The impact of hepatic pressurization on liver shear wave speed estimates in constrained versus unconstrained conditions

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.

Noninvasive ultrasound assessment of tissue internal pressure using dual mode elasticity imaging: a phantom study

Objective.Tissue internal pressure, such as interstitial fluid pressure in solid tumors and intramuscular pressure in compartment syndrome, is closely related to the pathological state of tissues. It is of great diagnostic value to measure and/or monitor the internal pressure of targeted tissues. Because most of the current methods for measuring tissue pressure are invasive, noninvasive methods are highly desired. In this study, we developed a noninvasive method for qualitative assessment of tissue internal pressure based on a combination of two ultrasound elasticity imaging methods: strain imaging and shear wave elasticity imaging.Approach.The method was verified through experimental investigation using two tissue-mimicking phantoms each having an inclusion confined by a membrane, in which hydrostatic pressures can be applied and maintained. To examine the sensitivity of the elasticity imaging methods to pressure variation, strain ratio and shear modulus ratio (SMR) between the inclusion and background of phantom were obtained.Main results.The results first experimentally prove that pressure, in addition to elasticity, is a contrast mechanism of strain imaging, and further demonstrate that a comparative analysis of strain ratio and SMR is an effective method for noninvasive tissue internal pressure detection.Significance.This work provides a new perspective in interpreting the strain ratio data in medical diagnosis, and it also provides a noninvasive alternative for assessing tissue internal pressure, which could be valuable for the diagnosis of pressure-related diseases.

Shear Wave Elastography: A Review on the Confounding Factors and Their Potential Mitigation in Detecting Chronic Kidney Disease

Early detection of chronic kidney disease is important to prevent progression of irreversible kidney damage, reducing the need for renal transplantation. Shear wave elastography is ideal as a quantitative imaging modality to detect chronic kidney disease because of its non-invasive nature, low cost and portability, making it highly accessible. However, the complexity of the kidney architecture and its tissue properties give rise to various confounding factors that affect the reliability of shear wave elastography in detecting chronic kidney disease, thus limiting its application to clinical trials. The objective of this review is to highlight the confounding factors presented by the complex properties of the kidney, in addition to outlining potential mitigation strategies, along with the prospect of increasing the versatility and reliability of shear wave elastography in detecting chronic kidney disease.

DeepLiverNet: a deep transfer learning model for classifying liver stiffness using clinical and T2-weighted magnetic resonance imaging data in children and young adults

BACKGROUND

Although MR elastography allows for quantitative evaluation of liver stiffness to assess chronic liver diseases, it has associated drawbacks related to additional scanning time, patient discomfort, and added costs.

OBJECTIVE

To develop a machine learning model that can categorically classify the severity of liver stiffness using both anatomical T2-weighted MRI and clinical data for children and young adults with known or suspected pediatric chronic liver diseases.

MATERIALS AND METHODS

We included 273 subjects with known or suspected chronic liver disease. We extracted data including axial T2-weighted fast spin-echo fat-suppressed images, clinical data (e.g., demographic/anthropomorphic data, particular medical diagnoses, laboratory values) and MR elastography liver stiffness measurements. We propose DeepLiverNet (a deep transfer learning model) to classify patients into one of two groups: no/mild liver stiffening (<3 kPa) or moderate/severe liver stiffening (≥3 kPa). We conducted internal cross-validation using 178 subjects, and external validation using an independent cohort of 95 subjects. We assessed diagnostic performance using accuracy, sensitivity, specificity and area under the receiver operating characteristic curve (AuROC).

RESULTS

In the internal cross-validation experiment, the combination of clinical and imaging data produced the best performance (AuROC=0.86) compared to clinical (AuROC=0.83) or imaging (AuROC=0.80) data alone. Using both clinical and imaging data, the DeepLiverNet correctly classified patients with accuracy of 88.0%, sensitivity of 74.3% and specificity of 94.6%. In our external validation experiment, this same deep learning model achieved an accuracy of 80.0%, sensitivity of 61.1%, specificity of 91.5% and AuROC of 0.79.

CONCLUSION

A deep learning model that incorporates clinical data and anatomical T2-weighted MR images might provide a means of risk-stratifying liver stiffness and directing the use of MR elastography.

On the Challenges Associated with Obtaining Reproducible Measurements Using SWEI in the Median Nerve

This work discusses challenges we have encountered in acquiring reproducible measurements of shear wave speed (SWS) in the median nerve and suggests methods for improving reproducibility. First, procedural acquisition challenges are described, including nerve echogenicity, transducer pressure and transmit focal depth. Second, we present an iterative, radon sum-based algorithm that was developed specifically for measuring the SWS in median nerves. SWSs were measured using single track location shear wave elasticity imaging (SWEI) in the median nerves of six healthy volunteers and six patients diagnosed with carpal tunnel syndrome. Unsuccessful measurements were associated with several challenges including reverberation artifacts, low signal-to-noise ratio and temporal window limitations for tracking the velocity wave. To address these challenges, an iterative convergence algorithm was implemented to identify an appropriate temporal processing window that removed the reverberation artifacts while preserving shear wave signals. Algorithmically, it was important to consider the lateral regression kernel size and position and the temporal window. Procedurally, both nerve echogenicity and transducer compression were determined to impact the measured SWS. Shear waves were successfully measured in the median nerve proximal to the carpal tunnel, but SWEI measurements were significantly compromised within the carpal tunnel itself. The velocity-based SWSs were statistically significantly higher than the displacement SWSs (p < 0.0001), demonstrating for the first time dispersion in the median nerve in vivo using SWEI.

Time-Resolved Response of Cerebral Stiffness to Hypercapnia in Humans

Cerebral blood flow, cerebral stiffness (CS) and intracranial pressure are tightly linked variables of cerebrovascular reactivity and cerebral autoregulation. Transtemporal ultrasound time-harmonic elastography was used for rapid measurement of CS changes in 10 volunteers before, during and after administration of a gas mixture of 95% O₂ and 5% CO₂ (carbogen). Within the first 2.2 ± 2.0 min of carbogen breathing, shear wave speed determined as a surrogate parameter of CS increased from 1.57 ± 0.04 to 1.66 ± 0.05 m/s (p < 0.01) in synchrony with end-tidal CO₂ while post-hypercapnic CS recovery was delayed by 2.7 ± 1.4 min in relation to end-tidal CO₂. Our results indicate that CS is highly sensitive to changes in CO₂ levels of inhaled air. Possible mechanisms underlying the observed CS changes might be associated with cerebrovascular reactivity, cerebral blood flow adaptation and intracranial regulation, all of which are potentially relevant for future diagnostic applications of transtemporal time-harmonic elastography in a wide spectrum of neurologic diseases.

Analyzing acoustoelastic effect of shear wave elastography data for perfused and hydrated soft tissues using a macromolecular network inspired model

Shear wave elastography (SWE) has enhanced our ability to non-invasively make in vivo measurements of tissue elastic properties of animal and human tissues. Recently, researchers have taken advantages of acoustoelasticity in SWE to extract nonlinear elastic properties from soft biological tissues. However, most investigations of the acoustoelastic effects of SWE data (AE-SWE) rely on classic hyperelastic models for rubber-like (dry) materials. In this paper, we focus solely on understanding acoustoelasticity in soft hydrated tissues using SWE data and propose a straightforward approach to modeling the constitutive behavior of soft tissue that has a direct microstructural/macromolecular interpretation. Our approach incorporates two constitutive features relevant to biological tissues into AE-SWE: static dilation of the medium associated with nonstructural components (e.g. tissue hydration and perfusion) and finite extensibility derived from an ideal network of biological filaments. We evaluated the proposed method using data from an in-house tissue-mimicking phantom experiment, and ex vivo and in vivo AE-SWE data available in the SWE literature. In conclusion, predictions made by our approach agreed well with measurements obtained from phantom, ex vivo and in vivo tissue experiments.

Magnetic resonance elastography of liver and spleen: Methods and applications

The viscoelastic properties of the liver and spleen can be assessed with magnetic resonance elastography (MRE). Several actuators, MRI acquisition sequences and reconstruction algorithms have been proposed for this purpose. Reproducible results are obtained, especially when the examination is performed in standard conditions with the patient fasting. Accurate staging of liver fibrosis can be obtained by measuring liver stiffness or elasticity with MRE. Moreover, emerging evidence shows that assessing the tissue viscous parameters with MRE is useful for characterizing liver inflammation, non-alcoholic steatohepatitis, hepatic congestion, portal hypertension, and hepatic tumors. Further advances such as multifrequency acquisitions and compression-sensitive MRE may provide novel quantitative markers of hepatic and splenic mechanical properties that may improve the diagnosis of hepatic and splenic diseases.

Elastographic Assessment of Xenograft Pancreatic Tumors

High tissue pressures prevent chemotherapeutics from reaching the parenchyma of pancreatic ductal adenocarcinoma, which makes it difficult to treat this aggressive disease. Researchers currently use invasive probes to monitor the effectiveness of pressure-reducing therapies, but this practice introduces additional complications. Here, we hypothesize that Young's modulus is a good surrogate for tissue pressure because collagen density and hyaluoronic acid, the key features of the tumor microenvironment responsible for high tissue pressures, also affect modulus elastograms. To corroborate this hypothesis, we used model-based quasi-static elastography to assess how the Young's modulus of naturally occurring AsPc-1 pancreatic tumors varies with collagen density and hyaluoronic acid concentration. We observed that Young's moduli of orthotopically grown xenograft tumors were 6 kPa (p < 0.05) higher than that of their subcutaneously grown counterparts. We also observed a strong correlation between Young's modulus and regions within the tumors with high collagen (R² ≈ 0.8) and hyaluoronic acid (R² ≈ 0.6) densities. These preliminary results indicate that hyaluronic acid and collagen density, features of the pancreatic ductal adenocarcinoma tumor microenvironment responsible for high tissue pressure, influence Young's modulus.

The impact of intraocular pressure on elastic wave velocity estimates in the crystalline lens

Intraocular pressure (IOP) is believed to influence the mechanical properties of ocular tissues including cornea and sclera. The elastic properties of the crystalline lens have been mainly investigated with regard to presbyopia, the age-related loss of accommodation power of the eye. However, the relationship between the elastic properties of the lens and IOP remains to be established. The objective of this study is to measure the elastic wave velocity, which represents the mechanical properties of tissue, in the crystalline lens ex vivo in response to changes in IOP. The elastic wave velocities in the cornea and lens from seven enucleated bovine globe samples were estimated using ultrasound shear wave elasticity imaging. To generate and then image the elastic wave propagation, an ultrasound imaging system was used to transmit a 600 µs pushing pulse at 4.5 MHz center frequency and to acquire ultrasound tracking frames at 6 kHz frame rate. The pushing beams were separately applied to the cornea and lens. IOP in the eyeballs was varied from 5 to 50 mmHg. The results indicate that while the elastic wave velocity in the cornea increased from 0.96  ±  0.30 m s^-1 to 6.27  ±  0.75 m s^-1 as IOP was elevated from 5 to 50 mmHg, there were insignificant changes in the elastic wave velocity in the crystalline lens with the minimum and the maximum speeds of 1.44  ±  0.27 m s^-1 and 2.03  ±  0.46 m s^-1, respectively. This study shows that ultrasound shear wave elasticity imaging can be used to assess the biomechanical properties of the crystalline lens noninvasively. Also, it was observed that the dependency of the crystalline lens stiffness on the IOP was significantly lower in comparison with that of cornea.

Effect of Pressure on Liver Stiffness During the Development of Liver Fibrosis in Rabbits

This study was designed to investigate whether hepatic arterial pressure and portal pressure have an effect on liver stiffness during the development of liver fibrosis. Liver fibrosis was induced in 50 healthy New Zealand white rabbits. Laparotomy was performed to measure liver stiffness, and the portal vein and hepatic artery were successively ligated to repeat the measurements. A significant difference was observed among liver stiffness values measured at different time points (F = 22.82, p < 0.001). Differences between original liver stiffness and liver stiffness measured after portal ligation were positively correlated with portal pressure (r = 0.801, p < 0.001). In animals with grade 4 liver fibrosis, the increase in liver stiffness caused by pressure was greater than that caused by extracellular matrix accumulation (p = 0.002). In conclusion, hepatic arterial pressure and portal pressure have a significant effect on liver stiffness during the development of liver fibrosis.

The mechanical microenvironment in cancer: How physics affects tumours

The tumour microenvironment contributes greatly to the response of tumour cells. It consists of chemical gradients, for example of oxygen and nutrients. However, a physical environment is also present. Apart from chemical input, cells also receive physical signals. Tumours display unique mechanical properties: they are a lot stiffer than normal tissue. This may be either a cause or a consequence of cancer, but literature suggests it has a major impact on tumour cells as will be described in this review. The mechanical microenvironment may cause malignant transformation, possibly through activation of oncogenic pathways and inhibition of tumour suppressor genes. In addition, the mechanical microenvironment may promote tumour progression by influencing processes such as epithelial-to-mesenchymal transition, enhancing cell survival through autophagy, but also affects sensitivity of tumour cells to therapeutics. Furthermore, multiple intracellular signalling pathways prove sensitive to the mechanical properties of the microenvironment. It appears the increased stiffness is unlikely to be caused by increased stiffness of the tumour cells themselves. However, there are indications that tumours display a higher cell density, making them more rigid. In addition, increased matrix deposition in the tumour, as well as increased interstitial fluid pressure may account for the increased stiffness of tumours. Overall, tumour mechanics are significantly different from normal tissue. Therefore, this feature should be further explored for use in cancer prevention, detection and treatment.

A multiresolution approach to shear wave image reconstruction

Shear wave imaging techniques build maps of local elasticity estimating the local group velocity of induced mechanical waves. Velocity estimates are formed using the time delay in the motion profile of the medium at two or more points offset from the shear wave source. Because the absolute time-of-flight between any pair of locations scales with the distance between them, there is an inherent trade-off between robustness to time-of-flight errors and lateral spatial resolution based on the number and spacing of the receive points used for each estimate. This work proposes a method of using the time delays measured between all combinations of locations to estimate a noise-robust, high-resolution image. The time-of-flight problem is presented as an overdetermined system of linear equations that can be directly solved with and without spatial regularization terms. Finite element method simulations of acoustic radiation force-induced shear waves are used to illustrate the method, demonstrating superior contrast-to-noise ratio and lateral edge resolution characteristics compared with linear regression of arrival times. This technique may improve shear wave imaging in situations where time-of-flight noise is a limiting factor.

Frequency adaptation for enhanced radiation force amplitude in dynamic elastography

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

WFUMB guidelines and recommendations for clinical use of ultrasound elastography: Part 1: basic principles and terminology

Conventional diagnostic ultrasound images of the anatomy (as opposed to blood flow) reveal differences in the acoustic properties of soft tissues (mainly echogenicity but also, to some extent, attenuation), whereas ultrasound-based elasticity images are able to reveal the differences in the elastic properties of soft tissues (e.g., elasticity and 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 pathologic lesions. Typically, all elasticity measurement and imaging methods introduce a mechanical excitation and monitor the resulting tissue response. Some of the most widely available commercial elasticity imaging methods are 'quasi-static' and use external tissue compression to generate images of the resulting tissue strain (or deformation). In addition, many manufacturers now provide shear wave imaging and measurement methods, which deliver stiffness images based upon the shear wave propagation speed. The goal of this review is to describe the fundamental physics and the associated terminology underlying these technologies. We have included a questions and answers section, an extensive appendix, and a glossary of terms in this manuscript. We have also endeavored to ensure that the terminology and descriptions, although not identical, are broadly compatible across the WFUMB and EFSUMB sets of guidelines on elastography (Bamber et al. 2013; Cosgrove et al. 2013).

Material characterization of in vivo and in vitro porcine brain using shear wave elasticity

Realistic computer simulation of closed head trauma requires accurate mechanical properties of brain tissue, ideally in vivo. A substantive deficiency of most existing experimental brain data is that properties were identified through in vitro mechanical testing. This study develops a novel application of shear wave elasticity imaging to assess porcine brain tissue shear modulus in vivo. Shear wave elasticity imaging is a quantitative ultrasound technique that has been used here to examine changes in brain tissue shear modulus as a function of several experimental and physiologic parameters. Animal studies were performed using two different ultrasound transducers to explore the differences in physical response between closed skull and open skull arrangements. In vivo intracranial pressure in four animals was varied over a relevant physiologic range (2-40 mmHg) and was correlated with shear wave speed and stiffness estimates in brain tissue. We found that stiffness does not vary with modulation of intracranial pressure. Additional in vitro porcine specimens (n = 14) were used to investigate variation in brain tissue stiffness with temperature, confinement, spatial location and transducer orientation. We observed a statistically significant decrease in stiffness with increased temperature (23%) and an increase in stiffness with decreasing external confinement (22-37%). This study determined the feasibility of using shear wave elasticity imaging to characterize porcine brain tissue both in vitro and in vivo. Our results underline the importance of temperature- and skull-derived boundary conditions to brain stiffness and suggest that physiologic ranges of intracranial pressure do not significantly affect in situ brain tissue properties. Shear wave elasticity imaging allowed for brain material properties to be experimentally characterized in a physiologic setting and provides a stronger basis for assessing brain injury in computational models.

Ultrasound shear wave elasticity imaging quantifies coronary perfusion pressure effect on cardiac compliance

Diastolic heart failure (DHF) is a major source of cardiac related morbidity and mortality in the world today. A major contributor to, or indicator of DHF is a change in cardiac compliance. Currently, there is no accepted clinical method to evaluate the compliance of cardiac tissue in diastolic dysfunction. Shear wave elasticity imaging (SWEI) is a novel ultrasound-based elastography technique that provides a measure of tissue stiffness. Coronary perfusion pressure affects cardiac stiffness during diastole; we sought to characterize the relationship between these two parameters using the SWEI technique. In this work, we demonstrate how changes in coronary perfusion pressure are reflected in a local SWEI measurement of stiffness during diastole. Eight Langendorff perfused isolated rabbit hearts were used in this study. Coronary perfusion pressure was changed in a randomized order (0-90 mmHg range) and SWEI measurements were recorded during diastole with each change. Coronary perfusion pressure and the SWEI measurement of stiffness had a positive linear correlation with the 95% confidence interval (CI) for the slope of 0.009-0.011 m/s/mmHg ( R(2) = 0.88 ). Furthermore, shear modulus was linearly correlated to the coronary perfusion pressure with the 95% CI of this slope of 0.035-0.042 kPa/mmHg ( R(2) = 0.83). In conclusion, diastolic SWEI measurements of stiffness can be used to characterize factors affecting cardiac compliance specifically the mechanical interaction (cross-talk) between perfusion pressure in the coronary vasculature and cardiac muscle. This relationship was found to be linear over the range of pressures tested.

Magnetic resonance elastography of the liver in patients status-post fontan procedure: feasibility and preliminary results

OBJECTIVE

The purpose of this study was to evaluate the feasibility of performing magnetic resonance elastography (MRE) as a screening tool for elevated liver stiffness in patients' status-post Fontan procedure.

BACKGROUND

With greater numbers of Fontan patients surviving far into adulthood, a factor increasingly affecting long-term prognosis is the presence of hepatic congestion and fibrosis. If detected early, steps can be taken to potentially slow or halt the progression of fibrosis. MRE is a relatively new, noninvasive imaging technique, which can quantitatively measure liver stiffness and provide an estimate of the extent of fibrosis.

METHODS

A retrospective study was conducted using MRE to evaluate liver stiffness in patients with a history of Fontan procedure. An MRE was performed in the same session as a clinical cardiac MRI. The liver was interrogated at four slice locations, and a mean liver stiffness value was calculated for each patient using postprocessing software. The medical records were reviewed for demographic and clinical characteristics.

RESULTS

During the time frame of this investigation, 17 MRE exams were performed on 16 patients. All patients had elevated liver stiffness values as defined by MRE standards. The median of the individual mean liver stiffness values was 5.1 kPa (range: 3.4-8.2 kPa). This range of liver stiffness elevation would suggest the presence of mild to severe fibrosis in a patient with standard cardiovascular anatomy. We found a significant trend toward higher liver stiffness values with greater duration of Fontan circulation (rs = 0.55, P = .02).

CONCLUSION

Our preliminary findings suggest that MRE is a feasible method for evaluating the liver in Fontan patients who are undergoing surveillance cardiac MRI. Further investigation with histologic correlation is needed to determine the contributions of hepatic congestion and fibrosis to the liver stiffness in this population.

Hepatic and splenic stiffness augmentation assessed with MR elastography in an in vivo porcine portal hypertension model

PURPOSE

To investigate the influence of portal pressure on the shear stiffness of the liver and spleen in a well-controlled in vivo porcine model with magnetic resonance elastography (MRE). A significant correlation between portal pressure and tissue stiffness could be used to noninvasively assess increased portal venous pressure (portal hypertension), which is a frequent clinical condition caused by cirrhosis of the liver and is responsible for the development of many lethal complications.

MATERIALS AND METHODS

During multiple intraarterial infusions of Dextran-40 in three adult domestic pigs in vivo, 3D abdominal MRE was performed with left ventricle and portal catheters measuring blood pressure simultaneously. Least-squares linear regressions were used to analyze the relationship between tissue stiffness and portal pressure.

RESULTS

Liver and spleen stiffness have a dynamic component that increases significantly following an increase in portal or left ventricular pressure. Correlation coefficients with the linear regressions between stiffness and pressure exceeded 0.8 in most cases.

CONCLUSION

The observed stiffness-pressure relationship of the liver and spleen could provide a promising noninvasive method for assessing portal pressure. Using MRE to study the tissue mechanics associated with portal pressure may provide new insights into the natural history and pathophysiology of hepatic diseases and may have significant diagnostic value in the future.

Ultrasonic characterization of the nonlinear properties of canine livers by measuring shear wave speed and axial strain with increasing portal venous pressure

Elevated hepatic venous pressure is the primary source of complications in advancing liver disease. Ultrasound imaging is ideal for potential noninvasive hepatic pressure measurements as it is widely used for liver imaging. Specifically, ultrasound based stiffness measures may be useful for clinically monitoring pressure, but the mechanism by which liver stiffness increases with hepatic pressure has not been well characterized. This study is designed to elucidate the nonlinear properties of the liver during pressurization by measuring both hepatic shear wave speed (SWS) and strain with increasing pressure. Tissue deformation during hepatic pressurization was tracked in 8 canine livers using successively acquired 3-D B-mode volumes and compared with concurrently measured SWS. When portal venous pressure was increased from clinically normal (0-5mmHg) to pressures representing highly diseased states at 20mmHg, the liver was observed to expand with axial strain measures up to 10%. At the same time, SWS estimates were observed to increase from 1.5-2m/s at 0-5mmHg (baseline) to 3.25-3.5m/s at 20mmHg.

In vivo swine kidney viscoelasticity during acute gradual decrease in renal blood flow: pilot study

Elasticity imaging methods have been used to study kidney mechanical properties and have demonstrated that the kidney elastic modulus increases with disease state. However, studies in swine suggests that kidney elastic modulus is also affected by hemodynamic variables. A newly emerging method called Shearwave Dispersion Ultrasound Vibrometry (SDUV) offers a tool to determine renal elasticity and viscosity in vivo. The purpose of this study is directed toward evaluating the feasibility of SDUV for in vivo measurements of healthy swine kidney during acute gradual decease of renal blood flow. In this study in vivo SDUV measurements were made on a group of 5 normal swine kidneys at baseline renal blood flow (RBF) and 25, 50, 75 and 100% decrease in RBF. The shear elastic modulus at full baseline was 7.04 ± 0.92 kPa and 3.48 ± 0.20 kPa at 100% decrease in RBF. The viscosity did not change between baseline (2.23 ± 0.33 Pa·s) and 100% decrease in RBF (2.03 ± 0.32 Pa·s). The data from this study indicates that other variables such as local blood flow, pressure and volume as well as method accuracy need to be measured to illustrate the relationship between shear elasticity and viscosity associated with acute kidney processes.