Chia-Lun Yeh

Investigation on anisotropy of elastic properties in tendon using shear wave elasticity imaging

Non-invasive evaluation of tendon structure and function is of great use clinically. We proposed that the shear wave elasticity imaging has a better potential in differentiating normal and destructed tendon tissue than high frequency sonography. Four in vitro porcine tendons were studied in this research. High frequency ultrasound could provide good spatial resolution to monitor the detail structure changes by collagenase alterations. By analyzing the speckle changes based on defining a signal to noise ratio (SNR), we could quantitatively estimate the structure differences. The SNR alteration in the region of interest (ROI) before and after collagenase injection is close to 4%. The changes induced by structure alterations are not obvious even through in high frequency ultrasound imaging. Further to analyze the changes of shear wave speed, the differences before and after collagenase injection in longitudinal and transverse section of tendon were 21.3% and 8.3%, respectively. From our results, we found the changes of shear wave speed were much more than speckle intensity alterations after collagenase injection. Moreover, the decrease ratio of shear wave speed in longitudinal section is much more than in transverse section. In other words, to diagnose the tendon disease could prior to investigate on the changes of mechanical property in longitudinal section of tendon. The changes of shear wave speed could provide a batter characteristic for differentiation of normal or diseased tissue.

Shear-wave elasticity imaging of a liver fibrosis mouse model using high-frequency ultrasound

The objective of this study was to develop a high-frequency imaging platform for evaluating liver fibrosis in mice based on shear-wave elasticity imaging (SWEI). Although SWEI has been used to diagnose hepatic fibrosis clinically, it is performed at relatively low frequencies (<;20 MHz). For preclinical ultrasound imaging in small animals, a high-frequency (>30 MHz) single-element transducer with mechanical scanning is often used. In this study we developed a new SWEI system based on a 40-MHz single-element transducer for imaging and a separate 20-MHz excitation transducer for producing the radiation force and the associated shear waves. Liver fibrosis was induced in ten C57BL/6 (B6) mice using carbon tetrachloride; the other ten mice served as the control group. Synchronizing the excitation beam (i.e., the beam from the excitation transducer) and the detection beam sequence (i.e., the beam from the imaging transducer) allows this mechanicalscanning setup to analyze the shear-wave dispersion relation. The liver viscoelastic properties were determined in vivo by measuring the shear-wave dispersion curve followed by fitting to the Voigt model. The mice were then killed and the fibrosis stage was evaluated (from F0 to F4) based on the METAVIR score. The measured mean values of liver elasticity and viscosity, respectively, ranged from 1.06 to 1.89 kPa and from 1.29 to 1.75 Pa·s for normal F0 and fibrosis stages of F3 and F4. The Spearman coefficients for the correlations between the measured elasticity and viscosity at various fibrosis stages as assessed by the METAVIR score were 0.73 (p <; 0.001) and 0.634 (p = 0.0013), respectively. We also found that the collagen content in the liver was linearly correlated with the measured elasticity (r2 = 0.54, p <; 0.001) and less strongly with the viscosity (r2 = 0.26, p = 0.022). Finally, the diagnosis performance of high-frequency SWEI was evaluated using multivariate receiver operating characteristic curve (ROC) analysis. The areas under the multivariate ROC curve for diagnosing fibrosis stages of F ≥ 3, F = 4, F0 vs. F3, F0 vs. F4, and F3 vs. F4 were 0.9, 0.98, 0.83, 1.0, and 0.96, respectively. Compared with traditional ROC analysis, an improved diagnosis performance was found for diagnosing fibrosis stages of F ≥ 3 and F0 vs. F3. These results demonstrate that the developed high-frequency SWEI platform can yield quantitative viscoelastic properties for diagnosing various fibrosis stages in mice. It is a promising tool for studying the progression of liver fibrosis in preclinical animal models both noninvasively and quantitatively.

Shear wave elastography of a liver fibrosis mouse model using a high frequency ultrasound system with mechanical scanning

Liver fibrosis is one of the most common chronic liver diseases and has long serious consequence for patients morbidity and mortality. As the liver fibrosis often involves the changes of mechanical properties, shear wave elastography (SWE) can be a promising tool for diagnosis of hepatic fibrosis. In clinical applications, SWE is generally performed with an array system at relative low frequencies (<;20MHz). For preclinical studies on small animals, on the other hand, a high frequency (> 30MHz) single element transducer with mechanical scanning may still be used. In addition, high frequency transducers (both array and single element transducers) may not be able to produce shear wave with sufficient amplitudes for detection and estimation. Consequently, the objective of this study is to develop a high frequency platform for evaluation of liver fibrosis on mice using SWE. The platform uses a 40MHz single element transducer for imaging and a separate 20MHz transducer for producing radiation force and shear waves. Liver fibrosis is induced in 10 mice using carbon tetrachloride (CCl4); 10 mice served as control group. Special pulse sequence was also designed so that shear wave propagation can be monitored and measured with this mechanical scanning setup. Liver elastic properties were measured in vivo by analyzing shear wave dispersion combined with a Voigt model fitting. The animals were then sacrificed and the stages of fibrosis were analyzed using METAVIR score. The measured mean values of shear elasticity ranged from 1.06 to 1.89 kPa for normal (stage F0) and fibrosis stages with METAVIR score of F3-F4, respectively. The spearman correlation coefficient presents a high correlation between the measured elasticity and fibrosis stages (0.73, p<;0.001). Based on these results, elasticity measurements using this high frequency SWE platform can provide quantitative assessment of liver fibrosis stages. In other words, this new imaging platform that combines the advantages of high frequency ultrasound and SWE can be a promising tool for studying the progression of liver fibrosis on pre-clinical animal models non-invasively and quantitatively.

Shear Wave Measurements for Evaluation of Tendon Diseases

This study investigated the feasibility of using supersonic shear wave measurements to quantitatively differentiate normal and damaged tendons based on their mechanical properties. Five freshly harvested porcine tendons excised from pig legs were used. Tendon damage was induced by incubating the tendons with a 1% w/v collagenase solution. Values of shear modulus were derived both by a time-of-flight (TOF) approach and by a transverse isotropic plate model (TI-model). Results show that as the preload applied to the tendon increased from 0 to 3 N, the mean shear modulus derived based on the TOF approach, the TI-model, and Youngs modulus estimated from mechanical testing increased from 14.6 to 89.9 kPa, 53.9 to 348 kPa, and from 1.45 to 10.36 MPa, respectively, in untreated tendons, and from 8.4 to 67 kPa, 28 to 258 kPa, and from 0.93 to 7.2 MPa in collagenase-treated tendons. Both the TOF approach and the TI-model correlated well with the changes in Youngs moduli. Although there is bias on estimation of shear modulus using the TOF approach, it still provides statistical significance to differentiate normal and damaged tendons. Our data indicate that the SSI is a valuable imaging technique to assess tendon stiffness dynamics and characterize normal and collagenase-damaged tendons.

Stiffness dynamics of rabbit's achilles tendons evaluated by shear wave elastography in vivo

The physiological function of tendon is to withstand the tension generated by muscles during various type of joint movement, and thus prevent muscle from violent damage. Hence, the dynamic change of tendon stiffness for adapting various external forces is highly related to its functional performance. In other words, monitoring the dynamic tendon stiffness at various stretching conditions can be used for assessment of tendon functions. Recently, a relatively new imaging technique called shear wave elastography (SWE) has emerged as a promising tool for estimation of tissue stiffness. However, because tendons are anisotropic materials with the highest stiffness along the longitudinal direction, the general relation between the shear wave speed and Youngs modulus derived from isotropic materials (i.e., E=3ρc2) does not hold. To precisely estimate the mechanical properties of tendon, it is necessary to analyze the velocity dispersion of the guided waves propagating inside tendons. The aim of this study is thus to evaluate SWE combined with dispersion analysis as a diagnostic tool for tendon functionality by monitoring the dynamics of tendon stiffness at various stretching conditions. SWE was in vivo applied to six Achilles tendons of three New Zealand rabbits. Assuming the tendons as a transverse isotropic material, the tendons were passively stretched at four ankle joint angles and the dispersion of shear waves running in parallel with the longitudinal direction of the tendons were analyzed to yield the elastic constants C55 in Christoffels tensor. The measured mean value of C55 at joint angles of 125°, 110°, 95° and 80°were 0.42MPa, 0.95MPa, 1.49MPa and 2.27MPa, respectively. Our results show that the change of C55 is highly correlated with the stretching conditions. This suggests that the dynamic stiffness of tendon at various stretching conditions can be monitored by elastic constant C55. In summary, SWE combined with dispersion analysis is a powerful tool to non-invasively monitor the stiffness dynamic changes of tendons and highly potential for diagnosis of tendon injury and monitoring of the treatment efficiency.

Correlation between the shear wave speed in tendon and its elasticity properties

Mechanical properties, such as the Youngs modulus, of the tendon are highly correlated to its pathological state. The isolated tensile testing is the conventional method for measuring the Youngs modulus of tendon. However, it is an invasive process and is not suitable for early diagnosis. A non-invasive method for measuring the Youngs modulus of tendon is thus highly desired. Recently, shear wave elasticity imaging has been widely used to quantify the elasticity property of soft tissue non-invasively. Up to date, the relation between the Youngs modulus (E) and the shear wave speed (Vs), (i.e., E-V relation), of tendon is still difficult to formulate. In this study, five porcine tendons were used to test the correlation between the relation between shear wave speed and Youngs modulus in a normal and a collagenase-induced diseased model. The measurement of the Youngs modulus was accomplished on a step motor with a load cell. Different degrees of pre-loading ranging from 0.5 to 3N were used to change the elastic properties both in normal and diseased models. For each loading condition, a fully preprogrammed array system was used to generate and detect the shear wave speed. The measured Youngs modulus under different degrees of preloading are highly correlated with the shear wave speed in both normal and diseased models. The averaged correlation coefficients between the wave speed and elastic modulus in normal and diseased models are 0.97±0.02 and 0.99 ± 0.005, respectively. We further use a second order polynomial to model the E-Vs relation. The scaling coefficient was found to be 0.104 and 0.117, respectively, for the normal and the diseased model. When all data points were adopted in the fitting, the scaling coefficient was 0.107. Based on these results, we found that the E-Vs relation is similar in both the normal and diseased model. The shear wave speed can be an index for quantifying Youngs modulus of tendon. Our findings may provide a new strategy for tendon function investigation in clinical practice.

Tissue shear viscosity measurements using a spectral ratio method

Mechanical properties such as elasticity and viscosity are highly related to tissue pathology state. Images that provide the geometry information of an object as well as its shear elasticity and viscosity are important in clinical applications. In the supersonic shear imaging (SSI) technique, image reconstruction in an inhomogeneous medium could be performed by varying the reconstruction kernel size, which in this article was regarded as the modified supersonic shear imaging (mSSI) method. In this study, we proposed a spectral ratio (SR) method for the reconstruction of shear elasticity and viscosity images. The reconstruction performance of an embedded object was evaluated for three different sizes of kernels, which were 0.308, 1.84 and 3.08 mm with comparison with the results obtained by the SSI method. The mean elasticity and viscosity error in the best reconstruction case using the SR method was 3.2% and 28.45%, respectively, while the mean elasticity and viscosity error in the best case using modify spectroscopy method is 19.56% and 26.36%, respectively. Both SR and mSSI method could provide boundary information of the embedded object. The SR method could achieve a spatial resolution of 0.308 mm in both elasticity and viscosity reconstruction.

Pulsed high-intensity focused ultrasound exposure decreases shear wave speed of rabbit's Achilles tendons

Tightening and contracture of dense fibrous tissues including tendons and ligaments are debilitating conditions frequently encountered in clinical practice. Conservative management for loosening of the tightened tissues usually has limited effectiveness and invasive interventions are inevitable in most protracted conditions. Pulsed high-intensity focused ultrasound (HIFU) holds the promise of being an alternative nonsurgical option for releasing of the tightened tissues. In this work, we evaluated the feasibility of combining pulsed-HIFU and supersonic shear imaging (SSI) as a novel platform for loosening of tightened tissues in a controllable manner. An ex vivo model using fresh porcine tendons was first employed to determine the acoustic parameters set for the delivered HIFU pulses that are optimized for decrease of shear wave speed (SWS) in the tendons measured by SSI. Pulsed-HIFU insonation was then applied to the Achilles tendons of New Zealand white rabbits using the acoustic setting optimized in the ex vivo model. We found that pulsed-HIFU exposure with a pulse repetition frequency of 500 Hz, a pulse cycle of 50, and an exposure duration of 1 min significantly reduced SWS of the Achilles tendons in vivo without rising of the tissue temperature. By contrast, application of conventional continuous-wave HIFU in the ex vivo samples markedly increased tissue SWS and temperature. These findings suggest that pulsed-HIFU may loosen the treated tissue via non-thermal mechanisms. Our results indicate that pulsed-HIFU combing SSI as a promising therapeutic alternative for contracture of dense fibrous tissues.

Joint capsule loosening by high-intensity pulsed ultrasound

High-intensity ultrasound in pulsed wave mode is investigated as a new treatment method for joint contracture. Joint contracture is characterized by an abnormally thickened, shortened joint capsule that tightly constrains joint movement. Non-invasive treatment typically involves lengthy physical therapy. Disappointing recovery is not uncommon and invasive interventions are frequently required. A new, non-invasive therapeutic strategy is highly desired. Of particular interest in this study is the application of high intensity pulsed ultrasound. Using an ex vivo model, here we investigate the effect of pulsed-waved ultrasound of high intensity (PUHI) on tendons and ligaments of swine. Changes of the B-mode images of the tissues before and after the application of PUHI were determined and correlated with the mechanical and histological examinations. Our preliminary results show that PUHI exerts significantly mechanical disruption on the tissues, while the changes can be successfully monitored by a B-mode-image-based approach.