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Viscoelastic Tissue Characterization Based on Harmonic and Transient Shear Wave Elastography

Juvenal Ormachea Quispe

Supervised by Professor Kevin J. Parker

Hopeman Building, Room 224

Elastography is a rapidly growing field in which imaging systems are used to estimate the viscoelastic properties of tissue. For example, elevated liver stiffness is an important indicator of fibrosis, and so the diagnostic value of elastography adds new information to the conventional radiology image. Within elastography techniques, shear waves play an important role, as they can be propagated by a source through the soft tissues and tracked by the imaging system.

The distinction between shear wave group and phase velocities is important in elastography, because diagnoses are made using a variety of techniques on different scanners: some rely on group velocity estimates, but others assess phase velocity. This document reviews the general definitions of group and phase velocity and examines their specific relations within an important general class of rheological models. For the class of tissues and materials exhibiting power law dispersion, group velocity is significantly greater than phase velocity, and simple expressions are shown to interrelate the commonly measured parameters. Examples are given from phantoms and tissues.

This thesis then considers the propagation of shear waves from acoustic radiation impulsive forces. Parameter estimation of the shear wave speed in tissues are based on some underlying models of shear wave propagation. The models typically include specific choices of the spatial and temporal shape of the force impulse and the elastic or viscoelastic properties of the medium. In this work, the analytical treatment of 2-D shear wave propagation in a biomaterial is presented. Estimators of attenuation and shear wave speed are derived from the analytical solutions, and these are applied to an elastic phantom, a viscoelastic phantom, and in vivo liver using a clinical ultrasound scanner. Additionally, it shows and examines the rheological models that can capture the dominant viscoelastic behaviors associated with fat and inflammation in the liver, and quantifies the resulting changes in shear wave speed and viscoelastic parameters. Theoretical results are shown to match measurements in phantoms and animal studies reported in the literature. Finally, the shear wave attenuation parameter, and its relation to diseased states of the liver, is studied. This work focused on the hypothesis that steatosis adds a viscous (lossy) component to the liver, which increases shear wave attenuation. Twenty patients’ livers were scanned and the resulting displacement profiles were analyzed to derive both the speed and attenuation of the shear waves within 6-cm2 regions of interest. The results were compared with pathology scores obtained from ultrasound-guided liver biopsies taken under ultrasound guidance. Across these cases, increases in shear wave attenuation were linked to increased steatosis score. This preliminary study supports the hypothesis and indicates the possible utility of the measurements for non-invasive and quantitative assessment of steatosis.

The shear wave speed estimators can be relatively simple if plane wave behavior is assumed with a known direction of propagation as it is considered in several elastography methods based on acoustic radiation force impulse. However, multiple reflections from organ boundaries and internal inhomogeneities and mode conversions can create a complicated field in time and space. Thus, this work also explores the mathematics of multiple component shear wave fields and derives the basic properties. It approaches this problem from the historic perspective of reverberant fields, a conceptual framework used in architectural acoustics and related fields. The reverberant shear wave field approach was evaluated and compared with another well-known elastography technique using two calibrated elastic and viscoelastic phantoms. The results indicate that it is possible to estimate the viscoelastic properties in each scanned medium. Moreover, the simultaneous multi-frequency application can be accomplished by applying an array of external sources that can be excited by multiple frequencies within a band- width, all contributing to the shear wave field produced in the liver or other target organ. This enables the analysis of the dispersion of shear wave speed as it increases with frequency, indicating the viscoelastic and lossy nature of the tissue under study. Furthermore, complete 2-D dispersion images can be created and displayed alongside the shear wave speed images. The author reports preliminary studies on in vivo breast and liver tissues, employing frequencies up to 700 Hz in breast tissue, and robust reverberant patterns of shear waves across the entire liver and kidney in obese patients. Dispersion images are shown to have contrast between tissue types and with quantitative values that align with previous studies. In addition to the shear wave speed and dispersion, this thesis also reports, numerically and experimentally, that it is possible to assess shear wave attenuation in tissues by using a reverberant shear wave field. The shear wave attenuation coefficient results are in agreement with those obtained in previous studies reported in the literature. In that sense, the R-SWE approach shows the potential to obtain a complete rheological characterization in in vivo tissue by measuring the shear wave speed, shear wave dispersion, and shear wave attenuation.

Finally, the specific conclusions of this research are summarized in the last chapter, with a special emphasis of next steps and future work that can be accomplished to improve the results presented in this work.