Poroelastography: Ultrasonic imaging of the poroelastic behavior of phantoms and tissues

Elastography is a well-established imaging modality that applies a quasi-static compression to estimate and image the mechanical properties of ultrasonically-scanned tissues. In the last several years, many studies have been carried out to understand the fundamental trade-offs in elastographic imaging and to demonstrate the feasibility of generating quality elastograms of tissues in vitro and in vivo. The majority of these studies were aimed at investigating the mechanical response of the tissues to quick loading and under the condition that the response of interest occurred instantaneously after the application of the load. The focus of the research developed in this dissertation is on establishing the feasibility of using elastography to image the time-dependent mechanical behavior of poroelastic soft-tissue-mimicking phantoms and tissues. This study involved the generation of poroelastograms to monitor the time-dependent changes of homogeneous and non-homogeneous poroelastic samples subjected to sustained unconfined compression. From the poroelastograms, two new types of elastograms, referred to as Poisson's ratio time constant elastogram and permeability elastogram, respectively, were generated to obtain information on the underlying permeability distribution of the samples that ostensibly cause the time-dependent changes observed in the poroelastograms. Independent mechanical measurements, simulations, and studies of the major issues related to image quality analysis, such as signal-to-noise ratio, contrast-to-noise ratio, and resolution, were performed and used for designing the poroelastography experiments and interpreting the results. The work reported in this dissertation led to the conclusion that it is feasible to use elastographic techniques to generate quality images of the spatial and temporal poroelastic behavior of certain materials and tissues in vitro and in vivo.