Characterizations of Viscoelastic Liver Tissue Based on Shear Wave Elastography and Ultrasound Wave Speckle Statistics

Sedigheh (Daria) S. Poul, PhD Defense, Advised by Professor Kevin Parker

Hopeman 224

The most prevalent chronic liver disease is non-alcoholic fatty liver disease (NAFLD), affecting approximately 30% of the population in the United States. NAFLD is expected to be the leading cause of liver transplantation globally in the future. There has always been an urgent need for potential biomarkers to diagnose liver disease at early stages so that the liver may have a chance to heal itself and recover. NAFLD starts with simple steatosis and can progress to clinically significant fibrosis and later to cirrhosis requiring liver transplantation. The current reference standard for diagnosing NAFLD is liver biopsy which is invasive.  

The focus of this research is on studying the biomechanics of liver tissues in normal and abnormal fatty conditions. In this dissertation, liver tissue characterization is investigated using both shear wave (SW) elastography approaches and also quantitative ultrasound approaches based on a novel framework for ultrasound first order speckle statistics. Specifically, in this research: 

• Rheological models for elastography are assessed comprehensively, suggesting the most appropriate one for liver elastography over a wide time/frequency range. Moreover, an important question is answered regarding the possibility of fat and fibrosis as confounding cofactors in liver elastography, suggesting fibrosis being a strong cofactor in the correlation of SW attenuation with fat % and steatosis (fat) being a weak cofactor in SW speed correlation with fibrosis. Further, a novel elastography approach (Reverberant SW technique) is implemented and investigated for characterization of fatty fibrotic conditions in 3D model of liver and in tissue-mimicking phantoms, focusing on proposing the effective RSW excitation method based on analyzing the RSW field. 

Also, Liver tissue characterization from the quantitative ultrasound aspect is studied under a novel framework for characterizing ultrasound first order speckle statistics, leading to the Burr distribution that has not been employed in field of medical imaging before. This novel framework showed promising results in characterizing various in vivo rat liver scan data in normal, fibrosis and steatosis conditions as well as in 3D models of scattering tissues, which suggest that the Burr underlying parameters may have potentials for liver tissue characterizations