Exploration of the use of X-ray scattering to characterize atherosclerotic plaque tissue

Every year an estimated 17 million people worldwide die of cardiovascular disease, making it responsible for one-third of all deaths. Even though vascular problems have been around for a long time there are still many factors that are not well understood. Atherosclerosis the most common form of cardiovascular disease. It is a complex biological process in which plaque gradually develops at susceptible locations in certain arteries (e.g., carotid, femoral). Plaque typically consists of a core containing lipids and cellular material separated from the blood stream by fibrous tissue (cap). Rupture of plaque tissue can lead to the formation of blood clots that often produce cardiovascular complications such as heart attacks, strokes and acute limb ischemia. Plaque development is believed to start with cholesterol deposition and foam cell formation. Further progression leads to the formation of mature plaques that often have a fibrous tissue containing smooth muscle cells, elastin, and collagen fibrils. Underneath the fibrous tissue are often softer materials containing cellular components and cholesterol crystals. The strength of fibrous cap is thought to contribute to plaque stability. Over time, bio-mineralization (calcified regions) of a plaque may occur and cholesterol crystals may penetrate into the fibrous tissue. Within prosthetic grafts, intimal hyperplasia can form fibrous tissue by a process that differs from atherosclerosis. The tissue may seriously impede blood flow. For the purposes of this dissertation, the key aspects of plaque tissue explored are the microstructural arrangement of collagen fibrils, the presence of the bio-mineral hydroxyapatite and cholesterol crystals and the use of collagen fibrils as micro strain sensors. The use of small X-ray scattering (SAXS) to characterize the orientation of collagen fibrils in human plaque tissue was explored, for what is believed to be the first time. A specimen tilting methodology was developed to find 3D collagen fibril orientation. Changes in collagen fibril orientation approaching macroscopic calcifications (millimeters in size) were observed in plaque tissue specimens from femoral and carotid arteries. In most cases, collagen fibrils were found to be oriented in the circumferential direction away from such calcifications. Spatial distribution maps of hydroxyapatite (HA) and cholesterol crystals (CC) in carotid and femoral plaque tissue specimens were obtained, for the first time. Wide angle X-ray scattering (WAXS) was shown to be suitable for that purpose. HA crystal sizes in tissue specimens were also determined and mapped. It was found that HA crystal sizes examined were comparable to sizes in bone. HA sizes within a given specimen varied by a factor of two to three in most cases and from specimen-to-specimen (patient-to-patient) by roughly 50%. Sizes were often greatest in regions of macroscopic calcification with high X-ray absorption and smaller near edges of calcifications having lower absorption. The use of SAXS to measure strains in collagen fibrils of specimens of human plaque tissue and porcine aorta was explored. Collagen fibril strains in aorta tissue were measured successfully and related to tissue-level macroscopic strains. The experiments provided useful insights in the possibility of using fibrils as microscopic "strain sensors" if changes in collagen fibril D-period can be monitored. Experimental difficulties prevented a similar assessment for plaque tissue specimens. An attempt was made for the first time to measure residual strains in cardiovascular tissue using SAXS. The results, although inconclusive, provide a starting point to spark interest in possible future work. Technology advancements have allowed for very precise measurement tools such as SAXS and WAXS to be developed, which can be utilized to help understand atherosclerotic plaque tissue. It is my hope that researchers can take what was learned in this study and continue the research further.

Keywords: Mechanical Engineering.
Advisor: Nelson Drew 
Sheppard S. (Sheri)