Strain Elastograms Research Articles

A Novel Elastographic Frame Quality Indicator and its use in Automatic Representative-Frame Selection from a Cine Loop

This study was aimed at developing a method for automatically selecting a few representative frames from several hundred axial-shear strain elastogram frames typically obtained during freehand compression elastography of the breast in vivo. This may also alleviate some inter-observer variations that arise at least partly because of differences in selection of representative frames from a cine loop for evaluation and feature extraction. In addition to the correlation coefficient and frame-average axial strain that have been previously used as quality indicators for axial strain elastograms, we incorporated the angle of compression, which has unique effects on axial-shear strain elastogram interpretation. These identified quality factors were computed for every frame in the elastographic cine loop. The algorithm identifies the section having N contiguous frames (N = 10) that possess the highest cumulative quality scores from the cine loop as the one containing representative frames. Data for total of 40 biopsy-proven malignant or benign breast lesions in vivo were part of this study. The performance of the automated algorithm was evaluated by comparing its selection against that by trained radiologists. The observer- identified frame that consisted of a sonogram, axial strain elastogram and axial-shear strain elastogram was compared with the respective images in the frames of the algorithm-identified section using cross-correlation as a similarity measure. It was observed that there was, on average (∼standard deviation), 82.2% (∼2.2%), 83.4% (∼3.8%) and 78.4% (∼3.6%) correlation between corresponding images of the observer-selected and algorithm-selected frames, respectively. The results indicate that the automatic frame selection method described here may provide an objective way to select a representative frame while saving time for the radiologist. Furthermore, the frame quality metric described and used here can be displayed in real time as feedback to guide elastographic data acquisition and for training purposes.

Ultrahigh-resolution optical coherence elastography.

Visualizing stiffness within the local tissue environment at the cellular and subcellular level promises to provide insight into the genesis and progression of disease. In this Letter, we propose ultrahigh-resolution optical coherence elastography (UHROCE), and demonstrate 3D imaging of local axial strain of tissues undergoing compressive loading. We combine optical coherence microscopy (OCM) and phase-sensitive detection of local tissue displacement to produce strain elastograms with resolution (x,y,z) of 2×2×15 μm. We demonstrate this performance on a freshly excised mouse aorta and reveal the mechanical heterogeneity of vascular smooth muscle cells and elastin sheets, otherwise unresolved in a typical, lower resolution optical coherence elastography (OCE) system.

Ultrasound elastography assessment of bone/soft tissue interface

We report on the use of elastographic imaging techniques to assess the bone/soft tissue interface, a region that has not been previously investigated but may provide important information about fracture and bone healing. The performance of axial strain elastograms and axial shear strain elastograms at the bone/soft tissue interface was studied ex vivo on intact and fractured canine and ovine tibias. Selected ex vivo results were corroborated on intact sheep tibias in vivo. The elastography results were statistically analyzed using elastographic image quality tools.The results of this study demonstrate distinct patterns in the distribution of the normalized local axial strains and axial shear strains at the bone/soft tissue interface with respect to the background soft tissue. They also show that the relative strength and distribution of the elastographic parameters change in the presence of a fracture and depend on the degree of misalignment between the fracture fragments. Thus, elastographic imaging modalities might be used in the future to obtain information regarding the integrity of bones and to assess the severity of fractures, alignment of bone fragments as well as to follow bone healing.

Influence of various breast factors on the quality of strain elastograms.

To determine the influence of the depth and size of breast lesions, the thickness of the adipose layer, and the thickness of the breast on the quality of strain elastograms obtained from Chinese women. Ultrasound elastography was conducted on 326 breast lesions in 272 women. The quality of the images was evaluated according to criteria for satisfactory and unsatisfactory elastograms. Univariate and multivariate logistic regression analyses were conducted to determine the relationship between measured factors and image quality. Among the 326 lesions, 300 (92%) had satisfactory images, and 26 (8%) had unsatisfactory images. Univariate analyses indicated that all of the data from the unsatisfactory group were higher than the data from the satisfactory group in terms of the distance from the deep and superficial edges of the lesion to the skin, the thickness of the adipose layer anterior to the lesion, and the breast thickness at the location of the lesion (all P < .05 ). Lesions in the unsatisfactory group were larger than those in the satisfactory group when the distance from the deep edge of the lesion to the skin was between 2.1 and 2.5 cm and the distance from the superficial edge of the lesion to the skin was between 1.6 and 2.3 cm (all P< .05). Multivariate logistic regression analysis revealed that both the distance from the superficial edge of the lesion to the skin (odds ratio, 18.335; P = .029) and the breast thickness at the location of the lesion (odds ratio, 20.447; P < .001) were the independent influencing factors that affected the quality of elastograms. Satisfactory images are more easily acquired if the lesion is relatively superficial or located in areas of thinner breast thickness. Furthermore, among deeper lesions, satisfactory images are more common in large lesions.

Development of a photoacoustic, ultrasound and fluorescence imaging catheter for the study of atherosclerotic plaque.

Atherosclerotic cardiovascular diseases are a major cause of death in industrialized countries. Molecular imaging modalities are increasingly recognized to be a promising avenue towards improved diagnosis and for the evaluation of new drug therapies. In this work, we present an acquisition system and associated catheter enabling simultaneous photoacoustic, ultrasound and fluorescence imaging of arteries designed for in vivo imaging. The catheter performance is evaluated in tissue-mimicking phantoms. Simultaneous imaging with three modalities is demonstrated at frame rates of 30 images per second for ultrasound and fluorescence and 1 image per 13 seconds for photoacoustic. Acquired radio-frequency ultrasound data could be processed to obtain radial strain elastograms. With motorized pullback, 3D imaging of phantoms was performed using the three modalities.

Measurement of muscle architecture concurrently with muscle hardness using ultrasound strain elastography

The B-mode ultrasound image that can measure muscle architecture is displayed side by side with the ultrasound strain elastogram that can assess muscle hardness. Consequently, muscle architecture can be measured concurrently with muscle hardness using ultrasound strain elastography. To demonstrate the measurement of muscle architecture concurrently with muscle hardness using ultrasound strain elastography. Concurrent measurements of muscle architectural parameters (muscle thickness, pennation angle, and fascicle length) and muscle hardness of the medial gastrocnemius were performed with ultrasound strain elastography. Separate measurements of the muscle architectural parameters were also performed for use as reference values for the concurrent measurements. Both types of measurements were performed twice at 20° dorsiflexion, neutral position, and 30° plantar flexion. Coefficients of variance of the muscle architectural parameters obtained from the concurrent measurements (≤7.6%) were significantly higher than those obtained from the separate measurements (≤2.4%) (all P < 0.05). Intraclass correlation coefficients of the architectural parameters were lower in the concurrent measurements (≥0.74) than in the separate measurements (≥0.97). However, there were no significant differences in any muscle architectural parameters between the concurrent and separate measurements (all P > 0.05). The use of ultrasound strain elastography for the concurrent measurement of muscle architecture and muscle hardness is feasible.

An analysis of the segmentation threshold used in axial–shear strain elastography

Axial–shear strain elastography was introduced recently to image the tumor–host tissue boundary bonding characteristics. The image depicting the axial–shear strain distribution in a tissue under axial compression was termed as an axial–shear strain elastogram (ASSE). It has been demonstrated through simulation, tissue-mimicking phantom experiments, and retrospective analysis of in vivo breast lesion data that metrics quantifying the pattern of axial–shear strain distribution on ASSE can be used as features for identifying the lesion boundary condition as loosely-bonded or firmly-bonded. Consequently, features from ASSE have been shown to have potential in non-invasive breast lesion classification into benign versus malignant. Although there appears to be a broad concurrence in the results reported by different groups, important details pertaining to the appropriate segmentation threshold needed for – (1) displaying the ASSE as a color-overlay on top of corresponding Axial Strain Elastogram (ASE) and/or sonogram for feature visualization and (2) ASSE feature extraction are not yet fully addressed. In this study, we utilize ASSE from tissue mimicking phantom (with loosely-bonded and firmly-bonded inclusions) experiments and freehand – acquired in vivo breast lesion data (7 benign and 9 malignant) to analyze the effect of segmentation threshold on ASSE feature value, specifically, the “fill-in” feature that was introduced recently. We varied the segmentation threshold from 20% to 70% (of the maximum ASSE value) for each frame of the acquisition cine-loop of every data and computed the number of ASSE pixels within the lesion that was greater than or equal to this threshold value. If at least 40% of the pixels within the lesion area crossed this segmentation threshold, the ASSE frame was considered to demonstrate a “fill-in” that would indicate a loosely-bonded lesion boundary condition (suggestive of a benign lesion). Otherwise, the ASSE frame was considered not to demonstrate a “fill-in” indicating a firmly-bonded lesion boundary condition (suggestive of a malignant lesion). The results demonstrate that in the case of in vivo breast lesion data the appropriate range for the segmentation threshold value seems to be 40–60%. It was noted that for a segmentation threshold within this range (for example, at 50%) all of the analyzed breast lesion cases can be correctly classified into benign and malignant, based on the percentage number of frames within the acquisition cine-loop that demonstrate a “fill-in”.

Analysis of image formation in optical coherence elastography using a multiphysics approach.

IMAGE FORMATION IN OPTICAL COHERENCE ELASTOGRAPHY (OCE) RESULTS FROM A COMBINATION OF TWO PROCESSES: the mechanical deformation imparted to the sample and the detection of the resulting displacement using optical coherence tomography (OCT). We present a multiphysics model of these processes, validated by simulating strain elastograms acquired using phase-sensitive compression OCE, and demonstrating close correspondence with experimental results. Using the model, we present evidence that the approximation commonly used to infer sample displacement in phase-sensitive OCE is invalidated for smaller deformations than has been previously considered, significantly affecting the measurement precision, as quantified by the displacement sensitivity and the elastogram signal-to-noise ratio. We show how the precision of OCE is affected not only by OCT shot-noise, as is usually considered, but additionally by phase decorrelation due to the sample deformation. This multiphysics model provides a general framework that could be used to compare and contrast different OCE techniques.

Method to estimate the deviation from ideal uniaxial compression during freehand elastography.

Quasi-static ultrasound elastography was introduced in the early 1990s to provide a way to visualize the mechanical properties of target tissue. Most commonly, only the axial strain is imaged and referred to as an Axial Strain Elastogram (ASE) or elastogram for simplicity. It has been shown that one can image the axial-shear strain distributions as well in addition to ASE. The image of the axial-shear strain is referred to as an axial-shear strain elastogram (ASSE). It has also been shown that the presence or absence of non-zero axial-shear strain values inside the inclusion (referred to as fill-in) along with contrasting margin at its boundary may serve as a potential feature from ASSE that can aid in non-invasive breast lesion classification. However, during freehand elastography, deviations from uniaxial compression often occur typically appearing in several of the frames of a cine-loop obtained during compression. It was shown recently that accounting for such deviations would be important for reliable interpretation of the "fill-in" observed in ASSE. In this article, we describe a method to estimate the angle of iso-displacement contour at a given depth and use this as a measure to quantify the deviation from the desired uniaxial compression during freehand elastography. We validate the estimated angle obtained from the axial-displacement map against the designed values in simulation and tissue-mimicking phantom experiments. The potential of the angle estimate to detect unreliable ASSE frames among the freehand-acquired data cine-loop is demonstrated using example cases of in vivo breast lesion data. Based on the results, we conclude that the angle of the iso-displacement contour from the axial-displacement map can be used as a metric to qualify an ASSE frame as reliable to interpret or not. Importantly, this metric can be obtained in real time and thus can provide operator feedback to guide and improve in vivo freehand elastography data acquisition quality.

Dynamic frame pairing in real-time freehand elastography.

Quasi-static ultrasound (US) elastography is now a well-established technique that involves acquiring US (RF/envelope) signals from an imaging plane before and after a small quasi-static compression to form axial strain elastograms (ASE). The image quality of the ASEs is a function of the applied axial strain. This relationship was extensively investigated and formalized in terms of strain filter in the literature. Most of the work in elastography formed elastograms by choosing pre- and post-compression frames separated by a desired compression strain. Although this approach is feasible in simulations and in vitro/in vivo experiments that involve controlled compression, it has been a challenge to do this during freehand compression in real time. In this work, we describe a one-prediction-one- correction method that dynamically selects pre- and post- compression frames to form an elastogram, based on the applied axial strain level. We validate the method using controlled compression experiments on phantoms and compare the performance of the dynamic frame pairing method against successive-frame pairing method in terms of the contrast-to-noise ratio (CNRe). Further, we demonstrate the advantages of the new method with the help of freehand acquired data from phantom experiments and in vivo breast data. The results demonstrate that the frame-pairing identified by the dynamic method matched the frame pairing that was designed to yield an applied axial strain of ~1%. The CNRe obtained by the traditional approach varied from as low as ~5 to as high as ~25, depending on the choice of skip number and compression rate. However, the dynamic frame pairing method provided elastograms with a CNRe that was consistently around ~20, irrespective of the compression rate. The results from analysis of 22 in vivo breast data demonstrated that the dynamic pairing method generated elastograms such that the frame-average axial strain (FAAS) of each frame in the cine-loop is consistently ~1% (0.011 ± 0.001).

Can quantitative synthetic aperture vascular elastography predict the stress distribution within the fibrous cap non-invasively

An imaging system that can detect and predict the propensity of an atherosclerotic plaque to rupture would reduce stroke. Radial and circumferential strain elastograms can reveal vulnerable regions within the fibrous cap. Circumferential stress imaging could predict the propensity of rupture. However, circumferential stress imaging demands either accurate knowledge of the geometric location of the fibrous cap or high quality strain information. We corroborated this hypothesis by performing studies on simulated vessel phantoms. More precisely, we computed stress elastograms with (1) precise knowledge of the fibrous cap, (2) no knowledge of the fibrous cap, (3) imprecise knowledge of the fibrous cap. We computed stress elastograms with accuracy of 8%, 15%, and 23% from high precision axial and lateral strain elastograms (i.e., 25 dB SNR) when precise, imprecise, and no geometric information was included the stress recovery method. The stress recovery method produced erroneous elastograms at lower noise level (i.e., 15 dB SNR), when no geometric information was included. Similarly, it produced elastograms with accuracy of 13% and 30% when precise and imprecise geometric information was included. The stress imaging method described in this paper performs well enough to warrant further studies with phantoms and ex-vivo samples.

Estimation of shear modulus ratio between inclusion and background using strain ratios in 2-D ultrasound elastography

The goal of this study is to assess the effects of region of interest (ROI) selection and lesion size on estimates of shear modulus ratio from strain ratios to quantify relative stiffness of breast tumors. A theoretical model and finite element method (FEM) simulations of lesions with various shear modulus ratios are created for a 2-D plane strain deformation. Both the lesion and the surrounding tissue are assumed to be linearly elastic, isotropic, homogenous, and incompressible. The results from the model and simulations are in agreement that the lesion-to-surrounding shear modulus ratio is linearly proportional to the axial normal strain ratio for small lesions when the ROI in the surrounding tissue is at least four lesion diameters away from the lesion. For larger lesions, FEM simulations show that the estimated strain ratio using the same ROI location increases with the lesion size and would overestimate the shear modulus ratio. Therefore, a correction factor is necessary for large breast lesions when strain ratios are used to estimate the shear modulus ratio. We also demonstrate that strain elastograms calculated using a speckle tracking method on simulated RF data are accurate enough to observe the same effect on strain ratio estimation. This result is confirmed using experimental data acquired from two tissue-mimicking phantoms. Our findings will help clinicians to estimate strain ratios and shear modulus ratios more accurately for more reliable comparison of one clinical examination to another.

Endovascular Shear Strain Elastography for the Detection and Characterization of the Severity of Atherosclerotic Plaques: In Vitro Validation and In Vivo Evaluation

This work explores the potential of shear strain elastograms to identify vulnerable atherosclerotic plaques. The Lagrangian speckle model estimator (LSME) elasticity imaging method was further developed to estimate shear strain elasticity (SSE). Three polyvinyl alcohol cryogel vessel phantoms were imaged with an intravascular ultrasound (IVUS) scanner. The estimated SSE maps were validated against finite-element results. Atherosclerosis was induced in carotid arteries of eight Sinclair mini-pigs using a combination of surgical techniques, diabetes and a high-fat diet. IVUS images were acquired in vivo in 14 plaques before euthanasia and histology. All plaques were characterized by high magnitudes in SSE maps that correlated with American Heart Association atherosclerosis stage classifications (r = 0.97, p < 0.001): the worse the plaque condition the higher was the absolute value of SSE, i.e. |SSE| (e.g., mean |SSE| was 3.70 ± 0.40% in Type V plaques, whereas it was reduced to 0.11 ± 0.01% in normal walls). This study indicates the feasibility of using SSE to highlight atherosclerotic plaque vulnerability characteristics.

Real-Time Monitoring of High-Intensity Focused Ultrasound Treatment Using Axial Strain and Axial-Shear Strain Elastograms

Axial strain elastograms (ASEs) have been found to help visualize sonographically invisible thermal lesions. However, in most studies involving high-intensity focused ultrasound (HIFU)-induced thermal lesions, elastography imaging was performed separately later, after the lesion was formed. In this article, the feasibility of monitoring, in real time, tissue elasticity variation during HIFU treatment and immediately thereafter is explored using quasi-static elastography. Further, in addition to ASEs, we also explore the use of simultaneously acquired axial-shear strain elastograms (ASSEs) for HIFU lesion visualization. Experiments were performed on commercial porcine liver samples in vitro. The HIFU experiments were conducted at two applied acoustic power settings, 35 and 20 W. The experimental setup allowed us to interrupt the HIFU pulse momentarily several different times during treatment to perform elastographic compression and data acquisition. At the end of the experiments, the samples were cut along the imaging plane and photographed to compare size and location of the formed lesion with those visualized on ASEs and ASSEs. Single-lesion and multiple-lesion experiments were performed to assess the contribution of ASEs and ASSEs to lesion visualization and treatment monitoring tasks. At both power settings, ASEs and ASSEs provided accurate location information during HIFU treatment. At the low-power setting case, ASEs and ASSEs provide accurate lesion size in real-time monitoring. Lesion appearance in ASEs and ASSEs was affected by the cavitation bubbles produced at the high-power setting. The results further indicate that the cavitation bubbles influence lesion appearance more in ASEs than in ASSEs. Both ASEs and ASSEs provided accurate size information after a waiting period that allowed the cavitation bubbles to disappear. The results indicate that ASSEs not only improve lesion visualization and size measurement of a single lesion, but, under certain conditions, also help to identify untreated gaps between adjacent lesions with high contrast.

Digital image correlation–based optical coherence elastography

Optical coherence elastography (OCE) provides deformation or material properties, mapping of soft tissue. We aim to develop a robust speckle tracking OCE technique with improved resolution and accuracy. A digital image correlation (DIC)-based OCE technique was developed by combining an advanced DIC algorithm with optical coherence tomography (OCT). System calibration and measurement error evaluation demonstrated that this DIC-based OCE technique had a resolution of ~0.6 μm displacement and <0.5% strain measurement in the axial scan direction. The measured displacement ranged from 0.6 to 150 μm, obtained via phantom imaging. The capability of the DIC-based OCE technique, for differentiation of stiffness, was evaluated by imaging a candle gel phantom with an irregularly shaped stiff inclusion. OCE imaging of a chicken breast sample differentiated the fat, membrane, and muscle layers. Strain elastograms of an aneurysm sample showed heterogeneity of the tissue and clear contrast between the adventitia and media. These promising results demonstrated the capability of the DIC-based OCE for the characterization of the various components of the tissue sample. Further improvement of the system will be conducted to make this OCE technique a practical tool for measuring and differentiating material properties of soft tissue.

Noninvasive vascular elastography using plane-wave and sparse-array imaging

Stroke may occur when an atherosclerotic plaque ruptures in the carotid artery. Noninvasive vascular elastography (NIVE) visualizes the strain distribution within the carotid artery, which is related to its mechanical properties that govern plaque rupture. Strain elastograms obtained from the transverse plane of the carotid artery are difficult to interpret, because strain is estimated in Cartesian coordinates. Sparsearray (SA) elastography overcomes this problem by transforming shear and normal strain to polar coordinates. However, the SA's transmit power may be too weak to produce useful elastograms in the clinical setting. Consequently, we are exploring other imaging methods to solve this potential problem. This study evaluated the quality of elastograms produced with SA imaging, plane-wave (PW) imaging, and compounded-plane-wave (CPW) imaging. We performed studies on simulated and physical vessel phantoms, and the carotid artery of a healthy volunteer. All echo imaging was performed with a linear transducer array that contained 128 elements, operating at 5 MHz. In SA imaging, 7 elements were fired during transmission, but all 128 elements were active during reception. In PW imaging, all 128 elements were active during both transmission and reception. We created CPW images by steering the acoustic beam within the range of -15° to 15° in increments of 5°. SA radial and circumferential strain elastograms were comparable to those produced using PW and CPW imaging. Additionally, side-lobe levels incurred during SA imaging were 20 dB lower than those produced during PW imaging, and 10 dB lower than those computed using CPW imaging. Overall, SA imaging performs well in vivo; therefore, we plan to improve the technique and perform preclinical studies.

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. Recently, we investigated and reported on the potential of the NASSA feature for breast lesion classification into fibroadenomas and cancers. In this article, we investigate the size distribution of the lesions that were part of the previous study and analyze classification performance specifically on small lesions (<10 mm diameter). A total of 33 biopsy-proven malignant tumors and 30 fibroadenomas were part of the study that involved three observers blinded to the Breast Imaging Reporting and Data System (BIRADS) ultrasound scores. The observers outlined the lesions on the sonograms and the lesion size (maximum circle-equivalent diameter in millimeters) was computed from this outline. The ASSE was automatically segmented and color-overlaid on the sonogram, and the NASSA feature from ASSE was computed semi-automatically. Receiver operating characteristic curves were then generated for the subset of cases involving small lesions. Box plots were produced for the two different lesion size groups, small and large, from a logistic regression classifier that was built previously. The results of our study show that approximately 38% and 22% of the fibroadenomas and cancers, respectively, were small. Furthermore, it was found that the NASSA feature resulted in a perfect classification of the small lesions, both in the training data and in the cross-validation. For lesions <10 mm the difference in fibroadenoma and cancer mean scores was 0.73 ± 0.13 (p < 0.001), whereas lesions >10 mm had a difference of 0.52 ± 0.24 (p < 0.001). The results also showed that the small lesions actually had better classification than the larger lesions (>10 mm). These results suggest that the ASSE feature can work equally well, even on small lesions, to improve the standard ultrasound BIRADS–based breast lesion classification of fibroadenoma and malignant tumors.

Non-Rigid Image Registration Based Strain Estimator for Intravascular Ultrasound Elastography

Intravascular ultrasound elastography (IVUSe) could improve the diagnosis of cardiovascular disease by revealing vulnerable plaques through their mechanical tissue properties. To improve the performance of IVUSe, we developed and implemented a non-rigid image-registration method to visualize the radial and circumferential component of strain within vascular tissues. We evaluated the algorithm's performance with four initialization schemes using simulated and experimentally acquired ultrasound images. Applying the registration method to radio-frequency (RF) echo frames improved the accuracy of displacements compared to when B-mode images were employed. However, strain elastograms measured from RF echo frames produce erroneous results when both the zero-initialization method and the mesh-refinement scheme were employed. For most strain levels, the cross-correlation-initialization method produced the best performance. The simulation study predicted that elastograms obtained from vessels with average strains in the range of 3%–5% should have high elastographic signal-to-noise ratio (SNRe)–on the order of 4.5 and 7.5 for the radial and circumferential components of strain, respectively. The preliminary in vivo validation study (phantom and an atherosclerotic rabbit) demonstrated that the non-rigid registration method could produce useful radial and circumferential strain elastograms under realistic physiologic conditions. The results of this investigation were sufficiently encouraging to warrant a more comprehensive in vivo validation.

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). Nevertheless, there has been a sustained effort in developing methods to improve the lateral motion tracking accuracy and thereby obtain better quality total shear strain elastogram (TSSE). We hypothesize that in some cases, even if good quality TSSE becomes possible, it may still be advantageous to utilize only the axial-shear strain (one of the components of the total shear strain) elastogram (ASSE). Specifically, we show through FEM and corroborating tissue-mimicking gelatin phantom experiments that the unique “fill-in” discriminant feature that was introduced recently for asymmetric breast lesion classification is depicted only in the ASSE and not in the TSSE. Note that the presence or conspicuous absence of this feature in ASSE was shown to characterize asymmetric inclusions' boundaries as either loosely-bonded or firmly-bonded to the surrounding, respectively. This might be an important observation because the literature suggests that benign breast lesions tend to be loosely-bonded, while malignant tumors are usually firmly-bonded. The results from the current study demonstrate that the use of shear strain lesion “fill-in” as a discriminant feature in the differentiation between asymmetric malignant and benign breast lesions is only possible when using the ASSEs and not the TSSEs.

Visualizing the radial and circumferential strain distribution within vessel phantoms using synthetic-aperture ultrasound elastography

Noninvasive elastography (NIVE) produces elastograms that are difficult to interpret because NIVE visualizes strain in the transducer coordinate system. In this paper, we hypothesized that transforming normal and shear strain elastograms to the vessel coordinate system will produce better strain elastograms. To corroborate this hypothesis, we acquired synthetic-aperture (SA) ultrasound data from simulated and physical vessel phantoms. In both studies, SA echo frames were reconstructed from data acquired with a sparse transducer array. The simulation study was performed with homogeneous and heterogenous phantoms, but in the experimental study we used a modified ultrasound scanner to acquire SA data from homogeneous (n = 1) and heterogeneous (n = 3) vessel phantoms. Axial and lateral displacements were estimated by performing two-dimensional cross-correlation analysis on the beamformed RF echo frames. We generated radial and circumferential strain elastograms by transforming normal and shear strain elastograms to the vessel coordinate system. The results revealed: 1) radial and circumferential strain elastograms acquired from simulated data had a relative root mean squared error on the order of 0.1%; 2) experimentally acquired radial and circumferential strain elastograms had elastographic contrast-to-noise ratio (CNRe) between 10 and 40 dB, and elastographic signal-to-noise ratio (SNRe) between 10 and 35 dB, depending on the number of active transmission elements employed during imaging; 3) radial and circumferential strain elastograms produced with fewer than 8 active transmission elements were inferior to those computed with a greater number of active elements; and 4) plaques were evident in the strain elastograms, except in those obtained with the sparsest transducer array. This study demonstrated that a syntheticaperture ultrasound system could visualize radial and circumferential strain noninvasively.