Vijay Shamdasani

Assessment of Liver Viscoelasticity by Using Shear Waves Induced by Ultrasound Radiation Force

PURPOSE To investigate the value of viscosity measured with ultrasonographic (US) elastography in liver fibrosis staging and to determine whether the use of a viscoelastic model to estimate liver elasticity can improve its accuracy in fibrosis staging. MATERIALS AND METHODS The study, which was performed from February 2010 to March 2011, was compliant with HIPAA and approved by the institutional review board. Written informed consent was obtained from each subject. Ten healthy volunteers (eight women and two men aged 27-55 years) and 35 patients with liver disease (17 women and 18 men aged 19-74 years) were studied by using US elasticity measurements of the liver (within 6 months of liver biopsy). US data were analyzed with the shear wave dispersion ultrasound vibrometry (SDUV) method, in which elasticity and viscosity are measured by evaluating dispersion of shear wave propagation speed, as well as with the time-to-peak (TTP) method, where tissue viscosity was neglected and only elasticity was estimated from the effective shear wave speed. The hepatic fibrosis stage was assessed histologically by using the METAVIR scoring system. The correlation of elasticity and viscosity was assessed with the Pearson correlation coefficient. The performances of SDUV and TTP were evaluated with receiver operating characteristic (ROC) curve analysis. RESULTS The authors found significant correlations between elasticity and viscosity measured with SDUV (r = 0.80) and elasticity measured with SDUV and TTP (r = 0.94). The area under the ROC curve for differentiating between grade F0-F1 fibrosis and grade F2-F4 fibrosis was 0.98 for elasticity measured with SDUV, 0.86 for viscosity measured with SDUV, and 0.95 for elasticity measured with TTP. CONCLUSION The results suggest that elasticity and viscosity measured between 95 Hz and 380 Hz by using SDUV are correlated and that elasticity measurements from SDUV and TTP showed substantially similar performance in liver fibrosis staging, although elasticity calculated from SDUV provided a better area under the ROC curve.

RSNA/QIBA: Shear wave speed as a biomarker for liver fibrosis staging

An interlaboratory study of shear wave speed (SWS) estimation was performed. Commercial shear wave elastography systems from Fibroscan, Philips, Siemens and Supersonic Imagine, as well as several custom laboratory systems, were involved. Fifteen sites were included in the study. CIRS manufactured and donated 11 pairs of custom phantoms designed for the purposes of this investigation. Dynamic mechanical tests of equivalent phantom materials were also performed. The results of this study demonstrate that there is very good agreement among SWS estimation systems, but there are several sources of bias and variance that can be addressed to improve consistency of measurement results.

Characterization of Carotid Plaques on 3-Dimensional Ultrasound Imaging by Registration With Multicontrast Magnetic Resonance Imaging

The ability of magnetic resonance imaging (MRI) in carotid plaque component identification has been well established. However, compared to the costly nature of MRI, 3‐dimensional (3D) ultrasound imaging is a more cost‐effective assessment tool. Thus, an attractive alternative for carotid disease monitoring would be to establish a strategy in which 3D ultrasound imaging is used as a screening tool that precedes MRI. To develop and validate such a protocol, registration between ultrasound and MR images is required. This article introduces a surface‐based algorithm for efficient ultrasound imaging‐MRI registration.

RSNA QIBA ultrasound shear wave speed Phase II phantom study in viscoelastic media

Using ultrasonic shear wave speed (SWS) estimates has become popular to noninvasively evaluate liver fibrosis, but significant inter-system variability in liver SWS measurements can preclude meaningful comparison of measurements performed with different systems. The RSNA Quantitative Imaging Biomarker Alliance (QIBA) ultrasound SWS committee has been developing elastic and viscoelastic (VE) phantoms to evaluate system dependencies of SWS estimates. The objective of this study is to compare SWS measurements between commercially-available systems using phantoms that have viscoelastic properties similar to those observed in normal and fibrotic liver. CIRS, Inc. fabricated three phantoms using a proprietary oil-water emulsion infused in a Zerdine® hydrogel that were matched in viscoelastic behavior to healthy and fibrotic human liver data. Phantoms were measured at academic, clinical, government and vendor sites using different systems with curvilinear arrays at multiple focal depths (3.0, 4.5 & 7.0 cm). The results of this study show that current-generation ultrasound SWS measurement systems are able to differentiate viscoelastic materials that span healthy to fibrotic liver. The deepest focal depth (7.0 cm) yielded the greatest inter-system variability for each phantom (maximum of 17.7%) as evaluated by IQR. Inter-system variability was consistent across all 3 phantoms and was not a function of stiffness. Median SWS estimates for the greatest outlier system for each phantom/focal depth combination ranged from 12.7-17.6%. Future efforts will include performing more robust statistical analyses of these data, comparing these phantom data trends with viscoelastic digital phantom data, providing vendors with study site data to refine their systems to have more consistent measurements, and integrating these data into the QIBA ultrasound shear wave speed measurement profile.

Noninvasive Assessment of Liver Fibrosis Using Ultrasound-Based Shear Wave Measurement and Comparison to Magnetic Resonance Elastography

Magnetic resonance elastography (MRE) has excellent performance in detecting liver fibrosis and is becoming an alternative to liver biopsy in clinical practice. Ultrasound techniques based on measuring the propagation speed of the shear waves induced by acoustic radiation force also have shown promising results for liver fibrosis staging. The objective of this study was to compare ultrasound‐based shear wave measurement to MRE.

Mapping skull attenuation for optimal probe placement in transcranial ultrasound applications

It takes skill and time to place an ultrasound probe on the optimal acoustic window for transcranial insonification. This hinders efficient ultrasound imaging and therapy of the brain. This paper presents two approaches for automatically identifying the best transtemporal window in order to facilitate clinical workflow. A mechanically translating matrix imaging probe is used in conjunction with a point source on the contralateral temple. Optimizing the probe position results in improved image sensitivity and limited aberration.

Phase aberration in Shear Wave Dispersion Ultrasound Vibrometry

Shearwave Dispersion Ultrasound Vibrometry (SDUV) is an acoustic radiation force based technique that measures tissue shear viscoelasticity by characterizing shear wave speed dispersion. An application of this technique is liver fibrosis staging. We previously reported findings from an animal study where shear modulus and viscosity reconstruction displayed larger variances for in vivo versus ex vivo cases. This study investigates two major causes of such increased variance, namely attenuation and phase aberration. Two sets of experiments were conducted using a custom phantom. In the first experiment, the phantom was imaged directly with varying pushing power by setting system transmit attenuation at different levels from 0 dB to 6 dB. The second set of experiments utilized different pieces of pork bellies as aberrators between the probe and the phantom, while maintaining the pushing power at 0 dB. For each data set, SDUV reconstruction algorithms yielded shear moduli within a region of interest (ROI) of 10 mm × 4 mm close to the pushing focus. The attenuation experiment showed that the variance in SDUV reconstruction results did not start to increase until the peak displacement dropped to 2.2 μm. On the other hand, insertion of an aberrator caused elevated variances even at a much higher peak displacement of 3.9 μm. The variances also swung greatly among different data sets with similar peak displacements. Moreover, thinner aberrators produced consistently better results even with lower peak displacements. All these observations indicate that phase aberration induced waveform distortion is more detrimental to SDUV than pure attenuation. It is beneficial to investigate phase aberration correction methods and apply them to improve SDUV performance.

A phantom study to cross-validate multimodality shear wave elastography techniques

As various quantitative shear wave elastography techniques are being implemented commercially and used clinically, it is important to understand what elastic modulus is measured by those techniques and to assess technical factors influencing measurement accuracy. Cross-validated in this study were four shear wave elastography techniques including ultrasound elastography point quantification (ElastPQ), magnetic resonance elastography (MRE), 1-D transient elastography (TE), and a Verasonics ultrasound system based elastography feature. ElastPQ is an acoustic radiation force (ARF) based technique implemented on an ultrasound system iU22 for a curvilinear probe C5-1. MRE measurements were made at a shear wave frequency of 100 Hz. The 1-D TE system uses an external mechanical vibrator at 100 Hz to generate shear waves and a 7.5 MHz single element transducer to track shear wave propagation. The Verasonics ultrasound system with a linear probe L7-4 uses a plane wave transmit imaging mode to measure propagation speed of shear waves induced by ARF. Six custom-made elasticity phantoms were measured selectively and were assumed purely elastic by the four techniques. The results show good agreement between two ARF based techniques: ElastPQ and Verasonics based feature. When comparing ElastPQ to 1-D TE, the discrepancy for the hardest phantom could be explained by the diffraction artifact in the latter system. Further improvements on the 1D-TE reconstruction algorithm are ongoing. The overestimation by ElastPQ and Verasonics against MRE could be attributed to frequency dispersion due to slight viscosity in phantoms. MRE measures shear wave speed at a single frequency 100 Hz, whereas the other two estimate an effective group velocity over a larger bandwidth for a broadband shear wave signal. 1-D TE and MRE measurements are in closest agreement due to similar shear wave frequencies. More validation of these techniques is required on real patients in clinical settings.

Finite Element Modeling for Shear Wave Elastography

Shear wave elastography is an important imaging modality to evaluate tissue mechanical properties and supplement conventional ultrasound diagnostic imaging. A 3D finite element model has been created in PZFlex for simulating and understanding shear wave generation by the acoustic radiation force, and its propagation through different media. The simulation settings were based on a shear wave elastography prototype using a Philips iU22 scanner with a C5-1 curvilinear probe. The modeling process was divided into two steps. In the first step, the acoustic field of the ultrasound probe was calculated and the output acoustic radiation stress (ARS) result in the 3D volume was saved. In the second step, the ARS data was applied as a boundary condition to generate the shear wave. The shear wave displacement time profiles in the region of interest were recorded at the end of the second step. The simulation was performed for different media, including uniform tissues with various shear moduli and viscosities, as well as uniform tissue background with an embedded stiffer inclusion. Clear differences were observed on the shear wave displacement time profiles, as the displacement peak was attenuated and widened by the higher shear modulus and viscosity. The simulation results were also cross-checked with elasticity reconstruction algorithms based on wave equation (WE), Voigt model (VM) and time-to-peak (TTP) methods. For a medium similar to normal liver tissue with 2KPa shear modulus, all three reconstruction methods reported shear modulus approximately the same as input value when the viscosity was negligible (WE: 2.05KPa, VM: 2.06KPa, TTP: 2.12KPa). With increased viscosity in the medium (2KPa, 2PaS), TTP seemed to under-estimate shear modulus in the near-field (WE: 2.41KPa, VM: 1.98KPa & 2.11PaS, TTP: 1.38KPa). For a uniform medium with an embedded spherical inclusion, all three methods successfully detected the inclusion and reconstructed stiffness maps. The results suggested that the finite element modeling could provide valuable insight in simulating and understanding shear wave generation and propagation. It could also be an important tool to evaluate and analyze stiffness reconstruction algorithms for shear wave elastography.

Exact viscoelastic Green's functions of the Voigt-model-based Navier's equation

Acoustic radiation force based ultrasound shear wave elastography (SWE) is a non-invasive tool for extracting quantitative tissue viscoelasticity information. Often, the first line of evaluation of an ultrasound SWE reconstruction technique is its performance on homogenous phantoms, either physical or numerical, with various viscoelastic properties. Here we address the numerical aspect by solving a governing equation of tissue response to force, the Naviers equation with viscoelastic Voigt model. Following a Lamés theorem approach, we have derived two exact Greens functions, one being a function of position and temporal frequency and the other of spatial frequency and time. The first Greens function can serve as a gold standard for evaluating other Greens functions at specific positions while the second allows for fast 3-D or 4-D (space and time) simulations on acoustic radiation force induced shear wave generation and propagation over an extended region. These Greens functions can help optimize shear wave induction and develop SWE reconstruction algorithms to achieve more accurate estimate of shear modulus and viscosity for clinical diagnosis.