Contrast-transfer efficiency for continuously varying tissue moduli: simulation and phantom validation.
TL;DR: The contrast-transfer efficiency (CTE) in elastography was extended to account for continuous changes of modulus distribution and it was shown that, for a finite size background, the strain contrast approaches the modulus contrast in the case of Gaussian distributions.
Abstract: This study consisted of two parts. In the first part, the contrast-transfer efficiency (CTE) in elastography was extended to account for continuous changes of modulus distribution. It was shown that, for a finite size background, the strain contrast approaches the modulus contrast in the case of Gaussian distributions. Thus, an increase in the CTE was obtained. For a fixed background size, it was shown that the CTE increases as the SD of the Gaussian distribution increases. This property was explained by the redistribution of strain concentrations at the inclusion/background interface. In the second part of the study, the CTE was verified experimentally. Six gelatin/agar/water-based phantoms embedding inclusions with modulus contrast varying between ± 6 dB were manufactured. It was shown that the modulus at the interface inclusion/background was continuous and, in turn, resulted in an increase of the CTE as compared to the known case of a sharp boundary. The continuous inclusion/background interface was explained by the existence of an osmotic pressure gradient. (E-mail: Faouzi.Kallel@uth.tmc.edu)
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TL;DR: Methods have been developed that utilize impulsive radiation force excitations, and ARFI images have spatial resolution comparable to that of B-mode, often with greater contrast, providing matched, adjunctive information, and SWEI images provide quantitative information about the tissue stiffness, typically with lower spatial resolution.
Abstract: Acoustic radiation force based elasticity imaging methods are under investigation by many groups. These methods differ from traditional ultrasonic elasticity imaging methods in that they do not require compression of the transducer, and are thus expected to be less operator dependent. Methods have been developed that utilize impulsive (i.e. < 1 ms), harmonic (pulsed), and steady state radiation force excitations. The work discussed herein utilizes impulsive methods, for which two imaging approaches have been pursued: 1) monitoring the tissue response within the radiation force region of excitation (ROE) and generating images of relative differences in tissue stiffness (Acoustic Radiation Force Impulse (ARFI) imaging); and 2) monitoring the speed of shear wave propagation away from the ROE to quantify tissue stiffness (Shear Wave Elasticity Imaging (SWEI)). For these methods, a single ultrasound transducer on a commercial ultrasound system can be used to both generate acoustic radiation force in tissue, and to monitor the tissue displacement response. The response of tissue to this transient excitation is complicated and depends upon tissue geometry, radiation force field geometry, and tissue mechanical and acoustic properties. Higher shear wave speeds and smaller displacements are associated with stiffer tissues, and slower shear wave speeds and larger displacements occur with more compliant tissues. ARFI images have spatial resolution comparable to that of B-mode, often with greater contrast, providing matched, adjunctive information. SWEI images provide quantitative information about the tissue stiffness, typically with lower spatial resolution. A review these methods and examples of clinical applications are presented herein.
322 citations
TL;DR: Current approaches to elastography in three areas are reviewed--quasi-static, harmonic and transient--and inversion schemes for each elastographic imaging approach are described, with a focus on first-order approximation methods for linear elastic methods and isotropic materials and advanced reconstruction methods for recovering parameters that characterize complex mechanical behavior.
Abstract: Elastography is emerging as an imaging modality that can distinguish normal versus diseased tissues via their biomechanical properties. This paper reviews current approaches to elastography in three areas—quasi-static, harmonic and transient—and describes inversion schemes for each elastographic imaging approach. Approaches include first-order approximation methods; direct and iterative inversion schemes for linear elastic; isotropic materials and advanced reconstruction methods for recovering parameters that characterize complex mechanical behavior. The paper's objective is to document efforts to develop elastography within the framework of solving an inverse problem, so that elastography may provide reliable estimates of shear modulus and other mechanical parameters. We discuss issues that must be addressed if model-based elastography is to become the prevailing approach to quasi-static, harmonic and transient elastography: (1) developing practical techniques to transform the ill-posed problem with a well-posed one; (2) devising better forward models to capture the complex mechanical behavior of soft tissues and (3) developing better test procedures to evaluate the performance of modulus elastograms.
295 citations
Cites methods from "Contrast-transfer efficiency for co..."
...The plane-strain approximation is typically used when structures above and below the ultrasound scan plane have motion confined to the elevation (z) direction (Kallel et al., 2001; Doyley et al., 2000; Skovoroda et al., 1995; Barbone and Bamber, 2002)....
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...The plane–strain approximation is typically used when structures above and below the ultrasound scan plane have motion confined to the elevation (z) direction (Kallel et al 2001, Doyley et al 2000, Skovoroda et al 1995, Barbone and Bamber 2002)....
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TL;DR: Interestingly, the smallest increase occurred in the phantom with the largest elastic contrast, while a small increase of about 10% in volume of the cylindrical inclusions occurred-a tolerable increase.
Abstract: Five 9 cm x 9 cm x 9 cm phantoms, each with a 2-cm-diameter cylindrical inclusion, were produced with various dry-weight concentrations of agar and gelatin. Elastic contrasts ranged from 1.5 to 4.6, and values of the storage modulus (real part of the complex Young's modulus) were all in the soft tissue range. Additives assured immunity from bacterial invasion and can produce tissue-mimicking ultrasound and NMR properties. Monitoring of strain ratios over a 7 to 10 month period indicated that the mechanical properties of the phantoms were stable, allowing about 1 month for the phantom to reach chemical equilibrium. The only dependable method for determining the storage moduli of the inclusions is to make measurements on samples excised from the phantoms. If it is desired to produce and accurately characterize a phantom with small inclusions with other shapes, such as an array of small spheres, an auxiliary phantom with the geometry of the cylindrical inclusion phantoms or the equivalent should be made at the same time using the same materials. The elastic contrast can then be determined using samples excised from the auxiliary phantom. A small increase of about 10% in volume of the cylindrical inclusions occurred-a tolerable increase. Interestingly, the smallest increase (about 5%) occurred in the phantom with the largest elastic contrast.
224 citations
Cites background from "Contrast-transfer efficiency for co..."
...When the dry-weight gelatin concentration is different in the inclusions than in the surroundings—as in the case of Kallel et al (2001)—it is reasonable that the size of an inclusion would change with time due to osmosis....
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...As in the case of the phantoms made by Kallel et al (2001), osmosis due to differences in dry-weight gelatin concentrations would preclude long-term stability....
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...Other investigators have produced heterogeneous phantoms using agar/gelatin mixtures (Kallel et al 2001)....
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TL;DR: The results suggest that elastography may have significant potential for quantitatively mapping the time-dependent mechanical behavior of poroelastic media, which is related to the dynamics of fluid flow and to the elasticity and permeability parameters of the media.
Abstract: The feasibility of using elastography for experimentally estimating and imaging the Poisson's ratio of porous media under drained and undrained conditions was investigated. Using standard elastographic procedures, static and time-sequenced poroelastograms (strain ratio images) of homogeneous cylindrical gelatin and commercially available tofu samples were generated under sustained applied axial strain. The experimental data show similar trends to those that were observed in finite-elements simulations, and to those that were calculated from classical theoretical models proposed for biphasic materials with similar mechanical properties. To demonstrate the applicability of elastography to monitor time-dependent changes in nonhomogeneous porous structures as well, preliminary time-sequenced poroelastograms were obtained from two-layer porous phantoms and porcine muscle samples in vitro. The results suggest that elastography may have significant potential for quantitatively mapping the time-dependent mechanical behavior of poroelastic media, which is related to the dynamics of fluid flow and to the elasticity and permeability parameters of the media.
132 citations
Cites methods from "Contrast-transfer efficiency for co..."
...The permeability is expressed in units of m/s and may be converted to units of m4/Ns, which is the unit measurement used to express the permeability in eqn (1), by taking into consideration the gravitational acceleration and the density of the water....
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...For the third set of experiments, gelatin phantoms having stiffness similar to that of the tofu samples were produced, as described in Kallel et al. (2001), from which cylindrical samples were cut....
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TL;DR: The results show that the upper bound of the axial resolution in elastography is controlled by the physical wave parameters of the ultrasound system used to acquire the data (transducer center frequency and band- width), however, an inappropriate choice of the parameters used to process the US data (cross-correlation window length and shift between consecutive windows) may compromise the best resolution attainable.
Abstract: The limits and trade-offs of the axial resolution in elastography were investigated using a controlled simulation study. The axial resolution in elastography was estimated as the distance between the full widths at half-maximum of the strain profiles of two equally stiff lesions embedded in a softer homogeneous background. The results show that the upper bound of the axial resolution in elastography is controlled by the physical wave parameters of the ultrasound (US) system used to acquire the data (transducer center frequency and band- width). However, an inappropriate choice of the parameters used to process the US data (cross-correlation window length and shift between consecutive windows) may compromise the best resolution attainable. The measured elastographic axial resolution was found to be on the order of the ultrasonic wavelength. (E-mail: Jonathan.Ophir@uth.tmc.edu) © 2002 World Federation for Ultrasound in Medicine & Biology.
120 citations
Cites background from "Contrast-transfer efficiency for co..."
...The use of a Gaussian distribution significantly reduces the presence of strain concentrations near the lesions (Kallel et al. 2001) and does not detract from the basic general result....
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...For instance, in healthy tissues, the change in tissue structure may be gradual, which may also result in a gradual change of modulus distribution (Kallel et al. 1998, 2001)....
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...A third group of parameters that affect elastographic performance includes the acoustic and mechanical properties of tissue (Kallel et al. 2001; Ophir et al. 1999)....
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References
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TL;DR: Initial results of several phantom and excised animal tissue experiments are reported which demonstrate the ability of this technique to quantitatively image strain and elastic modulus distributions with good resolution, sensitivity and with diminished speckle.
Abstract: We describe a new method for quantitative imaging of strain and elastic modulus distributions in soft tissues. The method is based on external tissue compression, with subsequent computation of the strain profile along the transducer axis, which is derived from cross-correlation analysis of pre- and post-compression A-line pairs. The strain profile can then be converted to an elastic modulus profile by measuring the stresses applied by the compressing device and applying certain corrections for the nonuniform stress field. We report initial results of several phantom and excised animal tissue experiments which demonstrate the ability of this technique to quantitatively image strain and elastic modulus distributions with good resolution, sensitivity and with diminished speckle. We discuss several potential clinical uses of this technique.
3,636 citations
TL;DR: Some of the previous work done in the related field of biomechanics is surveyed, and a lexicography for elastic imaging is introduced, hoped that this nomenclature will provide a meaningful categorization of various approaches and will make evident the inherent parameters displayed and conditions applied in deriving the resulting images.
Abstract: Recently, a number of methods have been developed that make it possible to image the elastic properties of soft tissues. Because certain types of tissues such as malignant lesions, for example, have elastic properties that are markedly different from surrounding tissues, elasticity imaging could provide a significant adjunct to current diagnostic ultrasonic methods. Further, elasticity imaging techniques could be used to augment the study of tissues that change their elastic properties, such as skeletal and cardiac muscle. In this paper, we survey some of the previous work done in the related field of biomechanics, and we review measurement techniques from the 1950s to the 1980s. Different approaches to elastic imaging and signal processing are then discussed and a lexicography for elastic imaging is introduced. It is hoped that this nomenclature will provide a meaningful categorization of various approaches and will make evident the inherent parameters displayed and conditions applied in deriving the resulting images. Key assumptions and signal processing approaches are also reviewed. Finally, directions for future work are suggested.
529 citations
Additional excerpts
...These techniques were carefully reviewed (Gao et al. 1996; Ophir et al. 1996; Plewes et al. 1999)....
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TL;DR: In this article, a device and procedure for measuring elastic properties of gelatin for elasticity imaging (elastography) was described. And the measured compression forces were comparable to results obtained from finite element analysis when linear elastic media are assumed.
Abstract: Acoustic and mechanical properties are reported for gelatin materials used to construct tissue-like phantoms for elasticity imaging (elastography). A device and procedure for measuring elastic properties are described. The measured compression forces were comparable to results obtained from finite element analysis when linear elastic media are assumed. Also measured were the stress relaxation, temporal stability, and melting point of the materials. Aldehyde concentration was used to increase the stiffness of the gelatin by controlling the amount of collagen cross-linking. A broad range of tissue-like elastic properties was achieved with these materials, although gels continued to stiffen for several weeks. The precision for elastic modulus measurements ranged from less than 0.1% for 100 kPa samples to 8.9% for soft (<10 kPa), sticky samples.
511 citations
"Contrast-transfer efficiency for co..." refers background in this paper
...However, formaldehyde-containing phantoms are known to be unstable, as well, because their moduli change over a long period of time due to steady polymerization of the gelatin fibers (Hall et al. 1997; Schott 1992)....
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TL;DR: A literature review of a variety of methods for the estimation of tissue elasticity that have been reported in the literature in the past 15 years, and of data on the elastic properties of soft tissues.
Abstract: We review the principles of elastography and some of its accomplishments to date in the area of breast imaging in vivo. We present a literature review of a variety of methods for the estimation of tissue elasticity that have been reported in the literature in the past 15 years, and of data on the elastic properties of soft tissues. This is followed by a description of elastography and its relationship to the theory of elasticity. The principles underlying the elastography imaging technique, and methods for time delay estimations and their tradeoffs are discussed. We then present results of computer simulations, phantom and tissue studies in vitro, and some breast imaging studies in vivo. After a discussion of the origin and appearance of some important elastographic artifacts, we conclude with some general observations and conclusions.
296 citations
"Contrast-transfer efficiency for co..." refers methods in this paper
...Prior to the estimation of the elastogram, global stretching (Ophir et al. 1996) of 0.85% was applied to the postcompression RF data....
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...These techniques were carefully reviewed (Gao et al. 1996; Ophir et al. 1996; Plewes et al. 1999)....
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TL;DR: Elastograms were consistently efficient in quantitatively depicting the elasticity contrast of hard lesions; however, they showed suboptimal contrast-transfer efficiency in cases of soft lesions in a hard background, and some of the errors associated with the assumption of homogeneity of the target are determined.
Abstract: Elastography is a new ultrasonic imaging technique that produces images (elastograms) of the elastic properties of compliant tissue. The ultrasonically measured quantity is the normal strain component in the direction of the applied load, and the three normal components of stress may be estimated using the modified Love’s analytical models while assuming a value close to 0.5 (incompressible) for Poisson’s ratio. The distribution of Young’s moduli can thus be computed and displayed in the form of two dimensional images called elastograms. The analytical models used for the estimation of the three normal components of stress assume that the target is semi-infinite and homogeneous in composition. The objective of this paper is to determine some of the errors associated with the assumption of homogeneity of the target. Experiments using finite-element simulations were performed to study the efficiency with which elastograms display the contrast in the Young’s modulus of a lesion or target, with respect to its background under certain conditions. It was observed (using the definition of contrasttransfer efficiency of elastography as the ratio of the elasticity contrast as measured from an elastogram, to the true contrast), that elastograms were consistently efficient in quantitatively depicting the elasticity contrast of hard lesions; however, they showed suboptimal contrast-transfer efficiency in cases of soft lesions in a hard background. In general, elastograms are efficient in displaying the elasticity contrast of hard or soft lesions which have a low contrast level with respect to the surround, irrespective of their size and location.
144 citations
"Contrast-transfer efficiency for co..." refers background or methods in this paper
...In other words, the CTE at low modulus contrasts is close to a value of 0 dB (Ponnekanti et al. 1995)....
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...D simulation results obtained by Ponnekanti et al. (1995) to 3-D....
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...For example, stress concentrations are known to occur at the interfaces of rapid changes of elastic modulus (Ponnekanti et al. 1995; Kallel et al. 1996; Bilgen and Insana 1998; Kallel and Ophir 1998; Bishop et al. 2000)....
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...D finite element simulation obtained by Ponnekanti et al. (1995)....
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...Different contrast-transfer efficiency (CTE) curves have been reported for uniform cylindrical and spherical inclusions (Ponnekanti et al. 1995; Kallel et al. 1996; Bilgen and Insana 1998)....
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