Sol-gel transition in agar-gelatin mixtures studied with transient elastography

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.

Hydrogels: The Next Generation Body Materials for Microfluidic Chips?

The integration of microfluidics with biomedical research is confronted with considerable limitations due to its body materials. With high content of water, hydrogels own superior biocompatibility and degradability. Can hydrogels become another material choice for the construction of microfluidic chips, particularly biofluidics? The present review aims to systematically establish the concept of hydrogel-based microfluidic chips (HMCs) and address three main concerns: i) why choosing hydrogels? ii) how to fabricate HMCs?, and iii) in which fields to apply HMCs? It is envisioned that hydrogels may be used increasingly as substitute for traditional materials and gradually act as the body material for microfluidic chips. The modifications of conventional process are highlighted to overcome issues arising from the incompatibility between the construction methods and hydrogel materials. Specifically targeting at the "soft and wet" hydrogels, an efficient flowchart of "i) high resolution template printing; ii) damage-free demolding; iii) twice-crosslinking bonding" is proposed. Accordingly, a broader microfluidic chip concept is proposed in terms of form and function. Potential biomedical applications of HMCs are discussed. This review also highlights the challenges arising from the material replacement, as well as the future directions of the proposed concept. Finally, the authors' viewpoints and perspectives for this emerging field are discussed.

Predictive Model for Designing Soft-Tissue Mimicking Ultrasound Phantoms With Adjustable Elasticity

The use of mechanically representative phantoms is important for experimental validation in ultrasound (US) imaging, elastography, and image registration. This article proposes a model to predict the elastic modulus of a soft tissue-mimicking phantom based on two very easily controllable parameters: gelatin concentration and refrigeration duration. The model has been validated on small- and large-scale phantoms; it provides a good prediction of the elastic modulus in both cases (error < 16.2%). The tissue-mimicking phantom is made following a low-cost and simple fabrication procedure using commercial household gelatin with psyllium hydrophilic mucilloid fiber to obtain echogenicity. A large range of elastic properties was obtained (15-100kPa) by adjusting the gelatin concentration between 5% and 20% (g/mL) and the refrigeration time of the sample between 2 and 168 h, allowing to mimic normal and pathological human soft tissues. The phantom's acoustic properties (velocity, attenuation, and acoustic impedance) are also assessed using the American Institute of Ultrasound in Medicine (AIUM) standard.

Acoustic radiation force induced resonance elastography of coagulating blood: theoretical viscoelasticity modeling and ex-vivo experimentation

Deep vein thrombosis is a common vascular disease that can lead to pulmonary embolism and death. The early diagnosis and clot age staging are important parameters for reliable therapy planning. This article presents an acoustic radiation force induced resonance elastography method for the viscoelastic characterization of clotting blood. The physical concept of this method relies on the mechanical resonance of the blood clot occurring at specific frequencies. Resonances are induced by focusing ultrasound beams inside the sample under investigation. Coupled to an analytical model of wave scattering, the ability of the proposed method to characterize the viscoelasticity of a mimicked venous thrombosis in the acute phase is demonstrated. Experiments with a gelatin-agar inclusion sample of known viscoelasticity are performed for validation and establishment of the proof of concept. In addition, an inversion method is applied in-vitro for the kinetic monitoring of the blood coagulation process of six human blood samples obtained from two volunteers. The computed elasticity and viscosity values of blood samples at the end of the 90 min kinetics were estimated at 411 ± 71 Pa and 0.25 ± 0.03 Pa.s for volunteer #1, and 387 ± 35 Pa and 0.23 ± 0.02 Pa.s for volunteer #2, respectively. The proposed method allowed reproducible time-varying thrombus viscoelastic measurements from samples having physiological dimensions.

Evaluation of the Effect of an Anisotropic Medium on Shear Wave Velocities of Intra-Muscular Gelatinous Inclusions

In highly anisotropic biological tissues such as muscle or tendons, calculating Young's modulus from the shear wave speed (c_sw) by using shear wave elastography (SWE) involves a complex transversally isotropic rheological model not yet used in common practice. To our knowledge, the effect of muscle anisotropy on c_sw of intra-muscular lesions has not yet been investigated. The objective of our study was to define the effect of an anisotropic medium on c_sw of intra-muscular gelatinous inclusions. We conducted a prospective monocentric, in vitro study in order to examine the quantitative and qualitative SWE behavior of a 9-mm gelatinous intra-muscular implant depending on the orientation of the probe relative to the muscle fibers. There were very significant differences in the prevalence of SWE signal void (p < 0.01) and in the c_sw (p < 0.01) in the gelatinous intra-muscular implants depending on the orientation of the probe relative to the fibers. Performing the c_sw measurements of centimetric intra-muscular lesions by orienting the probe perpendicular to the fibers decreases the probability of artifacts occurring at high intensity interfaces.

Ultrasound viscoelasticity assessment using an adaptive torsional shear wave propagation method

PURPOSE

Different approaches have been used in dynamic elastography to assess mechanical properties of biological tissues. Most techniques are based on a simple inversion based on the measurement of the shear wave speed to assess elasticity, whereas some recent strategies use more elaborated analytical or finite element method (FEM) models. In this study, a new method is proposed for the quantification of both shear storage and loss moduli of confined lesions, in the context of breast imaging, using adaptive torsional shear waves (ATSWs) generated remotely with radiation pressure.

METHODS

A FEM model was developed to solve the inverse wave propagation problem and obtain viscoelastic properties of interrogated media. The inverse problem was formulated and solved in the frequency domain and its robustness to noise and geometric constraints was evaluated. The proposed model was validated in vitro with two independent rheology methods on several homogeneous and heterogeneous breast tissue-mimicking phantoms over a broad range of frequencies (up to 400 Hz).

RESULTS

Viscoelastic properties matched benchmark rheology methods with discrepancies of 8%-38% for the shear modulus G' and 9%-67% for the loss modulus G″. The robustness study indicated good estimations of storage and loss moduli (maximum mean errors of 19% on G' and 32% on G″) for signal-to-noise ratios between 19.5 and 8.5 dB. Larger errors were noticed in the case of biases in lesion dimension and position.

CONCLUSIONS

The ATSW method revealed that it is possible to estimate the viscoelasticity of biological tissues with torsional shear waves when small biases in lesion geometry exist.

Oestreicher and elastography

A sphere moving back and forth in tissue generates the kinds of complex displacement fields that are used in elastography. The analytical solution of Hans Oestreicher for this phenomenon [(1951). J. Acoust. Soc. Am. 23, 704-714] gives an understanding of the transverse and longitudinal, fast and slow waves that are generated. The results suggest several ways to determine the absorption coefficients of tissues, which together with phase velocity permit the computation of both the real shear modulus and the shear viscosity as functions of frequency.

Frequency adaptation for enhanced radiation force amplitude in dynamic elastography

In remote dynamic elastography, the amplitude of the generated displacement field is directly related to the amplitude of the radiation force. Therefore, displacement improvement for better tissue characterization requires the optimization of the radiation force amplitude by increasing the push duration and/or the excitation amplitude applied on the transducer. The main problem of these approaches is that the Food and Drug Administration (FDA) thresholds for medical applications and transducer limitations may be easily exceeded. In the present study, the effect of the frequency used for the generation of the radiation force on the amplitude of the displacement field was investigated. We found that amplitudes of displacements generated by adapted radiation force sequences were greater than those generated by standard nonadapted ones (i.e., single push acoustic radiation force impulse and supersonic shear imaging). Gains in magnitude were between 20 to 158% for in vitro measurements on agar-gelatin phantoms, and 170 to 336% for ex vivo measurements on a human breast sample, depending on focus depths and attenuations of tested samples. The signal-to-noise ratio was also improved more than 4-fold with adapted sequences. We conclude that frequency adaptation is a complementary technique that is efficient for the optimization of displacement amplitudes. This technique can be used safely to optimize the deposited local acoustic energy without increasing the risk of damaging tissues and transducer elements.

Mechanical induction of the tumorigenic β-catenin pathway by tumour growth pressure

The tumour microenvironment may contribute to tumorigenesis owing to mechanical forces such as fibrotic stiffness or mechanical pressure caused by the expansion of hyper-proliferative cells. Here we explore the contribution of the mechanical pressure exerted by tumour growth onto non-tumorous adjacent epithelium. In the early stage of mouse colon tumour development in the Notch(+)Apc(+/1638N) mouse model, we observed mechanistic pressure stress in the non-tumorous epithelial cells caused by hyper-proliferative adjacent crypts overexpressing active Notch, which is associated with increased Ret and β-catenin signalling. We thus developed a method that allows the delivery of a defined mechanical pressure in vivo, by subcutaneously inserting a magnet close to the mouse colon. The implanted magnet generated a magnetic force on ultra-magnetic liposomes, stabilized in the mesenchymal cells of the connective tissue surrounding colonic crypts after intravenous injection. The magnetically induced pressure quantitatively mimicked the endogenous early tumour growth stress in the order of 1,200 Pa, without affecting tissue stiffness, as monitored by ultrasound strain imaging and shear wave elastography. The exertion of pressure mimicking that of tumour growth led to rapid Ret activation and downstream phosphorylation of β-catenin on Tyr654, imparing its interaction with the E-cadherin in adherens junctions, and which was followed by β-catenin nuclear translocation after 15 days. As a consequence, increased expression of β-catenin-target genes was observed at 1 month, together with crypt enlargement accompanying the formation of early tumorous aberrant crypt foci. Mechanical activation of the tumorigenic β-catenin pathway suggests unexplored modes of tumour propagation based on mechanical signalling pathways in healthy epithelial cells surrounding the tumour, which may contribute to tumour heterogeneity.

Mechanical induction of the tumorigenic β-catenin pathway by tumour growth pressure

The tumour microenvironment may contribute to tumorigenesis owing to mechanical forces such as fibrotic stiffness or mechanical pressure caused by the expansion of hyper-proliferative cells. Here we explore the contribution of the mechanical pressure exerted by tumour growth onto non-tumorous adjacent epithelium. In the early stage of mouse colon tumour development in the Notch(+)Apc(+/1638N) mouse model, we observed mechanistic pressure stress in the non-tumorous epithelial cells caused by hyper-proliferative adjacent crypts overexpressing active Notch, which is associated with increased Ret and β-catenin signalling. We thus developed a method that allows the delivery of a defined mechanical pressure in vivo, by subcutaneously inserting a magnet close to the mouse colon. The implanted magnet generated a magnetic force on ultra-magnetic liposomes, stabilized in the mesenchymal cells of the connective tissue surrounding colonic crypts after intravenous injection. The magnetically induced pressure quantitatively mimicked the endogenous early tumour growth stress in the order of 1,200 Pa, without affecting tissue stiffness, as monitored by ultrasound strain imaging and shear wave elastography. The exertion of pressure mimicking that of tumour growth led to rapid Ret activation and downstream phosphorylation of β-catenin on Tyr654, imparing its interaction with the E-cadherin in adherens junctions, and which was followed by β-catenin nuclear translocation after 15 days. As a consequence, increased expression of β-catenin-target genes was observed at 1 month, together with crypt enlargement accompanying the formation of early tumorous aberrant crypt foci. Mechanical activation of the tumorigenic β-catenin pathway suggests unexplored modes of tumour propagation based on mechanical signalling pathways in healthy epithelial cells surrounding the tumour, which may contribute to tumour heterogeneity.

A novel breast ultrasound system for providing coronal images: system development and feasibility study

Breast ultrasound images along coronal plane contain important diagnosis information. However, conventional clinical 2D ultrasound cannot provide such images. In order to solve this problem, we developed a novel ultrasound system aimed at providing breast coronal images. In this system, a spatial sensor was fixed on an ultrasound probe to obtain the image spatial data. A narrow-band rendering method was used to form coronal images based on B-mode images and their corresponding spatial data. Software was developed for data acquisition, processing, rendering and visualization. In phantom experiments, 20 inclusions with different size (5-20 mm) were measured using this new system. The results obtained by the new method well correlated with those measured by a micrometer (y=1.0147x, R(2)=0.9927). The phantom tests also showed that this system had excellent intra- and inter-operator repeatability (ICC>0.995). Three subjects with breast lesions were scanned in vivo using this new system and a commercially available three-dimensional (3D) probe. The average scanning times for the two systems were 64 s and 74 s, respectively. The results revealed that this new method required shorter scanning time. The tumor sizes measured on the coronal plane provided by the new method were smaller by 5.6-11.9% in comparison with the results of the 3D probe. The phantom tests and preliminary subject tests indicated the feasibility of this system for clinical applications by providing additional information for clinical breast ultrasound diagnosis.

Rheology over five orders of magnitude in model hydrogels: agreement between strain-controlled rheometry, transient elastography, and supersonic shear wave imaging

Shear wave elastography helps physicians to characterize pathologies by assessing biomechanical properties of soft tissues. Compared with classical rheology, these techniques allow the quantification of the mechanical properties of tissues in the frequency range of hundreds of hertz. In this paper, ultrasound elastographic measurements and classical rheology are compared over a frequency range spanning five orders of magnitude [0.01 to 1200 Hz] to characterize model gels at multiple scales. Hybrid hydrogels were specially synthesized to get a fine tuning of the material dissipative response. Strain-controlled rheology (SCR) experiments were performed to get the elastic moduli G" and loss moduli G" from 0.01 Hz to 10 Hz and were confirmed by tensile tests. Transient elastography (TE from 50 to 400 Hz) and supersonic shear imaging (SSI from 200 to 1200 Hz) were used to characterize polymers at high frequency. Two different hydrogels were tested in the ultrasound setup with different concentration of scatterers. From low-frequency measurements, elastic moduli were extrapolated at high frequency and a very good correlation was obtained between SCR and TE and between SCR and SSI (r = 0.92 and r = 0.95, respectively). This paper demonstrates the capability of shear wave elastography to accurately image rheological properties of soft tissues, to differentiate soft elastic domains from viscous ones. It also gives new insights into soft material science because it provides a rheological tool in a high-frequency domain complementary to conventional rheometry.

Rheological assessment of a polymeric spherical structure using a three-dimensional shear wave scattering model in dynamic spectroscopy elastography

With the purpose of assessing localized rheological behavior of pathological tissues using ultrasound dynamic elastography, an analytical shear wave scattering model was used in an inverse problem framework. The proposed method was adopted to estimate the complex shear modulus of viscoelastic spheres from 200 to 450 Hz. The inverse problem was formulated and solved in the frequency domain, allowing assessment of the complex viscoelastic shear modulus at discrete frequencies. A representative rheological model of the spherical obstacle was determined by comparing storage and loss modulus behaviors with Kelvin-Voigt, Maxwell, Zener, and Jeffrey models. The proposed inversion method was validated by using an external vibrating source and acoustic radiation force. The estimation of viscoelastic properties of three-dimensional spheres made softer or harder than surrounding tissues did not require a priori rheological assumptions. The proposed method is intended to be applied in the context of breast cancer imaging.

Correlation between classical rheometry and supersonic shear wave imaging in blood clots

The assessment of coagulating blood elasticity has gained importance as a result of several studies that have correlated it to cardiovascular pathologic conditions. In this study we use supersonic shear wave imaging (SSI) to measure viscoelastic properties of blood clots. At the same time, classical rheometry experiments were carried out on the same blood samples taken within the first few seconds of coagulation. Using SSI, phase velocities of the shear wave indicated increasing dispersion with time. In all cases, the frequency bandwidth of propagating shear waves changed from 20-50 Hz at the first few min of coagulation to around 300 Hz toward the end of experiments. Using the values of G' and G″ from the rheometry studies, the theoretical shear wave velocities were calculated and correlated with SSI measurements. Results of the two techniques were in very good agreement, confirming that SSI provides accurate measurements of viscoelastic properties as corroborated by conventional rheometric measurements.

Liver fibrosis assessment using transient elastography guided with real-time B-mode ultrasound imaging: a feasibility study

Liver fibrosis is a kind of chronic damage of the liver and can lead to cirrhosis, one of the top 10 causes of death in the Western world. However, there is still a lack of noninvasive methods for diagnosing liver fibrosis. Fibroscan (Echosens, Paris, France), a device based on A-mode transient elastography, has shown promising results. In this study, a transient elastography system with real-time B-mode imaging for non-invasive liver fibrosis assessment, named Liverscan, was developed; its performance was tested and compared with that of the Fibroscan. A specific measurement probe was designed and fabricated with a B-mode ultrasound transducer fixed along the axis of a mechanical vibrator. It was integrated with the Liverscan to measure liver stiffness based on the shear wave propagation in liver tissues. The system was validated by mechanical indentation test using custom-made agar-gelatin phantoms with different stiffness. To further test its feasibility, in vivo measurements were conducted in 67 volunteers (age, 34 ± 3 years; body mass index, 21.3 ± 2.8 kg/m(2); Mean ± SD., 34 male and 33 female), including 20 patients with various liver diseases, and 28 (19 male and 9 female) being tested by both Liverscan and Fibroscan. A significant linear correlation between the stiffness measured by the mechanical indentation test and that by the Liverscan (r = 0.973; p < 0.001) was obtained. The in vivo liver stiffness measured by Liverscan was also correlated with that by Fibroscan significantly (r = 0.886; p < 0.001). There was a significant difference in liver stiffness between the 20 patients and the other healthy subjects (14.1 ± 3.4 kPa vs. 10.5 ± 2.1 kPa; p = 0.001). The intra- and inter-observer tests indicated that the measurements were repeatable with intra-class correlation coefficients being 0.987 (p < 0.001) and 0.988 (p < 0.001), respectively. This study demonstrated that Liverscan with a specifically designed probe was able to measure and differentiate liver of different stiffness using the established measurement protocol under the guidance of real-time B-mode ultrasound imaging.

Imaging feedback of histotripsy treatments using ultrasound shear wave elastography

Histotripsy is a cavitation-based ultrasound therapy that mechanically fractionates soft solid tissues into fluid-like homogenates. This paper investigates the feasibility of imaging the tissue elasticity change during the histotripsy process as a tool to provide feedback for the treatments. The treatments were performed on agar tissue phantoms and ex vivo kidneys using 3-cycle ultrasound pulses delivered by a 750-kHz therapeutic array at peak negative/positive pressure of 17/108 MPa and a repetition rate of 50 Hz. Lesions with different degrees of damage were created with increasing numbers of therapy pulses from 0 to 2000 pulses per treatment location. The elasticity of the lesions was measured with ultrasound shear wave elastography, in which a quasi-planar shear wave was induced by acoustic radiation force generated by the therapeutic array, and tracked with ultrasound imaging at 3000 frames per second. Based on the shear wave velocity calculated from the sequentially captured frames, the Young's modulus was reconstructed. Results showed that the lesions were more easily identified on the shear wave velocity images than on B-mode images. As the number of therapy pulses increased from 0 to 2000 pulses/location, the Young's modulus decreased exponentially from 22.1 ± 2.7 to 2.1 ± 1.1 kPa in the tissue phantoms (R2 = 0.99, N = 9 each), and from 33.0 ± 7.1 to 4.0 ± 2.5 kPa in the ex vivo kidneys (R2 = 0.99, N = 8 each). Correspondingly, the tissues transformed from completely intact to completely fractionated as examined via histology. A good correlation existed between the lesions' Young's modulus and the degree of tissue fractionation as examined with the percentage of remaining structurally intact cell nuclei (R2 = 0.91, N = 8 each). These results indicate that lesions produced by histotripsy can be detected with high sensitivity using shear wave elastography. Because the decrease in the tissue elasticity corresponded well with the morphological and histological change, this study provides a basis for predicting the local treatment outcomes from tissue elasticity change.

AN OVERVIEW OF ELASTOGRAPHY - AN EMERGING BRANCH OF MEDICAL IMAGING

From times immemorial manual palpation served as a source of information on the state of soft tissues and allowed detection of various diseases accompanied by changes in tissue elasticity. During the last two decades, the ancient art of palpation gained new life due to numerous emerging elasticity imaging (EI) methods. Areas of applications of EI in medical diagnostics and treatment monitoring are steadily expanding. Elasticity imaging methods are emerging as commercial applications, a true testament to the progress and importance of the field.In this paper we present a brief history and theoretical basis of EI, describe various techniques of EI and, analyze their advantages and limitations, and overview main clinical applications. We present a classification of elasticity measurement and imaging techniques based on the methods used for generating a stress in the tissue (external mechanical force, internal ultrasound radiation force, or an internal endogenous force), and measurement of the tissue response. The measurement method can be performed using differing physical principles including magnetic resonance imaging (MRI), ultrasound imaging, X-ray imaging, optical and acoustic signals.Until recently, EI was largely a research method used by a few select institutions having the special equipment needed to perform the studies. Since 2005 however, increasing numbers of mainstream manufacturers have added EI to their ultrasound systems so that today the majority of manufacturers offer some sort of Elastography or tissue stiffness imaging on their clinical systems. Now it is safe to say that some sort of elasticity imaging may be performed on virtually all types of focal and diffuse disease. Most of the new applications are still in the early stages of research, but a few are becoming common applications in clinical practice.

Characterization of blood clot viscoelasticity by dynamic ultrasound elastography and modeling of the rheological behavior

Dynamic elastography (DE) is a new tool to study mechanical behavior of soft tissues via their motion response to propagating shear waves. This technique characterized viscoelasticity of 9 porcine whole blood samples (3 animals) during coagulation for a shearing frequency of 70Hz, and after complete clot formation between 50 and 160Hz. Clot storage (G') and loss (G″) moduli were calculated from shear wave velocity and attenuation. Temporal evolutions of G' and G″ during coagulation were typified with 4 parameters: maximum change in elasticity (G' slope(max)), elasticity after 120min of coagulation (G'(max)), time occurrence of G″ maximum (t(e)) and G″ at the plateau (G″(plateau)). G' and G″ frequency dependence of completely formed blood clots was fitted with 5 standard rheological models: Maxwell, Kelvin-Voigt, Jeffrey, Zener and third-order generalized Maxwell. DE had sufficient sensitivity to follow the coagulation kinetics described by a progressive increase in G', while G″ transitory increased followed by a rapid stabilization. Inter- and intra-animal dispersions (InterAD and IntraAD) of G'(max) (InterAD=15.9%, IntraAD=9.1%) showed better reproducibility than G' slope(max) (InterAD=40.4%, IntraAD=21.9%), t(e) (InterAD=27.4%, IntraAD=18.7%) and G″(plateau) (InterAD=58.6%, IntraAD=40.2%). G' evolution within the considered range of frequency exhibited an increase, followed by stabilization to a plateau, whereas G″ presented little variations with convergence at a quasi-constant value at highest frequencies. Residues χ(⁎), describing the goodness of fit between models and experimental data, showed statistically (p<0.05) that the Kelvin-Voigt model was less in agreement with experimental data than other models. The Zener model is recommended to predict G' and G″ dispersion of coagulated blood over the explored frequency range.

Three-dimensional transient and harmonic shear-wave scattering by a soft cylinder for dynamic vascular elastography

With the objective of characterizing biological soft tissues with dynamic elastography, a three-dimensional (3D) analytical model is proposed to simulate the scattering of plane shear waves by a soft cylinder embedded in an infinite soft medium. The 3D problem of harmonic plane shear-wave scattering is first formulated and solved, and the monochromatic solution is employed to simulate transient wave scattering. Both harmonic and transient simulations are compared with experimental 3D acquisitions. The good agreements obtained between measured and calculated displacement fields allowed to conclude on the validity of the proposed 3D harmonic and transient models. The spatial distribution of displacements (diffraction lobes, displacement oscillations, wave diffraction angles, etc.) and their relative amplitudes in both inclusion and surrounding materials depended on the contrast between the viscoelastic properties of the different media. The possibility of solving an inverse problem to assess soft heterogeneous medium viscoelasticity is discussed and some future theoretical and experimental developments are proposed.

Acoustoelasticity in soft solids: assessment of the nonlinear shear modulus with the acoustic radiation force

The assessment of viscoelastic properties of soft tissues is enjoying a growing interest in the field of medical imaging as pathologies are often correlated with a local change of stiffness. To date, advanced techniques in that field have been concentrating on the estimation of the second order elastic modulus (mu). In this paper, the nonlinear behavior of quasi-incompressible soft solids is investigated using the supersonic shear imaging technique based on the remote generation of polarized plane shear waves in tissues induced by the acoustic radiation force. Applying a theoretical approach of the strain energy in soft solid [Hamilton et al., J. Acoust. Soc. Am. 116, 41-44 (2004)], it is shown that the well-known acoustoelasticity experiment allowing the recovery of higher order elastic moduli can be greatly simplified. Experimentally, it requires measurements of the local speed of polarized plane shear waves in a statically and uniaxially stressed isotropic medium. These shear wave speed estimates are obtained by imaging the shear wave propagation in soft media with an ultrafast echographic scanner. In this situation, the uniaxial static stress induces anisotropy due to the nonlinear effects and results in a change of shear wave speed. Then the third order elastic modulus (A) is measured in agar-gelatin-based phantoms and polyvinyl alcohol based phantoms.

Assessment by transient elastography of the viscoelastic properties of blood during clotting

Blood clotting is a natural process that can be both beneficial and life-threatening for the human body. It allows the maintenance of hemostasis after vascular injury, but it can also cause deep vein thrombosis and heart stroke. This study aimed better to understand the clotting process from a biomechanical point of view by using an acoustic method. The long-term objective is the staging of the age of clots in deep veins for therapy planning. The transient elastography method using a shear elasticity probe served to evaluate the shear wave velocity (V(S)) and shear wave attenuation (alpha(S)) of porcine whole blood during in vitro clot formation. By solving an inverse problem, it was then possible to provide images of the elasticity (mu(B)) and of the viscosity (eta(B)) from clotting blood. The time-varying elasticity and viscosity were very similar to what has been observed for the sol-gel transition of polymers. The mechanical properties of blood clot, which were modified by varying the hematocrit and by adding heparin or fibrinogen, were clearly assessed by the transient elastography technique. It is concluded that the shear elasticity probe is an appropriate tool to quantify and follow the sol-gel transition of blood during clotting.