Christopher D Prihoda

Bio: Christopher D Prihoda is an academic researcher from University of Texas at Austin. The author has contributed to research in topics: Elastic modulus. The author has an hindex of 1, co-authored 1 publications receiving 82 citations.

Contrast-transfer efficiency for continuously varying tissue moduli: simulation and phantom validation.

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)

Acoustic Radiation Force Impulse (ARFI) Imaging: a Review.

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.

Model-based elastography: a survey of approaches to the inverse elasticity problem

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.

Tissue-mimicking agar/gelatin materials for use in heterogeneous elastography phantoms

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.

The feasibility of using elastography for imaging the Poisson's ratio in porous 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.

Axial resolution in elastography.

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.