define the modulation of specific ultrasonic parameters as a means to guide the differentiation of a specific set of stem cells towards adult and fully differentiated cell types. Herein, we outline the applications of LIPUS in regenerative medicine and the in vivo and in vitro studies that have confirmed the unbounded clinical potential of this platform. We highlight the latest developments aimed at investigating the physical and biological mechanisms of action of LIPUS, outlining the most recent efforts in using this technology to aid tissue engineering strategies for repairing tissue or modelling specific diseases. Ultimately, we detail tissue-specific applications harnessing LIPUS stimuli, offering insights over the engineering of new constructs and therapeutic modalities. Overall, we aim to lay the foundation for a deeper understanding of the mechanisms governing LIPUS-based therapy, to inform the development of safer and more effective tissue regeneration strategies in the field of regenerative medicine.

Between-Visit Reproducibility of Shear Wave Viscoelastography in Volunteers and Patients With Metabolic Dysfunction-Associated Steatotic Liver Disease

OBJECTIVE

To assess the reproducibility of six ultrasound (US)-determined shear wave (SW) viscoelastography parameters for assessment of mechanical properties of the liver in volunteers and patients with biopsy-proven metabolic dysfunction-associated steatotic liver disease (MASLD) or metabolic dysfunction-associated steatohepatitis (MASH).

METHODS

This prospective, cross-sectional, institutional review board-approved study included 10 volunteers and 20 patients with MASLD or MASH who underwent liver US elastography twice, at least 2 weeks apart. SW speed (SWS), Young's modulus (E), shear modulus (G), SW attenuation (SWA), SW dispersion (SWD), and viscosity were computed from radiofrequency data recorded on a research US scanner. Linear mixed models were used to consider the sonographer on duty as a confounder. The reproducibility of measurements was assessed by intraclass correlation coefficient (ICC), coefficient of variation (CV), reproducibility coefficient (RDC), and Bland-Altman analyses.

RESULTS

The sonographer performing the exam had no impact on viscoelastic parameters (P > .05). ICCs of SWS, E, G, SWA, SWD, and viscosity were, respectively, 0.89 (95% confidence intervals [CI]: 0.79-0.95), 0.81 (95% CI: 0.79-0.95), 0.90 (95% CI: 0.80-0.95), 0.96 (95% CI: 0.93-0.98), 0.78 (95% CI: 0.60-0.89), and 0.90 (95% CI: 0.80-0.95); CVs were 11.9, 23.3, 24.2, 10.1, 29.0, and 32.2%; RDCs were 33.0, 64.5, 66.9, 27.7, 80.3, and 89.2%, and Bland-Altman mean biases and 95% limits of agreement were -0.05 (-0.45, 0.35) m/s, -0.61 (-5.33, 4.10) kPa, -0.25 (-2.06, 1.56) kPa, -0.01 (-0.27, 0.26) Np/m/Hz, -0.09 (-7.09, 6.91) m/s/kHz, and -0.33 (-2.60, 1.94) Pa/s, between the two visits.

CONCLUSION

US-determined viscoelastography parameters can be measured with high reproducibility and consistency between two visits 2 weeks apart on the same ultrasound machine.

Characterizing dispersion in bovine liver using ARFI-based shear wave rheometry

Background:Dispersion presents both a challenge and a diagnostic opportunity in shear wave elastography (SWE).Shear Wave Rheometry(SWR) is an inversion technique for processing SWE data acquired using an acoustic radiation force impulse (ARFI) excitation. The main advantage of SWR is that it can characterize the shear properties of homogeneous soft media over a wide frequency range. Assumptions associated with SWR include tissue homogeneity, tissue isotropy, and axisymmetry of the ARFI excitation).Objective:Evaluate the validity of the SWR assumptions in ex vivo bovine liver.Approach:SWR was used to measure the shear properties of bovine liver tissue as function of frequency over a large frequency range. Assumptions associated with SWR (tissue homogeneity, tissue isotropy, and axisymmetry of the ARFI excitation) were evaluated through measurements performed at multiple locations and probe orientations. Measurements focused on quantities that would reveal violations of the assumptions.Main results:Measurements of shear properties were obtained over the 25-250 Hz range, and showed a 4-fold increase in shear storage modulus (from 1 to 4 kPa) and over a 10-fold increase in the loss modulus (from 0.2 to 3 kPa) over that decade-wide frequency range. Measurements under different conditions were highly repeatable, and model error was low in all cases.Significance and Conclusion:SWR depends on modeling the ARFI-induced shear wave as a full vector viscoelastic shear wave resulting from an axisymmetric source; it is agnostic to any specific rheological model. Despite this generality, the model makes three main simplifying assumptions. These results show that the modeling assumptions used in SWR are valid in bovine liver over a wide frequency band.

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.

Effect of rt-PA on Shear Wave Mechanical Assessment and Quantitative Ultrasound Properties of Blood Clot Kinetics In Vitro

OBJECTIVE

The consequences associated with blood clots are numerous and are responsible for many deaths worldwide. The assessment of treatment efficacy is necessary for patient follow-up and to detect treatment-resistant patients. The aim of this study was to characterize the effect of treatment on blood clots in vitro using quantitative ultrasound parameters.

METHODS

Blood from 10 pigs was collected to form three clots per pig in gelatin phantoms. Clots were subjected to 1) no treatment, 2) rt-PA (recombinant tissue plasminogen activator) treatment after 20 minutes of clotting, and 3) rt-PA treatment after 60 minutes of clotting. Clots were weighted before and after the experiment to assess the treatment effect by the mass loss. The clot kinetics was studied over 100 minutes using elastography (Young's modulus, shear wave dispersion, and shear wave attenuation). Homodyne K-distribution (HKD) parameters derived from speckle statistics were also studied during clot formation and dissolving (diffuse-to-total signal power ratio and intensity parameters).

RESULTS

Treated clots loosed significantly more mass than non-treated ones (P < .005). A significant increase in Young's modulus was observed over time (P < .001), and significant reductions were seen for treated clots at 20 or 60 minutes compared with untreated ones (P < .001). The shear wave dispersion differed for treated clots at 60 minutes versus no treatments (P < .001). The shear wave attenuation decreased over time (P < .001), and was different for clots treated at 20 minutes versus no treatments (P < .031). The HKD intensity parameter varied over time (P < .032), and was lower for clots treated at 20 and 60 minutes than those untreated (P < .001 and P < .02).

CONCLUSION

The effect of rt-PA treatment could be confirmed by a decrease in Young's modulus and HKD intensity parameter. The shear wave dispersion and shear wave attenuation were sensitive to late and early treatments, respectively. The Young's modulus, shear wave attenuation, and HKD intensity parameter varied over time despite treatment.

Ultrasound Shear Wave Propagation Modeling in General Tissue-Like Viscoelastic Materials

OBJECTIVE

This study aims to present an approach for the simulation of ultrasound elastic waves propagation in a diverse range of heterogeneous tissue-like viscoelastic materials, including, but not limited to, Kelvin-Voigt, Zener, Maxwell, Burger's, and Maxwell-Wiechert models, while also allowing for modeling highly viscous fluids.

METHODS

Ultrasound shear wave elastography (SWE) serves as a cost-effective modality for noninvasive, quantitative assessment of soft tissue viscoelastic mechanical properties. To explore tissue viscoelasticity, measuring the shear wave phase velocity in the frequency domain is a common method. In this paper, we employ modeling and numerical simulations to enhance the development of SWE methods. The study employs the staggered grid finite difference (SGFD) method along with recursive calculations of convolution integrals pertinent to linear viscoelastic models.

RESULTS

The presented numerical method demonstrates its capability to simulate the propagation of ultrasound elastic waves, both longitudinal and shear, across a broad spectrum of tissue-like viscoelastic heterogeneous materials. The approach successfully accommodates various viscoelastic models without requiring additional modifications in the numerical model, thus enabling a comprehensive exploration of different viscoelastic behaviors commonly observed in diverse tissue types.

CONCLUSION

The developed combination of the SGFD method and recursive calculation of convolution integrals presents a novel and versatile approach in modeling linear viscoelastic tissue-like materials for SWE applications. This method eliminates the need for model-specific adaptations in numerical simulations, thereby offering flexibility for exploring and understanding diverse viscoelastic behaviors inherent in different heterogeneous tissue types, contributing significantly to the advancement of ultrasound SWE for diagnostic purposes.

Transcranial ultrasound estimation of viscoelasticity and fluidity in brain tumors aided by transcranial shear waves

Cerebral diseases, such as brain tumors, are intricately linked to the mechanical properties of brain tissues. Estimating the mechanical properties of brain tumors using transcranial ultrasound is a promising approach. However, the complexity of cranial features introduces challenges, such as ultrasound attenuation and interference from multidirectional transcranial shear waves induced by impact vibrations. To address these issues, this study proposes a transcranial ultrasound estimation method assisted by transcranial shear vibrations. Transcranial vibrations apply shear forces on the parietal bone, inducing unidirectional transcranial shear waves within brain tissue, as validated through simulations. Shear waves at different frequencies were captured via transcranial ultrasound, which were used to assess the viscoelasticity and fluidity of brain tumors. Transcranial experimental validations were conducted in 3D-printed models with tumor phantoms and ex vivo animal tumors. Vibration safety assessments were also performed. The results demonstrate that transcranial ultrasound can detect micron displacements induced by transcranial shear waves. In phantom and ex vivo animal experiments, speed distribution maps were employed to determine the size and location of one or two tumors enclosed in the skull model. The results revealed that the proposed approach could detect tumors with a minimum diameter of 0.8 cm and an inter-tumor distance of 0.8 cm. Notably, significant differences in viscoelasticity and fluidity between normal brain tissue and brain tumors were found (p<0.001). The maximum assessment errors for the elasticity, viscosity, and fluidity using transcranial ultrasound were 11.90%, 4.82%, and 0.73%, respectively, indicating that fluidity was more robust than viscoelasticity. The maximum accelerations of the skull were only 3.21 ms-2.

Resonance, Velocity, Dispersion, and Attenuation of Ultrasound-Induced Shear Wave Propagation in Blood Clot In Vitro Models

OBJECTIVE

Improve the characterization of mechanical properties of blood clots. Parameters derived from shear wave (SW) velocity and SW amplitude spectra were determined for gel phantoms and in vitro blood clots.

METHODS

Homogeneous phantoms and phantoms with gel or blood clot inclusions of different diameters and mechanical properties were analyzed. SW amplitude spectra were used to observe resonant peaks. Parameters derived from those resonant peaks were related to mimicked blood clot properties. Three regions of interest were tested to analyze where resonances occurred the most. For blood experiments, 20 samples from different pigs were analyzed over time during a 110-minute coagulation period using the Young modulus, SW frequency dispersion, and SW attenuation.

RESULTS

The mechanical resonance was manifested by an increase in the number of SW spectral peaks as the inclusion diameter was reduced (P < .001). In blood clot inclusions, the Young modulus increased over time during coagulation (P < .001). Descriptive spectral parameters (frequency peak, bandwidth, and distance between resonant peaks) were linearly correlated with clot elasticity values (P < .001) with R2  = .77 for the frequency peak, .60 for the bandwidth, and .48 for the distance between peaks. The SW dispersion and SW attenuation reflecting the viscous behavior of blood clots decreased over time (P < .001), mainly in the early stage of coagulation (first minutes).

CONCLUSION

The confined soft inclusion configuration favored SW mechanical resonances potentially challenging the computation of spectral-based parameters, such as the SW attenuation. The impact of resonances can be reduced by properly selecting the region of interest for data analysis.

A novel ultrasonic method for measuring minute sinusoidal displacement by network analyzer

We developed a method for generating continuous sinusoidal displacements of an object to estimate viscoelastic parameters. However, the amplitude of the displacement caused by the ultrasonic excitation force under safe guidelines was small (a few micrometers), and it was difficult to stably measure the displacement. Therefore, to stably measure the amplitude of sinusoidal displacement as small as the order of micrometers, we proposed a novel method using a network analyzer. Ultrasonic waves were irradiated using an ultrasonic transducer on an object vibrating sinusoidally. The S parameter of the first reflected wave received from the surface of the object was measured using a network analyzer. The S parameter and the inverse Fourier transform were formulated theoretically, and the amplitude of the sinusoidal displacement of the object was estimated from the amplitude characteristics of the inverse Fourier-transformed signal. The proposed method was applied to measure sinusoidal displacements on the order of micrometers from 10 to 300 Hz on an object using a water tank experiment. The obtained sinusoidal displacement agreed well with the reference values measured using a laser displacement meter. The proposed method can accurately measure minute sinusoidal displacements that occur on an object.

Feasibility of Phase Velocity Imaging Using Multi Frequency Oscillation-Shear Wave Elastography

OBJECTIVE

To assess viscoelasticity, a pathologically relevant biomarker, shear wave elastography (SWE) generally uses phase velocity (PV) dispersion relationship generated via pulsed acoustic radiation force (ARF) excitation pulse. In this study, a multi-frequency oscillation (MFO)- excitation pulse with higher weight to higher frequencies is proposed to generate PV images via the generation of motion with energy concentrated at the target frequencies in contrast to the broadband frequency motion generated in pulsed SWE (PSWE).

METHODS

The feasibility of MFO-SWE to generate PV images at 100 to 1000 Hz in steps of 100 Hz was investigated by imaging 6 and 70 kPa inclusions with 6.5 and 10.4 mm diameter and ex vivo bovine liver with and without the presence of an aberration layer and chicken muscle ex vivo, and 4T1 mouse breast tumor, in vivo with comparisons to PSWE.

RESULTS

MFO-SWE-derived CNR was statistically higher than PSWE for 6 kPa (both with and without aberration) and 70 kPa (with aberration) inclusions and derived SNR of the liver was statistically higher than PSWE at higher frequency (600-1000 Hz). Quantitatively, at 600-1000 Hz, MFO-SWE improved CNR of inclusions (without and with) aberration on an average by (8.2 and 156)% and of the tumor by 122%, respectively, and improved SNR of the liver (without and with) aberration by (20.2 and 51.5)% and of chicken muscle by 72%, respectively compared to the PSWE.

CONCLUSIONS AND SIGNIFICANCE

These results indicate the advantages of MFO-SWE to improve PV estimation at higher frequencies which could improve viscoelasticity quantification and feature delineation.

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.

Load sharing between synergistic muscles characterized by a ligand-binding approach and elastography

The skeletal muscle contraction is determined by cross-bridge formation between the myosin heads and the actin active sites. When the muscle contracts, it shortens, increasing its longitudinal shear elastic modulus ([Formula: see text]). Structurally, skeletal muscle can be considered analogous to the molecular receptors that form receptor-ligand complexes and exhibit specific ligand-binding dynamics. In this context, this work aims to apply elastography and the ligand-binding framework to approach the possible intrinsic mechanisms behind muscle synergism. Based on the short-range stiffness principle and the acoustic-elasticity theory, we define the coefficient [Formula: see text], which is directly related to the fraction saturation of molecular receptors and links the relative longitudinal deformation of the muscle to its [Formula: see text]. We show that such a coefficient can be obtained directly from [Formula: see text] estimates, thus calculating it for the biceps brachii, brachioradialis, and brachialis muscles during isometric elbow flexion torque (τ) ramps. The resulting [Formula: see text] curves were analyzed by conventional characterization methods of receptor-ligand systems to study the dynamical behavior of each muscle. The results showed that, depending on muscle, [Formula: see text] exhibits typical ligand-binding dynamics during joint torque production. Therefore, the above indicates that these different behaviors describe the longitudinal shortening pattern of each muscle during load sharing. As a plausible interpretation, we suggested that this could be related to the binding kinetics of the cross-bridges during their synergistic action as torque increases. Likewise, it shows that elastography could be useful to assess contractile processes at different scales related to the change in the mechanical properties of skeletal muscle.

Clinical Application of Quantitative MR Imaging in Nonalcoholic Fatty Liver Disease

Viral hepatitis was previously the most common cause of chronic liver disease. However, in recent years, nonalcoholic fatty liver disease (NAFLD) cases have been increasing, especially in developed countries. NAFLD is histologically characterized by fat, fibrosis, and inflammation in the liver, eventually leading to cirrhosis and hepatocellular carcinoma. Although biopsy is the gold standard for the assessment of the liver parenchyma, quantitative evaluation methods, such as ultrasound, CT, and MRI, have been reported to have good diagnostic performances. The quantification of liver fat, fibrosis, and inflammation is expected to be clinically useful in terms of the prognosis, early intervention, and treatment response for the management of NAFLD. The aim of this review was to discuss the basics and prospects of MRI-based tissue quantifications of the liver, mainly focusing on proton density fat fraction for the quantification of fat deposition, MR elastography for the quantification of fibrosis, and multifrequency MR elastography for the evaluation of inflammation.

Comb Detection for Measuring Shear Wave Propagation

Plane wave compounding (PWC) is widely used to measure the propagation of shear waves. Implementing PWC on most commercial ultrasound scanners is challenging because all channel (>128) data must be processed or transferred to the host computing unit in real time. Comb detection transmits multiple focused beams simultaneously and results in a reduced number of receive lines to be processed in parallel. These comb beams are scanned laterally to acquire receive lines at different lateral positions in order to obtain data over a large region of interest (ROI). One of the potential issues with using multiple simultaneously transmitted beams is the issue of crosstalk between the beams. Crosstalk is analyzed through simulated beam patterns, simulated B-mode images, and motion data from shear wave elastography (SWE) experiments. Using a Hamming window on transmit and receive can suppress crosstalk to 1.2% root-mean-square error (RMSE, normalized RMSE to the peak magnitude of the reference signal) for shear wave motion signals. Four comb beams with three laterally scanned locations cover almost the entire field of view (FOV) and achieve the same frame rate as PWC with three angles. Phantom and in vivo studies demonstrate comparable motion data of comb detection to PWC in terms of motion signal quality and measured phase velocity. In addition, comb detection provides motion with lower noise and stronger signals than PWC, which is believed to be due to the advantages of transmitting focused beams rather than plane waves (PWs).

Reliability of Continuous Shear Wave Elastography in the Pathological Patellar Tendon

OBJECTIVES

Patellar tendon injuries occur via various mechanisms such as overuse, or due to surgical graft harvest for anterior cruciate ligament reconstruction (ACLR). Quantified patellar tendon stiffness after injury may help guide clinical care. Continuous shear wave elastography (cSWE) allows for the assessment of viscosity and shear modulus in tendons. The reliability of the measure, however, has not been established in the patellar tendon. The purpose of this study was to investigate the interrater reliability, intrarater reliability, and between-day stability of cSWE in both healthy and pathological patellar tendons.

METHODS

Participants with patellar tendinopathy (n = 13), history of ACLR using bone-patellar tendon-bone autograft (n = 9), and with no history of patellar tendon injury (n = 13) were recruited. cSWE was performed 4 times by multiple raters over 2 days. Intraclass correlations (ICC) and minimum detectable change (MDC_95% ) were calculated.

RESULTS

Good to excellent between-day stability were found for viscosity (ICC = 0.905, MDC_95%  = 8.3 Pa seconds) and shear modulus (ICC = 0.805, MDC_95%  = 27.4 kPa). The interrater reliability measures, however, were not as reliable (ICC = 0.591 and 0.532).

CONCLUSIONS

cSWE is a reliable assessment tool for quantifying patellar tendon viscoelastic properties over time. It is recommended, however, that a single rater performs the measure as the interrater reliability was less than ideal.

Clinical Application of Shear Wave Elastography With Shear Wave Dispersion Imaging in the Preoperative Evaluation of Hepatic Parenchyma in Patients With Liver Tumors

OBJECTIVES

This study aimed to compare the diagnostic accuracy of shear wave elastography (SWE) with that of shear wave dispersion (SWD) in evaluation of hepatic parenchyma in patients with liver tumors before resection.

METHODS

A total of 174 patients with liver tumors were prospectively enrolled. SWE and SWD examinations were performed. Fibrosis stage and necroinflammatory activity were determined histopathologically according to the Scheuer standard. We compared the diagnostic accuracy of SWE and SWD.

RESULTS

Both SWE and SWD values of the liver were highly correlated with liver fibrosis stage (P < .05, respectively). Both SWE and SWD values of the liver were moderately correlated with necroinflammatory activity (P < .05, respectively). Both SWE and SWD values of the liver were not correlated with steatosis (P > .05, respectively). Both SWE and SWD values were significantly different among the patients with different stages of liver fibrosis (P < .001, respectively). The area under the receiver operating characteristic (ROC) curve of SWE value was 0.982, 0.977, 0.969, and 0.984 for predicting S ≥ 1, S ≥ 2, S ≥ 3, and S = 4, respectively. The optimal cutoff SWE values were 6.9, 7.9, 8.7, and 10.6 kPa for S ≥ 1, S ≥ 2, S ≥ 3, and S = 4, respectively. The area under the ROC curve of SWD value was 0.967, 0.960, 0.925, and 0.954 for predicting S ≥ 1, S ≥ 2, S ≥ 3, and S = 4, respectively. The optimal cutoff SWD values were 11.2, 12.0, 13.2, and 16.0 m/s/kHz for S ≥ 1, S ≥ 2, S ≥ 3, and S = 4, respectively.

CONCLUSIONS

SWE and SWD could be noninvasive and accurate for predicting the stage of liver fibrosis in patients with liver tumors before surgery. SWE was more accurate than SWD in predicting severe fibrosis (S ≥ 3) and cirrhosis (S = 4).

Axial stress determines the velocity of shear wave propagation in passive but not active muscles in vivo

Ultrasound shear wave elastography can be used to characterize mechanical properties of unstressed tissue by measuring shear wave velocity (SWV), which increases with increasing tissue stiffness. Measurements of SWV have often been assumed to be directly related to the stiffness of muscle. Some have also used measures of SWV to estimate stress, since muscle stiffness and stress covary during active contractions, but few have considered the direct influence of muscle stress on SWV. Rather, it is often assumed that stress alters the material properties of muscle, and in turn, shear wave propagation. The objective of this study was to determine how well the theoretical dependency of SWV on stress can account for measured changes of SWV in passive and active muscles. Data were collected from six isoflurane-anesthetized cats; three soleus muscles and three medial gastrocnemius muscles. Muscle stress and stiffness were measured directly along with SWV. Measurements were made across a range of passively and actively generated stresses, obtained by varying muscle length and activation, which was controlled by stimulating the sciatic nerve. Our results show that SWV depends primarily on the stress in a passively stretched muscle. In contrast, the SWV in active muscle is higher than would be predicted by considering only stress, presumably due to activation-dependent changes in muscle stiffness. Our results demonstrate that while SWV is sensitive to changes in muscle stress and activation, there is not a unique relationship between SWV and either of these quantities when considered in isolation.NEW & NOTEWORTHY Ultrasound shear wave elastography may be an inexpensive way to measure muscle stress in passive muscle. Here, using a cat model we directly measured shear wave velocity (SWV), muscle stress, and muscle stiffness. Our results show that SWV depends primarily on the stress in a passively stretched muscle. In contrast, the SWV in active muscle is higher than would be predicted by considering only stress, presumably due to activation-dependent changes in muscle stiffness.

A New Estimation Scheme for Improving the Performance of Shear Wave Elasticity Imaging

Shear wave velocity (SWV) reconstruction based on time-of-flight (TOF) is widely adopted to realize shear wave elasticity imaging (SWEI). It typically breaks down the reconstruction of a SWV image into many kernels and treats them independently. We hypothesized that information exchange among kernels improves the performance of SWEI. Therefore, we propose the approach of iterative re-weighted least squares based on inter-kernel communication (IKC-IRLS). We also hypothesized that time-to-peak (TTP) is superior to cross-correlation (CC) in visualizing small targets because TTP uses higher shear wave frequencies than CC. To examine the hypotheses, IKC-IRLS was combined with TTP data and compared with four established methods. The five methods were tested by imaging several small-size stiff targets (2.5, 4.0 and 6.4 mm in diameter) using different kernel sizes in the simulation and real experiments. The results indicate that the IKC-IRLS approach can mitigate speckle noise and is robust to TTP outliers. Consequently, the proposed method achieves the highest contrast-to-noise ratio and the lowest mean absolute percentage error of target in almost all tested cases.

Noninvasive Assessment of Liver Fibrosis and Inflammation in Chronic Hepatitis B: A Dual-task Convolutional Neural Network (DtCNN) Model Based on Ultrasound Shear Wave Elastography

BACKGROUND AND AIMS

Liver stiffness (LS) measured by shear wave elastography (SWE) is often influenced by hepatic inflammation. The aim was to develop a dual-task convolutional neural network (DtCNN) model for the simultaneous staging of liver fibrosis and inflammation activity using 2D-SWE.

METHODS

A total of 532 patients with chronic hepatitis B (CHB) were included to develop and validate the DtCNN model. An additional 180 consecutive patients between December 2019 and April 2021 were prospectively included for further validation. All patients underwent 2D-SWE examination and serum biomarker assessment. A DtCNN model containing two pathways for the staging of fibrosis and inflammation was used to improve the classification of significant fibrosis (≥F2), advanced fibrosis (≥F3) as well as cirrhosis (F4).

RESULTS

Both fibrosis and inflammation affected LS measurements by 2D-SWE. The proposed DtCNN performed the best among all the classification models for fibrosis stage [significant fibrosis AUC=0.89 (95% CI: 0.87-0.92), advanced fibrosis AUC=0.87 (95% CI: 0.84-0.90), liver cirrhosis AUC=0.85 (95% CI: 0.81-0.89)]. The DtCNN-based prediction of inflammation activity achieved AUCs of 0.82 (95% CI: 0.78-0.86) for grade ≥A1, 0.88 (95% CI: 0.85-0.90) grade ≥A2 and 0.78 (95% CI: 0.75-0.81) for grade ≥A3, which were significantly higher than the AUCs of the single-task groups. Similar findings were observed in the prospective study.

CONCLUSIONS

The proposed DtCNN improved diagnostic performance compared with existing fibrosis staging models by including inflammation in the model, which supports its potential clinical application.

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.

Viscoelasticity assessment for in vivo quantification of muscle contusion injury in rats using shear wave elastography

The aim of the study described here was to investigate the role of viscoelasticity in assessing muscle fibrosis and inflammation in a rat model of contusion using quantitative shear wave elastography (SWE). Unilateral gastrocnemius muscle contusion was induced in 32 male rats using an impactor apparatus. The contralateral muscles served as the control group. SWE was applied to the control group and rats 1, 3, 14 and 21 d after successful modeling (each time point group, n = 8). Histologic features were used as reference standards. The degree of fibrosis was moderately correlated with shear wave speed (r = 0.53), whereas the degree of inflammation was well correlated with shear wave dispersion (SWD) slope (r = 0.74). The area under the receiver operating characteristic curve (AUC) for the dispersion slope for muscle inflammation and fibrosis assessment was 0.87 (95% confidence interval: 0.705-0.963), which exceeded that of the shear wave speed (0.68, 95% confidence interval: 0.494-0.834). The larger decline in dispersion slope in the fibrotic stage than in the inflammation stage (1-d group vs. 14-d group or 21-d group, p &lt; 0.05) indicated better predictive performance than the shear wave speed.

Improved two-point frequency shift power method for measurement of shear wave attenuation

Quantitative assessment of mechanical properties of biological soft tissues is frequently evaluated using a noninvasive modality, called ultrasound shear wave elastography (SWE). SWE typically exerts an acoustic radiation force (ARF) to produce shear waves propagating in the lateral direction for which velocities and attenuations are measured. The tissue viscoelasticity is commonly studied by investigating the shear wave phase velocity curves. Viscoelastic tissue properties can also be characterized through utilizing various shear wave attenuation techniques. In this study, we propose an improved method for measuring the shear wave attenuation, called two-point frequency shift power (2P-FSP), which is an improved version of the two-point frequency shift (2P-FS) method. The technique is fully data driven and does not use a rheological model for mathematical modeling. The 2P-FSP method utilizes the power spectra frequency shift of shear waves measured at two spatial positions, which provides robustness to noise. The conceptual basis for the 2P-FSP is provided and tested with numerical and experimental data. We investigated how the location of the first signal and the distance interval between the two locations influence the shear wave attenuation measurement in the 2P-FSP technique. We utilized the 2P-FSP method on numerical phantom data generated using a finite-difference-based method in tissue-mimicking viscoelastic media. Moreover, we tested the 2P-FSP method with data from custom-made tissue-mimicking viscoelastic phantom experiments, and ex vivo porcine liver. We compared results from the proposed technique with results from 2P-FS and analytical values in the case of simulations. The results showed that the 2P-FSP method provides improved results over the 2P-FS technique for lower signal-to-noise ratio (SNR) and locations farther from the push location considered, and can be used to measure attenuation of viscoelastic soft tissues.

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.

Quantitative Estimation of Mechanical Anisotropy Using Acoustic Radiation Force (ARF)-Induced Peak Displacements (PD): In Silico and Experimental Demonstration

Elastic degree of anisotropy (DoA) is a diagnostically relevant biomarker in muscle, kidney, breast, and other organs. Previously, elastic DoA was qualitatively assessed as the ratio of peak displacements (PD) achieved with the long-axis of a spatially asymmetric Acoustic Radiation Force Impulse (ARFI) excitation point spread function (PSF) aligned along versus across the axis of symmetry (AoS) in transversely isotropic materials. However, to better enable longitudinal and cross-sectional analyses, a quantitative measure of elastic DoA is desirable. In this study, qualitative ARFI PD ratios are converted to quantitative DoA, measured as the ratio of longitudinal over transverse shear elastic moduli, using a model empirically derived from Field II and finite element method (FEM) simulations. In silico, the median absolute percent error (MAPE) in ARFI-derived shear moduli ratio (SMR) was 1.75%, and predicted SMRs were robust to variations in transverse shear modulus, Young's moduli ratio, speed of sound, attenuation, density, and ARFI excitation PSF dimension. Further, ARFI-derived SMRs distinguished two materials when the true SMRs of the compared materials differed by as little as 10%. Experimentally, ARFI-derived SMRs linearly correlated with the corresponding ratios measured by Shear Wave Elasticity Imaging (SWEI) in excised pig skeletal muscle ( [Formula: see text], MAPE = 13%) and in pig kidney, in vivo ( [Formula: see text], MAPE = 5.3%). These results demonstrate the feasibility of using the ARFI PD to quantify elastic DoA in biological tissues.

Nonlinear Characterization of Tissue Viscoelasticity With Acoustoelastic Attenuation of Shear Waves

Shear-wave elastography (SWE) measures shear-wave speed (SWS), which is related to the underlying shear modulus of soft tissue. SWS in soft tissue changes depending on the amount of external strain that soft tissue is subjected to due to the acoustoelastic (AE) phenomenon. In the literature, variations of SWS as a function of applied uniaxial strain were used for nonlinear characterization, assuming soft tissues to be elastic, although soft tissues are indeed viscoelastic in nature. Hence, nonlinear characterization using SWS alone is insufficient. In this work, we use SWS together with shear-wave attenuation (SWA) during incremental quasi-static compressions in order to derive biomechanical characterization based on the AE theory in terms of well-defined storage and loss moduli. As part of this study, we also quantify the effect of applied strain on measurements of SWS and SWA since such confounding effects need to be taken into account when using SWS and/or SWA, e.g., for staging a disease state, while such effects can also serve as an additional imaging biomarker. Our results from tissue-mimicking phantoms with varying oil percentages and ex vivo porcine liver experiments demonstrate the feasibility of our proposed methods. In both experiments, SWA was observed to decrease with applied strain. For 10% compression in ex vivo livers, shear-wave attenuation decreased, on average, by 28% (93 Np/m), while SWS increased, on average, by 20% (0.26 m/s).

Plantar Soft Tissue Characterization Using Reverberant Shear Wave Elastography: A Proof-of-Concept Study

Plantar soft tissue stiffness provides relevant information on biomechanical characteristics of the foot. Therefore, appropriate monitoring of foot elasticity could be useful for diagnosis, treatment or health care of people with complex pathologies such as a diabetic foot. In this work, the reliability of reverberant shear wave elastography (RSWE) applied to plantar soft tissue was investigated. Shear wave speed (SWS) measurements were estimated at the plantar soft tissue at the first metatarsal head, the third metatarsal head and the heel from both feet in five healthy volunteers. Experiments were repeated for a test-retest analysis with and without the use of gel pad using a mechanical excitation frequency range between 400 and 600 Hz. Statistical analysis was performed to evaluate the reliability of the SWS estimations. In addition, the results were compared against those obtained with a commercially available shear wave-based elastography technique, supersonic imaging (SSI). The results indicate a low coefficient of variation for test-retest experiments with gel pad (median: 5.59%) and without gel pad (median: 5.83%). Additionally, the values of the SWS measurements increase at higher frequencies (median values: 2.11 m/s at 400 Hz, 2.16 m/s at 450 Hz, 2.24 m/s at 500 Hz, 2.21 m/s at 550 Hz and 2.31 m/s at 600 Hz), consistent with previous reports at lower frequencies. The SWSs at the plantar soft tissue at the first metatarsal head, third metatarsal head and heel were found be significantly (p<0.05) different, with median values of 2.42, 2.16 and 2.03 m/s, respectively which indicates the ability of the method to differentiate between shear wave speeds at different anatomical locations. The results indicated better elastographic signal-to-noise ratios with RSWE compared to SSI because of the artifacts presented in the SWS generation. These preliminary results indicate that the RSWE approach can be used to estimate the plantar soft tissue elasticity, which may have great potential to better evaluate changes in biomechanical characteristics of the foot.

Shear-Wave Dispersion Slope of the Liver: Effect of Study Protocol and Ascites on the Measurement Applicability

This study aimed to evaluate the shear-wave dispersion (SWD) scanning protocol including the minimum number of measurements and better size of the region of interest (ROI), as well as the influence of ascites on the measurement applicability. Patients who had undergone serial SWD examinations between July 2019 and December 2020 were included. In patients with chronic liver disease (group A), two different ROI sizes were applied, and at least 10 measurements were repeated to determine the minimum number of measurements and better ROI size. In patients with liver failure (group B), failure and unreliable results were compared between patients with and without ascites. A minimum of five measurements when using a 20-mm ROI and six measurements when using a 10-mm ROI were required. Compared with using a 20-mm ROI, a 10-mm ROI showed a higher unreliable rate. The failure and unreliable rates of SWD in patients with ascites were significantly higher than those in patients without ascites. SWD examination required at least five measurements when using a 20-mm ROI and six measurements when using a 10-mm ROI. A larger ROI was associated with higher reliability, and ascites influenced the failure and reliability of the SWD measurement.

Assessing cardiac stiffness using ultrasound shear wave elastography

Shear wave elastography offers a new dimension to echocardiography: it measures myocardial stiffness. Therefore, it could provide additional insights into the pathophysiology of cardiac diseases affecting myocardial stiffness and potentially improve diagnosis or guide patient treatment. The technique detects fast mechanical waves on the heart wall with high frame rate echography, and converts their propagation velocity into a stiffness value. A proper interpretation of shear wave data is required as the shear wave interacts with the intrinsic, yet dynamically changing geometrical and material characteristics of the heart under pressure. This dramatically alters the wave physics of the propagating wave, demanding adapted processing methods compared to other shear wave elastography applications as breast tumor and liver stiffness staging. Furthermore, several advanced analysis methods have been proposed to extract supplementary material features such as viscosity and anisotropy, potentially offering additional diagnostic value. This review explains the general mechanical concepts underlying cardiac shear wave elastography and provides an overview of the preclinical and clinical studies within the field. We also identify the mechanical and technical challenges ahead to make shear wave elastography a valuable tool for clinical practice.

Sources of Variability in Shear Wave Speed and Dispersion Quantification with Ultrasound Elastography: A Phantom Study

There is a growing interest in quantifying shear-wave dispersion (SWD) with ultrasound shear-wave elastography (SWE). Recent studies suggest that SWD complements shear-wave speed (SWS) in diffuse liver disease diagnosis. To accurately interpret these metrics in clinical practice, we analyzed the impact of operator-dependent acquisition parameters on SWD and SWS measurements. Considered parameters were the acquisition depth, lateral position and size of the region of interest (ROI), as well as the size of the SWE acquisition box. Measurements were performed using the Canon Aplio i800 system (Canon Medical Systems, Otawara, Tochigi, Japan) and four homogeneous elasticity phantoms with certified stiffness values ranging from 3.7 to 44 kPa. In general, SWD exhibited two to three times greater variability than SWS. The acquisition depth was the main variance-contributing factor for both SWS and SWD, which decayed significantly with depth. The lateral ROI position contributed as much as the acquisition depth to the total variance in SWD. Locations close to the initial shear-wave excitation pulse were more robust to biases because of inaccurate probe-phantom coupling. The size of the ROI and acquisition box did not introduce significant variations. These results suggest that future guidelines on multiparametric elastography should account for the depth- and lateral-dependent variability of measurements.

Evaluation of Robustness of Local Phase Velocity Imaging in Homogenous Tissue-Mimicking Phantoms

Shear wave elastography (SWE) is a method of evaluating mechanical properties of soft tissues. Most current implementations of SWE report the group velocity for shear wave velocity, which assumes an elastic, isotropic, homogenous and incompressible tissue. Local phase velocity imaging (LPVI) is a novel method of phase velocity reconstruction that allows for accurate evaluation of shear wave velocity at specified frequencies. This method's robustness was evaluated in 11 elastic and 8 viscoelastic phantoms using linear and curvilinear arrays. We acquired data with acoustic radiation force push beams with different focal depths and F-numbers and reconstructed phase velocity images over a wide range of frequencies. Regardless of phantom, push beam focal depth and reconstruction frequency, an F-number around 3.0 was found to produce the largest usable area in the phase velocity reconstructions. For elastic phantoms scanned with a linear array, the optimal focal depth, frequency range and maximum region of interest (ROI) were 20-30 mm, 100-400 Hz and 2.70 cm², respectively. For viscoelastic phantoms scanned with a linear array, the optimal focal depth, frequency and maximum ROI were 20-30 mm, 100-300 Hz and 1.54 cm², respectively. For the curvilinear array in the same phantoms, optimal focal depth, frequency range and maximum ROIs were 45-60 mm, 100-400 and 100-300 Hz and 1.54 cm², respectively. In further work, LPVI reconstructions from inclusion phantoms will be evaluated to simulate non-homogeneous tissues. Additionally, LPVI will be evaluated in larger-volume phantoms to account for wave reflection from the containers when using the curvilinear array.

Acoustic Radiation Force: A Review of Four Mechanisms for Biomedical Applications

Radiation force is a universal phenomenon in any wave motion where the wave energy produces a static or transient force on the propagation medium. The theory of acoustic radiation force (ARF) dates back to the early 19th century. In recent years, there has been an increasing interest in the biomedical applications of ARF. Following a brief history of ARF, this article describes a concise theory of ARF under four physical mechanisms of radiation force generation in tissue-like media. These mechanisms are primarily based on the dissipation of acoustic energy of propagating waves, the reflection of the incident wave, gradients of the compressional wave speeds, and the spatial variations of energy density in standing acoustic waves. Examples describing some of the practical applications of ARF under each mechanism are presented. This article concludes with a discussion on selected ideas for potential future applications of ARF in biomedicine.

Using rheology to quantify the effects of localized collagenase treatments on uterine fibroid digestion

Uterine fibroids are stiff, benign tumors containing excessive, disordered collagens that occur in 70-80% of women before age 50 and cause bleeding and pain. Collagenase Clostridium histolyticum (CCH) is a bacterial enzyme capable of digesting the collagens present in fibroids. By combining CCH with injectable drug delivery systems to enhance effectiveness, a new class of treatments could be developed to reduce the stiffness of fibroids, preventing the need for surgical removal and preserving fertility. In this work, we achieved localization of CCH via physical entrapment by co-injecting a thermoresponsive pNIPAM-based polymeric delivery system called LiquoGel (LQG), which undergoes a sol-gel transition upon heating. Toxicity study results for LQG injected subcutaneously into mice demonstrate that LQG does not induce lesions or other adverse effects. We then used rheology to quantify the effects of localized CCH injections on the modulus and viscoelasticity of uterine fibroids, which exhibit gel-like behavior, through ex vivo and in vivo digestion studies. Ex vivo CCH injections reduce the tissue modulus by over two orders of magnitude and co-injection of LQG enhances this effect. Rheological results from an in vivo digestion study in mice show a significant reduction in tissue modulus and increase in tissue viscoelasticity 7 days after a single injection of LQG+CCH. Parallel histological staining validates that the observed rheological changes correspond to an increase in collagen lysis after treatment by LQG+CCH. These results show promise for development of injectable and localized enzymatic therapies for uterine fibroids and other dense tumors. STATEMENT OF SIGNIFICANCE: Uterine fibroids are stiff, benign tumors containing high collagen levels that cause bleeding and pain in women. Fertility-preserving and minimally-invasive treatments to soften fibroids are needed as an alternative to surgical removal via hysterectomy. We demonstrate through ex vivo and in vivo studies that co-injecting a thermoresponsive polymer delivery system (LQG) alongside a bacterial collagenase (CCH) enzyme significantly increases treatment effectiveness at softening fibroids through CCH localization. We use rheology to measure the modulus and viscoelasticity of fibroids and histology to show that fibroid softening corresponds to a decrease in collagen after treatment with LQG+CCH. These results highlight the utility of rheology at quantifying tissue properties and present a promising injectable therapy for fibroids and other dense tumors.

The impact of body position and exercise on the measurement of liver Young's modulus by real-time shear wave elastography

BACKGROUND

In the past ten years, liver biopsies have been used as a method to accurately diagnose the stage of fibrosis.

OBJECTIVE

This study aimed to evaluate whether body position and exercise affect the measurement of liver Young's modulus of healthy volunteers by real-time shear wave elastography (RT-SWE).

METHODS

RT-SWE was used to measure liver Young's modulus in the supine and left lateral positions of 70 healthy volunteers at rest and measure the liver Young's modulus in the lying position before exercise, and at zero, five, and ten minutes of rest after exercise.

RESULTS

The liver Young's modulus in the left lateral position was significantly higher than in the supine position (P< 0.05), and the measured value in the supine position was more stable than the left lateral position. The liver Young's modulus measured at zero minutes after exercise was significantly higher than that measured before exercise (P< 0.05). The liver Young's modulus measured at five minutes after exercise was significantly higher than that measured at zero minutes after exercise (P<0.05) and was not statistically different from the measured value before exercise (P> 0.05). The liver Young's modulus measured at ten minutes after exercise was significantly higher from that measured at zero minutes after exercise (P< 0.05) and was not statistically different from the measured value at five minutes after exercise (P> 0.05).

CONCLUSION

Body position and exercise have a significant impact on the measurement of liver Young's modulus. It is recommended that the examinees take a supine position during the measurement, and measurement should be conducted at least ten minutes after exercise.

Surface wave elastography is a reliable method to correlate muscle elasticity, torque, and electromyography activity level

The shear elastic modulus is one of the most important parameters to characterize the mechanical behavior of soft tissues. In biomechanics, ultrasound elastography is the gold standard for measuring and mapping it locally in skeletal muscle in vivo. However, their applications are limited to the laboratory or clinic. Thus, low-frequency elastography methods have recently emerged as a novel alternative to ultrasound elastography. Avoiding the use of high frequencies, these methods allow obtaining a mean value of bulk shear elasticity. However, they are frequently susceptible to diffraction, guided waves, and near field effects, which introduces biases in the estimates. The goal of this work is to test the performance of the non-ultrasound surface wave elastography (NU-SWE), which is portable and is based on new algorithms designed to correct the incidence of such effects. Thus, we show its first application to muscle biomechanics. We performed two experiments to assess the relationships of muscle shear elasticity versus joint torque (experiment 1) and the electromyographic activity level (experiment 2). Our results were comparable regarding previous works using the reference ultrasonic methods. Thus, the NU-SWE showed its potentiality to get wide the biomechanical applications of elastography in many areas of health and sports sciences.

Shear Wave Elastography: A Review on the Confounding Factors and Their Potential Mitigation in Detecting Chronic Kidney Disease

Early detection of chronic kidney disease is important to prevent progression of irreversible kidney damage, reducing the need for renal transplantation. Shear wave elastography is ideal as a quantitative imaging modality to detect chronic kidney disease because of its non-invasive nature, low cost and portability, making it highly accessible. However, the complexity of the kidney architecture and its tissue properties give rise to various confounding factors that affect the reliability of shear wave elastography in detecting chronic kidney disease, thus limiting its application to clinical trials. The objective of this review is to highlight the confounding factors presented by the complex properties of the kidney, in addition to outlining potential mitigation strategies, along with the prospect of increasing the versatility and reliability of shear wave elastography in detecting chronic kidney disease.

Acousto-elasticity of transversely isotropic incompressible soft tissues: characterization of skeletal striated muscle

Using shear wave elastography, we measure the changes in the wave speed with the stress produced by a striated muscle during isometric voluntary contraction. To isolate the behaviour of an individual muscle from complementary or antagonistic actions of adjacent muscles, we select theflexor digiti minimimuscle, whose sole function is to extend the little finger. To link the wave speed to the stiffness, we develop an acousto-elastic theory for shear waves in homogeneous, transversely isotropic, incompressible solids subject to an uniaxial stress. We then provide measurements of the apparent shear elastic modulus along, and transversely to, the fibre axis for six healthy human volunteers of different age and sex. The results display a great variety across the six subjects. We find that the slope of the apparent shear elastic modulus along the fibre direction changes inversely to the maximum voluntary contraction (MVC) produced by the volunteer. We propose an interpretation of our results by introducing the S (slow) or F (fast) nature of the fibres, which harden the muscle differently and accordingly, produce different MVCs. A natural follow-up on this study is to apply the method to patients with musculoskeletal disorders or neurodegenerative diseases.

Time-Aligned Plane Wave Compounding Methods for High-Frame-Rate Shear Wave Elastography: Experimental Validation and Performance Assessment on Tissue Phantoms

Shear wave elastography (SWE) is an ultrasonic technique able to quantitatively assess the mechanical properties of tissues by combining acoustic radiation force and ultrafast imaging. While utilizing coherent plane wave compounding enhances echo and shear wave motion signal-to-noise ratio (SNR), it also reduces the effective pulse repetition frequency (PRF_e), affecting the accuracy of the measurements of motion and, consequently, of material properties. It is important to maintain both high-motion SNR and PRF_e, particularly for the characterization of (material and/or geometrical) dispersive tissues such as arteries. This work proposes a method for SWE measurements with high SNR, while maintaining a high PRF_e, using conventional clinical ultrasound scanners. A time alignment process is applied after acquiring data from plane wave transmissions at different angles. The time alignment uses interpolation to obtain data points at higher frame rates, and the time-aligned data are compounded to increase the SNR. The method is used for SWE in tissue-mimicking phantoms of different stiffness and is compared with traditional plane wave compounding. Increases of 58% and 36% in spatial and temporal bandwidth compared with conventional plane wave compounding, respectively, can be achieved for SWE measurements of representative arterial stiffness values. Improvements in phase velocity accuracy and bandwidth in an arterial phantom are also described, to emphasize the beneficial advantage in dispersive cases.

Prostate Cancer Detection Using 3-D Shear Wave Elasticity Imaging

Transrectal ultrasound (TRUS) B-mode imaging provides insufficient sensitivity and specificity for prostate cancer (PCa) targeting when used for biopsy guidance. Shear wave elasticity imaging (SWEI) is an elasticity imaging technique that has been commercially implemented and is sensitive and specific for PCa. We have developed a SWEI system capable of 3-D data acquisition using a dense acoustic radiation force (ARF) push approach that leads to enhanced shear wave signal-to-noise ratio compared with that of the commercially available SWEI systems and facilitates screening of the entire gland before biopsy. Additionally, we imaged and assessed 36 patients undergoing radical prostatectomy using 3-D SWEI and determined a shear wave speed threshold separating PCa from healthy prostate tissue with sensitivities and specificities akin to those for multiparametric magnetic resonance imaging fusion biopsy. The approach measured the mean shear wave speed in each prostate region to be 4.8 m/s (Young's modulus E = 69.1 kPa) in the peripheral zone, 5.3 m/s (E = 84.3 kPa) in the central gland and 6.0 m/s (E = 108.0 kPa) for PCa with statistically significant (p < 0.0001) differences among all regions. Three-dimensional SWEI receiver operating characteristic analyses identified a threshold of 5.6 m/s (E = 94.1 kPa) to separate PCa from healthy tissue with a sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and area under the curve (AUC) of 81%, 82%, 69%, 89% and 0.84, respectively. Additionally, a shear wave speed ratio was assessed to normalize for tissue compression and patient variability, which yielded a threshold of 1.11 to separate PCa from healthy prostate tissue and was accompanied by a substantial increase in specificity, PPV and AUC, where the sensitivity, specificity, PPV, NPV and AUC were 75%, 90%, 79%, 88% and 0.90, respectively. This work illustrates the feasibility of using 3-D SWEI data to detect and localize PCa and demonstrates the benefits of normalizing for applied compression during data acquisition for use in biopsy targeting studies.

MRI and muscle imaging for idiopathic inflammatory myopathies

Although idiopathic inflammatory myopathies (IIM) are a heterogeneous group of diseases nearly all patients display muscle inflammation. Originally, muscle biopsy was considered as the gold standard for IIM diagnosis. The development of muscle imaging led to revisiting not only the IIM diagnosis strategy but also the patients' follow-up. Different techniques have been tested or are in development for IIM including positron emission tomography, ultrasound imaging, ultrasound shear wave elastography, though magnetic resonance imaging (MRI) remains the most widely used technique in routine. Whereas guidelines on muscle imaging in myositis are lacking here we reviewed the relevance of muscle imaging for both diagnosis and myositis patients' follow-up. We propose recommendations about when and how to perform MRI on myositis patients, and we describe new techniques that are under development.

Phase Velocity Estimation With Expanded Bandwidth in Viscoelastic Phantoms and Tissues

Ultrasound shear wave elastography (SWE) is a technique used to measure mechanical properties to evaluate healthy and pathological soft tissues. SWE typically employs an acoustic radiation force (ARF) to generate laterally propagating shear waves that are tracked in the spatiotemporal domains, and algorithms are used to estimate the wave velocity. The tissue viscoelasticity is often examined through analyzing the shear wave phase velocity dispersion curves, which is the variation of phase velocity with frequency or wavelength. A number of available methods to estimate dispersion exist, which can differ in resolution and variance. Moreover, most of these techniques reconstruct dispersion curves for a limited frequency band. In this work, we propose a novel method used for dispersion curve calculation. Our unique approach uses a generalized Stockwell transformation combined with a slant frequency-wavenumber analysis (GST-SFK). We tested the GST-SFK method on numerical phantom data generated using a finite-difference-based method in tissue-mimicking viscoelastic media. In addition, we evaluated the method on numerical shear wave motion data with different amounts of white Gaussian noise added. Additionally, we performed tests on data from custom-made tissue-mimicking viscoelastic phantom experiments, ex vivo porcine liver measurements, and in vivo liver tissue experiments. We compared results from our method with two other techniques used for estimating shear wave phase velocity: the two-dimensional Fourier transform (2D-FT) and the eigenvector (EV) method. Tests carried out revealed that the GST-SFK method provides dispersion curve estimates with lower errors over a wider frequency band in comparison to the 2D-FT and EV methods. In addition, the GST-SFK provides expanded bandwidth by a factor of two or more to be used for phase velocity estimation, which is meaningful for a tissue dispersion analysis in vivo.

Viscoelastic parameter estimation using simulated shear wave motion and convolutional neural networks

Ultrasound shear wave elastography (SWE) techniques have been very useful for the analysis of tissue rheological properties, but there are still obstacles for robust evaluation of viscoelastic tissue properties. In this proof-of-concept study, we investigate whether convolutional neural networks (CNN) are capable of retrieving the elasticity and viscosity parameters from simulated shear wave motion images. Staggered-grid finite difference simulations based on a Kelvin-Voigt rheological model were used to generate data for this study. The wave motion datasets were created using Kelvin-Voigt shear elasticity values ranging from 1 to 25 kPa, shear viscosities ranging from 0 to 10 Pa⋅s, and two different push profiles using f-numbers of 1 and 2. The CNN architectures, optimized using mean squared error loss, were then trained to retrieve a specific viscoelastic parameter. Both elasticity and viscosity values were successfully retrieved, with regression R² values above 0.99 when correlating the estimated mechanical properties versus the true mechanical properties. The CNN performance was also compared to estimation of shear elasticity and viscosity from fitting dispersion curves estimated from two-dimensional Fourier transform analysis. The results demonstrated that the CNN models were robust to noise, vertical position and partially to f-number. The architecture was proven to be robust to multiple push profiles if trained properly. The CNN results showed higher accuracy over the full viscoelastic parameter range compared to the Fourier-based analysis. The overall results showed the CNNs' potential to be an alternative to complex mathematical analyses such as Fourier analysis and dispersion curve estimation used currently for shear wave viscoelastic parameter estimation.

Multiparametric in vivo ultrasound shear wave viscoelastography on farm-raised fatty duck livers: human radiology imaging applied to food sciences

Nine mulard ducks that were being raised for foie gras (steatosis) production went through in vivo shear wave (SW) elastography imaging of their liver during the force-feeding period to investigate changes in liver tissue characteristics. A total of 4 imaging sessions at an interval of 3 to 4 d were conducted at the farm on each animal. Three ducks were sacrificed at the second, third, and fourth imaging sessions for histopathology analysis of all animals at these time points. Six SW elastography parameters were evaluated: SW speed, SW attenuation, SW dispersion, Young's modulus, viscosity, and shear modulus. Shear waves of different frequencies propagate with different phase velocities. Thus, SW speed and other dependent parameters such as Young's modulus, viscosity, and shear modulus were computed at 2 frequencies: 75 and 202 Hz. Each parameter depicted a statistically significant trend along the force-feeding process (P-values between 0.001 and 0.0001). The fat fraction of the liver increased over the 12-day period of feeding. All parameters increased monotonically over time at 75 Hz, whereas modal relations were seen at 202 Hz. Shear wave dispersion measured between 75 and 202 Hz depicted a plateau from day 5. Based on this validation, proposed imaging methods are aimed to be used in the future on naturally fed ducks and geese.

Severity of liver fibrosis using shear wave elastography is influenced by hepatic necroinflammation in chronic hepatitis patients, but not in cirrhotic patients

AIM

Shear wave elastography (SWE) in patients with chronic liver diseases is a noninvasive useful method for the diagnosis of liver fibrosis severity, which can be an alternative to liver biopsy. However, the liver stiffness measurement using SWE can be affected by various factors including hepatic inflammation, extrahepatic cholestasis, heart failure, and underlying liver diseases. The aim of this study is to clarify the correlation between liver stiffness using SWE and hepatic necroinflammation serologically and pathologically.

METHODS

A total of 843 patients with chronic liver disease who received liver biopsy were analyzed. Liver stiffness measurement using transient elastography (TE) and virtual touch quantification (VTQ) were carried out on the same day as the liver biopsy. The correlation between SWE and hepatic inflammation was analyzed serologically and pathologically.

RESULTS

The liver stiffness values increased significantly with the progression of liver fibrosis and inflammation (overall p < 0.001). In patients with F0-1, F2, and F3, TE and VTQ values of A2 or A3 were significantly higher than those of A0 or A1 (p value, all <0.05), but not in patients with F4. The median alanine aminotransferase (ALT) values increased significantly with the progression of liver inflammation (p < 0.001). Moreover, TE and VTQ in patients with ALT ≥70 IU/L were significantly higher than those in patients with ALT <70 IU/L (p < 0.01), but not in patients with F4.

CONCLUSION

Shear wave elastography can be affected by hepatic necroinflammation in F0-F3 fibrosis, but not in F4.

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.

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.

Shear wave dispersion as a potential biomarker for cervical remodeling during pregnancy: evidence from a non-human primate model

Shear wave dispersion (variation of phase velocity with frequency) occurs in tissues with layered and anisotropic microstructure and viscous components, such as the uterine cervix. This phenomenon, mostly overlooked in previous applications of cervical Shear Wave Elasticity Imaging (SWEI) for preterm birth risk assessment, is expected to change drastically during pregnancy due to cervical remodeling. Here we demonstrate the potential of SWEI-based descriptors of dispersion as potential biomarkers for cervical remodeling during pregnancy. First, we performed a simulation-based pre-selection of two SWEI-based dispersion descriptors: the ratio R of group velocities computed with particle-velocity and particle-displacement, and the slope S of the phase velocity vs. frequency. The pre-selection consisted of comparing the contrast-to-noise ratio (CNR) of dispersion descriptors in materials with different degrees of dispersion with respect to a low-dispersive medium. Shear waves induced in these media by SWEI were simulated with a finite-element model of Zener viscoelastic solids. The pre-selection also considered two denoising strategies to improve CNR: a low-pass filter with automatic frequency cutoff determination, and singular value decomposition of shear wave displacements. After pre-selection, the descriptor-denoising combination that produced the largest CNR was applied to SWEI cervix data from 18 pregnant Rhesus macaques acquired at weeks 10 (mid-pregnancy stage) and 23 (late pregnancy stage) of the 24.5-week full pregnancy. A maximum likelihood linear mixed-effects model (LME) was used to evaluate the dependence of the dispersion descriptor on pregnancy stage, maternal age, parity and other experimental factors. The pre-selection study showed that descriptor S combined with singular value decomposition produced a CNR 11.6 times larger than the other descriptor and denoising strategy combinations. In the Non-Human Primates (NHP) study, the LME model showed that descriptor S significantly decreased from mid to late pregnancy (-0.37 ± 0.07 m/s-kHz per week, p <0.00001) with respect to the base value of 15.5 ± 1.9 m/s-kHz. This change was more significant than changes in other SWEI features such as the group velocity previously reported. Also, S varied significantly between the anterior and posterior portions of the cervix (p =0.02) and with maternal age (p =0.008). Given the potential of shear wave dispersion to track cervical remodeling, we will extend its application to ongoing longitudinal human studies.

An analytical model of full-field displacement and strain induced by amplitude-modulated focused ultrasound in harmonic motion imaging

The majority of disease processes involves changes in the micro-structure of the affected tissue, which can translate to changes in the mechanical properties of the corresponding tissue. Harmonic motion imaging (HMI) is an elasticity imaging technique that allows the study of the mechanical parameters of tissue by detecting the tissue response by a harmonic motion field, which is generated by oscillatory acoustic radiation force (ARF). HMI has been demonstrated in tumor detection and characterization as well as monitoring of ablation procedures. In this study, an analytical HMI model is demonstrated and compared with a finite element model (FEM), allowing rapid and accurate computation of the displacement, strain, and shear wave velocity (SWV) at any location in a homogeneous linear elastic material. Average absolute differences between the analytical model and the FEM were respectively 1.2 % for the displacements and 0.5 % for the strains for 41940 force voxels at 0.22 seconds per displacement evaluation. A convergence study showed that the average difference could be further decreased to 1.0 % and 0.15 % for the displacements and strains, respectively, if force resolution is increased. SWV fields, as calculated with the FEM and the analytical model, have regional differences in velocities up to 0.57 m/s with an average absolute difference of 0.11±0.07 m/s, primarily due to imperfections in the non-reflecting FEM boundary conditions. The apparent SWV differed from the commonly used plane-wave approximation by up to 1.2 m/s due to near and intermediate field effects. Maximum displacement amplitudes for a model with an inclusion stabilize within 10 % of the homogeneous model at an inclusion radius of 10 mm while the maximum strain reacts faster, stabilizing at an inclusion radius of 3 mm. In conclusion, an analytical model for HMI stiffness estimation is presented in this paper. The analytical model has advantages over FEM as the full-field displacements do not need to be calculated to evaluate the model at a single measurement point. This advantage, together with the computational speed, makes the analytical model useful for real-time imaging applications. However, the analytical model was found to have restrictive assumptions on tissue homogeneity and infinite dimensions, while the FEM approaches were shown adaptable to variable geometry and non-homogeneous properties.