Imaging and Measurement of the Poroelastic Behavior of Materials using New Ultrasound Elastography Techniques

Ultrasound elastography (USE) is a well-established technique used to non-invasively map tissue stiffness. More recently, a novel branch of USE, called poroelastography, has been proposed, which aims at estimating the poroelastic response of a tissue. The hypothesis of poroelastography is that underlying pathology, such as cancer, alters the fluid transport mechanisms in the tissue, which alters its poroelastic response. Poroelastography techniques estimate the temporal behavior of axial strain and effective Poisson’s ratio (EPR) of a poroelastic media under compression. While these methods have been successfully proven to estimate the poroelastic response of poroelastic media and tissues, there are some major limitations which need to be addressed. There is a lack of tissue mimicking poroelastic phantoms with tunable poroelastic properties. Also, while US poroelastography techniques aim to estimate the temporal behavior of EPR, the estimation of the EPR using USE is challenging due to known image quality limitations of lateral strain elastography techniques. Also, the relationship between the spatio-temporal behavior of interstitial fluid pressure (IFP), axial strain and EPR is currently not known. IFP is an important parameter, which is known to help in the diagnosis and characterization of soft tissue cancers. In this dissertation, I attempt to solve some of these current issues in the field of ultrasound poroelastography imaging, taking the field one step forward. In this work I investigated the use of polyacrylamide gel for creating new class of phantoms for poroelastography. Results of the study indicate that by using polyacrylamide gel, tissue mimicking poroelastic phantoms with controlled fluid flow can be generated. This new class of phantom material can be used to conceptualize and validate techniques in poroelastography and for temporal ultrasound elastography imaging in general. For reliable estimation of EPR, I proposed a new US poroelastography technique which uses two ultrasound transducers. By using a simulation module, image quality from this new technique was statistically compared from previously used methods. The feasibility to experimentally estimate EPR using two-transducers was also demonstrated. The results indicate that the two-transducer poroelastography technique has superior image quality than the existing methods. In this study, I also studied the spatio-temporal behavior of axial strain, EPR and IFP with change in interstitial permeability. A 2D poroelastic finite element model was used, followed by an ultrasound simulation algorithm. The temporal behavior was estimated by finding the time constant of the temporal curves. Results of the analysis indicate that increased IFP creates a new contrast mechanism in both the axial strain and EPR elastograms. Also, the spatio-temporal patterns of IFP are closely related to that of the EPR and, hence, a reliable estimation of EPR may aid in a non-invasive assessment of underlying IFP

Effect of Boundary Conditions on Performance of Poroelastographic Imaging Techniques in Non Homogenous Poroelastic Media

In the study of the mechanical behavior of biological tissues, many complex tissues are often modeled as poroelastic systems due to their high fluid content and mobility. Fluid content and fluid transport mechanisms in tissues are known to be highly correlated with several pathologies. Thus, imaging techniques capable of providing accurate information about these mechanisms can potentially be of great diagnostic value. Ultrasound elastography is an imaging modality that is currently used as a complement to sonographic methods to detect a variety of tissue pathologies. Poroelastography is a new elastographic technique that has been recently proposed to image the mechanical behavior of tissues that can be modeled as poroelastic media. The few poroelastographic studies retrievable focus primarily on homogeneous poroelastic media. In this study, a statistical analysis of the performance of poroelastographic techniques in a non-homogeneous poroelastic simulation model under different loading conditions was carried out. The two loading conditions simulated were stress relaxation (application of constant strain) and creep compression (application of constant stress), both of which have been commonly used in the field of poroelastography. Simulations were performed using a FE poroelastic simulation software combined with ultrasound simulation software techniques and poroelastography processing algorithms developed in our laboratory. The non-homogeneous poroelastic medium was modeled as a cube (background) containing a cylindrical inclusion (target). Different permeability, Young’s modulus and Poisson’s ratio contrasts between the underlying matrix of the background and the target were considered. Both stress relaxation and creep compression loading conditions were simulated. The performance of poroelastography techniques was quantified in terms of accuracy, elastographic contrast–to–noise ratio and contrast transfer efficiency. The results of this study show that, in general, image quality of both axial strain and effective Poisson’s ratio poroelastograms is a complex function of time, which depends on the contrast between the poroelastic material properties of the background and the poroelastic material properties of the target and the boundary conditions. The results of this study could have important implications in defining the clinical range of applications of poroelastographic techniques and in the methodologies currently deployed

Assessment of Ultrasound Elastography for Orthopedic Applications

Ultrasound imaging is emerging as an attractive alternative modality to standard x-ray and CT methods for bone assessment applications. The high reflectivity at the bone/soft tissue interface that occurs due to high acoustic impedance mismatch presents an important diagnostic opportunity affording the detection of abnormalities at bone surfaces with high accuracy and contrast-to-noise ratios. Furthermore, the mechanical properties of the soft tissue surrounding the bones undergo changes depending on the integrity of the underlying bone, viz. intact, fractured or healing. Unlike other imaging modalities, ultrasound elastography techniques, with their sensitivity to variations in soft tissue stiffness, are able to assist with monitoring bone regrowth. However, there is presently a lack of systematic studies that investigate the performance of diagnostic ultrasound techniques in bone imaging applications. This dissertation aims at understanding the performance limitations of new ultrasound techniques for assessing intact and fractured bones in vitro as well as in vivo. Ultrasound based 2D, 3D and elastography imaging experiments were performed on in vitro and in vivo samples of mammalian as well as non-mammalian bones. Ultrasound measurements of controlled defects were statistically compared with those obtained from the same samples using alternate imaging modalities. The performance of axial strain elastograms and axial shear strain elastograms at the soft tissue/bone interface was also studied in intact and fractured bones, and statistical analysis was carried out using elastographic image quality tools. The results of this study demonstrate that it is feasible to use diagnostic ultrasound imaging techniques to assess bone defects in real time and with high accuracy and precision. The relative strength of the axial strains and the axial shear strains at the bone/soft tissue interface with respect to the background soft tissue reduce in the presence of a fracture. Consequently, the study concluded that a combination of these imaging modalities might provide information regarding the integrity of the underlying bone and also an insight into the severity of the fractures, alignment of bone fragments and the progress of bone healing. In the future, ultrasound imaging techniques might provide a cost-effective, real-time, safe and portable diagnostic tool for bone imaging applications

Assessment of Ultrasound Elastography for Orthopedic Applications

Ultrasound imaging is emerging as an attractive alternative modality to standard x-ray and CT methods for bone assessment applications. The high reflectivity at the bone/soft tissue interface that occurs due to high acoustic impedance mismatch presents an important diagnostic opportunity affording the detection of abnormalities at bone surfaces with high accuracy and contrast-to-noise ratios. Furthermore, the mechanical properties of the soft tissue surrounding the bones undergo changes depending on the integrity of the underlying bone, viz. intact, fractured or healing. Unlike other imaging modalities, ultrasound elastography techniques, with their sensitivity to variations in soft tissue stiffness, are able to assist with monitoring bone regrowth. However, there is presently a lack of systematic studies that investigate the performance of diagnostic ultrasound techniques in bone imaging applications. This dissertation aims at understanding the performance limitations of new ultrasound techniques for assessing intact and fractured bones in vitro as well as in vivo. Ultrasound based 2D, 3D and elastography imaging experiments were performed on in vitro and in vivo samples of mammalian as well as non-mammalian bones. Ultrasound measurements of controlled defects were statistically compared with those obtained from the same samples using alternate imaging modalities. The performance of axial strain elastograms and axial shear strain elastograms at the soft tissue/bone interface was also studied in intact and fractured bones, and statistical analysis was carried out using elastographic image quality tools. The results of this study demonstrate that it is feasible to use diagnostic ultrasound imaging techniques to assess bone defects in real time and with high accuracy and precision. The relative strength of the axial strains and the axial shear strains at the bone/soft tissue interface with respect to the background soft tissue reduce in the presence of a fracture. Consequently, the study concluded that a combination of these imaging modalities might provide information regarding the integrity of the underlying bone and also an insight into the severity of the fractures, alignment of bone fragments and the progress of bone healing. In the future, ultrasound imaging techniques might provide a cost-effective, real-time, safe and portable diagnostic tool for bone imaging applications