Design and Testing of a Single-Element Ultrasound Viscoelastography System for Point-of-Care Edema Quantification.

Management of fluid overload in patients with end-stage renal disease represents a unique challenge to clinical practice because of the lack of accurate and objective measurement methods. Currently, peripheral edema is subjectively assessed by palpation of the patient's extremities, ostensibly a qualitative indication of tissue viscoelastic properties. New robust quantitative estimates of tissue fluid content would allow clinicians to better guide treatment, minimizing reactive treatment decision making.

Small breast lesion classification performance using the normalized axial-shear strain area feature.

Breast cancers that are found and confirmed because they are causing symptoms tend to be larger and are more likely to have already spread to the lymph nodes and beyond. Thus, early detection and confirmation are of paramount importance. The normalized axial-shear strain area (NASSA) feature from the axial-shear strain elastogram (ASSE) has been shown to be a feature that can identify the boundary-bonding conditions that are indicative of the presence of cancer.

On the advantages of imaging the axial-shear strain component of the total shear strain in breast tumors.

Axial-shear strain elastography was described recently as a method to visualize the state of bonding at an inclusion boundary. Although total shear strain elastography was initially proposed for this purpose, it did not evolve beyond the initial reported finite element model (FEM) and simulation studies. One of the major reasons for this was the practical limitation in estimating the tissue motion perpendicular (lateral) to the ultrasound (US) beam as accurately as the motion along the US beam (axial).

Visualization of HIFU-induced lesion boundaries by axial-shear strain elastography: a feasibility study.

In this paper, we report on a study that investigated the feasibility of reliably visualizing high-intensity focused ultrasound (HIFU) lesion boundaries using axial-shear strain elastograms (ASSE). The HIFU-induced lesion cases used in the present work were selected from data acquired in a previous study. The samples consisted of excised canine livers with thermal lesions produced by a magnetic resonance-compatible HIFU system (GE Medical System, Milwaukee, WI, USA) and were cast in a gelatin block for the elastographic experiment.

Axial-shear strain elastography for breast lesion classification: further results from in vivo data.

The purpose of this work was to investigate the potential of the normalized axial-shear strain area (NASSA) feature, derived from axial-shear strain elastograms (ASSE), for breast lesion classification of fibroadenoma and cancer. This study consisted of previously acquired in vivo digital radiofrequency data of breast lesions. A total of 33 biopsy-proven malignant tumors and 30 fibroadenoma cases were included in the study, which involved three observers blinded to the original BIRADS-ultrasound scores.

Importance of axial compression verification to correct interpretation of axial-shear strain elastograms in breast lesions.

We have recently shown that the appearance of Axial-Shear Strain Elastograms (ASSEs) for the case of loosely-bonded, elliptical inclusions (like fibroadenomas in the breast) is unique and therefore has the potential to distinguish benign fibroadenomas from malignant tumors in the breast. The ASSEs were obtained using quasi-static axial compressions, in a like manner as in normal axial-strain elastography. However, strict axial compression is achieved most often only by computer-controlled acquisitions and not by more practical freehand acquisitions.

Axial-shear strain distributions in an elliptical inclusion model: experimental validation and in vivo examples with implications to breast tumor classification.

Recently, we reported on the axial-shear strain fill-in of the interior of loosely bonded stiff elliptical inclusions in a soft background at non-normal orientations, and the lack of fill-in in firmly bonded inclusions at any orientation. In this paper, we report on the experimental validation of the simulation studies using tissue-mimicking gelatin-based phantoms. We also show a few confirmatory examples of the existence of these phenomena in benign vs.

Breast tumor classification using axial shear strain elastography: a feasibility study.

Recently, the feasibility of visualizing the characteristics of bonding at an inclusion-background boundary using axial-shear strain elastography was demonstrated. In this paper, we report a feasibility study on the utility of the axial-shear strain elastograms in the classification of in vivo breast tumor as being benign or malignant. The study was performed using data sets obtained from 15 benign and 15 malignant cases that were biopsy proven.

On the differences between two-dimensional and three-dimensional simulations for assessing elastographic image quality: a simulation study.

In this work, we introduced an elastographic simulation framework, which estimates upper bounds on elastographic image quality by accounting for three-dimensional (3D) tissue motion and the 3D nature of the ultrasound beam. For the boundary conditions and the range of applied strains considered in this study, it was observed that for applied strains smaller than 0.7%, fast two-dimensional (2D) simulations and 3D simulations predicted similar upper bounds on elastographic signal-to-noise (SNR(e)) and contrast-to-noise ratios (CNR(e)); however, for applied strains greater than 0.

The feasibility of using poroelastographic techniques for distinguishing between normal and lymphedematous tissues in vivo.

Lymphedema is a common condition involving an abnormal accumulation of lymphatic fluid in the interstitial space that causes swelling, most often in the arm(s) and leg(s). Lymphedema is a significant lifelong concern that can be congenital or develop following cancer treatment or cancer metastasis. Common methods of evaluation of lymphedema are mostly qualitative making it difficult to reliably assess the severity of the disease, a key factor in choosing the appropriate treatment.

Effect of lesion boundary conditions on axial strain elastograms: a parametric study.

Ultrasound elastography produces strain images of compliant tissues under quasi-static compression. When a material is compressed, there are several parameters that affect the stress-distribution and, hence, the strain distribution in the material. The state of bonding of an inclusion to the background material is a critical parameter.

The feasibility of estimating and imaging the mechanical behavior of poroelastic materials using axial strain elastography.

In this paper, we have investigated the feasibility of imaging the mechanical behavior of poroelastic materials using axial strain elastography. Cylindrical samples obtained from poroelastic materials having different elastic and permeability properties were subjected to a constant compression force (a classical creep experiment), during which poroelastographic data were acquired. For comparison, we also tested a few gelatin phantoms and non-homogeneous poroelastic phantoms constructed by combining different poroelastic materials.

Visualization of bonding at an inclusion boundary using axial-shear strain elastography: a feasibility study.

Ultrasound elastography produces strain images of compliant tissues under quasi-static compression. In axial-shear strain elastography, the local axial-shear strain resulting from application of quasi-static axial compression to an inhomogeneous material is imaged. The overall hypothesis of this work is that the pattern of axial-shear strain distribution around the inclusion/background interface is completely determined by the bonding at the interface after normalization for inclusion size and applied strain levels, and that it is feasible to extract certain features from the axial-shear strain elastograms to quantify this pattern.

Assessing image quality in effective Poisson's ratio elastography and poroelastography: II.

Poroelastography is a novel elastographic technique for imaging the time variation of the mechanical behaviour of poroelastic materials. Poroelastograms are generated as a series of time-sequenced effective Poisson's ratio (EPR) elastograms, obtained from the imaged material under sustained compression. In the companion report (Righetti et al 2007 Phys.

Assessing image quality in effective Poisson's ratio elastography and poroelastography: I.

The quality of strain estimates in elastography is typically quantified by several quality factors such as the elastographic signal-to-noise ratio, the elastographic contrast-to-noise ratio and the spatial axial and lateral resolutions. While theoretical and simulation works have led to established upper bounds of these image quality factors in axial strain elastography, the performance limitations of lateral strain elastography, effective Poisson's ratio elastography and poroelastography are still not well understood. In this paper, we investigate the theoretical upper bounds of image quality of effective Poisson's ratio elastography starting from an analysis of the performance limitations of axial strain and lateral strain elastography.

Signal-to-noise ratio, contrast-to-noise ratio and their trade-offs with resolution in axial-shear strain elastography.

In axial-shear strain elastography, the local axial-shear strain resulting from the application of quasi-static axial compression to an inhomogeneous material is imaged. In this paper, we investigated the image quality of the axial-shear strain estimates in terms of the signal-to-noise ratio (SNR(asse)) and contrast-to-noise ratio (CNR(asse)) using simulations and experiments. Specifically, we investigated the influence of the system parameters (beamwidth, transducer element pitch and bandwidth), signal processing parameters (correlation window length and axial window shift) and mechanical parameters (Young's modulus contrast, applied axial strain) on the SNR(asse) and CNR(asse).

Resolution of axial shear strain elastography.

The technique of mapping the local axial component of the shear strain due to quasi-static axial compression is defined as axial shear strain elastography. In this paper, the spatial resolution of axial shear strain elastography is investigated through simulations, using an elastically stiff cylindrical lesion embedded in a homogeneously softer background. Resolution was defined as the smallest size of the inclusion for which the strain value at the inclusion/background interface was greater than the average of the axial shear strain values at the interface and inside the inclusion.

A new method for generating poroelastograms in noisy environments.

Poroelastography has been recently introduced as a new elastographic technique that may be used to describe the spatial and temporal behavior of poroelastic materials. The experimental methodology proposed thus far for phantoms and tissues in vitro requires the acquisition of a precompression rf frame, the application of a unit step strain compression to the sample and the acquisition of subsequent post-compression frames from the material. Elastograms and poroelastograms are generated by cross-correlating the sequentially-acquired postcompression frames with the reference precompression frame.

Noise performance and signal-to-noise ratio of shear strain elastograms.

In this paper, we develop a theoretical expression for the signal-to-noise ratio (SNR) of shear strain elastograms. The previously-developed ideas for the axial strain filter (ASF) and lateral strain filter (LSF) are extended to define the concept of the shear strain filter (SSF). Some of our theoretical results are verified using simulations and phantom experiments.

Analysis of a hybrid spectral strain estimation technique in elastography.

Conventional spectral elastographic techniques estimate strain using cross-correlation methods. Despite promising results, decorrelation effects compromise the accuracy of these techniques and, subsequently, the tissue strain estimates. Since tissue compression in the time-domain corresponds to upscaling in the frequency-domain, decorrelation effects become more pronounced as tissue strains increase and are a fundamental concern in spectral cross-correlation elastography.

Enhancing the performance of model-based elastography by incorporating additional a priori information in the modulus image reconstruction process.

Model-based elastography is fraught with problems owing to the ill-posed nature of the inverse elasticity problem. To overcome this limitation, we have recently developed a novel inversion scheme that incorporates a priori information concerning the mechanical properties of the underlying tissue structures, and the variance incurred during displacement estimation in the modulus image reconstruction process. The information was procured by employing standard strain imaging methodology, and introduced in the reconstruction process through the generalized Tikhonov approach.

Comparison of shift estimation strategies in spectral elastography.

This paper compares the performance of various spectral shift estimators for use in spectral elastography, namely, the normalized cross-correlation (NCC), sum squared difference (SSD) and sum absolute difference (SAD). Simulation and experimental results demonstrate that the spectral SSD-based elastographic method exhibits no marked difference in performance compared to the more computationally costly NCC-based approach, which has conventionally been the preferred estimator in spectral elastography. The spectral SAD-based strain estimator, despite being computationally less burdening, failed to exhibit performance comparable to that of the NCC- and SSD-based techniques.

Investigation of parametric spectral estimation techniques for elasticity imaging.

Several autoregressive (AR) and autoregressive moving average (ARMA) parametric spectral estimators were evaluated for use in tissue strain estimation. Using both 1-D simulations and in vitro phantom experiments, the performance of these parametric spectral strain estimators were compared against both a nonparametric discrete Fourier transform (DFT) spectral strain estimator and a coherent elastographic technique. Parametric spectral estimator model orders were selected based on a modified strain filter approach.

A method for generating permeability elastograms and Poisson's ratio time-constant elastograms.

The feasibility of imaging the permeability and Poisson's ratio time-constant of porous media was investigated. The study involved the following steps. First, poroelastograms were generated from porous tofu phantoms under sustained compression.

Comparative evaluation of strain-based and model-based modulus elastography.

Elastography based on strain imaging currently endures mechanical artefacts and limited contrast transfer efficiency. Solving the inverse elasticity problem (IEP) should obviate these difficulties; however, this approach to elastography is often fraught with problems because of the ill-posed nature of the IEP. The aim of the present study was to determine how the quality of modulus elastograms computed by solving the IEP compared with those produced using standard strain imaging methodology.