Kathy Nightingale

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.

4K-5 Shear Wave Velocity Estimation Using Acoustic Radiation Force Impulsive Excitation in Liver In Vivo

Acoustic radiation force can be used to mechanically excite tissue in remote, focused locations, and the tissue response can be monitored using ultrasonic correlation based methods. The speed with which the resulting shear waves propagate away from the focal region can be estimated and used to quantify the material shear modulus, as originally proposed by Sarvazyan et. al. (1998). This imaging approach has been implemented by Bercoff et. al. (2004), using a highly parallel custom ultrasound system, and Helmholtz reconstructions. We have developed a system that is implemented on a commercial scanner using 4:1 parallel processing, and a new algorithm for estimating shear wave speed, which does not require 2nd order temporal and spatial differentiation of displacement data. The method is robust and generates consistent measurements over multiple acquisitions. The goal of our work is to develop this system for the purpose of staging liver fibrosis. The method was used to measure elastic moduli of liver in vivo in healthy human volunteers, and in a rat model, and the moduli obtained with this method are consistent with those reported in the literature

Finite element modeling of impulsive excitation and shear wave propagation in an incompressible, transversely isotropic medium

Elastic properties of materials can be measured by observing shear wave propagation following localized, impulsive excitations and relating the propagation velocity to a model of the material. However, characterization of anisotropic materials is difficult because of the number of elasticity constants in the material model and the complex dependence of propagation velocity relative to the excitation axis, material symmetries, and propagation directions. In this study, we develop a model of wave propagation following impulsive excitation in an incompressible, transversely isotropic (TI) material such as muscle. Wave motion is described in terms of three propagation modes identified by their polarization relative to the material symmetry axis and propagation direction. Phase velocities for these propagation modes are expressed in terms of five elasticity constants needed to describe a general TI material, and also in terms of three constants after the application of two constraints that hold in the limit of an incompressible material. Group propagation velocities are derived from the phase velocities to describe the propagation of wave packets away from the excitation region following localized excitation. The theoretical model is compared to the results of finite element (FE) simulations performed using a nearly incompressible material model with the five elasticity constants chosen to preserve the essential properties of the material in the incompressible limit. Propagation velocities calculated from the FE displacement data show complex structure that agrees quantitatively with the theoretical model and demonstrates the possibility of measuring all three elasticity constants needed to characterize an incompressible, TI material.

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

Increased hepatic venous pressure can be observed in patients with advanced liver disease and congestive heart failure. This elevated portal pressure also leads to variation in acoustic radiation-force-derived shear wave-based liver stiffness estimates. These changes in stiffness metrics with hepatic interstitial pressure may confound stiffness-based predictions of liver fibrosis stage. The underlying mechanism for this observed stiffening behavior with pressurization is not well understood and is not explained with commonly used linear elastic mechanical models. An experiment was designed to determine whether the stiffness increase exhibited with hepatic pressurization results from a strain-dependent hyperelastic behavior. Six excised canine livers were subjected to variations in interstitial pressure through cannulation of the portal vein and closure of the hepatic artery and hepatic vein under constrained conditions (in which the liver was not free to expand) and unconstrained conditions. Radiation-force-derived shear wave speed estimates were obtained and correlated with pressure. Estimates of hepatic shear stiffness increased with changes in interstitial pressure over a physiologically relevant range of pressures (0-35 mmHg) from 1.5 to 3.5 m s(-1). These increases were observed only under conditions in which the liver was free to expand while pressurized. This behavior is consistent with hyperelastic nonlinear material models that could be used in the future to explore methods for estimating hepatic interstitial pressure noninvasively.

Finite element analysis of radiation force induced tissue motion with experimental validation

An ultrasonic radiation force-based method for remote palpation of tissue is investigated. The use of radiation force to image tissue stiffness has been proposed by several researchers. In this paper, the potential for using a diagnostic ultrasound system to both apply radiation force and track the resulting tissue displacements is investigated using Finite Element Methods (FEM), and the results are compared with experimental results. Remote palpation is accomplished by interspersing high intensity pushing beams with low intensity tracking beams. This generates localized radiation forces which can be applied throughout the tissue, with the resulting displacement patterns determined using correlation techniques. An area that is stiffer than the surrounding medium distributes the force, resulting in larger regions of displacement, and smaller maximum displacements. The resulting displacement maps provide information as to the location and size of regions of increased stiffness. The authors have developed an FEM model that predicts displacements resulting from acoustic radiation force fields generated by diagnostic transducers in various complex media. They perform a parametric analysis of varying tissue and acoustic beam characteristics on radiation force induced tissue displacements. Displacements are on the order of microns, with considerable differences in displacement patterns in the presence and absence of a lesion (or stiff inclusion). Initial experimental results are presented that support the findings in the model.

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.

Transcending the traditional: Using tablet PCs to enhance engineering and computer science instruction

Traditional instructional methods present many obstacles to effective teaching and learning in engineering and computer science courses. These include a reliance on text-based or static mediums to convey equation- and graphics-heavy concepts, a disconnect between theoretical lecture presentations and applied laboratory or homework exercises, and a difficulty in promoting collaborative activities that more accurately reflect an engineering approach to problem solving. Additionally, technical courses can suffer, like any other course, when students are not actively engaged in the learning and when instructors cannot gauge student understanding. This project has explored the utility of Tablet PCs for overcoming these challenges within a sample of courses in engineering and computer science. There were three primary questions: which knowledge domains benefit from the use of Tablet PCs; whether observed benefits are derived from Tablet PC-specific activities; and what problems limit the effectiveness of Tablet PCs in educational settings? The evaluation of assessment data using regression approaches demonstrated that Tablet-PC-specific activities had a consistent, meaningful, and positive impact upon engineering and computer science courses.

Comparison between 3D ARFI imaging and mpMRI in detecting clinically-significant prostate cancer lesions

Current prostate cancer screening methods involve non-targeted needle biopsies and detection of clinically-insignificant lesions that receive excessive treatments, exposing patients to unnecessary adverse side effects and placing a burden on our health care systems. There is a strong clinical need for improved prostate imaging methods that are sensitive and specific for clinically-significant prostate cancer lesions to guide needle biopsies, target focal treatments, and improve overall patient outcomes. In this study, we compared 3D in vivo Acoustic Radiation Force Impulse (ARFI) imaging with 3 Tesla, endorectal coil, multi-parametric magnetic resonance imaging (mpMRI) to correlate the ability for each modality to identify clinically-significant prostate cancer lesions. We also correlated Apparent Diffusion Coefficient (ADC) values from Diffusion Weighted Imaging (DWI) MR sequences with ARFI indices of suspicion and MR Prostate Imaging - Reporting and Data Systems (PI-RADS) scores, testing the hypothesis that increased cellular density is associated with regions suspicious for prostate cancer in ARFI images. Overall, ARFI and mpMR imaging were well-correlated in identifying clinically-significant prostate cancer lesions. There were several cases where only one of the imaging modalities was able to identify the prostate cancer lesion, highlighting the potential to further improve prostate cancer lesion detection and localization with a fused ARFI:mpMRI imaging system. ADC values were decreased in all prostate cancer lesions identified with mpMRI, but there were no obvious trends between the absolute ADC values and the ARFI image indices of suspicion.

Undergraduate biomedical imaging education.

Biomedical engineers, with their training in the life sciences as well as engineering, mathematics and physical sciences, are uniquely poised to both support and advance biomedical imaging technologies, which noninvasively capture the structure and function of living organisms. Biomedical imaging, born of Roentgen’s 1895 discovery of X-rays, now incorporates a wide variety of modalities that produce images reflecting the distribution and interaction of energy with the body’s tissues. Such images are acquired and used in many different ways including traditional medical imaging for diagnosis, therapy planning and assessment. Biomedical engineers also acquire and use images in the forms of standard videos to study gait analysis, endoscopic videos to guide therapeutic interventions, and still camera optical images to quantify biometrics and to read DNA microarrays. Strong undergraduate imaging curricula are critical to developing the essential human infrastructure needed to support biomedical imaging. This is especially true given the rapidly growing demand for engineers trained in the field of biomedical imaging. 6 This paper reviews current imaging curricula in biomedical engineering undergraduate programs and describes the best practices that should guide the development and refinement of biomedical imaging curricula. These curricula must capitalize on available resources—persons with expertise to teach imaging, computers with imaging software, imaging equipment if available and hands-on alternatives and simulations otherwise, as well as resources available on the internet as described in this paper. Incorporation of imaging examples to teach concepts already taught in basic science and mathematics courses helps address the lim

Utilization of acoustic streaming to classify breast lesions in vivo

Results from a clinical study in which Streaming Detection was used to successfully differentiate fluid-filled lesions (cysts) from solid lesions in the breast are presented. Streaming Detection is an ultrasonic technique in which high intensity ultrasound pulses are used to induce acoustic streaming in cyst fluid, and this motion is detected using flow estimation methods. In fourteen of fifteen simple cysts acoustic streaming was generated and detected. Evidence of nonlinear enhancement of acoustic streaming was observed in several cysts. Streaming Detection was also performed on fourteen sonographically indeterminate breast lesions. Acoustic streaming was generated and detected in four of these lesions, each of which were relatively small (average size of 0.4/spl times/0.5 cm). It seems that Streaming Detection is particularly suited for diagnosis of small, possibly newer cysts which may appear indeterminate on conventional sonography due to their small size. These results indicate that Streaming Detection would be a useful adjunct to conventional sonography for the purpose of breast lesion classification.