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ABSTRACT: Three-dimensional impedance maps (3DZMs) are virtual volumes of acoustic impedance values constructed from histology to represent tissue microstructure acoustically. From the 3DZM, the ultrasonic backscattered power spectrum can be predicted and model based scatterer properties, such as effective scatterer diameter (ESD), can be estimated. Additionally, the 3DZM can be exploited to visualize and identify possible scattering sites, which may aid in the development of more effective scattering models to better represent the ultrasonic interaction with underlying tissue microstructure. In this study, 3DZMs were created from a set of human fibroadenoma samples. ESD estimates were made assuming a fluid-filled sphere form factor model from 3DZMs of volume 300×300×300 μm. For a collection of 33 independent human fibroadenoma tissue samples, the ESD was estimated to be 111±40.7 μm. The 3DZMs were then investigated visually to identify possible scattering sources which conformed to the estimated model scatterer dimensions. This estimation technique allowed a better understanding of the spatial distribution and variability of the estimates throughout the volume.
IEEE Transactions on Medical Imaging 07/2011; · 3.64 Impact Factor
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ABSTRACT: A hybrid approach that inherits both the robustness of the regularized motion tracking approach and the efficiency of the predictive search approach is reported. The basic idea is to use regularized speckle tracking to obtain high-quality seeds in an explorative search that can be used in the subsequent intelligent predictive search. The performance of the hybrid speckle-tracking algorithm was compared with three published speckle-tracking methods using in vivo breast lesion data. We found that the hybrid algorithm provided higher displacement quality metric values, lower root mean squared errors compared with a locally smoothed displacement field, and higher improvement ratios compared with the classic block-matching algorithm. On the basis of these comparisons, we concluded that the hybrid method can further enhance the accuracy of speckle tracking compared with its real-time counterparts, at the expense of slightly higher computational demands.
IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 05/2011; · 1.69 Impact Factor
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ABSTRACT: In vivo attenuation slope measurements usually utilize the backscattered signal from pulse/echo ultrasound. In this work the down shift of the center frequency of an emitted ultrasound pulse with penetration depth is utilized to estimate the attenuation slope. A diffraction correction of the focused ultrasound source is performed by measuring the reflection from a planar surface positioned throughout the depth of focus. A focused single element transducer with a measured center frequency of 8.2 MHz and a fractional band width of 72% was used to interrogate four tissue mimicking phantoms. The scatterers in the tissue mimicking phantoms were glass spheres embedded in a gelatin/milk matrix. In one set of the phantoms, the backscattering strength was varied; in the other set of phantoms the attenuation slope was varied. The attenuation slope (AS<sub>BS</sub>) was estimated using pulse/echo data obtained by scanning the phantoms. The ¿true¿ attenuation slope (AS<sub>Thru</sub>) was obtained from two independent insertion loss measurements performed at two different laboratories. The relative error of AS<sub>BS</sub> was investigated for different regions of interest (ROI) for all phantoms. Three different axial and lateral ROI sizes were tested. It was observed that the average relative error (average over all four phantoms) changed by less than three percent when the lateral size of the ROI was decreased by seventy percent. The axial size of the ROI was changed by thirty percent whereas the average error changed by less then three percent.
Ultrasonics Symposium (IUS), 2009 IEEE International; 10/2009
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ABSTRACT: Three-dimensional impedance maps (3DZMs) are virtual volumes of acoustic impedance values constructed from histology to represent tissue microstructure acoustically. From the 3DZM, estimations can be made for ultrasonic backscatter and scatterer properties, such as effective scatterer diameter (ESD). Additionally, the 3DZM can be exploited to visualize and identify possible scattering sites, which may aid in the development of more effective scattering models to better represent the ultrasonic interaction with underlying tissue microstructure. In this study, 3DZMs were created from several human fibroadenoma samples. ESD estimates were obtained using the fluid-filled sphere form factor model. These estimates were made using two regions of interest (ROIs) sizes: cubes of side length 300 ¿m and 150 ¿m. This estimation technique allowed a better understanding of the spatial distribution and variability of the estimates throughout the volume. For a collection of 33 3DZMs, the ESD was estimated to be 99±43 ¿m with the large ROI and 65±30 ¿m when using the small ROI. The 3DZMs were then investigated visually to identify possible scattering sources, which conformed to the estimated characteristic scatterer dimensions. This visualization and comparison resulted in the identification of possible ultrasonic scattering sources within human fibroadenomas.
Ultrasonics Symposium (IUS), 2009 IEEE International; 10/2009
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ABSTRACT: Quantitative ultrasound (QUS) imaging is a model-based approach aimed at lesion detection and classification. In this study, the RF backscattered signals from rat fibroadenomas were fit to various mathematical models to yield effective scatterer diameter (ESD) estimates which are tied to tissue microstructure. The goal of these experiments was to understand potential sources of scattering in live tissue across a wide frequency range and how results from different models compare to one another.
Ultrasonics Symposium (IUS), 2009 IEEE International; 10/2009
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ABSTRACT: Radiofrequency (RF) ablation is the most common minimally invasive therapy used in the United States to treat primary (hepatocellular carcinoma) and metastatic liver cancers. The ability to accurately evaluate the thermal ablation zone while a patient is undergoing radiofrequency ablation may help reduce the high recurrence rates (as high as 55%) following radiofrequency ablation therapy. In this paper we demonstrate the feasibility of performing ultrasound-based elastic modulus imaging (EMI) to evaluate thermal ablation zones in a porcine model. A total of 14 in vivo RF and microwave ablation zones (1-3 cm in diameter) created in 5 porcine animals with normal livers were studied. After open-abdominal ablation, RF ultrasound echo data were acquired under the guidance of a real-time strain imaging system. Imaging results of ablation areas obtained by the EMI and strain imaging methods were then compared with gross pathology section obtained in the corresponding imaging planes. When comparing ablation area measurements, EMI had higher correlation with gross pathology measurements than strain imaging (EMI Pearson coefficient = 0.95, p<0.0001; strain Pearson coefficient = 0.85, p<0.0001). Furthermore, the EMI method improved contrast-to-noise ratios by approximately a factor of 2 compared to strain images. These preliminary results support the hypothesis that EMI might potentially enhance the ability to visualize thermal ablation zones, thereby improving assessment of ablative therapies. Our future work will be directed to test the EMI method in tumor-bearing animal models and to rigorously validate our results with histology in these models.
Ultrasonics Symposium (IUS), 2009 IEEE International; 10/2009
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ABSTRACT: Previous work has demonstrated improved diagnostic performance of highly trained breast radiologists when provided with B-mode plus elastography images over B-mode images alone. In those studies we have observed that elasticity imaging can be difficult to perform if there is substantial motion of tissue out of the image plane. So we are extending our methods to 3D/4D elasticity imaging with 2D arrays. Further, we have also documented the fact that some breast tumors change contrast with increasing deformation and those observations are consistent with in vitro tissue measurements. Hence, we are investigating imaging tissue stress-strain nonlinearity. These studies will require relatively large tissue deformations (e.g., > 20%) which will induce out of plane motion further justifying 3D/4D motion tracking. To further enhance our efforts, we have begun testing the ability to perform modulus reconstructions (absolute elastic parameter) imaging of in vivo breast tissues. The reconstructions are based on high quality 2D displacement estimates from strain imaging. Piecewise linear (secant) modulus reconstructions demonstrate the changes in elasticity image contrast seen in strain images but, unlike the strain images, the contrast in the modulus images approximates the absolute modulus contrast. Nonlinear reconstructions assume a reasonable approximation to the underlying constitutive relations for the tissue and provide images of the (near) zero-strain shear modulus and a nonlinearity parameter that describes the rate of tissue stiffening with increased deformation. Limited data from clinical trials are consistent with in vitro measurements of elastic properties of tissue samples and suggest that the nonlinearity of invasive ductal carcinoma exceeds that of fibroadenoma and might be useful for improving diagnostic specificity. This work is being extended to 3D.
Engineering in Medicine and Biology Society, 2009. EMBC 2009. Annual International Conference of the IEEE; 10/2009
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ABSTRACT: Radio-frequency (RF) ablation is a minimally invasive treatment for tumors in various abdominal organs. It is effective if good tumor localization and intraprocedural monitoring can be done. In this paper, we investigate the feasibility of using an ultrasound-based Young's modulus reconstruction algorithm to image an ablated region whose stiffness is elevated due to tissue coagulation. To obtain controllable tissue deformations for abdominal organs during and/or intermediately after the RF ablation, the proposed modulus imaging method is specifically designed for using tissue deformation fields induced by the RF electrode. We have developed a new scheme under which the reconstruction problem is simplified to a 2-D problem. Based on this scheme, an iterative Young's modulus reconstruction technique with edge-preserving regularization was developed to estimate the Young's modulus distribution. The method was tested in experiments using a tissue-mimicking phantom and on ex vivo bovine liver tissues. Our preliminary results suggest that high contrast modulus images can be successfully reconstructed. In both experiments, the geometries of the reconstructed modulus images of thermal ablation zones match well with the phantom design and the gross pathology image, respectively.
IEEE Transactions on Medical Imaging 09/2009; · 3.64 Impact Factor
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ABSTRACT: Compared to conventional piezoelectric transducers, new capacitive microfabricated ultrasonic transducer (CMUT) technology is expected to offer a broader bandwidth, higher resolution and advanced 3D/4D imaging inherent in a 2D array. For ultrasound scatterer size imaging, a broader frequency range provides more information on frequency-dependent backscatter, and therefore, generally more accurate size estimates. Elevational compounding, which can significantly reduce the large statistical fluctuations associated with parametric imaging, becomes readily available with a 2D array. In this work, we show phantom and in vivo breast tumor scatterer size image results using a prototype 2D CMUT transducer (9 MHz center frequency) attached to a clinical scanner. A uniform phantom with two 1 cm diameter spherical inclusions of slightly smaller scatterer size was submerged in oil and scanned by both the 2D CMUT and a conventional piezoelectric linear array transducer. The attenuation and scatterer sizes of the sample were estimated using a reference phantom method. RF correlation analysis was performed using the data acquired by both transducers. The 2D CMUT results indicate that at a 2 cm depth (near the transmit focus for both transducers) the correlation coefficient reduced to less than 1/e for 0.2 mm lateral or 0.25 mm elevational separation between acoustic scanlines. For the conventional array this level of decorrelation requires a 0.3 mm lateral or 0.75 mm elevational translation. Angular and/or elevational compounding is used to reduce the variance of scatterer size estimates. The 2D array transducer acquired RF signals from 140 planes over a 2.8 cm elevational direction. If no elevational compounding is used, the fractional standard deviation of the size estimates is about 12% of the mean size estimate for both the spherical inclusion and the background. Elevational compounding of 11 adjacent planes reduces it to 7% for both media. Using an experimentally estimated attenuation of 0.6 dB cm(-1) MHz(-1), scatterer size estimates for an in vivo breast tumor also demonstrate improvements using elevational compounding with data from the 2D CMUT transducer.
Physics in Medicine and Biology 08/2008; 53(15):4169-83. · 2.83 Impact Factor
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ABSTRACT: This study compares 3D strain volumes formed from RF echo data that was acquired using the same phantom with a commercially available C7F2 mechanically rocked array (MRA) operating at 6.15 MHz and a prototype 2-D capacitive micromachined ultrasound transducer (CMUT) operating at 9 MHz. Multiple motion tracking techniques were used in both cases to compare resulting strain images. The different tracking methods produced images with varying degrees of quality. Normalized cross correlation between the motion-compensated deformed RF data and the reference RF data was used as a measure of the accuracy of the motion tracking. The contrast to noise ratio of the inclusion and background were used as a measure of strain image quality.
Ultrasonics Symposium, 2007. IEEE; 11/2007
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ABSTRACT: We have begun testing the ability to perform modulus reconstructions (absolute elastic parameter) imaging of in vivo breast tissues. The reconstructions are based on high quality 2D displacement estimates from strain imaging. Piecewise linear (secant) modulus reconstructions demonstrate the changes in elasticity image contrast seen in strain images but, unlike the strain images, the contrast in the modulus images approximates the absolute modulus contrast. Nonlinear reconstructions assume a reasonable approximation to the underlying constitutive relations for the tissue and provide images of the (near) zero-strain shear modulus and a nonlinearity parameter that describes the rate of tissue stiffening with increased deformation. Limited data from clinical trials are consistent with in vitro measurements of elastic properties of tissue samples and suggest that the nonlinearity of invasive ductal carcinoma exceeds that of fibroadenoma and might be useful for improving diagnostic specificity.
Ultrasonics Symposium, 2007. IEEE; 11/2007
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ABSTRACT: Because ablation therapy alters the elastic modulus of tissues, emerging strain imaging methods may enable clinicians for the first time to have readily available, cost effective, real-time guidance to identify the location and boundaries of thermal lesions. Electrode displacement elastography is a method of strain imaging tailored specifically to ultrasound-guided electrode-based ablative therapies (eg. radiofrequency or microwave ablation). Here tissue deformation is achieved by applying minute perturbations to the unconstrained end of the treatment electrode, resulting in localized motion around the end of the electrode embedded in tissue. In this paper, we present a method for 3D elastographic reconstruction from volumetric data acquired using the C7F2 fourSight 4D ultrasound transducer, provided by Siemens Medical Solutions. Lesion reconstruction is demonstrated for a spherical inclusion centered in a tissue-mimicking phantom, which simulates a thermal lesion embedded in a normal tissue background. Elastographic reconstruction is also performed for a thermal lesion created in vitro in canine liver using radiofrequency ablation. Post-processing is done on the 2D strain images to form surface-rendered 3D elastograms of the inclusion. Elastographic volume estimates of the inclusion compare reasonably well with the actual known inclusion volume, with 3D electrode displacement elastography slightly underestimating the true inclusion volume.
Ultrasonics Symposium, 2007. IEEE; 11/2007
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ABSTRACT: Ultrasound-based mechanical strain imaging systems utilize signals from conventional diagnostic ultrasound systems to image tissue elasticity contrast that provides new diagnostically valuable information. Previous works (Hall et al 2003 Ultrasound Med. Biol. 29 427, Zhu and Hall 2002 Ultrason. Imaging 24 161) demonstrated that uniaxial deformation with minimal elevation motion is preferred for breast strain imaging and real-time strain image feedback to operators is important to accomplish this goal. The work reported here enhances the real-time speckle tracking algorithm with two significant modifications. One fundamental change is that the proposed algorithm is a column-based algorithm (a column is defined by a line of data parallel to the ultrasound beam direction, i.e. an A-line), as opposed to a row-based algorithm (a row is defined by a line of data perpendicular to the ultrasound beam direction). Then, displacement estimates from its adjacent columns provide good guidance for motion tracking in a significantly reduced search region to reduce computational cost. Consequently, the process of displacement estimation can be naturally split into at least two separated tasks, computed in parallel, propagating outward from the center of the region of interest (ROI). The proposed algorithm has been implemented and optimized in a Windows system as a stand-alone ANSI C++ program. Results of preliminary tests, using numerical and tissue-mimicking phantoms, and in vivo tissue data, suggest that high contrast strain images can be consistently obtained with frame rates (10 frames s(-1)) that exceed our previous methods.
Physics in Medicine and Biology 08/2007; 52(13):3773-90. · 2.83 Impact Factor
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ABSTRACT: We are objectively investigating lesion contrast with ultrasound strain imaging of in vivo solid breast lesions. Tissue deformation is performed during real-time strain imaging. A modified block matching algorithm is used to estimate the displacement between frames of radiofrequency (RF) echo data and the gradient of the displacements are computed to form strain images. Displacement estimates from strain image formation are used to compensate for motion, lesion segmentation is performed on each stationary strain image, and strain image contrast is calculated as a function of accumulated strain. Strain image contrast of fibroadenomas are compared to strain image contrast of invasive ductal carcinomas to determine if strain image contrast can aide in distinguishing between most common benign and malignant solid breast lesions
Ultrasonics Symposium, 2006. IEEE; 11/2006
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ABSTRACT: Ultrasonic strain imaging systems are gaining rapid attention for breast tumor differentiation, despite the fact that consistently obtaining high quality in vivo strain images is a persistent challenge. To enhance the clinical usability of such systems, much effort has been devoted to developing more sophisticated motion tracking algorithms. This study takes an alternate route to investigate a new strain formation scheme for improving in vivo strain image quality. This method is a retrospective processing technique that is not restricted to a particular motion tracking algorithm. A block-matching algorithm was used in this study for our convenience. Radiofrequency (RF) echo data acquired from a Siemens Elegra with freehand scanning of in vivo breast tissue were used to validate this method. Through processing of in vivo breast tissue data (7 data sets with different types of lesions and roughly 700 RF echo frames in total), our findings demonstrate that higher quality strain images can be obtained through the proposed retrospective pairing technique
Ultrasonics Symposium, 2006. IEEE; 11/2006
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ABSTRACT: This study developed a robust algorithm for realtime ultrasonic strain imaging that employs dynamic programming techniques. Since tissue motion under external mechanical stimuli should be highly continuous, the speckle tracking problem fits well within the general framework of the hidden Markov model and can be solved as an optimization problem. A cost function combining correlation and motion continuity was used to regularize motion tracking. We found that the new algorithm provides more accurate displacement estimates than our previous algorithm for in vivo clinical data. In particular, the new algorithm is capable of tracking larger frame-average tissue deformation (1-2%) and increasing strain image consistency in a sequence of images. The new algorithm can also tolerate larger local strain (approx. 10%). Preliminary results also suggest that a significantly longer sequence of high contrast strain images (e.g. 45 vs. 15 in one cancer dataset) could be obtained with the new algorithm compared to the previous algorithm. We also achieved more than 10 frames/second with a 3 cm by 3 cm region of interest, which is sufficient to provide real-time feedback during in vivo elasticity imaging
Ultrasonics Symposium, 2006. IEEE; 11/2006
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ABSTRACT: In a previous study, theoretical expressions were derived for the correlation between ultrasonic scatterer-size estimates and their associated spectral measures when echo data are acquired from the same location but at different angles. The results were verified using simulations. In the present work, we further analyze simulation data for these conditions; in addition, we measure the correlations using a cylindrical tissue-mimicking phantom. Experimental and theoretical results show that the relationship of scatterer-size correlation to insonification angle depends on gate duration, gate type and beam profile. Some discrepancies are noted between experimental results and theoretical predictions, particularly when using smaller gated windows. The sources of the discrepancies are discussed. Experimental results using a 6-MHz linear array suggest that, to save acquisition and processing time while reducing variance, a 2 degree-3 degree angular increment step provides efficient angular compounding for scatterer-size imaging with this array. Theoretical predictions can provide estimates of expected correlations between angular acquisitions when compounding with other transducers.
Ultrasonic Imaging 11/2006; 28(4):230-44. · 1.54 Impact Factor
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ABSTRACT: Ultrasonic strain imaging that uses signals from conventional diagnostic ultrasound systems is capable of showing the contrast of tissue elasticity, which provides new diagnostically valuable information. To assess and improve the diagnostic performance of ultrasonic strain imaging, it is essential to have a quantitative measure of image quality. Moreover, it is useful if the image quality measure is simple to interpret and can be used for visual feedback while scanning and as a training tool for operator performance evaluation. This report describes the development of a novel quantitative method for systematic performance assessment that is based on the combination of measures of the accuracy of motion tracking and consistency among consecutive strain fields. The accuracy of motion tracking assesses the reliability of strain images. The consistency among consecutive strain images assesses the signal quality in strain images. The clinical implications of the proposed method to differentiate good or poor strain images are discussed. Results of experiments with tissue-mimicking phantoms and in vivo breast-tissue data demonstrate that the performance measure is a useful method for automatically rating elasticity image quality.
IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 07/2006; · 1.69 Impact Factor
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ABSTRACT: We are developing a method for imaging the elastic properties of tissue using unmodified clinical equipment and techniques that are similar to standard clinical exams. Our work with in vivo data from human subjects suggests that elasticity imaging provides new diagnostically significant information. For example, we can observe a nonlinear stress-strain relationship among tissues. Both the accuracy and variance of the displacement estimates must be understood to verify that observation. A significant body of work in algorithm development, computer simulation and phantom experiments precedes this effort. Much of that work addressed the variance in 1-D displacement estimates. The displacement estimate variance for a two-dimensional (2D) search with a 2-D data kernel can be adequately studied using simulated echo data. The accuracy of displacement estimates when the true displacement is unknown, as with biological tissue experiments, is more difficult to mimic and is studied using data acquired from in vivo breast imaging. Methods to reduce displacement estimate variance and verify displacement estimate accuracy are presented.
Biomedical Imaging: Nano to Macro, 2004. IEEE International Symposium on; 05/2004
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ABSTRACT: We are developing a clinical ultrasonic imaging system for real-time estimation and display of tissue elastic properties. We have demonstrated that real-time feedback of elasticity images is essential for obtaining high-quality data (consecutive images with high spatial coherence). The key element to successful scanning is real-time visual feedback which guides the patient positioning and compression direction. Our data have clearly demonstrated nonlinearity in the strain properties of different tissue types. We have also demonstrated that a comparison of the area of a breast lesion observed in strain images versus B-mode images is a sensitive criterion for differentiating malignant from benign tumors. Frame-to-frame variability in strain images somewhat degrades the ability to observe these phenomena. Three strategies for reducing frame-to-frame strain image noise are described. The combination of these post-processing strategies provides a significant improvement in the quality of long sequences of strain images.
Ultrasonics Symposium, 2002. Proceedings. 2002 IEEE; 11/2002
