Shear Wave Speed Estimation Research Articles

Evaluating the Benefit of Elevated Acoustic Output in Harmonic Motion Estimation in Ultrasonic Shear Wave Elasticity Imaging

Harmonic imaging techniques have been applied in ultrasonic elasticity imaging to obtain higher-quality tissue motion tracking data. However, harmonic tracking can be signal-to-noise ratio and penetration depth limited during clinical imaging, resulting in decreased yield of successful shear wave speed measurements. A logical approach is to increase the source pressure, but the in situ pressures used in diagnostic ultrasound have been subject to a de facto upper limit based on the Food and Drug Administration guideline for the mechanical index (MI <1.9). A recent American Institute of Ultrasound in Medicine report concluded that an in situ MI up to 4.0 could be warranted without concern for increased risk of cavitation in non-fetal tissues without gas bodies if there were a concurrent clinical benefit. This work evaluates the impact of using an elevated MI in harmonic motion tracking for hepatic shear wave elasticity imaging. The studies indicate that high-MI harmonic tracking increased shear wave speed estimation yield by 27% at a focal depth of 5 cm, with larger yield increase in more difficult-to-image patients. High-MI tracking improved harmonic tracking data quality by increasing the signal-to-noise ratio and decreasing jitter in the tissue motion data. We conclude that there is clinical benefit to use of elevated acoustic output in shear wave tracking, particularly in difficult-to-image patients.

EP22.12: A multi‐biomarker approach to evaluation of the human pregnant cervix using quantitative ultrasound (QUS) imaging biomarkers

To integrate quantitative ultrasound (QUS) biomarkers of changes in cervical softness and extracellular matrix (ECM) organisation from 1st trimester (1T) to 3rd trimester (3T). In pregnant women (n=30) we used shear wave speed estimation (SWS) to measure softness, acoustic backscatter to evaluate ECM microstructural organisation via mean backscattered power difference (mBSPD) and effective scatterer diameter (ESD), and the Nakagami “M” to measure scatterer number density. Parametric and non-parametric tests were used as appropriate to compare differences between parameters and groups. SWS decreased from 1T to 3T [4.09±1.19 m/s (CI: 3.42-4.80) vs 2.13±0.66 m/s (CI: 1.84-2.45), p < 0.0001], suggesting softer tissue in 3T; backscatter anisotropy (mBSPD), was greater in 1T than 3T [4.65±1.72 dB (CI: 3.86-5.62) vs 2.70 ± 1.15 dB(CI: 2.30-3.31), p=0.0061], suggesting less ECM organisation in 3T; effective scatterer diameter (ESD), was larger in 1T than 3T [90±24 μm (CI: 78-97) vs 46±24 μm (CI: 35-56), p < 0.0001], suggesting less ECM organisation in 3T; Nakagami “M” did not vary from 1T to 3T [0.86±0.12, (CI: 0.79-0.92) vs 0.84±0.10, (CI: 0.80-0.89), p=0.80] suggesting no change in collagen concentration from 1T to 3T; and, in 3T ripened cervixes compared to untreated, SWS was lower [1.54±0.31 m/s (CI: 1.37-1.72) vs 2.13±0.66 m/s (CI: 1.84-2.45), p=0.002] but mBSPD unchanged [2.96±1.43 dB (CI: 2.35-3.77) vs 3.14±2.24 dB (CI: 2.51-4.73), p=0.8545], suggesting increased softness without change in ECM organisation. These preliminary results suggest that QUS biomarkers can simultaneously and comprehensively evaluate ECM organisation and tissue softness. They seem consistent with molecular studies in humans and animals that suggest mechanisms of cervical change are different in early compared to late pregnancy. In summary, combined QUS biomarkers may prove valuable for assessing in vivo cervical remodelling in women.

Effects of age and pathology on shear wave speed of the human rotator cuff.

Rotator cuff tears are common and often repaired surgically, but post-operative repair tissue healing, and shoulder function can be unpredictable. Tear chronicity is believed to influence clinical outcomes, but conventional clinical approaches for assessing tear chronicity are subjective. Shear wave elastography (SWE) is a promising technique for assessing soft tissue via estimates of shear wave speed (SWS), but this technique has not been used extensively on the rotator cuff. Specifically, the effects of age and pathology on rotator cuff SWS are not well known. The objectives of this study were to assess the association between SWS and age in healthy, asymptomatic subjects, and to compare measures of SWS between patients with a rotator cuff tear and healthy, asymptomatic subjects. SWE images of the supraspinatus muscle and intramuscular tendon were acquired from 19 asymptomatic subjects and 11 patients with a rotator cuff tear. Images were acquired with the supraspinatus under passive and active (i.e., minimal activation) conditions. Mean SWS was positively associated with age in the supraspinatus muscle and tendon under passive and active conditions (p ≤ 0.049). Compared to asymptomatic subjects, patients had a lower mean SWS in their muscle and tendon under active conditions (p ≤ 0.024), but no differences were detected under passive conditions (p ≥ 0.783). These findings identify the influences of age and pathology on SWS in the rotator cuff. These preliminary findings are an important step toward evaluating the clinical utility of SWE for assessing rotator cuff pathology. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:282-288, 2018.

Interface wave studies on a laboratory test facility

Rayleigh wave measurements were made in the Interface Wave Test Facility at the University of Rhode Island to develop techniques to estimate the shear wave properties of near surface sediment. Repeating source events at various ranges spaced equally at 0.15 m from a fixed receiver (accelerometer) created a virtual source array. The source events consisted of dropping a tennis ball thereby exciting Rayleigh waves. A monitoring accelerometer was used to record each source event at a fixed distance from the source location in order to time it. This enabled to calculate the travel time of the Rayleigh waves from the monitoring receiver to the fixed receiver. The phase velocity dispersion is calculated using high resolution frequency-wavenumber processing. The shear wave speed of the sediment layers in the “sand tank” is estimated using an inversion scheme. The shear wave speed estimates is compared to direct measurements using a calibrated bender element system at selected depths. The bender element system was placed in a test tank initially filled with mason sand and the Rayleigh wave inversion system was deployed on the surface of the sand in the tank. [Work supported by Army Research Office and ONR.]

Improved Shear Wave Group Velocity Estimation Method Based on Spatiotemporal Peak and Thresholding Motion Search.

Quantitative ultrasound elastography is increasingly being used in the assessment of chronic liver disease. Many studies have reported ranges of liver shear wave velocity values for healthy individuals and patients with different stages of liver fibrosis. Nonetheless, ongoing efforts exist to stabilize quantitative ultrasound elastography measurements by assessing factors that influence tissue shear wave velocity values, such as food intake, body mass index, ultrasound scanners, scanning protocols, and ultrasound image quality. Time-to-peak (TTP) methods have been routinely used to measure the shear wave velocity. However, there is still a need for methods that can provide robust shear wave velocity estimation in the presence of noisy motion data. The conventional TTP algorithm is limited to searching for the maximum motion in time profiles at different spatial locations. In this paper, two modified shear wave speed estimation algorithms are proposed. The first method searches for the maximum motion in both space and time [spatiotemporal peak (STP)]; the second method applies an amplitude filter [spatiotemporal thresholding (STTH)] to select points with motion amplitude higher than a threshold for shear wave group velocity estimation. The two proposed methods (STP and STTH) showed higher precision in shear wave velocity estimates compared with TTP in phantom. Moreover, in a cohort of 14 healthy subjects, STP and STTH methods improved both the shear wave velocity measurement precision and the success rate of the measurement compared with conventional TTP.

Ultrasonic Shear Wave Elasticity Imaging Sequencing and Data Processing Using a Verasonics Research Scanner.

Ultrasound elasticity imaging has been developed over the last decade to estimate tissue stiffness. Shear wave elasticity imaging (SWEI) quantifies tissue stiffness by measuring the speed of propagating shear waves following acoustic radiation force excitation. This work presents the sequencing and data processing protocols of SWEI using a Verasonics system. The selection of the sequence parameters in a Verasonics programming script is discussed in detail. The data processing pipeline to calculate group shear wave speed (SWS), including tissue motion estimation, data filtering, and SWS estimation is demonstrated. In addition, the procedures for calibration of beam position, scanner timing, and transducer face heating are provided to avoid SWS measurement bias and transducer damage.

Guidelines for Finite-Element Modeling of Acoustic Radiation Force-Induced Shear Wave Propagation in Tissue-Mimicking Media.

Ultrasound shear wave elastography is emerging as an important imaging modality for evaluating tissue material properties. In its practice, some systematic biases have been associated with ultrasound frequencies, focal depths and configuration, and transducer types (linear versus curvilinear), along with displacement estimation and shear wave speed estimation algorithms. Added to that, soft tissues are not purely elastic, so shear waves will travel at different speeds depending on their spectral content, which can be modulated by the acoustic radiation force (ARF) excitation focusing, duration, and the frequency-dependent stiffness of the tissue. To understand how these different acquisition and material property parameters may affect the measurements of shear wave velocity, the simulations of the propagation of shear waves generated by ARF excitations in viscoelastic media are a very important tool. This paper serves to provide an in-depth description of how these simulations are performed. The general scheme is broken into three components: 1) simulation of the 3-D ARF push beam; 2) applying that force distribution to a finite-element model; and 3) extraction of the motion data for post-processing. All three components will be described in detail and combined to create a simulation platform that is powerful for developing and testing algorithms for academic and industrial researchers involved in making quantitative shear-wave-based measurements of tissue material properties.

A quantitative imaging biomarker alliance

The Radiological Society of North America (RSNA) created the Quantitative Imaging Biomarker Alliance (QIBA) to convert “imaging systems” with subjective image interpretation to “measurement systems” with objective measurands having actionable numerical values. The goal is to create QIBA protocols and profiles that specify methods for data acquisition, analysis, and interpretation. QIBA profiles also provide specific claims of what can be accomplished by following the QIBA protocol. The intent is to validate the profile across imaging systems in a collaboration between industry, government (NIH, FDA, NIST), Pharma, clinicians, and academics. The RSNA/QIBA effort includes over 850 volunteers worldwide with Biomarker Committees for all major imaging modalities. The effort is expanding with collaborations in Japan (the Japan Radiological Society’s QIBA-Japan) and in Europe (European Institute for Biomedical Imaging Research and the European Imaging Biomarker Alliance; EIBIR—EIBALL). The QIBA ultrasound group began with a committee to investigate and minimize sources of bias and variance in shear wave speed estimates for liver fibrosis assessment. The group has grown to about 200 members and includes committees to advance biomarkers based on measurement of volume blood flow and parameters derived from contrast-enhanced ultrasound. The status and implications of these efforts will be presented.

Comparison of Robust Methods for Shear Wave Speed Estimation by Simulation

Mechanical properties of tissue are often related to the pathological state of tissue. Therefore, non-invasively measuring tissue stiffness has important clinical applications. With the assumption of isotropy, incompressibility and linearity, the shear modulus of tissue is related to its shear wave propagation speed. Acoustic radiation force from a focused ultrasound beam can be used to generate shear waves at the focal region within tissue, which propagate orthogonally to the direction of the pushing ultrasound beam. The shear wave speed can be estimated based on the so called time-to-flight principle. The shear wave arrival time determined at several lateral positions along the shear wave propagation path can be measured by the displacement profiles, which can be tracked using correlation-based method by pulse-echo ultrasound. This approach has been successively used with various modifications by several groups. The purpose of this study is to design a simulation method to generate the pulse-echo ultrasound signal, calculate the displacement profile in the spatial and time domain, and estimate the shear wave speed using RANSAC, Radon Sum and robust linear regression method, compare and analyze the algorithm performance of these methods.

Estimation of Shear Wave Speed in the Rhesus Macaques' Uterine Cervix.

Cervical softness is a critical parameter in pregnancy. Clinically, preterm birth is associated with premature cervical softening and postdates birth is associated with delayed cervical softening. In practice, the assessment of softness is subjective, based on digital examination. Fortunately, objective, quantitative techniques to assess softness, and other parameters associated with microstructural cervical change are emerging. One of these is shear wave speed (SWS) estimation. In principle, this allows objective characterization of stiffness because waves travel more slowly in softer tissue. We are studying SWS in humans and rhesus macaques, the latter in order to accelerate translation from bench to bedside. For the current study, we estimated SWS in ex vivo cervices of rhesus macaques, n=24 nulliparous (never given birth) and n=9 multiparous (delivered at least one baby). Misoprostol (a prostaglandin used to soften human cervices prior to gynecological procedures) was administered to 13 macaques prior to necropsy (nulliparous: 7; multiparous: 6). SWS measurements were made at predetermined locations from the distal to proximal end of the cervix on both the anterior and posterior cervix, with five repeat measures at each location. The intent was to explore macaque cervical microstructure, including biological and spatial variability, to elucidate the similarities and differences between the macaque and the human cervix in order to facilitate future in vivo studies. We found that SWS is dependent on location in the normal nonpregnant macaque cervix, as in the human cervix. Unlike the human cervix, we detected no difference between ripened and unripened rhesus macaque cervix samples, nor nulliparous versus multiparous samples, although we observed a trend toward stiffer tissue in nulliparous samples. We found rhesus macaque cervix to be much stiffer than human, which is important for technique refinement. These findings are useful for guiding study of cervical microstructure in both humans and macaques.

Quasi-plane shear wave propagation induced by acoustic radiation force with a focal line region: a simulation study

Shear wave propagation speed has been regarded as an attractive indicator for quantitatively measuring the intrinsic mechanical properties of soft tissues. While most existing techniques use acoustic radiation force (ARF) excitation with focal spot region based on linear array transducers, we try to employ a special ARF with a focal line region and apply it to viscoelastic materials to create shear waves. First, a two-dimensional capacitive micromachined ultrasonic transducer with 64 × 128 fully controllable elements is realised and simulated to generate this special ARF. Then three-dimensional finite element models are developed to simulate the resulting shear wave propagation through tissue phantom materials. Three different phantoms are explored in our simulation study using: (a) an isotropic viscoelastic medium, (b) within a cylindrical inclusion, and (c) a transverse isotropic viscoelastic medium. For each phantom, the ARF creates a quasi-plane shear wave which has a preferential propagation direction perpendicular to the focal line excitation. The propagation of the quasi-plane shear wave is investigated and then used to reconstruct shear moduli sequentially after the estimation of shear wave speed. In the phantom with a transverse isotropic viscoelastic medium, the anisotropy results in maximum speed parallel to the fiber direction and minimum speed perpendicular to the fiber direction. The simulation results show that the line excitation extends the displacement field to obtain a large imaging field in comparison with spot excitation, and demonstrate its potential usage in measuring the mechanical properties of anisotropic tissues.

On Shear Wave Speed Estimation for Agar-Gelatine Phantom

Conventional imaging of diagnostic ultrasound is widely used. Although it makes the differences in the soft tissues echogenicities’ apparent and clear, it fails in describing and estimating the soft tissue mechanical properties. It cannot portray their mechanical properties, such as the elasticity and stiffness. Estimating the mechanical properties increases chances of the identification of lesions or any pathological changes. Physicians are now characterizing the tissue’s mechanical properties as diagnostic metrics. Estimating the tissue’s mechanical properties is achieved by applying a force on the tissue and calculating the resulted shear wave speed. Due to the difficulty of calculating the shear wave speed precisely inside the tissue, it is estimated by analyzing ultrasound images of the tissue at a very high frame rate. In this paper, the shear wave speed is estimated using finite element analysis. A model is constructed to simulate the tissue’s mechanical properties. For a generalized soft tissue model, Agar-gelatine model is used because it has properties similar to that of the soft tissue. A point force is applied at the center of the proposed model. As a result of this force, a deformation is caused. Peak displacements are tracked along the lateral dimension of the model for estimating the shear wave speed of the propagating wave using the Time-To-Peak displacement (TTP) method. Experimental results have shown that the estimated speed of the shear wave is 5.2 m/sec. The speed value is calculated according to shear wave speed equation equals about 5.7 m/sec; this means that our speed estimation system’s accuracy is about 91 %, which is reasonable shear wave speed estimation accuracy with a less computational power compared to other tracking methods.

A Hilbert Transform Based Shear Wave Speed Estimation Method for Shear Wave Elasticity Imaging

In this paper, a Hilbert transform based method is used to estimate shear wave speed. Shear wave speed can be measured from the lateral propagation of particle displacements or velocities within the tissue, these particle displacements or velocities are generated by simulating radio frequency signals measured from ultrasound probe elements that image the displaced particles generated from a finite-element model (FEM) that simulates the dynamic response of tissues to acoustic radiation forces. The proposed shear wave speed estimation method is based on locating a zero-amplitude crossing in the Hilbert transform of the cross-correlation function between the particle displacement measured at first lateral location greater than the lateral width of shear wave and the displacement measured and subsequent lateral lines under examination at a certain depth. The results obtained from this method is compared to the lateral Time to Peak (TTP) method that finds the instance at which the maximum displacement is detected at each lateral location under examination at a certain depth. The proposed algorithm reveals a reconstruction of materials having shear modulus of 1.43±0.20, 2.92±0.39, 4.09±0.54, 8.13±1.06, 12.11±1.52, 16.16±2.08 kPa on particle displacement signals for 1.33, 2.835, 4, 8, 12, and 16 kPa shear moduli materials respectively and 1.34±0.40, 2.66±0.41, 3.77±0.58, 7.45±1.14, 11.34±1.51, and 14.90±2.08 kPa on particle velocity signals for 1.33, 2.835, 4, 8, 12, and 16 kPa shear moduli materials respectively. Finally, the proposed method is based on locating a zero-amplitude crossing in the Hilbert transform of the cross-correlation function appears to provide more accurate results than Lateral TTP method.

A new method for shear wave speed estimation in shear wave elastography.

Visualization of mechanical properties of tissue can aid in noninvasive pathology diagnosis. Shear wave elastography (SWE) measures the elastic properties of soft tissues by estimation of local shear wave propagation speed. In this paper, a new robust method for estimation of shear wave speed is introduced which has the potential for simplifying continuous filtering and real-time elasticity processing. Shear waves were generated by external mechanical excitation and imaged at a high frame rate. Three homogeneous phantoms of varying elastic moduli and one inclusion phantom were imaged. Waves propagating in separate directions were filtered and shear wave speed was estimated by inversion of the 1-D first-order wave equation. Final 2-D shear wave speed maps were constructed by weighted averaging of estimates from opposite traveling directions. Shear wave speed results for phantoms with gelatin concentrations of 5%, 7%, and 9% were 1.52 ± 0.10 m/s, 1.86 ± 0.10 m/s, and 2.37 ± 0.15 m/s, respectively, which were consistent with estimates computed from three other conventional methods, as well as compression tests done with a commercial texture analyzer. The method was shown to be able to reconstruct a 2-D speed map of an inclusion phantom with good image quality and variance comparable to conventional methods. Suggestions for further work are given.

Toward Standardized Acoustic Radiation Force (ARF)-Based Ultrasound Elasticity Measurements With Robotic Force Control.

Acoustic radiation force (ARF)-based approaches to measure tissue elasticity require transmission of a focused high-energy acoustic pulse from a stationary ultrasound probe and ultrasound-based tracking of the resulting tissue displacements to obtain stiffness images or shear wave speed estimates. The method has established benefits in biomedical applications such as tumor detection and tissue fibrosis staging. One limitation, however, is the dependence on applied probe pressure, which is difficult to control manually and prohibits standardization of quantitative measurements. To overcome this limitation, we built a robot prototype that controls probe contact forces for shear wave speed quantification. The robot was evaluated with controlled force increments applied to a tissue-mimicking phantom and in vivo abdominal tissue from three human volunteers. The root-mean-square error between the desired and measured forces was 0.07 N in the phantom and higher for the fatty layer of in vivo abdominal tissue. The mean shear wave speeds increased from 3.7 to 4.5 m/s in the phantom and 1.0 to 3.0 m/s in the in vivo fat for compressive forces ranging from 2.5 to 30 N. The standard deviation of shear wave speeds obtained with the robotic approach were low in most cases ( 0.2 m/s) and comparable to that obtained with a semiquantitative landmark-based method. Results are promising for the introduction of robotic systems to control the applied probe pressure for ARF-based measurements of tissue elasticity. This approach has potential benefits in longitudinal studies of disease progression, comparative studies between patients, and large-scale multidimensional elasticity imaging.

Changes in shear wave speed pre- and post-induction of labor: a feasibility study.

To explore the feasibility of using shear wave speed (SWS) estimates to detect differences in cervical softening pre- and post-ripening in women undergoing induction of labor. Subjects at 37-41 weeks' gestation undergoing cervical ripening before induction of labor were recruited (n = 20). Examinations, performed prior to administration of misoprostol and 4 h later included Bishop score, transvaginal ultrasound measurement of cervical length, and 10 replicate SWS measurements using an ultrasound system equipped with a prototype transducer (128 element, 3 mm diameter, 14 mm aperture) attached to the clinician's hand. Subjects were divided into two groups, 'not-in-labor' and 'marked-progression', based on cervical evaluation at the second examination. Measurements were compared via individual paired hypotheses tests and using a linear mixed model, with the latter also used to compare groups. Spearman's rank correlation coefficient was used to compare SWS with Bishop score. The linear mixed model can take into account clustered data and accommodate multiple predictors simultaneously. The Wilcoxon signed-rank paired test established a significant difference in pre- and post-ripening SWS, with mean SWS estimates of 2.53 ± 0.75 and 1.54 ± 0.31 m/s, respectively (P < 0.001) in the not-in-labor group (decrease in stiffness) and 1.58 ± 0.33 and 2.35 ± 0.65 m/s for the marked-progression group (increase in stiffness). The linear mixed model corroborated significant differences in pre- and post-ripening measurements in individual subjects (P < 0.001) as well as between groups (P < 0.0001). SWS estimates were significantly correlated with digitally-assessed cervical softness and marginally correlated with Bishop score as assessed by Spearman's rank correlation coefficient. In-vivo SWS estimates detected stiffness differences before and after misoprostol-induced softening in term pregnancies. This ultrasonic shear elasticity imaging technique shows promise for assessing cervical softness.

Analyzing the Impact of Increasing Mechanical Index and Energy Deposition on Shear Wave Speed Reconstruction in Human Liver

Shear wave elasticity imaging (SWEI) has found success in liver fibrosis staging. This work evaluates hepatic SWEI measurement success as a function of push pulse energy using two mechanical index (MI) values (1.6 and 2.2) over a range of pulse durations. Shear wave speed (SWS) was measured in the livers of 26 study subjects with known or potential chronic liver diseases. Each measurement consisted of eight SWEI sequences, each with different push energy configurations. The rate of successful SWS estimation was linearly proportional to the push energy. SWEI measurements with higher push energy were successful in patients for whom standard push energy levels failed. The findings also suggest that liver capsule depth could be used prospectively to identify patients who would benefit from elevated output. We conclude that there is clinical benefit to using elevated acoustic output for hepatic SWS measurement in patients with deeper livers.

Quantitative shear elasticity imaging from a complex elastic wavefield in soft solids with application to passive elastography.

In passive elastography, the complex physiological noise present in the human body is used to conduct an elastography experiment. In the present work, quantitative shear elasticity imaging from a complex elastic wavefield is demonstrated in soft solids. By correlating the elastic field at different positions, which can be interpreted as a time-reversal experiment, shear waves are virtually focused on any point of the imaging plane. According to the Rayleigh criterion, the focus size is directly related to the shear wave speed and thus to the shear elasticity. To locally retrieve a shear wave speed estimation, analytical and empirical expressions that relate the focus size with the shear wave speed and the frequency band used in the correlation computation are derived. The validity of such expressions is demonstrated numerically and experimentally on a tissue-mimicking phantom consisting of two different elastic layers. The obtained results were in complete agreement with a prior shear wave speed estimation demonstrating the potential of the technique to quantitative shear elasticity assessment using a complex elastic wavefield. Finally, an ultraslow experiment at an imaging rate of 10 Hz shows the technique to be compatible with slow imaging devices such as standard echographs or MRI scanners.

On the use of local speckle field as a correction factor for shear modulus estimates based on multiple-track-locations methods

Acoustic-radiation-force impulse (ARFI) imaging for characterization of shear modulus of biological tissues employs either multiple-track-locations (MTL) methods or single-track-location (STL) methods. MTL estimates of shear modulus at different depths suffer from more variability compared to those of STL estimates. Our studies have shown a significant correlation between bias in shear-wave arrival time estimates and local speckle field lateral statistics. We propose using the local speckle field as a surrogate for the unknown bias apparent in MTL shear-wave speed estimates. This local speckle field is determined using a “swept-receive acquisition” that is produced by holding the transmit beam fixed while laterally translating the receive aperture. Application of various lateral weighting functions to the swept-receive image results in an approximate compensation to the tracking bias. In this study, we implement our technique on simulation and experimental data from gelatin phantoms. Additionally, the effect of varying transmit-receive aperture f/# on the accuracy of our compensation method is investigated in simulation. Finally, the quality of compensated MTL shear modulus images are quantified and compared to those of the uncompensated MTL and of STL methods in matched tissue mimicking phantom experiments.

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.