Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Science

Program

Medical Biophysics

Supervisor

Pickering, Geoffrey J.

2nd Supervisor

Bartha, R

Co-Supervisor

Abstract

Thoracic aortic aneurysms (TAAs) are a life-threatening dilation of the thoracic aorta that can lead to catastrophic dissection or rupture. In TAAs, the structure of the aortic wall is perturbed. Current imaging approaches assess lumen dimensions but lack the ability to delineate aortic wall architecture, a key determinant of aortic integrity. Diffusion tensor imaging (DTI) is an MRI modality which can reveal tissue microstructure through diffusion characteristics. In this thesis I investigated the potential for DTI to evaluate TAA architecture. A porcine model of aortic wall degeneration was developed, and DTI scans were performed on healthy and degenerated porcine ascending aortas. Here, DTI scalar indices correlated with histological markers of aortic damage. Subsequently, DTI assessment of human ascending aortic aneurysms revealed that abnormalities in DTI scalar indices correlated with medial degeneration including regional cellularity and regional glycosaminoglycan deposition.

Summary for Lay Audience

The aorta is the largest artery in the body and is responsible for trafficking all the body’s oxygenated blood as it is pumped out of the left ventricle of the heart. This vessel is subject to tremendous pressure and its wall must possess great tensile strength. To cope with this task, the aorta is highly organized, with around 40 layers of densely packed smooth muscle cells interwoven with elastin fibres and supportive collagen. However, the aorta can deteriorate and its function can fail. Strategies to image the aorta are critical. Currently, these strategies include echocardiography, computed tomography, and standard magnetic resonance imaging (MRI). These strategies can inform on the shape of the aorta, aortic diameter, and aortic wall thickness. However, these imaging methods lack the ability to assess the structural organization of the aortic wall itself. This limitation is an important gap, because the primary determinant of aortic structure and function is the integrity of the layered components of the wall. This thesis explored the potential of diffusion tensor imaging (DTI) as a strategy to characterize the internal organization of aortic wall elements. DTI is an MRI-based imaging method that tracks the movement of water and can be used to map fiber organizations in highly structured tissues such as white matter tracts in the brain. Here I investigate the ability of DTI to inform on aortic structural elements such as elastin fibre disruptions, smooth muscle cell density, and the presence of glycosaminoglycans. This analysis could set the stage for the application of DTI to evaluate the internal architecture of the aortic wall.

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