Description and implementation of the Supersonic Shear Imaging method on the Verasonics research system

Supersonic shear imaging (SSI) is a new ultrasound based technique for real time visualization of soft tissue viscoelastic properties. Using ultrasound focused beams, it is possible to remotely generate inside the body mechanical sources radiating low frequency shear waves. SSI is based on the ultrasonic generation of a shear source moving at a supersonic speed inside the body. In a complete analogy with the "sonic boom" created by a supersonic aircraft, the resulting shear waves will constructively interfere along a Mach cone, creating two intense plane waves. These plane shear waves propagate through the medium and are progressively distorted by tissue mechanical inhomogeneities. The ultrafast scanner developed in our laboratory (5000 images/s) is able to generate this supersonic source and image, in real time, the propagation of the resulting shear waves. Using inversion algorithms, viscosity and elasticity maps of the medium can be deduced from this shear wave propagation movie. Creating such a supersonic regime enables quantitative tissue elasticity mapping in less than 20 ms, even in strongly viscous medium like breast or liver. Results validating SSI for quantitative shear elasticity mapping in heterogeneous tissue mimicking phantoms are presented. Detection of in vitro thermally-induced lesions on fresh tissue samples is shown. In vivo tests made on healthy volunteers show the potential clinical applicability of SSI for breast cancer detection. Finally, viscosity mapping using SSI has been studied theoretically and experimentally. Based on a Voigt model, simulations in different viscous and elastic media were compared and fitted to in vitro experiments. Using this theoretical background, viscosity maps using SSI were for the first time derived in viscoelastic phantoms.

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Dynamic elastography holds great promise for biological tissues characterization. Resulting from the radiation force induced by focused ultrasound beam, plane shear waves are generated within the medium and imaged with an ultrafast ultrasound scanner. Known as Supersonic Shear Imaging (SSI) technique, this method allows, from the measurements of shear wave velocities, to compute shear modulus (mu) maps. Beside, in order to improve tissue diagnostic, the evaluation of the nonlinear elastic moduli could be of some interest. Recently a new formulation of the nonlinear equation describing the propagation of plane shear waves in isotropic soft incompressible solids have been developed using a new expression, up to the fourth order, of the strain energy density (e): e = muI2 + A/3I3, + DI2 2. Where I2, I3 are invariants defined by Landau of the strain tensor and A, D the third and fourth order shear elastic constants. It has been shown that the nonlinearity parameter depends only on three coefficients betas = betas (mu, A, D). To date, no measurement of the parameter D have been carried out in incompressible media. In order to estimate the nonlinear parameter A, this theoretical background on soft incompressible solids is applied to the acoustoelasticity theory. Such analysis gives the variations of shear wave speed as a function of the applied stress and leads to measure both the linear shear modulus (mu) and the third order shear modulus (A). Taking advantages of the SSI technique, an acoustoelasticity experiment is performed in different incompressible soft media (agar-gelatin based phantoms). In addition, to create finite amplitude plane shear waves, the SSI technique is replaced by a vibrator applied at the surface of the phantoms. Thanks to the ultrasound ultrafast imaging system, the third harmonic component is generated by nonlinearity is measured as a function of the propagation distance. Then by comparing experiments and analytical expression of the third harmonic component given by a perturbation method, the nonlinear parameter betas is deduced. Finally, the combination of these experiments with results obtained in acoustoelasticity leads to the determination of the fourth order elastic modulus (D). First, measurements of the A modulus reveal that while the behavior of phantoms is quite close from a linear point of view, their nonlinear modulus A are quite different. Applied to acoustoelasticity, the SSI technique provides potential medical applications in in vivo conditions for nonlinear characterization of biological tissues. Second, results from the complete procedure reveal a variation of the nonlinear behavior as a function of the gelatin concentration increasing. This set of experiments provides the characterization, up to the fourth order, of the nonlinear shear elastic moduli in incompressible soft media.

Supersonic shear imaging (SSI) is a new ultrasound-based technique for real-time visualization of soft tissue viscoelastic properties. Using ultrasonic focused beams, it is possible to remotely generate mechanical vibration sources radiating low-frequency, shear waves inside tissues. Relying on this concept, SSI proposes to create such a source and make it move at a supersonic speed. In analogy with the "sonic boom" created by a supersonic aircraft, the resulting shear waves will interfere constructively along a Mach cone, creating two intense plane shear waves. These waves propagate through the medium and are progressively distorted by tissue heterogeneities. An ultrafast scanner prototype is able to both generate this supersonic source and image (5000 frames/s) the propagation of the resulting shear waves. Using inversion algorithms, the shear elasticity of medium can be mapped quantitatively from this propagation movie. The SSI enables tissue elasticity mapping in less than 20 ms, even in strongly viscous medium like breast. Modalities such as shear compounding are implementable by tilting shear waves in different directions and improving the elasticity estimation. Results validating SSI in heterogeneous phantoms are presented. The first in vivo investigations made on healthy volunteers emphasize the potential clinical applicability of SSI for breast cancer detection.

The diagnostic performance of supersonic shear imaging (SSI) in comparison with those of transient elastography (TE) and acoustic radiation force impulse imaging (ARFI) for staging fibrosis in nonalcoholic fatty liver disease (NAFLD) patients has not been fully assessed, especially in Asian populations with relatively lean NAFLD compared to white populations. Thus, we focused on comparing the diagnostic performances of TE, ARFI, and SSI for staging fibrosis in a head-to-head manner, and identifying the clinical, anthropometric, biochemical, and histological features which might affect liver stiffness measurement (LSM) in our prospective biopsy-proven NAFLD cohort. In this study, ninety-four patients with biopsy-proven NAFLD were included prospectively. Liver stiffness was measured using TE, SSI, and ARFI within 1 month of liver biopsy. The diagnostic performance for staging fibrosis was assessed using receiver operating characteristic (ROC) analysis. Anthropometric data were evaluated as covariates influencing LSM by regression analyses. Liver stiffness correlated with fibrosis stage (p < 0.05); the area under the ROC curve of TE (kPa), SSI (kPa), and ARFI (m/s) were as follows: 0.757, 0.759, and 0.657 for significant fibrosis and 0.870, 0.809, and 0.873 for advanced fibrosis. Anthropometric traits were significant confounders affecting SSI, while serum liver injury markers significantly confounded TE and ARFI. In conclusion, the LSM methods had similar diagnostic performance for staging fibrosis in patients with NAFLD. Pre-LSM anthropometric evaluation may help predict the reliability of SSI.

Characterization of the nonlinear elastic properties of soft tissues using the supersonic shear imaging (SSI) technique: inverse method, ex vivo and in vivo experiments.

Several ultrasound-based techniques for the estimation of soft tissue elasticity are currently being investigated. Most of them study the medium response to dynamic excitations. Such responses are usually modeled in a purely elastic medium using a Green's function solution of the motion equation. However, elasticity by itself is not necessarily a discriminant parameter for malignancy diagnosis. Modeling viscous properties of tissues could also be of great interest for tumor characterization. We report in this paper an explicit derivation of the Green's function in a viscous and elastic medium taking into account shear, bulk, and coupling waves. From this theoretical calculation, 3D simulations of mechanical waves in viscoelastic soft tissues are presented. The relevance of the viscoelastic Green's function is validated by comparing simulations with experimental data. The experiments were conducted using the supersonic shear imaging (SSI) technique which dynamically and remotely excites tissues using acoustic radiation force. We show that transient shear waves generated with SSI are modeled very precisely by the Green's function formalism. The combined influences of out-of-plane diffraction, beam shape, and shear viscosity on the shape of transient waves are carefully studied as they represent a major issue in ultrasound-based viscoelasticity imaging techniques.

This article presents corrections to J. Bercoff et al., "The role of viscosity in the impulse diffraction field of elastic waves induced by the acoustic radiation force," IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 51, no. 11, pp. 1523-1536, Nov. 2004, and J. Bercoff et al., "Supersonic shear imaging: A new technique for soft tissue elasticity mapping," IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 51, no. 4, pp. 396-409, Apr. 2004.

Observation of transient shear-wave propagation in soft tissue is of great interest for the study of tissue viscoelastic properties. In previous work, we introduced a technique, called Supersonic Shear Imaging (SSI), able to generate transient shear waves using the acoustic radiation force and image their propagation in real time in soft tissues. In this work, a local inverse problem of the shear wave propagation permitting the recovery of shear elasticity and viscosity is presented. Compressional and shear waves are decoupled by applying the curl operator to the experimental 2-D displacement field. The role of viscosity on the accuracy of the elasticity estimation is studied. The influence of out of plane shear propagation on the inversion algorithm is discussed. Finally, in media presenting shear viscoelasticity heterogeneities, finite difference simulations are used to study the spatial resolution of the algorithm and its sensitivity to the signal-to-noise ratio. Experiments on calibrated tissue-mimicking phantoms presenting different viscoelastic properties are presented validating the simulation results. First in vivo results on female breasts are presented.

Increasing interest is given to elasticity to monitor HIFU treatments, since elasticity changes during thermal ablation. Moreover, the shear modulus of in vitro soft tissue samples was proven to be linked to the thermal dose. Hence, the study aims to evaluate the link of in vivo elasticity changes, assessed by supersonic shear imaging, with temperature and thermal dose. Seven male rats (Sprague Dawley, 250 g) were anesthetized. The right leg was depilated and immersed in a thermo-regulated bath. Two optical temperature sensors were placed into the biceps femoris. The leg was warmed at a targeted temperature, from 38°C to 50°C, for a time corresponding to the theoretical threshold of necrosis in muscle (thermal dose ∼ 240 minutes at 43°C). Every 40 seconds, shear plane waves were created in the biceps femoris, in a plane neighboring the thermocouples, using the Supersonic Shear Imaging method based on the radiation force. The shear wave propagation was acquired with 180 images at 10 000 frames/s. The changes of elasticity were assessed for the different temperature-time profiles in order to highlight its dependence with the thermal dose. The leg swelled when the temperature reached 39°C. Then, contraction of the muscular fibers was observed on the Bmode images at 44–45°C (thermal dose from 2 to 140 min), followed by a flow of fluid around the muscle. The effect of the thermal dose on the in vivo elasticity of the muscles was the same for all the rats. The shear modulus exponentially increased at a thermal dose was equal to 220 ± 78 min. The cumulative thermal dose did not significantly change when the shear modulus reached 2.5 fold the initial value or more. Thus, this threshold value (220 min) was assumed to be the threshold of necrosis. This value is consistent with the necrosis of the skin and the theoretical threshold of necrosis of muscular tissues (240 min). This study gives promising prospects for the use of the elasticity to monitor thermal effects on tissues.

Due to the hierarchical structure of tendon tissue (fascicles→fibers→fibrils), it is essential to investigate morphology and mechanics at the nano-scopic level to better understand whole body function. Previous studies investigating changes in tendon mechanics after exercise/disease/unloading have focused on the whole tendon and fascicle levels, but limited data exists on adaptations at the collagen fibril level. Objective: Provide a new method to study previously frozen tendon tissue by investigating collagen fibril 1) morphology, 2) nano-mechanical properties, and 3) elasticity maps using atomic force microscopy (AFM). Hypothesis: This method will provide a reliable way to assess tendon structure and function at the nano-scale. Methods: C57BL/6J mouse tendons from NASA Rodent Research-1 (-80°C) were chemically separated and mounted onto glass slides then mechanically separated into tendon subunit sheets and left to air-dry. Collagen fibril morphology was tested with a JPK Nanowizard 4a in tapping mode with a Tap300Al-G cantilever (tip radius=10nm, k=40N/m). Samples were then rehydrated for mechanical testing in Quantitative Imaging (QI) mode with an SNL-10D cantilever (tip radius=2nm, k=0.06N/m). Force data acquisition took place in liquid with a set point of 2nN and Z-speed of 40μm/s. Force-microscopy (FM) and structural data were processed using JPK Data Processing Software and Gwyddion, respectively. Data: Tendon collagen fibrils exhibited uniform D-periods across 38 fibril sections (D-period: 66nm±2.1, diameter: 63nm±23.5). Quantitative analysis of the force curves (n=30) along the highest point of the fibril revealed an average Young’s Modulus (YM) of 2.53MPa±0.31. Elasticity maps (n=3) highlighting the distribution of mechanical properties were generated using FM data. Results Summary: Our tissue preparation protocol combined with AFM reliably determines collagen fibril morphology and nano-mechanical properties of mouse tendons that were previously frozen. As evidence, we present systematic YM within and across different data sets. In addition, the YM values were found to be consistent with previous measurements in tendon tissue under similar conditions. Elasticity maps produced by analysis of FM data indicate the similarity of YM values across the tendon surface. Tendon fibrils were found to have similar D-periodicity despite the variation in fibril size, which is consistent with previous literature. Conclusions: This research provides a new method to test tendon collagen fibril structure and mechanical properties at the nano-scale, presenting a novel analysis (elasticity maps) showing contrast in YM across the fibril surface. Funding: CSUPERB New Investigator Grant to JRB This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Pain and tenderness of the upper trapezius are the major complaints among people with chronic neck and shoulder disorders. Hyper-activation and increased muscle tension of the upper trapezius during arm elevation will cause imbalance of the scapular muscle force and contribute to neck and shoulder disorders. Assessing the elasticity of the upper trapezius in different arm positions is therefore important for identifying people at risk so as to give preventive programmes or for monitoring the effectiveness of the intervention programmes for these disorders. This study aimed to establish the reliability of supersonic shear imaging (SSI) in quantifying upper trapezius elasticity/shear elastic modulus and its ability to measure the modulation of muscle elasticity during arm elevation. Twenty-eight healthy adults (15 males, 13 females; mean age = 29.6 years) were recruited to participate in the study. In each participant, the shear elastic modulus of the upper trapezius while the arm was at rest and at 30° abduction was measured by two operators and twice by operator 1 with a time interval between the measurements. The results showed excellent within- and between-session intra-operator (ICC = 0.87–0.97) and inter-observer (ICC = 0.78–0.83) reliability for the upper trapezius elasticity with the arm at rest and at 30° abduction. An increase of 55.23% of shear elastic modulus from resting to 30° abduction was observed. Our findings demonstrate the possibilities for using SSI to quantify muscle elasticity and its potential role in delineating the modulation of upper trapezius elasticity, which is essential for future studies to compare the differences in shear elastic modulus between normal elasticity and that of individuals with neck and shoulder disorders.

Reliability of Abdominal Muscle Stiffness Measured Using Elastography during Trunk Rehabilitation Exercises

Taping techniques that aim to protect and reduce stress on an injured tissue during rehabilitation are common in sport. Called deloading tape, the mechanism of action is hypothesized to involve effects ranging from direct mechanical deloading of the underlying soft tissues to psychological effects on confidence. There is no evidence that deloading tape has direct mechanical effects. This study used an elastographic technique (supersonic shear imaging) to test the hypothesis that deloading tape applied to the skin over the rectus femoris would reduce stress within the taped area of this muscle. Thirteen healthy volunteers participated in this experiment. Muscle shear elastic modulus was compared between three treatments (no tape, deloading tape, and sham tape) in four conditions: three conditions without muscle contraction at different rectus femoris muscle-tendon unit lengths (moderately stretched, highly stretched, and shortened) and during submaximal isometric leg extension. Although there was no effect of treatment when the muscle was shortened (P = 0.99), the shear elastic modulus was significantly affected by treatment for the three other conditions (all P < 0.002). Muscle shear elastic modulus was significantly less during application of deloading tape than that during both the no tape and sham tape conditions (all P ≤ 0.001; e.g., vs no tape: moderately stretched, 8.4 ± 2.7 vs 6.7 ± 1.7 kPa; highly stretched, 25.2 ± 8.2 vs 14.4 ± 4.3 kPa; submaximal contraction, 21.3 ± 4.8 vs 14.2 ± 4.3 kPa). Through the use of elastography, this is the first study to support the hypothesis that deloading tape reduces stress in the underlying muscle region, thereby providing a biomechanical explanation for the effect observed during rehabilitation in clinical practice (reduce pain, restore function, and aid recovery). Further investigations are necessary to confirm these results in injured tissues.

马尔文激光粒度分布仪始于本世纪初应用于河流泥沙粒度分析，虽然与传统的沉降法分析原理有着本质的区别，但其仪器所具备的操作简便、分析数据稳定和处理自动化程度高、成果获取快等优点，深受使用者的青睐，目前已在河流泥沙颗粒分析中得到了应用和普及。然而，用户在仪器使用中会遇到一些具体问题：如基础参数设定和检查不方便、测试数据的好坏无法自动检测和提示处理，测试的颗粒级配数据特征值及极值挑选比较麻烦、分析报告格式不统一、数据读取和输出处理繁琐等。为解决这些问题，并统一各泥沙监测站的激光仪操作方法和要求，在充分利用激光仪现有功能的基础上，开发研制出了一套“马尔文激光粒度仪泥沙数据处理模板”，该模板能自动配置基础参数、对分析数据进行质量检查并提示，按测站编码打印标准格式报告单、形成批量分析数据的文本输出文件等，可直接提供给南方片的整汇编软件进行整理、整编等。经过近几年的运行取得良好效果，提高了工作效率，减少了出错机率，保障了成果质量。 Malvern laser particle size analyzer has been applied in the particle analysis of river sediments since the beginning of this century, and has been highly praised by users for simple operation, stable data from analysis, high automation of processing and quick achievements, etc. regardless of its significant deviation from the principles of traditional sedimentation analysis approach. Now, it has been applied and popularized in the particle analysis of river sediments. Nevertheless, users may run into some problems in the use of this equipment, such as, inconvenient setting and check of fundamental parameters, lack of automatic detection and warning about quality of test data, troublesome selection of characteristic and extreme values in the data of tested particle grading, inconsistent form of analysis report, and cumber-some reading and output of data, etc. To solve these problems and unify the principles and requirements for operation of laser analyzer at the sediment monitoring stations, a “sediment data processing template for Malvern laser particle size analyzer” is developed while making full use of the existing functions of laser analyzer. This template is able to configure the fundamental parameters automatically, check the quality of analytic data and give warnings, print the reports in standard form according to the code of each monitoring station, and generate the textual output of analytic data in batches, etc., which can be directly sorted out and edited in the Southern Hydrological Data Processing (SHDP) software. The operation for several years has achieved favorable effects, improved working efficiency, reduced error rate and ensured the quality of achievements.