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

Relationship between the liver tissue shear modulus and histopathologic findings analyzed by intraoperative shear wave elastography and digital microscopically assisted morphometry in patients with hepatocellular carcinoma

OBJECTIVES

Shear wave elastography is a novel noninvasive method for assessing liver fibrosis by measuring liver stiffness. This study was conducted to evaluate how pathologic changes could have an impact on measured elasticity values in both resected hepatocellular carcinomas and adjacent liver tissue.

METHODS

Intraoperative shear wave elastography was performed in 7 patients who underwent liver resection at our institution; 7 hepatocellular carcinomas and adjacent liver tissue were subjected to elastographic measurements. A total of 48 circular regions of interest (ROIs; 3-8 mm in diameter) were located in the hepatocellular carcinomas (n = 37) and adjacent liver tissue (n = 11), and mean stiffness values were obtained from each ROI. All of the histologic images corresponding to the 48 ROIs after surgery were transformed into digital microscopic images by a scanning system, and histologic parameters, such as the proportions of nuclear areas, fatty areas, fibrous areas, and vessel areas, were quantitatively assessed. Relationships between the mean stiffness and the histologic parameters were investigated by the mixed effects model.

RESULTS

By univariate analysis, the proportions of collagen fiber areas (P = .039), fibrous areas (P = .045), hepatocellular nuclear areas (P = .045), and nuclear areas other than hepatocellular and lymphoplasmacytic areas (P = .039) showed statistically positive associations with mean stiffness values. Multivariate analysis indicated that the proportion of collagen fiber areas was the strongest pathologic determinant of mean stiffness (P = .008), with hepatocellular nuclear areas also having a significant effect (P = .010).

CONCLUSIONS

Fibrosis predictably affects elastographic estimation, but hepatocellular density (ie, hepatocellular nuclear areas) also alters elastographic assessment.

Ultrafast imaging in biomedical ultrasound

Although the use of ultrasonic plane-wave transmissions rather than line-per-line focused beam transmissions has been long studied in research, clinical application of this technology was only recently made possible through developments in graphical processing unit (GPU)-based platforms. Far beyond a technological breakthrough, the use of plane or diverging wave transmissions enables attainment of ultrafast frame rates (typically faster than 1000 frames per second) over a large field of view. This concept has also inspired the emergence of completely novel imaging modes which are valuable for ultrasound-based screening, diagnosis, and therapeutic monitoring. In this review article, we present the basic principles and implementation of ultrafast imaging. In particular, present and future applications of ultrafast imaging in biomedical ultrasound are illustrated and discussed.

Viscoelastic response (VisR) imaging for assessment of viscoelasticity in Voigt materials

Viscoelastic response (VisR) imaging is presented as a new acoustic radiation force (ARF)-based elastographic imaging method. Exploiting the Voigt model, VisR imaging estimates displacement in only the ARF region of excitation from one or two successive ARF impulses to estimate τσ, the relaxation time for constant stress. Double-push VisR τσ estimates were not statistically significantly different (p < 0.02) from those of shearwave dispersion ultrasound vibrometry (SDUV) or monitored steady-state excitation recovery (MSSER) ultrasound in six homogeneous viscoelastic tissue mimicking phantoms with elastic moduli ranging from 3.92 to 15.34 kPa and coefficients of viscosity ranging from 0.87 to 14.06 Pa·s. In two-dimensional imaging, double-push VisR τσ images discriminated a viscous spherical inclusion in a structured phantom with higher CNR over a larger axial range than single-push VisR or conventional acoustic radiation force impulse (ARFI) ultrasound. Finally, 2-D in vivo double-push VisR images in normal canine semitendinosus muscle were compared with spatially matched histochemistry to corroborate lower double-push VisR τσ values in highly collagenated connective tissue than in muscle, suggesting double-push VisR's in vivo relevance to diagnostic imaging, particularly in muscle. The key advantages and disadvantages to VisR, including lack of compensation for inertial terms, are discussed.

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.

Quantification of liver viscoelasticity with acoustic radiation force: a study of hepatic fibrosis in a rat model

Ultrasound elastography, based on shear wave propagation, enables the quantitative and non-invasive assessment of liver mechanical properties such as stiffness and has been found to be feasible for and useful in the diagnosis of hepatic fibrosis. Most ultrasound elastographic methods use a purely elastic model to describe liver mechanical properties. However, to describe tissue that is dispersive and to obtain an accurate measure of tissue elasticity, the viscoelasticity of the tissue should be examined. The objective of this study was to investigate the shear viscoelastic characteristics, as measured by ultrasound elastography, of liver fibrosis in a rat model and to evaluate the diagnostic accuracy of viscoelasticity for staging liver fibrosis. Liver fibrosis was induced in 37 rats using carbon tetrachloride (CCl4); 6 rats served as controls. Liver viscoelasticity was measured in vitro using shear waves induced by acoustic radiation force. The measured mean values of liver elasticity and viscosity ranged from 0.84 to 3.45 kPa and from 1.12 to 2.06 Pa·s for fibrosis stages F0-F4, respectively. Spearman correlation coefficients indicated that stage of fibrosis was well correlated with elasticity (0.88) and moderately correlated with viscosity (0.66). The areas under receiver operating characteristic curves were 0.97 (≥F2), 0.91 (≥F3) and 1.00 (F4) for elasticity and 0.91 (≥F2), 0.79 (≥F3) and 0.74 (F4) for viscosity, respectively. The results confirmed that shear wave velocity was dispersive in frequency, suggesting a viscoelastic model to describe liver fibrosis. The study finds that although viscosity is not as good as elasticity for staging fibrosis, it is important to consider viscosity to make an accurate estimation of elasticity; it may also provide other mechanical insights into liver tissues.

JSUM ultrasound elastography practice guidelines: basics and terminology

Ten years have passed since the first commercial equipment for elastography was released; since then clinical utility has been demonstrated. Nowadays, most manufacturers offer an elastography option. The most widely available commercial elastography methods are based on strain imaging, which uses external tissue compression and generates images of the resulting tissue strain. However, imaging methods differ slightly among manufacturers, which results in different image characteristics, for example, spatial and temporal resolution, and different recommended measurement conditions. In addition, many manufacturers have recently provided a shear wave-based method, providing stiffness images based on shear wave propagation speed. Each method of elastography is designed on the basis of assumptions of measurement conditions and tissue properties. Thus, we need to know the basic principles of elastography methods and the physics of tissue elastic properties to enable appropriate use of each piece of equipment and to obtain more precise diagnostic information from elastography. From this perspective, the basic section of this guideline aims to support practice of ultrasound elastography.

Meta-analysis: ARFI elastography versus transient elastography for the evaluation of liver fibrosis

AIMS

This meta-analysis aims to compare the diagnostic performance of acoustic radiation force impulse (ARFI) elastography and transient elastography (TE) in the assessment of liver fibrosis using liver biopsy (LB) as 'gold-standard'.

METHODS

PubMed, Medline, Lilacs, Scopus, Ovid, EMBASE, Cochrane and Medscape databases were searched for all studies published until 31 May 2012 that evaluated the liver stiffness by means of ARFI, TE and LB. Information abstracted from each study according to a fixed protocol included study design and methodological characteristics, patient characteristics, interventions, outcomes and missing outcome data.

RESULTS

Thirteen studies (11 full-length articles and 2 abstracts) including 1163 patients with chronic hepatopathies were included in the analysis. Inability to obtain reliable measurements was more than thrice as high for TE as that of ARFI (6.6% vs. 2.1%, P < 0.001). For detection of significant fibrosis, (F ≥ 2) the summary sensitivity (Se) was 0.74 (95% CI: 0.66-0.80) and specificity (Sp) was 0.83 (95% CI: 0.75-0.89) for ARFI, while for TE the Se was 0.78 (95% CI: 0.72-0.83) and Sp was 0.84 (95% CI: 0.75-0.90). For the diagnosis of cirrhosis, the summary Se was 0.87 (95% CI: 0.79-0.92) and Sp was 0.87 (95% CI: 0.81-0.91) for ARFI elastography, and, respectively, 0.89 (95% CI: 0.80-0.94) and 0.87 (95% CI: 0.82-0.91) for TE. The diagnostic odds ratio of ARFI and TE did not differ significantly in the detection of significant fibrosis [mean difference in rDOR = 0.27 (95% CI: 0.69-0.14)] and cirrhosis [mean difference in rDOR = 0.12 (95% CI: 0.29-0.52)].

CONCLUSION

Acoustic radiation force impulse elastography seems to be a good method for assessing liver fibrosis, and shows higher rate of reliable measurements and similar predictive value to TE for significant fibrosis and cirrhosis.

Imaging transverse isotropic properties of muscle by monitoring acoustic radiation force induced shear waves using a 2-D matrix ultrasound array

A 2-D matrix ultrasound array is used to monitor acoustic radiation force impulse (ARFI) induced shear wave propagation in 3-D in excised canine muscle. From a single acquisition, both the shear wave phase and group velocity can be calculated to estimate the shear wave speed (SWS) along and across the fibers, as well as the fiber orientation in 3-D. The true fiber orientation found using the 3-D radon transform on B-mode volumes of the muscle was used to verify the fiber direction estimated from shear wave data. For the simplified imaging case when the ARFI push can be oriented perpendicular to the fibers, the error in estimating the fiber orientation using phase and group velocity measurements was 3.5 ± 2.6° and 3.4 ± 1.4° (mean ± standard deviation), respectively, over six acquisitions in different muscle samples. For the more general case when the push is oblique to the fibers, the angle between the push and the fibers is found using the dominant orientation of the shear wave displacement magnitude. In 30 acquisitions on six different muscle samples with oblique push angles up to 40°, the error in the estimated fiber orientation using phase and group velocity measurements was 5.4 ± 2.9° and 5.3 ± 3.2°, respectively, after estimating and accounting for the additional unknown push angle. Either the phase or group velocity measurements can be used to estimate fiber orientation and SWS along and across the fibers. Although it is possible to perform these measurements when the push is not perpendicular to the fibers, highly oblique push angles induce lower shear wave amplitudes which can cause inaccurate SWS measurements.

Multi-push (MP) acoustic radiation force (ARF) ultrasound for assessing tissue viscoelasticity, in vivo

Acoustic radiation force (ARF) ultrasound is a method of elastographic imaging in which micron-scale tissue displacements, induced and tracked by ultrasound, reflect clinically relevant tissue mechanical properties. Our laboratory has recently shown that tissue viscoelasticity is assessed using the novel Multi-Push (MP) ARF method. MP ARF applies the Voigt model for viscoelastic materials and compares the displacements achieved by successive ARF excitations to qualitatively or quantitatively represent the relaxation time for constant stress, which is a direct descriptor of the viscoelastic response of the tissue. We have demonstrated MP ARF in custom viscoelastic tissue mimicking materials and implemented the method in vivo in canine muscle and human renal allografts, with strong spatial correlation between MP ARF findings and histochemical features and previously reported mechanical changes with renal disease. These data support that noninvasive MP ARF is capable of clinically relevant assessment of tissue viscoelastic properties.

Acoustic waves in medical imaging and diagnostics

Up until about two decades ago acoustic imaging and ultrasound imaging were synonymous. The term ultrasonography, or its abbreviated version sonography, meant an imaging modality based on the use of ultrasonic compressional bulk waves. Beginning in the 1990s, there started to emerge numerous acoustic imaging modalities based on the use of a different mode of acoustic wave: shear waves. Imaging with these waves was shown to provide very useful and very different information about the biological tissue being examined. We discuss the physical basis for the differences between these two basic modes of acoustic waves used in medical imaging and analyze the advantages associated with shear acoustic imaging. A comprehensive analysis of the range of acoustic wavelengths, velocities and frequencies that have been used in different imaging applications is presented. We discuss the potential for future shear wave imaging applications.

Inter- and intra-operator reliability and repeatability of shear wave elastography in the liver: a study in healthy volunteers

This study assessed the reproducibility of shear wave elastography (SWE) in the liver of healthy volunteers. Intra- and inter-operator reliability and repeatability were quantified in three different liver segments in a sample of 15 subjects, scanned during four independent sessions (two scans on day 1, two scans 1 wk later) by two operators. A total of 1440 measurements were made. Reproducibility was assessed using the intra-class correlation coefficient (ICC) and a repeated measures analysis of variance. The shear wave speed was measured and used to estimate Young's modulus using the Supersonics Imagine Aixplorer. The median Young's modulus measured through the inter-costal space was 5.55 ± 0.74 kPa. The intra-operator reliability was better for same-day evaluations (ICC = 0.91) than the inter-operator reliability (ICC = 0.78). Intra-observer agreement decreased when scans were repeated on a different day. Inter-session repeatability was between 3.3% and 9.9% for intra-day repeated scans, compared with to 6.5%-12% for inter-day repeated scans. No significant difference was observed in subjects with a body mass index greater or less than 25 kg/m(2).

Acoustic radiation force elasticity imaging in diagnostic ultrasound

The development of ultrasound-based elasticity imaging methods has been the focus of intense research activity since the mid-1990s. In characterizing the mechanical properties of soft tissues, these techniques image an entirely new subset of tissue properties that cannot be derived with conventional ultrasound techniques. Clinically, tissue elasticity is known to be associated with pathological condition and with the ability to image these features in vivo; elasticity imaging methods may prove to be invaluable tools for the diagnosis and/or monitoring of disease. This review focuses on ultrasound-based elasticity imaging methods that generate an acoustic radiation force to induce tissue displacements. These methods can be performed noninvasively during routine exams to provide either qualitative or quantitative metrics of tissue elasticity. A brief overview of soft tissue mechanics relevant to elasticity imaging is provided, including a derivation of acoustic radiation force, and an overview of the various acoustic radiation force elasticity imaging methods.

On the precision of time-of-flight shear wave speed estimation in homogeneous soft solids: initial results using a matrix array transducer

A system capable of tracking radiation-force-induced shear wave propagation in a 3-D volume using ultrasound is presented. In contrast to existing systems, which use 1-D array transducers, a 2-D matrix array is used for tracking shear wave displacements. A separate single-element transducer is used for radiation force excitation. This system allows shear wave propagation in all directions away from the push to be observed. It is shown that for a limit of 64 tracking beams, by placing the beams at the edges of the measurement region of interest (ROI) at multiple directions from the push, time-of- flight (TOF) shear wave speed (SWS) measurement uncertainty can theoretically be reduced by 40% compared with equally spacing the tracking beams within the ROI along a single plane, as is typical when using a 1-D array for tracking. This was verified by simulation, and a reduction of 30% was experimentally observed on a homogeneous phantom. Analytical expressions are presented for the relationship between TOF SWS measurement uncertainty and various shear wave imaging parameters. It is shown that TOF SWS uncertainty is inversely proportional to ROI size, and inversely proportional to the square root of the number of tracking locations for a given distribution of beam locations relative to the push. TOF SWS uncertainty is shown to increase with the square of the SWS, indicating that TOF SWS measurements are intrinsically less precise for stiffer materials.

Shear wave induced resonance elastography of venous thrombi: a proof-of-concept

Shear wave induced resonance elastography (SWIRE) is proposed for deep venous thrombosis (DVT) elasticity assessment. This new imaging technique takes advantage of properly polarized shear waves to induce resonance of a confined mechanical heterogeneity. Realistic phantoms (n = 9) of DVT total and partial clot occlusions with elasticities from 406 to 3561 Pa were built for in vitro experiments. An ex vivo study was also performed to evaluate the elasticity of two fresh porcine venous thrombi in a pig model. Transient shear waves at 45-205 Hz were generated by the vibration of a rigid plate (plane wavefront) or by a needle to simulate a radiation pressure on a line segment (cylindrical wavefront). Induced propagation of shear waves was imaged with an ultrafast ultrasound scanner and a finite element method was developed to simulate tested experimental conditions. An inverse problem was then formulated considering the first resonance frequency of the DVT inclusion. Elasticity agreements between SWIRE and a reference spectroscopy instrument (RheoSpectris) were found in vitro for total clots either in plane (r(2) = 0.989) or cylindrical (r(2) = 0.986) wavefront configurations. For total and partial clots, elasticity estimation errors were 9.0 ±4.6% and 9.3 ±11.3%, respectively. Ex vivo, the blood clot elasticity was 498 ±58 Pa within the inferior vena cava and 436 ±45 Pa in the right common iliac vein (p = 0.22). To conclude, the SWIRE technique seems feasible to quantitatively assess blood clot elasticity in the context of DVT ultrasound imaging.

Incorporating endorectal MR elastography into multi-parametric MRI for prostate cancer imaging: Initial feasibility in volunteers

PURPOSE

To investigate the tolerability and technical feasibility of performing endorectal MR elastography (eMRE) in human volunteers within the representative age group commonly affected by prostate cancer.

MATERIALS AND METHODS

Endorectal MRE was conducted on seven volunteers in a 1.5 Tesla (T) MR imager using a rigid endorectal coil. Another five volunteers were imaged on a 3T MR imager using an inflatable balloon type endorectal coil. Tolerability was accessed for vibration amplitudes of ±1-50 μm and for frequencies of 100-300 Hz.

RESULTS

All 12 volunteers tolerated the displacements necessary to successfully perform eMRE. Shear waves with frequencies up to 300 Hz could propagate across the entire prostate using both coil designs.

CONCLUSION

The results of this study motivate further investigation of eMRE in prostate cancer patients to help determine if there is an added value of integrating eMRE into existing multi-parametric prostate MRI exams.

Monodomain shear wave propagation and bidomain shear wave dispersion in an elastic model of cardiac tissue

Cardiac tissue elastically deforms under an applied stress, permitting shear waves to propagate through the heart. Traditionally, this behavior has been modeled with a monodomain approach, in which the mechanical properties of the intracellular and extracellular spaces are averaged together. We consider a mechanical bidomain model of cardiac tissue in which the mechanics of the intracellular and extracellular spaces are considered individually with the two spaces coupled by a spring constant. We find two normal modes of oscillation: one in which the intracellular and extracellular spaces oscillate together (a monodomain mode) and the other in which they oscillate in opposition (a bidomain mode). These two modes have unique dispersion relationships. In the extreme approximation of equal shear moduli and mass densities of the intracellular and extracellular spaces, the dispersion in the bidomain mode depends on the spring constant, while it does not in the monodomain mode.

Estimating the local viscoelastic properties from dispersive shear waves using time-frequency ridge analysis

Modulated low-frequency shear waves can be non-invasively generated locally within a medium, by the oscillatory acoustic radiation force resulting from the interference of two focused quasi-CW ultrasound beams of slightly different frequencies. The propagation of such shear waves within a viscoelastic medium is known to be affected by the dispersive effects of viscosity. Specifically, a low-frequency (LF) spectral component was shown to arise with increased viscosities and higher modulation frequencies and appear as a 'slow' wave at the end of the shear waveform. In this paper, the shear dispersion characteristics are studied based on the Pseudo-Wigner-Ville distribution (PWVD) in the time-frequency domain. The ridges of the PWVD are then extracted and used to calculate the frequency-dependent shear speed, by identifying the LF dispersive component both in time and frequency. Using numerical simulations, it is shown that this way of estimating the shear dispersion is more efficient and robust than the conventional phase-delay Fourier method. Thus, more accurate estimates of the local shear modulus and viscosity of the propagating medium could be achieved. The effects of noise on the proposed method are also discussed.

Evaluation of neoadjuvant chemotherapy response in women with locally advanced breast cancer using ultrasound elastography

PURPOSE

Ultrasound elastography is a new imaging technique that can be used to assess tissue stiffness. The aim of this study was to investigate the potential of ultrasound elastography for monitoring treatment response of locally advanced breast cancer patients undergoing neoadjuvant therapy.

METHODS

Fifteen women receiving neoadjuvant chemotherapy had the affected breast scanned before, 1, 4, and 8 weeks following therapy initiation, and then before surgery. Changes in elastographic parameters related to tissue biomechanical properties were then determined and compared to clinical and pathologic tumor response after mastectomy.

RESULTS

Patients who responded to therapy demonstrated a significant decrease (P < .05) in strain ratios and strain differences 4 weeks after treatment initiation compared to non-responding patients. Mean strain ratio and mean strain difference for responders was 81 ± 3% and 1 ± 17% for static regions of interest (ROIs) and 81 ± 3% and 6 ± 18% for dynamic ROIs, respectively. In contrast, these parameters were 102±2%, 110±17%, 101±4%, and 109±30% for non-responding patients, respectively. Strain ratio using static ROIs was found to be the best predictor of treatment response, with 100% sensitivity and 100% specificity obtained 4 weeks after starting treatment.

CONCLUSIONS

These results suggest that ultrasound elastography can be potentially used as an early predictor of tumor therapy response in breast cancer patients.

Assessment of liver viscoelasticity by using shear waves induced by ultrasound radiation force

PURPOSE

To investigate the value of viscosity measured with ultrasonographic (US) elastography in liver fibrosis staging and to determine whether the use of a viscoelastic model to estimate liver elasticity can improve its accuracy in fibrosis staging.

MATERIALS AND METHODS

The study, which was performed from February 2010 to March 2011, was compliant with HIPAA and approved by the institutional review board. Written informed consent was obtained from each subject. Ten healthy volunteers (eight women and two men aged 27-55 years) and 35 patients with liver disease (17 women and 18 men aged 19-74 years) were studied by using US elasticity measurements of the liver (within 6 months of liver biopsy). US data were analyzed with the shear wave dispersion ultrasound vibrometry (SDUV) method, in which elasticity and viscosity are measured by evaluating dispersion of shear wave propagation speed, as well as with the time-to-peak (TTP) method, where tissue viscosity was neglected and only elasticity was estimated from the effective shear wave speed. The hepatic fibrosis stage was assessed histologically by using the METAVIR scoring system. The correlation of elasticity and viscosity was assessed with the Pearson correlation coefficient. The performances of SDUV and TTP were evaluated with receiver operating characteristic (ROC) curve analysis.

RESULTS

The authors found significant correlations between elasticity and viscosity measured with SDUV (r = 0.80) and elasticity measured with SDUV and TTP (r = 0.94). The area under the ROC curve for differentiating between grade F0-F1 fibrosis and grade F2-F4 fibrosis was 0.98 for elasticity measured with SDUV, 0.86 for viscosity measured with SDUV, and 0.95 for elasticity measured with TTP.

CONCLUSION

The results suggest that elasticity and viscosity measured between 95 Hz and 380 Hz by using SDUV are correlated and that elasticity measurements from SDUV and TTP showed substantially similar performance in liver fibrosis staging, although elasticity calculated from SDUV provided a better area under the ROC curve.

Shear elastic modulus can be used to estimate an index of individual muscle force during a submaximal isometric fatiguing contraction

The present study was designed to determine whether fatigue alters the ability to estimate an index of individual muscle force from shear elastic modulus measurements ( experiment I), and to test the ability of this technique to highlight changes in load sharing within a redundant muscle group during an isometric fatiguing task ( experiment II). Twelve subjects participated in experiment I, which consisted of smooth linear torque ramps from 0 to 80% of maximal voluntary contraction (MVC) performed before and after an isometric fatigue protocol, beginning at 40% of MVC and stopped when the force production dropped below 30% of MVC. Although the relationships between modulus and torque were very similar for pre- and postfatigue [root mean square deviation (RMSdeviation) = 3.7 ± 2.6% of MVC], the relationships between electromyography activity level and torque were greatly altered by fatigue (RMSdeviation = 10.3 ± 2.6% of MVC). During the fatiguing contraction, shear elastic modulus provided a significantly lower RMSdeviation between measured torque and estimated torque than electromyography activity level (5.7 ± 0.9 vs. 15.3 ± 3.8% of MVC). Experiment II performed with eight participants consisted of an isometric knee extension at 25% of MVC sustained until exhaustion. Opposite changes in shear elastic modulus were observed between synergists (vastus medialis, vastus lateralis, and rectus femoris) of some participants, reflecting changes in load sharing. In conclusion, despite the fact that we did not directly estimate muscle force (in Newtons), this is the first demonstration of an experimental technique to accurately quantify relative changes in force in an individual human muscle during a fatiguing contraction.

The variance of quantitative estimates in shear wave imaging: theory and experiments

In this paper, we investigate the relationship between the estimated shear modulus produced in shear wave imaging and the acquisition parameters. Using the framework of estimation theory and the Cramer-Rao lower bound applied both to the estimation of the velocity field variance and to the estimation of the shear wave travel time, we can derive the analytical formulation of the shear modulus variance σ(2)(μ) using relevant physical parameters such as the shear wave frequency, bandwidth, and ultrasonic parameters. This variance corresponds to the reproducibility of shear modulus reconstruction for a deterministic, quasi-homogeneous, and purely elastic medium. We thus consider the shear wave propagation as a deterministic process which is then corrupted during its observation by electronic noise and speckle decorrelation caused by shearing. A good correlation was found between analytical, numerical, and experimental results, which indicates that this formulation is well suited to understand the parameters' influence in those cases. The analytical formula stresses the importance of high-frequency and wideband shear waves for good estimation. Stiffer media are more difficult to assess reliably with identical acquisition signal-to-noise ratios, and a tradeoff between the reconstruction resolution of the shear modulus maps and the shear modulus variance is demonstrated. We then propose to use this formulation as a physical ground for a pixel-based quality measure that could be helpful for improving the reconstruction of real-time shear modulus maps for clinical applications.

Review of MR elastography applications and recent developments

The technique of MR elastography (MRE) has emerged as a useful modality for quantitatively imaging the mechanical properties of soft tissues in vivo. Recently, MRE has been introduced as a clinical tool for evaluating chronic liver disease, but many other potential applications are being explored. These applications include measuring tissue changes associated with diseases of the liver, breast, brain, heart, and skeletal muscle including both focal lesions (e.g., hepatic, breast, and brain tumors) and diffuse diseases (e.g., fibrosis and multiple sclerosis). The purpose of this review article is to summarize some of the recent developments of MRE and to highlight some emerging applications.

Office-based elastographic technique for quantifying mechanical properties of skeletal muscle

OBJECTIVES

Our objectives were to develop a new, efficient, and easy-to-administer approach to ultrasound elastography and assess its ability to provide quantitative characterization of viscoelastic properties of skeletal muscle in an outpatient clinical environment. We sought to show its validity and clinical utility in assessing myofascial trigger points, which are associated with myofascial pain syndrome.

METHODS

Ultrasound imaging was performed while the muscle was externally vibrated at frequencies in the range of 60 to 200 Hz using a handheld vibrator. The spatial gradient of the vibration phase yielded the shear wave speed, which is related to the viscoelastic properties of tissue. The method was validated using a calibrated experimental phantom, the biceps brachii muscle in healthy volunteers (n = 6), and the upper trapezius muscle in symptomatic patients with axial neck pain (n = 13) and asymptomatic (pain-free) control participants (n = 9).

RESULTS

Using the experimental phantom, our method was able to quantitatively measure the shear moduli with error rates of less than 20%. The mean shear modulus ± SD in the normal biceps brachii measured 12.5 ± 3.4 kPa, within the range of published values using more sophisticated methods. Shear wave speeds in active myofascial trigger points and the surrounding muscle tissue were significantly higher than those in normal tissue at high frequency excitations (>100 Hz; P < .05).

CONCLUSIONS

Off-the-shelf office-based equipment can be used to quantitatively characterize skeletal muscle viscoelastic properties with estimates comparable to those using more sophisticated methods. Our preliminary results using this method indicate that patients with spontaneous neck pain and symptomatic myofascial trigger points have increased tissue heterogeneity at the trigger point site and the surrounding muscle tissue.

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.

Muscle crush injury of extremity: quantitative elastography with supersonic shear imaging

The aim of this study was to determine the characteristic of muscle crush injury at quantitative ultrasonographic elastography using supersonic shear imaging (SSI). Twenty-three New Zealand rabbits underwent crush injury to left hind leg caused by a special balloon cuff device. Conventional ultrasonography and SSI quantitative elastography were performed at both crushed and uncrushed regions of the left hind legs. Quantitative lesion elasticity was measured using the Young's modulus (in kilopascals) at 0.5 h, 2 h, 6 h, 24 h and 72 h after the release of the crushing pressure. Compared with those from the uncrushed regions, both the maximum and mean elasticity values at these time points from the crushed regions were significantly higher (p < 0.001). A receiver operating characteristic (ROC) analysis was employed to assess diagnostic performance. ROC curves showed that extremity crush injury was diagnosed using elasticity value and the greater the elasticity value, the greater the diagnostic value. SSI provides quantitative elasticity measurements, thus, adding complementary information that potentially could help in crush injury characterization with conventional ultrasonography.

Application of 1-D transient elastography for the shear modulus assessment of thin-layered soft tissue: comparison with supersonic shear imaging technique

Elasticity estimation of thin-layered soft tissues has gained increasing interest propelled by medical applications like skin, corneal, or arterial wall shear modulus assessment. In this work, the authors propose one-dimensional transient elastography (1DTE) for the shear modulus assessment of thin-layered soft tissue. Experiments on three phantoms with different elasticities and plate thicknesses were performed. First, using 1DTE, the shear wave speed dispersion curve inside the plate was obtained and validated with finite difference simulation. No dispersive effects were observed and the shear wave speed was directly retrieved from time-of-flight measurements. Second, the supersonic shear imaging (SSI) technique (considered to be a gold standard) was performed. For the SSI technique, the propagating wave inside the plate is guided as a Lamb wave. Experimental SSI dispersion curves were compared with finite difference simulation and fitted using a generalized Lamb model to retrieve the plate bulk shear wave speed. Although they are based on totally different mechanical sources and induce completely different diffraction patterns for the shear wave propagation, the 1DTE and SSI techniques resulted in similar shear wave speed estimations. The main advantage of the 1DTE technique is that bulk shear wave speed can be directly retrieved without requiring a dispersion model.

Supersonic shear imaging provides a reliable measurement of resting muscle shear elastic modulus

The aim of the present study was to assess the reliability of shear elastic modulus measurements performed using supersonic shear imaging (SSI) in nine resting muscles (i.e. gastrocnemius medialis, tibialis anterior, vastus lateralis, rectus femoris, triceps brachii, biceps brachii, brachioradialis, adductor pollicis obliquus and abductor digiti minimi) of different architectures and typologies. Thirty healthy subjects were randomly assigned to the intra-session reliability (n = 20), inter-day reliability (n = 21) and the inter-observer reliability (n = 16) experiments. Muscle shear elastic modulus ranged from 2.99 (gastrocnemius medialis) to 4.50 kPa (adductor digiti minimi and tibialis anterior). On the whole, very good reliability was observed, with a coefficient of variation (CV) ranging from 4.6% to 8%, except for the inter-operator reliability of adductor pollicis obliquus (CV = 11.5%). The intraclass correlation coefficients were good (0.871 ± 0.045 for the intra-session reliability, 0.815 ± 0.065 for the inter-day reliability and 0.709 ± 0.141 for the inter-observer reliability). Both the reliability and the ease of use of SSI make it a potentially interesting technique that would be of benefit to fundamental, applied and clinical research projects that need an accurate assessment of muscle mechanical properties.

Loss tangent and complex modulus estimated by acoustic radiation force creep and shear wave dispersion

Elasticity imaging methods have been used to study tissue mechanical properties and have demonstrated that tissue elasticity changes with disease state. In current shear wave elasticity imaging methods typically only shear wave speed is measured and rheological models, e.g. Kelvin-Voigt, Maxwell and Standard Linear Solid, are used to solve for tissue mechanical properties such as the shear viscoelastic complex modulus. This paper presents a method to quantify viscoelastic material properties in a model-independent way by estimating the complex shear elastic modulus over a wide frequency range using time-dependent creep response induced by acoustic radiation force. This radiation force induced creep method uses a conversion formula that is the analytic solution of a constitutive equation. The proposed method in combination with shearwave dispersion ultrasound vibrometry is used to measure the complex modulus so that knowledge of the applied radiation force magnitude is not necessary. The conversion formula is shown to be sensitive to sampling frequency and the first reliable measure in time according to numerical simulations using the Kelvin-Voigt model creep strain and compliance. Representative model-free shear complex moduli from homogeneous tissue mimicking phantoms and one excised swine kidney were obtained. This work proposes a novel model-free ultrasound-based elasticity method that does not require a rheological model with associated fitting requirements.

Integration of crawling waves in an ultrasound imaging system. Part 1: system and design considerations

An ultrasound system (GE Logiq 9) was modified to produce a synthetic crawling wave using shear wave displacements generated by the radiation force of focused beams formed at the left and the right edge of the region of interest (ROI). Two types of focusing, normal and axicon, were implemented. Baseband (IQ) data was collected to determine the left and right displacements, which were then used to calculate an interference pattern. By imposing a variable delay between the two pushes, the interference pattern moves across the ROI to produce crawling waves. Also temperature and pressure measurements were made to assess the safety issues. The temperature profiles measured in a veal liver along the focal line showed the maximum temperature rise less than 0.8°C, and the pressure measurements obtained in degassed water and derated by 0.3 dB/cm/MHz demonstrate that the system can operate within FDA safety guidelines.

A Review of Shearwave Dispersion Ultrasound Vibrometry (SDUV) and its Applications

Measurement of tissue elasticity has emerged as an important advance in medical imaging and tissue characterization. However, soft tissue is inherently a viscoelastic material. One way to characterize the viscoelastic material properties of a material is to measure shear wave propagation velocities within the material at different frequencies and use the dispersion of the velocities, or variation with frequency, to solve for the material properties. Shearwave Dispersion Ultrasound Vibrometry (SDUV) is an ultrasound-based technique that uses this feature to characterize the viscoelastic nature of soft tissue. This method has been used to measure the shear elasticity and viscosity in various types of soft tissues including skeletal muscle, cardiac muscle, liver, kidney, prostate, and arterial vessels. This versatile technique provides measurements of viscoelastic material properties with high spatial and temporal resolution, which can be used for assessing these properties in normal and pathologic tissues. The goals of this paper are to 1) give an overview of viscoelasticity and shear wave velocity dispersion, 2) provide a history of the development of the SDUV method, and 3) survey applications for SDUV that have been previously reported.

Integration of crawling waves in an ultrasound imaging system. Part 2: signal processing and applications

This paper introduces methods to generate crawling wave interference patterns from the displacement fields generated from radiation force pushes on a GE Logiq 9 scanner. The same transducer and system provides both the pushing pulses to generate the shear waves and the tracking pulses to measure the displacements. Acoustic power and system limitations result in largely impulsive displacement fields. Measured displacements from pushes on either side of a region-of-interest (ROI) are used to calculate continuously varying interference patterns. This technique is explained along with a brief discussion of the conventional mechanical source-driven crawling waves for comparison. We demonstrate the method on three example cases: a gelatin-based phantom with a cylindrical inclusion, an oil-gelatin phantom and mouse livers. The oil-gelatin phantom and the mouse livers demonstrate not only shear speed estimation, but the frequency dependence of the shear wave speeds.

Estimation of individual muscle force using elastography

Background

Estimation of an individual muscle force still remains one of the main challenges in biomechanics. In this way, the present study aimed: (1) to determine whether an elastography technique called Supersonic Shear Imaging (SSI) could be used to estimate muscle force, (2) to compare this estimation to that one provided by surface electromyography (EMG), and (3) to determine the effect of the pennation of muscle fibers on the accuracy of the estimation.

Methods and Results

Eleven subjects participated in two experimental sessions; one was devoted to the shear elastic modulus measurements and the other was devoted to the EMG recordings. Each session consisted in: (1) two smooth linear torque ramps from 0 to 60% and from 0 to 30% of maximal voluntary contraction, for the first dorsal interosseous and the abductor digiti minimi, respectively (referred to as “ramp contraction”); (2) two contractions done with the instruction to freely change the torque (referred to as “random changes contraction”). Multi-channel surface EMG recordings were obtained from a linear array of eight electrodes and the shear elastic modulus was measured using SSI. For ramp contractions, significant linear relationships were reported between EMG activity level and torque (R² = 0.949±0.036), and between shear elastic modulus and torque (R² = 0.982±0.013). SSI provided significant lower RMS_deviation between measured torque and estimated torque than EMG activity level for both types of contraction (1.4±0.7 vs. 2.8±1.4% of maximal voluntary contraction for “ramp contractions”, p<0.01; 4.5±2.3 vs. 7.9±5.9% of MVC for “random changes contractions”, p<0.05). No significant difference was reported between muscles.

Conclusion

The shear elastic modulus measured using SSI can provide a more accurate estimation of individual muscle force than surface EMG. In addition, pennation of muscle fibers does not influence the accuracy of the estimation.

Estimating material viscoelastic properties based on surface wave measurements: a comparison of techniques and modeling assumptions

Previous studies of the first author and others have focused on low audible frequency (<1 kHz) shear and surface wave motion in and on a viscoelastic material comprised of or representative of soft biological tissue. A specific case considered has been surface (Rayleigh) wave motion caused by a circular disk located on the surface and oscillating normal to it. Different approaches to identifying the type and coefficients of a viscoelastic model of the material based on these measurements have been proposed. One approach has been to optimize coefficients in an assumed viscoelastic model type to match measurements of the frequency-dependent Rayleigh wave speed. Another approach has been to optimize coefficients in an assumed viscoelastic model type to match the complex-valued frequency response function (FRF) between the excitation location and points at known radial distances from it. In the present article, the relative merits of these approaches are explored theoretically, computationally, and experimentally. It is concluded that matching the complex-valued FRF may provide a better estimate of the viscoelastic model type and parameter values; though, as the studies herein show, there are inherent limitations to identifying viscoelastic properties based on surface wave measurements.

Shearwave dispersion ultrasound vibrometry (SDUV) on swine kidney

Shearwave dispersion ultrasound vibrometry (SDUV) is used to quantify both tissue shear elasticity and shear viscosity by evaluating dispersion of shear wave propagation speed over a certain bandwidth (50 to 500 Hz). The motivation for developing elasticity imaging techniques is the desire to diagnose disease processes. However, it is important to study the mechanical properties of healthy tissues; such data can enhance clinical knowledge and improve understanding of the mechanical properties of tissue. The purpose of this study is to evaluate the feasibility of using SDUV for in vitro measurements of renal cortex shear elasticity and shear viscosity in healthy swine kidneys. Eight excised kidneys from female pigs were used in these in vitro experiments and a battery of tests was performed to gain insight into the material proper ties of the renal cortex. In these 8 kidneys, the overall renal cortex elasticity and viscosity were 1.81 ± 0.17 kPa and 1.48 ± 0.49 Pa-s, respectively. In an analysis of the material properties over time after excision, there was not a statistically significant difference in shear elasticity over a 24-h period, but a statistically significant difference in shear viscosity was found. Homogeneity of the renal cortex was examined and it was found that shear elasticity and shear viscosity were statistically different within a kidney, suggesting global tissue inhomogeneity. In creases of more than 30% in shear elasticity and shear viscosity were observed after immersion in 10% formaldehyde. Finally, it was found that the renal cortex is rather anisotropic. Two values for shear elasticity and shear viscosity were measured depending on shear wave propagation direction. These various tests elucidated different aspects of the material properties and the structure of the ex vivo renal cortex.

Bias observed in time-of-flight shear wave speed measurements using radiation force of a focused ultrasound beam

Measurement of shear wave propagation speed has important clinical applications because it is related to tissue stiffness and health state. Shear waves can be generated in tissues by the radiation force of a focused ultrasound beam (push beam). Shear wave speed can be measured by tracking its propagation laterally from the push beam focus using the time-of-flight principle. This study shows that shear wave speed measurements with such methods can be transducer, depth and lateral tracking range dependent. Three homogeneous phantoms with different stiffness were studied using curvilinear and linear array transducer. Shear wave speed measurements were made at different depths, using different aperture sizes for push and at different lateral distance ranges from the push beam. The curvilinear transducer shows a relatively large measurement bias that is depth dependent. The possible causes of the bias and options for correction are discussed. These bias errors must be taken into account to provide accurate and precise time-of-flight shear wave speed measurements for clinical use.

Assessment of viscous and elastic properties of sub-wavelength layered soft tissues using shear wave spectroscopy: theoretical framework and in vitro experimental validation

In elastography, quantitative imaging of soft tissue elastic properties is provided by local shear wave speed estimation. Shear wave imaging in a homogeneous medium thicker than the shear wavelength is eased by a simple relationship between shear wave speed and local shear modulus. In thin layered organs, the shear wave is guided and thus undergoes dispersive effects. This case is encountered in medical applications such as elastography of skin layers, corneas, or arterial walls. In this work, we proposed and validated shear wave spectroscopy as a method for elastic modulus quantification in such layered tissues. Shear wave dispersion curves in thin layers were obtained by finite-difference simulations and numerical solving of the boundary conditions. In addition, an analytical approximation of the dispersion equation was derived from the leaky Lamb wave theory. In vitro dispersion curves obtained from phantoms were consistent with numerical studies (deviation <1.4%). The least-mean-squares fitting of the dispersion curves enables a quantitative and accurate (error < 5% of the transverse speed) assessment of the elasticity. Dispersion curves were also found to be poorly influenced by shear viscosity. This phenomenon allows independent recovery of the shear modulus and the viscosity, using, respectively, the dispersion curve and the attenuation estimation along the propagation axis.

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.

Estimation of mechanical properties of a viscoelastic medium using a laser-induced microbubble interrogated by an acoustic radiation force

An approach to assess the mechanical properties of a viscoelastic medium using laser-induced microbubbles is presented. To measure mechanical properties of the medium, dynamics of a laser-induced cavitation microbubble in viscoelastic medium under acoustic radiation force was investigated. An objective lens with a 1.13 numerical aperture and an 8.0 mm working distance was designed to focus a 532 nm wavelength nanosecond pulsed laser beam and to create a microbubble at the desired location. A 3.5 MHz ultrasound transducer was used to generate acoustic radiation force to excite a laser-induced microbubble. Motion of the microbubble was tracked using a 25 MHz imaging transducer. Agreement between a theoretical model of bubble motion in a viscoelastic medium and experimental measurements was demonstrated. Young's modulii reconstructed using the laser-induced microbubble approach were compared with those measured using a direct uniaxial method over the range from 0.8 to 13 kPa. The results indicate good agreement between methods. Thus, the proposed approach can be used to assess the mechanical properties of a viscoelastic medium.

Principles of ultrasound elastography

Many manufacturers of ultrasound systems offer elastography capabilities. Though the research literature describes techniques involving external actuators, commercial systems have preferred to adopt techniques that may be used with a single transducer. Techniques may be divided into those which measure strain and those which measure shear wave velocity. Strain elastography involves deformation of the tissue followed by imaging of the degree of compression or extension of the tissue. Strain elastography does not estimate tissue stiffness; however, the strain ratio may be used as a surrogate index of stiffness. Shear wave elastography provides true quantitative information on elastic modulus. This involves induction of shear waves, estimation of shear wave velocity c s and conversion to elastic modulus E using the equation E = 3 ρ cs2 where ρ is the density. The description of tissues as being purely elastic is simplistic. In practice they may also exhibit time-dependent viscous behaviour. Recent literature describe methods that have been developed for estimation of the viscoelastic behaviour from the change in strain with time or by estimation of the shear wave dispersion, a technique known as ‘shear wave spectroscopy’. These methods may become commercially available in the medium term, offering a new quantity (tissue viscosity) for diagnostic use.

Noninvasive in vivo liver fibrosis evaluation using supersonic shear imaging: a clinical study on 113 hepatitis C virus patients

Supersonic shear imaging (SSI) has recently been demonstrated to be a repeatable and reproducible transient bidimensional elastography technique. We report a prospective clinical evaluation of the performances of SSI for liver fibrosis evaluation in 113 patients with hepatitis C virus (HCV) and a comparison with FibroScan (FS). Liver elasticity values using SSI and FS ranged from 4.50 kPa to 33.96 kPa and from 2.60 kPa to 46.50 kPa, respectively. Analysis of variance (ANOVA) shows a good agreement between fibrosis staging and elasticity assessment using SSI and FS (p < 10(-5)). The areas under receiver operating characteristic (ROC) curves for elasticity values assessed from SSI were 0.948, 0.962 and 0.968 for patients with predicted fibrosis levels F ≥ 2, F ≥ 3 and F = 4, respectively. These values are compared with FS area under the receiver operating characteristic curve (AUROC) of 0.846, 0.857 and 0.940, respectively. This comparison between ROC curves is particularly significant for mild and intermediate fibrosis levels. SSI appears to be a fast, simple and reliable method for noninvasive liver fibrosis evaluation.

Acoustic radiation force-based elasticity imaging methods

Conventional diagnostic ultrasound images portray differences in the acoustic properties of soft tissues, whereas ultrasound-based elasticity images portray differences in the elastic properties of soft tissues (i.e. stiffness, viscosity). The benefit of elasticity imaging lies in the fact that many soft tissues can share similar ultrasonic echogenicities, but may have different mechanical properties that can be used to clearly visualize normal anatomy and delineate pathological lesions. Acoustic radiation force-based elasticity imaging methods use acoustic radiation force to transiently deform soft tissues, and the dynamic displacement response of those tissues is measured ultrasonically and is used to estimate the tissue's mechanical properties. Both qualitative images and quantitative elasticity metrics can be reconstructed from these measured data, providing complimentary information to both diagnose and longitudinally monitor disease progression. Recently, acoustic radiation force-based elasticity imaging techniques have moved from the laboratory to the clinical setting, where clinicians are beginning to characterize tissue stiffness as a diagnostic metric, and commercial implementations of radiation force-based ultrasonic elasticity imaging are beginning to appear on the commercial market. This article provides an overview of acoustic radiation force-based elasticity imaging, including a review of the relevant soft tissue material properties, a review of radiation force-based methods that have been proposed for elasticity imaging, and a discussion of current research and commercial realizations of radiation force based-elasticity imaging technologies.

Monitoring of thermal therapy based on shear modulus changes: II. Shear wave imaging of thermal lesions

The clinical applicability of high-intensity focused ultrasound (HIFU) for noninvasive therapy is currently hampered by the lack of robust and real-time monitoring of tissue damage during treatment. The goal of this study is to show that the estimation of local tissue elasticity from shear wave imaging (SWI) can lead to a precise mapping of the lesion. HIFU treatment and monitoring were respectively performed using a confocal setup consisting of a 2.5-MHz single element transducer focused at 34 mm on ex vivo samples and an 8-MHz ultrasound diagnostic probe. Ultrasound-based strain imaging was combined with shear wave imaging on the same device. The SWI sequences consisted of 2 successive shear waves induced at different lateral positions. Each wave was created with pushing beams of 100 μs at 3 depths. The shear wave propagation was acquired at 17,000 frames/s, from which the elasticity map was recovered. HIFU sonications were interleaved with fast imaging acquisitions, allowing a duty cycle of more than 90%. Thus, elasticity and strain mapping was achieved every 3 s, leading to real-time monitoring of the treatment. When thermal damage occurs, tissue stiffness was found to increase up to 4-fold and strain imaging showed strong shrinkages that blur the temperature information. We show that strain imaging elastograms are not easy to interpret for accurate lesion characterization, but SWI provides a quantitative mapping of the thermal lesion. Moreover, the concept of shear wave thermometry (SWT) developed in the companion paper allows mapping temperature with the same method. Combined SWT and shear wave imaging can map the lesion stiffening and temperature outside the lesion, which could be used to predict the eventual lesion growth by thermal dose calculation. Finally, SWI is shown to be robust to motion and reliable in vivo on sheep muscle.

Shear elastic modulus estimation from indentation and SDUV on gelatin phantoms

Tissue mechanical properties such as elasticity are linked to tissue pathology state. Several groups have proposed shear wave propagation speed to quantify tissue mechanical properties. It is well known that biological tissues are viscoelastic materials; therefore, velocity dispersion resulting from material viscoelasticity is expected. A method called shearwave dispersion ultrasound vibrometry (SDUV) can be used to quantify tissue viscoelasticity by measuring dispersion of shear wave propagation speed. However, there is not a gold standard method for validation. In this study, we present an independent validation method of shear elastic modulus estimation by SDUV in three gelatin phantoms of differing stiffness. In addition, the indentation measurements are compared to estimates of elasticity derived from shear wave group velocities. The shear elastic moduli from indentation were 1.16, 3.40, and 5.6 kPa for a 7%, 10%, and 15% gelatin phantom, respectively. SDUV measurements were 1.61, 3.57, and 5.37 kPa for the gelatin phantoms, respectively. Shear elastic moduli derived from shear wave group velocities were 1.78, 5.2, and 7.18 kPa for the gelatin phantoms, respectively. The shear elastic modulus estimated from the SDUV, matched the elastic modulus measured by indentation. On the other hand, shear elastic modulus estimated by group velocity did not agree with indentation test estimations. These results suggest that shear elastic modulus estimation by group velocity will be bias when the medium being investigated is dispersive. Therefore, a rheological model should be used in order to estimate mechanical properties of viscoelastic materials.

In vivo mapping of brain elasticity in small animals using shear wave imaging

A combination of radiation force and ultrafast ultrasound imaging is used to both generate and track the propagation of a shear wave in the brain whose local speed is directly related to stiffness, characterized by the dynamic shear modulus G*. When performed on trepanated rats, this approach called shear wave imaging (SWI) provides 3-D brain elasticity maps reaching a spatial resolution of 0.7 mm×1 mm×0.4 mm with a good reproducibility (<13%). The dynamic shear modulus of brain tissues exhibits values in the 2-25 kPa range with a mean value of 12 kPa and is quantified for different anatomical regions. The anisotropy of the shear wave propagation is studied and the first in vivo anisotropy map of brain elasticity is provided. The propagation is found to be isotropic in three gray matter regions but highly anisotropic in two white matter regions. The good temporal resolution (~10 ms per acquisition) of SWI also allows a dynamic estimation of brain elasticity to within a single cardiac cycle, showing that brain pulsatility does not transiently modify local elasticity. SWI proves its potential for the study of pathological modifications of brain elasticity both in small animal models and in clinical intra-operative imaging.

Wide-range dynamic magnetic resonance elastography

Tissue mechanical parameters have been shown to be highly sensitive to disease by elastography. Magnetic resonance elastography (MRE) in the human body relies on the low-dynamic range of tissue mechanics <100 Hz. In contrast, MRE suited for investigations of mice or small tissue samples requires vibration frequencies 10-20 times higher than those used in human MRE. The dispersion of the complex shear modulus (G(⁎)) prevents direct comparison of elastography data at different frequency bands and, consequently, frequency-independent viscoelastic models that fit to G(*) over a wide dynamic range have to be employed. This study presents data of G(*) of samples of agarose gel, liver, brain, and muscle measured by high-resolution MRE in a 7T-animal scanner at 200-800 Hz vibration frequency. Material constants μ and α according to the springpot model and related to shear elasticity and slope of the G(*)-dispersion were determined. Both μ and α of calf brain and bovine liver were found to be similar, while a sample of fibrotic human liver (METAVIR score of 3) displayed about fifteen times higher shear elasticity, similar to μ of bovine muscle measured in muscle fiber direction. α was the highest in fibrotic liver, followed by normal brain and liver, while muscle had the lowest α-values of all biological samples investigated in this study. As expected, the least G(*)-dispersion was seen in soft gel. The proposed technique of wide-range dynamic MRE can provide baseline data for both human MRE and high-dynamic MRE for better understanding tissue mechanics of different tissue structures.

Robust estimation of time-of-flight shear wave speed using a radon sum transformation

Time-of-flight methods allow quantitative measurement of shear wave speed (SWS) from ultrasonically tracked displacements following impulsive excitation in tissue. However, application of these methods to in vivo data are challenging because of the presence of gross outlier data resulting from sources such as physiological motion or spatial inhomogeneities. This paper describes a new method for estimating SWS by considering a solution space of trajectories and evaluating each trajectory using a metric that characterizes wave motion along the entire trajectory. The metric used here is found by summing displacement data along the trajectory as in the calculation of projection data in the Radon transformation. The algorithm is evaluated using data acquired in calibrated phantoms and in vivo human liver. Results are compared with SWS estimates using a random sample consensus (RANSAC) algorithm described by Wang et al. Good agreement is found between the Radon sum and RANSAC SWS estimates with a correlation coefficient of greater than 0.99 for phantom data and 0.91 for in vivo liver data. The Radon sum transformation is suitable for use in situations requiring real-time feedback and is comparably robust to the RANSAC algorithm with respect to outlier data.

Viscoelasticity-based MR elastography of skeletal muscle

An in vivo multifrequency magnetic resonance elastography (MRE) protocol was developed for studying the viscoelastic properties of human skeletal muscle in different states of contraction. Low-frequency shear vibrations in the range of 25-62.5 Hz were synchronously induced into the femoral muscles of seven volunteers and measured in a cross-sectional view by encoding the fast-transverse shear wave component parallel to the muscle fibers. The so-called springpot model was used for deriving two viscoelastic constants, μ and α, from the dispersion functions of the complex shear modulus in relaxed and in loaded muscle. Representing the shear elasticity parallel to the muscle fibers, μ increased in all volunteers upon contraction from 2.68 ± 0.23 kPa to 3.87 ± 0.50 kPa. Also α varied with load, indicating a change in the geometry of the mechanical network of muscle from relaxation (α = 0.253 ± 0.009) to contraction (α = 0.270 ± 0.009). These results provide a reference for a future assessment of muscular dysfunction using rheological parameters.