Induced Shear Waves Research Articles

Pulse-Echo Ultrasound for Quantitative Measurements of Two Uncorrelated Elastic Parameters

ObjectiveThis paper describes the relationship between elastic tissue properties and strain and presents an initial investigation of pulse-echo ultrasound to measure two uncorrelated elastic parameters in tissue-mimicking phantoms. The two elastic parameters are the shear modulus, related to deformation of shape, and what we in the paper define as the nonlinear compressibility, related to deformation of volume. MethodsWe prepared tissue-mimicking phantoms containing lesions of variable shear modulus and variable nonlinear compressibility. An in-house framework for shear wave imaging was developed using ultrasound radiation force at 4.5 MHz to induce shear waves and plane wave imaging with pulses in a frequency band centered around 12.5 MHz to track the shear waves. For measurements of nonlinear compressibility, co-propagating dual-frequency pulse complexes at 0.7 MHz and 14 MHz were applied. Algorithms were implemented on a Verasonics Vantage ultrasound scanner and a custom-made multi-frequency ultrasound transducer was used. Mechanical indentation measurements were performed to validate ultrasound measurements of the shear modulus. For the nonlinear compressibility, ultrasound measurements were compared to results derived from the literature. ResultsWe found good agreement in elasticity results from ultrasound measurements and mechanical indentation as well as when comparing with results derived from the literature. ConclusionResults of the current investigation were promising. We plan patient studies involving thyroid lesions and liver steatosis to explore whether measurements of elastic parameters related both to shape deformation and volume deformation are useful in clinical practice.

Acoustic radiation force-induced longitudinal shear wave for ultrasound-based viscoelastic evaluation

Acoustic radiation force (ARF) is widely used to induce shear waves for evaluating the mechanical properties of biological tissues. Two shear waves can be generated when exciting with ARF: a transverse shear wave, also simply called shear wave (SW), and a longitudinal shear wave (LSW). Shear waves (SWs) have been broadly used to assess the mechanical properties. Some articles have reported that the LSW can be used to evaluate mechanical properties locally. However, existing LSW studies are mainly focused on the group velocity evaluation using optical coherence tomography (OCT). Here, we report that a LSW generated with ARF can be used to probe viscoelastic properties, including shear modulus and viscosity, using ultrasound. We took advantage of the surface boundary effect to reflect the LSW, named RLSW, to address the energy deficiency of LSW induced by ARF. We systematically evaluated the experiments with tissue-mimicking viscoelastic phantoms and validated by numerical simulations. Phase velocity and dispersion comparison between the results induced by a RLSW and a SW exhibit good agreement in both the numerical simulations and experimental results. The Kelvin-Voigt (KV) model was used to determine the shear modulus and viscosity. RLSW shows great potential to evaluate localized viscoelastic properties, which could benefit various biomedical applications such as evaluating the viscoelasticity of heterogeneous materials or microscopic lesions of tissues.

Effect of interstitial fluid pressure on shear wave elastography: an experimental and computational study

Objective. An elevated interstitial fluid pressure (IFP) can lead to strain-induced stiffening of poroelastic biological tissues. As shear wave elastography (SWE) measures functional tissue stiffness based on the propagation speed of acoustically induced shear waves, the shear wave velocity (SWV) can be used as an indirect measurement of the IFP. The underlying biomechanical principle for this stiffening behavior with pressurization is however not well understood, and we therefore studied how IFP affects SWV through SWE experiments and numerical modeling. Approach. For model set-up and verification, SWE experiments were performed while dynamically modulating IFP in a chicken breast. To identify the confounding factors of the SWV-IFP relationship, we manipulated the material model (linear poroelastic versus porohyperelastic), deformation assumptions (geometric linearity versus nonlinearity), and boundary conditions (constrained versus unconstrained) in a finite element model mimicking the SWE experiments. Main results. The experiments demonstrated a statistically significant positive correlation between the SWV and IFP. The model was able to reproduce a similar SWV-IFP relationship by considering an unconstrained porohyperelastic tissue. Material nonlinearity was identified as the primary factor contributing to this relationship, whereas geometric nonlinearity played a smaller role. The experiments also highlighted the importance of the dynamic nature of the pressurization procedure, as indicated by a different observed SWV-IFP for pressure buildup and relaxation, but its clinical relevance needs to be further investigated. Significance. The developed model provides an adaptable framework for SWE of poroelastic tissues and paves the way towards non-invasive measurements of IFP.

Wearable bioadhesive ultrasound shear wave elastography.

Acute liver failure (ALF) is a critical medical condition defined as the rapid development of hepatic dysfunction. Conventional ultrasound elastography cannot continuously monitor liver stiffness over the course of rapidly changing diseases for early detection due to the requirement of a handheld probe. In this study, we introduce wearable bioadhesive ultrasound elastography (BAUS-E), which can generate acoustic radiation force impulse (ARFI) to induce shear waves for the continuous monitoring of modulus changes. BAUS-E contains 128 channels with a compact design with only 24 mm in the azimuth direction for comfortable wearability. We further used BAUS-E to continuously monitor the stiffness of in vivo rat livers with ALF induced by d-galactosamine over 48 hours, and the stiffness change was observed within the first 6 hours. BAUS-E holds promise for clinical applications, particularly in patients after organ transplantation or postoperative care in the intensive care unit (ICU).

Continuous shear wave measurements for dynamic cardiac stiffness evaluation in pigs

Ultrasound-based shear wave elastography is a promising technique to non-invasively assess the dynamic stiffness variations of the heart. The technique is based on tracking the propagation of acoustically induced shear waves in the myocardium of which the propagation speed is linked to tissue stiffness. This measurement is repeated multiple times across the cardiac cycle to assess the natural variations in wave propagation speed. The interpretation of these measurements remains however complex, as factors such as loading and contractility affect wave propagation. We therefore applied transthoracic shear wave elastography in 13 pigs to investigate the dependencies of wave speed on pressure–volume derived indices of loading, myocardial stiffness, and contractility, while altering loading and inducing myocardial ischemia/reperfusion injury. Our results show that diastolic wave speed correlates to a pressure–volume derived index of operational myocardial stiffness (R = 0.75, p < 0.001), suggesting that both loading and intrinsic properties can affect diastolic wave speed. Additionally, the wave speed ratio, i.e. the ratio of systolic and diastolic speed, correlates to a pressure–volume derived index of contractility, i.e. preload-recruitable stroke work (R = 0.67, p < 0.001). Measuring wave speed ratio might thus provide a non-invasive index of contractility during ischemia/reperfusion injury.

Skeletal Muscle Assessment Using Quantitative Ultrasound: A Narrative Review.

Ultrasound (US) is an important imaging tool for skeletal muscle analysis. The advantages of US include point-of-care access, real-time imaging, cost-effectiveness, and absence of ionizing radiation. However, US can be highly dependent on the operator and/or US system, and a portion of the potentially useful information carried by raw sonographic data is discarded in image formation for routine qualitative US. Quantitative ultrasound (QUS) methods provide analysis of the raw or post-processed data, revealing additional information about normal tissue structure and disease status. There are four QUS categories that can be used on muscle and are important to review. First, quantitative data derived from B-mode images can help determine the macrostructural anatomy and microstructural morphology of muscle tissues. Second, US elastography can provide information about muscle elasticity or stiffness through strain elastography or shear wave elastography (SWE). Strain elastography measures the induced tissue strain caused either by internal or external compression by tracking tissue displacement with detectable speckle in B-mode images of the examined tissue. SWE measures the speed of induced shear waves traveling through the tissue to estimate the tissue elasticity. These shear waves may be produced using external mechanical vibrations or internal "push pulse" ultrasound stimuli. Third, raw radiofrequency signal analyses provide estimates of fundamental tissue parameters, such as the speed of sound, attenuation coefficient, and backscatter coefficient, which correspond to information about muscle tissue microstructure and composition. Lastly, envelope statistical analyses apply various probability distributions to estimate the number density of scatterers and quantify coherent to incoherent signals, thus providing information about microstructural properties of muscle tissue. This review will examine these QUS techniques, published results on QUS evaluation of skeletal muscles, and the strengths and limitations of QUS in skeletal muscle analysis.

Simultaneous proton resonance frequency T1 - MR shear wave elastography for MR-guided focused ultrasound multiparametric treatment monitoring.

To develop an efficient MRI pulse sequence to simultaneously measure multiple parameters that have been shown to correlate with tissue nonviability following thermal therapies. A 3D segmented EPI pulse sequence was used to simultaneously measure proton resonance frequency shift (PRFS) MR thermometry (MRT), T1 relaxation time, and shear wave velocity induced by focused ultrasound (FUS) push pulses. Experiments were performed in tissue mimicking gelatin phantoms and ex vivo bovine liver. Using a carefully designed FUS triggering scheme, a heating duty cycle of approximately 65% was achieved by interleaving FUS ablation pulses with FUS push pulses to induce shear waves in the tissue. In phantom studies, temperature increases measured with PRFS MRT and increases in T1 correlated with decreased shear wave velocity, consistent with material softening with increasing temperature. During ablation in ex vivo liver, temperature increase measured with PRFS MRT initially correlated with increasing T1 and decreasing shear wave velocity, and after tissue coagulation with decreasing T1 and increasing shear wave velocity. This is consistent with a previously described hysteresis in T1 versus PRFS curves and increased tissue stiffness with tissue coagulation. An efficient approach for simultaneous and dynamic measurements of PRSF, T1 , and shear wave velocity during treatment is presented. This approach holds promise for providing co-registered dynamic measures of multiple parameters, which correlates to tissue nonviability during and following thermal therapies, such as FUS.

Influence of the Field of View on Shear Wave Velocity Estimation

Abstract Tissue elasticity contains important information for physicians in diagnosis and treatment, and, e.g., can help in tumor detection because tumors are stiffer than healthy tissue. Ultrasound shear wave elastography imaging (US-SWEI) can be used to estimate tissue stiffness by measuring the velocity of induced shear waves. Commonly, a linear US probe is used to track shear waves at a high imaging frequency in 2D. Realtime US-SWEI is limited by the required time for data processing. Hence, reducing the imaging field of view (FOV) is beneficial as it decreases the size of the acquired data and thereby the acquisition, transfer and processing time. However, a decrease in the FOV has the disadvantage that shear waves are tracked over a smaller distance and thus, also fewer sampling points are available for velocity estimation. This trade-off between a smaller FOV and thus, a smaller data size, and the accuracy of shear wave velocity estimation is investigated in this work. For this purpose, shear waves were tracked with a linear US probe in gelatin phantoms with four different stiffness values. During data processing, we reduced the FOV virtually from 38.1mm to 2.1mm. It was found that a reduction of the FOV to 4.5mm leads to an overestimation of up to five times larger shear wave velocities but still allows to distinguish between phantoms of different stiffness. However, not all estimated velocity values could be clearly assigned to the correct stiffness value. The smallest studied FOV of 2.1mm was not sufficient for distinguishing between the phantoms anymore.

Contribution of UltraFast™ Ultrasound and Shear Wave Elastography in the Imaging of Carotid Artery Disease.

Carotid artery disease is one of the main global causes of disability and premature mortality in the spectrum of cardiovascular diseases. One of its main consequences, stroke, is the second biggest global contributor to disability and burden via Disability Adjusted Life Years after ischemic heart disease. In the last decades, B-mode and Doppler-based ultrasound imaging techniques have become an indispensable part of modern medical imaging of carotid artery disease. However, they have limited abilities in carotid artery plaque and wall characterization and are unable to provide simultaneous quantitative and qualitative flow information while the images are burdened by low framerates. UltraFast™ ultrasound is able to overcome these obstacles by providing simultaneous quantitative and qualitative flow analysis information in high frame rates via UltraFast™ Doppler. Another newly developed ultrasound technique, shear wave elastography, is based on the visualization of induced shear waves and the measurement of the shear wave propagation speed in the examined tissues which enables real-time carotid plaque and wall analysis. These newly developed ultrasound modalities have potential to significantly improve workflow efficiency and are able to provide a plethora of additional imaging information of carotid artery disease in comparison to conventional ultrasound techniques.

On the interplay of loading, myocardial stiffness and contractility in transthoracic acoustic radiation force-induced shear wave measurements in pigs

Abstract Funding Acknowledgements Type of funding sources: Public Institution(s). Main funding source(s): Research Foundation Flanders (FWO grant number 1211620N) &amp; TTW-Dutch Heart Foundation partnership program "Early recognition of cardiovascular diseases" (project number 14740) Background Acoustic radiation force-based shear wave elastography (SWE) is a promising technique to non-invasively assess mechanical properties of the heart based on the propagation speed of acoustically induced shear waves. However, the interpretation of cardiac SWE measurements remains complex, and it is unclear how other factors such as loading affect shear wave propagation speed (SWS) measurements in diastole and systole. Purpose We applied transthoracic SWE in a pig model to investigate the dependencies of diastolic and systolic SWS on pressure-volume (PV) loop derived indices of loading, myocardial stiffness and contractility. Methods In 7 pigs, loading conditions were altered (increasing or decreasing preload; increasing afterload) and myocardial stiffness was changed (LAD occlusion for 60-100 minutes followed by 40 minutes of reperfusion). For each intervention, transthoracic SWE measurements were performed in a parasternal long-axis view with a high frame rate ultrasound system (&amp;gt; 6.2 kHz). Recordings of 28 ms were repeated at 34 Hz during 1.5 s to track shear waves throughout the cardiac cycle. To determine systolic and diastolic SWS in a robust manner, a piecewise linear model was fitted to the SWS data of each intervention representing multiple acquisitions, heartbeats and M-lines (fig. 1a). PV loops were recorded simultaneously with SWE measurements to estimate end-diastolic pressure (EDP), end-systolic pressure (ESP), end-diastolic pressure volume relation (EDPVR with exponential coefficient β in fig. 1b) and preload-recruitable stroke work (PRSW). Passive chamber stiffness was evaluated as the local slope of the EDPVR, i.e. β·EDP. Linear regressions and Pearson’s correlation coefficients were calculated. Results Diastolic SWS was significantly correlated to EDP when altering loading (blue in fig. 2a: R = 0.55; p &amp;lt; 0.01) and stiffness (orange in fig. 2a: R = 0.66; p &amp;lt; 0.01). A similar correlation is found between SWS and passive chamber stiffness β·EDP (fig. 2b). Diastolic SWS is more sensitive to changes in stiffness than in loading, as reflected by the larger slope of the regression line (0.79 vs. 0.28 in fig. 2b). Furthermore, systolic SWS significantly correlated with measures of contractility during loading alterations: ESP in fig. 2c (R = 0.69; p &amp;lt; 0.001) and PRSW in fig. 2d (R = 0.63; p = 0.02). However, no significant correlation was found between systolic SWS and contractility during infarct/reperfusion. Conclusion This study shows that diastolic SWS reflects the instantaneous stiffness of the myocardium, but is not a load-independent measure of the intrinsic passive mechanical properties of the heart. Instantaneous stiffness, and thus diastolic SWS, might be altered by loading (due to material non-linearity) or intrinsic mechanical changes. Furthermore, loading experiments suggest that systolic SWS is related to contractility. The relation between SWS and contractility in the presence of myocardial infarct deserves further study. Abstract Figure. Fig. 1: SWS and PV analysis. Abstract Figure. Fig. 2: SWS vs. PV-derived indices.

On the steerability of phased array EMATs: The dipole element

Shear wave phased array EMATs have a range of properties that make them a desirable solution compared to their piezoelectric counterparts: They do not require direct contact with the specimen and they can operate at elevated temperatures. Shear wave phased array EMATs do not need a Perspex wedge or couplant, making them easy to deploy across industrial applications even at elevated temperatures. Nonetheless, they are not commonly used because of concerns over the strength of signal excitation and lack of knowledge about potential changes in the emitted wavefields as a result of changes in the magnetic properties of the inspected material. In this work, a shear wave phased array EMAT designed to induce shear waves into paramagnetic, diamagnetic, ferromagnetic and ferromagnetic and magnetostrictive metals without compromising its angle coverage or steerability is proposed. The phased array EMAT was designed based on a dipole element wave generation source. The proposed dipole element source is formed by the interaction of the bias magnetic field of the PA EMAT and the electromagnetic field induced by the race-track coils of the PA EMAT. For each race-track coil this results in a combined source that consists of two parallel and slightly separated shear line sources of opposite polarity. A detailed study of the wave generation based on the surface loads induced by the dipole element is presented. The surface loads of the three excitation mechanisms involved in the EMAT transduction -Lorentz force, magnetisation force and magnetostriction force-were estimated in a 2D finite element model. The waves generated by the induced surface loads were estimated in a second 2D finite element model. A shear wave steerability analysis of the phased array EMAT on different metallic materials based on the 2D finite element results is presented in combination with an experimental evaluation on Aluminium 6082 (diamagnetic), mild steel EN1A (ferromagnetic), stainless steel 316 (paramagnetic), stainless steel 304 (paramagnetic) and nickel 201 (ferromagnetic and magnetostrictive). The experimental results showed good agreement with the simulation study. The simulation and experimental results concluded that the angle coverage of the phased array EMAT is almost insensitive to the inspected metallic material.

Phase velocity imaging using acoustic radiation force-induced shear wave with a multi-frequency excitation pulse

The viscoelastic properties of tissues are often assessed by analyzing the phase velocity dispersion curve computed using a 2-D Fourier transform onthe single fixed duration acoustic radiation force excitation pulse (FD-ARFEP)-induced shear wave with broadband frequencies. This study proposes a multi-frequency ARFEP (MF-ARFEP) using a single imaging transducer to generate shear waves at target frequencies of 100:100:1000 Hz to reduce noise in the phase velocity images. The MF-ARFEP is generated by summing sinusoids with target frequencies and then sampled to collect tracking pulses in-between the sampled MF-ARFEP. The MF-ARFEP is validated by imaging 6 and 70 kPa inclusions in a 18 kPa background, exvivo canine liver, and in vivo 4T1 breast cancer mouse tumor with comparison to the FD-ARFEP. The median phase velocity was similar between MF-ARFEP versus FD-ARFEP. However, the CNR was on an average 1.3 times higher in MF-ARFEP versus FD-ARFEP -derived images of inclusions and the coefficient of variation was on an average 1.1 and 2.2 times lower in MF-ARFEP versus FD-ARFEP-derived images of tumor and liver. These results demonstrate the feasibility of the MF-ARFEP excitation pulse to generate phase velocity images at 100:100:1000 Hz frequencies with reduced noise compared to the FD-ARFEP. Ongoing studies entail the translation of this new viscoelasticity imaging method for the characterization of breast cancer tumors in patients.

Externally induced shear waves in the right ventricular free wall during the cardiac cycle

Increased right ventricular (RV) diastolic stiffness is linked to adverse prognosis in pulmonary arterial hypertension. Currently, a non-invasive method for measuring RV stiffness is lacking. In the left ventricle, shear wave (SW) speed is related to diastolic stiffness. We show for the first time that SWs in the RV free wall (RVFW) can be induced and imaged transthoracically with ultrasound. SW imaging was performed using an L7-4 array (ATL, USA) with a Vantage 256 system (Verasonics, USA) in a 5-weeks-old Yorkshire-Landrace pig (±120 beats/min). SWs were induced by a push beam (f0 = 4.5MHz, push duration = 800μs) on the RVFW and its propagation was imaged (f0 = 5.2MHz) using plane wave compounding (-12 deg, 0 deg, +12 deg) at 3 kHz framerate. Three acquisitions of 1 s were performed, where fourteen SWs were sequentially induced during each acquisition. SW propagation speeds were calculated using a semi-automatic pipeline that includes tissue velocity estimation along a manually traced spline on the RVFW, and Radon transform to estimate SW speed. At least 85% of the waves were tracked successfully for all acquisitions. The SW velocities varied during the heart cycle as expected, ranging from 0.58 ± 0.09 m/s at end-diastole to 1.8 ± 0.2 m/s during systole. Pathological increase in stiffness may be demonstrated in longitudinal case/control studies.

Lorentz force induced shear waves for magnetic resonance elastography applications

Quantitative mechanical properties of biological tissues can be mapped using the shear wave elastography technique. This technology has demonstrated a great potential in various organs but shows a limit due to wave attenuation in biological tissues. An option to overcome the inherent loss in shear wave magnitude along the propagation pathway may be to stimulate tissues closer to regions of interest using alternative motion generation techniques. The present study investigated the feasibility of generating shear waves by applying a Lorentz force directly to tissue mimicking samples for magnetic resonance elastography applications. This was done by combining an electrical current with the strong magnetic field of a clinical MRI scanner. The Local Frequency Estimation method was used to assess the real value of the shear modulus of tested phantoms from Lorentz force induced motion. Finite elements modeling of reported experiments showed a consistent behavior but featured wavelengths larger than measured ones. Results suggest the feasibility of a magnetic resonance elastography technique based on the Lorentz force to produce an shear wave source.

Ultrasonic elastography to assess biomechanical properties of the optic nerve head and peripapillary sclera of the eye

PurposeTo quantitatively investigate both optic nerve head (ONH) and peripapillary sclera (PPS) biomechanical properties of porcine eyes through an ultrasonic elastography imaging system in response to both increasing and decreasing intraocular pressure (IOP). MethodsThe Young’s modulus of the ONH and PPS were assessed using our high resolution ultrasonic imaging system which utilized a mechanical shaker to induce shear waves and an off-axis aligned 40 MHz needle transducer to track micron-level displacement along the direction of wave propagation. In this study, imaging on a total of 8 ex vivo porcine eyes preloaded with IOPs from 6 mmHg to 30 mmHg was performed. To have a better understanding of the effect of varying IOP on biomechanics, both increasing and decreasing IOPs were investigated. ResultsThe increase of the Young’s modulus of ONH (92.4 ± 13.9 kPa at 6 mmHg to 224.7 ± 71.1 kPa at 30 mmHg) and PPS (176.8 ± 14.3 kPa at 6 mmHg to 573.5 ± 64.4 kPa at 30 mmHg) following IOP elevation could be observed in the reconstructed Young’s modulus of the shear wave elasticity (SWE) imaging while the B-mode structural images remained almost unchanged. In addition, for the same IOP level, both ONH and PPS have a tendency to be stiffer with decreasing IOP as compared to increasing IOP. ConclusionsOur results demonstrate the feasibility of using our ultrasonic elastography system to investigate the stiffness mapping of posterior eye with high resolution in both increasing and decreasing IOPs, making this technique potentially useful for glaucoma.

In vivo assessment of the mechanical properties of crystalline lenses in a rabbit model using ultrasound elastography: Effects of ultrasound frequency and age

PurposeThe occurrence of presbyopia and cataract is closely related to changes in the mechanical properties of the crystalline lens. There are no established methods so far for in vivo assessment. By introducing ultrasound elastography, we proposed group velocity (Vg) of an induced shear wave as a new biomarker to characterize the mechanical properties of the lens in our previous study. Here, we investigated the effect of the ultrasound frequency on measurement accuracy and validated the results with a conventional ex vivo compression testing. We also demonstrated a change trend in Vg and its correlation with age in a rabbit model. MethodsEight New Zealand white rabbits were fed normally from the fourth to seventh month. An ultrasound elastography system was developed to measure Vgin vivo on every eye once per month. The performances when using a high-frequency (L22-11v) and low-frequency (L11-4v) probe were compared. Rabbits were sacrificed after in vivo measurements by the end of the seventh month and this was followed by ex vivo ultrasound measurements and conventional compression tests on the extracted lenses. ResultsThe results demonstrated that there were no significance differences in Vg between measurements with high-frequency (USE-HF) and low-frequency (USE-LF) probes in the same month-age group. The mean Vg and the standard deviation of four rabbits that were 7 months old were 2.37 ± 0.24 m/s, 2.36 ± 0.25 m/s, 2.43 ± 0.26 m/s and 2.44 ± 0.38 m/s, with USE-HF for ex vivo and in vivo measurements and USE-LF for ex vivo and in vivo measurements, respectively. There were no significant differences (p > 0.05) and they were all in agreement with the results of compression tests, which was 16.16 ± 1.84 kPa in Young's modulus. The results also showed that Vg increased with age. In combination with the results of our previous study, Vg showed a relatively sharp increase from 2 to 5 months, while it had a slight increase from 5 to 7 months. ConclusionsThe USE-HF and USE-LF has comparable accuracy in Vg measurements while USE-HF had an advantage regarding better spatial resolution. The change trend of Vg was in accord with the growth phase of New Zealand white rabbits, which usually results in sexual maturity at 5 months old. This implies that Vg can be used as a biomarker parameter for evaluating the mechanical properties of the lens undergoing physiological changes.

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Covering the Cover

Attenuation of Shear Waves in Normal and Steatotic Livers

Shear wave propagation in the liver has been a robust subject of research, with shear wave speed receiving the most attention. The correlation between increased shear wave speed and increased fibrosis in the liver has been established as a useful diagnostic tool. In comparison, the precise mechanisms of shear wave attenuation, and its relation to diseased states of the liver, are less well-established. This study focused on the hypothesis that steatosis adds a viscous (lossy) component to the liver, which increases shear wave attenuation. Twenty patients’ livers were scanned with ultrasound and with induced shear wave propagation, and the resulting displacement profiles were analyzed using recently developed estimators to derive both the speed and attenuation of the shear waves within 6-cm2 regions of interest. The results were compared with pathology scores obtained from liver biopsies taken under ultrasound guidance. Across these cases, increases in shear wave attenuation were linked to increased steatosis score. This preliminary study supports the hypothesis and indicates the possible utility of the measurements for non-invasive and quantitative assessment of steatosis.

The Evolving Role of Shear Wave Elastography in the Diagnosis and Treatment of Prostate Cancer.

To forego misdiagnosis and biopsy-related complications, we explore a novel technique that embraces tissue elasticity to circumvent the current limitations in the accurate, non-invasive detection of prostate cancer. A MEDLINE® database search of literature pertinent to prostate elastography published in the last five years was guided using the following terms: tissue elasticity, ultrasound elastography, strain elastography, magnetic resonance elastography, and shear wave elastography. Ultrasound shear wave elastography (SWE) utilizes the propagation of induced shear waves to provide a quantitative measurement of elasticity of soft tissue. Studies attempting to prove SWE as a strong predictor of malignant prostatic tissue have validated its high specificity and sensitivity compared to the sub-50% range observed with magnetic resonance imaging or transrectal ultrasound. Because its median positive predictive value is also much higher than the sub-40% range attributed to more conventional techniques, SWE yields reduced false-negative biopsy rates. It furthermore exhibits higher intraobserver reproducibility compared to conventional ultrasound. Recent treatments have also utilized SWE for noninvasive prostate cancer tissue ablation. Future improvements include standardized cutoff ranges for quantifying elasticity of malignant tissue. The implementation of a scoring system to predict cancer aggressiveness and prognosis may eliminate the need for unwarranted repeat biopsies postdiagnosis.

Shear Waves Reveal Viscoelastic Changes in Skeletal Muscles After Hemispheric Stroke.

We investigated alterations in material properties such as elasticity and viscoelasticity of stroke-affected muscles using ultrasound induced shear waves and mechanical models. We used acoustic radiation force to generate shear waves along fascicles of biceps muscles and measured their propagation velocity. The shear wave data were collected in muscles of 13 hemiplegic stroke survivors under passive conditions at 90°, 120°, and 150° elbow flexion angles. In a viscoelastic medium, as opposed to a purely elastic medium, the shear wave propagation velocity depends on the frequency content of the induced wave. Therefore, in addition to the shear wave group velocity (GpV), we also estimated a frequency-dependent phase velocity (PhV). We found significantly higher GpVs and PhVs in stroke-affected muscles ( ). The velocity data were used to estimate shear elasticity and viscosity using an elastic and viscoelastic material models. A pure elastic model showed increased shear elasticity in stroke-affected muscles ( ). The Voigt model estimates of viscoelastic properties were also significantly different between the stroke-impaired and non-impaired muscles. We observed significantly larger model-estimated viscosity values on the stroke-affected side at elbow flexion angles of 120° and 150°. Furthermore, the creep behavior (tissue strain resulting from the application of sudden constant stress) of the model was also different between muscles of the paretic and non-paretic side. We speculate that these changes are associated with the structural disruption of muscles after stroke and may potentially affect force generation from muscle fibers as well as transmission of force to tendons.