X-ray Phase-contrast CT: Quantitative Analysis Of Microvascular Changes And Differentiation Of Different Blood Vessels In Liver Fibrosis

X-ray phase-contrast imaging(PCI) can substantially enhance contrast, and is particularly useful in differentiating biological soft tissues with small density differences. It has approximately a 1000-fold higher sensitivity than conventional radiography. PCI shows promise for improved biomedical imaging because it enables the production of images with high resolution and simultaneously requires lower radiation doses. Combined with computed tomography(C T), PCI-CT can exhibit the superior properties over conventional absorption-based C T, promising to provide high-resolution micro-CT images, especially for soft tissues. Thus, the research on PCI-CT has significant value and perspective in biomedical imaging.Liver fibrosis is a reversible wound-healing response to chronic liver injury, and it is characterized by the accumulation of extracellular matrix. Ultimately, liver fibrosis may lead to cirrhosis, characterized by destruction of liver lobule, pseudolobule and nodule formation and portal hypertension. The development and variation of liver fibrosis always correlates with changes in micro vascular structure and morphology. The microvascular changes are regarded as one of the most important and recently discovered pathophysiological features of liver fibrosis. Microvasculature imaging in the liver can be employed to assess fibrosis progression and assist in the early finding of liver fibrosis and the objective evaluation of the degree of fibrosis. In this thesis, by means of PCI-CT technique, imaging of liver fibrosis in rat is performed at x-ray imaging and biomedical application beamline(BL13W1) of Shanghai Synchrotron Radiation Facility(SSRF) in China. Combined with PCI-CT with three-dimensional(3D) technique, this thesis dedicates to study the anatomical and pathological features of the microvasculature in liver fibrosis. The main contributions of this thesis are summarized as follows:(1) In this study, twenty-four Sprague-Dawley rats were induced by carbon tetrachloride(CC l4) as the models of liver fibrosis. All the samples were divided into four groups, each group with six rats, and they represented normal, mild, moderate and severe liver fibrosis, respectively. In order to investigate themicrovascular changes of the liver during the development of fibrosis, the samples from different stages of liver fibrosis were imaged using PCI-CT without contrast agents. Combined with the 3D visualization technique, the 3D microvascular morphology from different stages of liver fibrosis were presented using PCI-CT. Q uantitative evaluation of vascular morphology features, such as vascular distortion deformation, texture features on the inner wall of the vessel, fractal dimension, and Murray’s deviation were performed, and the development of liver fibrosis was quantitatively analyzed by use of the morphological changes in microvasculature. Based on the reconstructed 3D microvasculature in liver fibrosis, the microvascular density was obtained, which was also confirmed by immunohistochemical analysis of CD34. In addition, the portal pressures were measured and recorded. Correlations between portal pressures and microvascular changes in all rats were performed. The study showed that PCI-CT could clearly depict the changes of the microvascular morphology and density from different stages of liver fibrosis and could better characterize the various stages of fibrosis progression using 3D microvascular changes. Furthermore, the statistical analyses further identified that microvascular changes significantly correlated with portal pressures, which may have a significant potential in the evaluation of portal hypertension.(2) In liver fibrosis induced by bile duct ligation(BDL), vascular lesions and bile duct lesions influence each other, and they promo te the progression of the liver disease together. The effective visualization of vascular and bile ductular structures facilitates the study of microcirculation mechanisms and pathological changes associated with liver fibrosis induced by BDL. In this study, liver microvasculature was visualized without contrast agents in vitro with PCI-CT using liver fibrosis samples induced by BDL in rats. The histological section examination confirmed the correspondence of C T images with the microvascular morphology of the samples. By means of the PCI-CT and 3D visualization technique, 3D microvascular structures in samples from different stages of liver fibrosis were clearly revealed. The 3D vessel microstructure waspresented with regular shapes in the normal liver sample. In the liver fibrosis samples, vascular deformations, such as vascular disorders and abnormal vascular morphology, became apparent due to compression caused by fibrosis tissues, and the bile ducts were clearly visualized because the bile ducts expanded with the ligation operation. Ductular proliferation, which presented cluster distributions near the edge of the liver surface, was detected in the severe liver fibrosis sample. Different types of blood vessels(portal veins, hepatic veins) were distinguished from ductular proliferation and bile ducts with good sensitivity, excellent specificity, excellent accuracy, and good interreader agreement based on 3D microvascular structure characteristics in liver fibrosis samples. The study showed that PCI-CT could assess the morphological and pathological changes in liver microvasculature that result from fibrosis.