Magnetic Resonance Imaging Assessment of Cardiovascular Function during Exercise A Clinical Study Investigating Treatment Effects in Aortic Disease

Abstract

The aorta is an integral component of the cardiovascular system, damping pressure fluctuations, and determining the systemic pressure and afterload on the heart left ventricle. Arteries, especially the aorta, stiffen with age and disease which alters this dynamic relationship. Increased aortic stiffness has been associated with risk of cardiovascular events. The cardiovascular system can be evaluated by Magnetic Resonance Imaging (MRI) which provides detailed anatomical and functional information. Exercise stress testing is also an essential clinical tool to evaluate cardiovascular function. Cardiovascular changes induced by the additional metabolic demand under exercise, together with the exercise capacity and symptomatic responses provide clinicians with diagnostic and prognostic information that can aid clinical decision making. Combining MR imaging with exercise testing is difficult to achieve due to the high magnetic fields inherent to MRI environments and their susceptibility to interference from external electronic signals. An exercise bicycle compatible with the MRI environment has been developed which enables subjects to exercise lying inside the scanner, allowing images to be acquired immediately after an appropriate exercise capacity is achieved. Imaging procedures to evaluate left ventricular function were optimised to accommodate the elevated heart rates associated with moderate exercise. Additionally, two imaging methods estimating aortic stiffness were implemented and validated against aortic phantoms. Incorporated into a pump system developed specifically for validation, pulsatile aortic blood flow was simulated through these compliant phantoms. These methods were applied to patients with aortic disease at rest and during moderate exercise. The efficacy of two treatments for hypertension was assessed in patients with aortic regurgitation, ascending aortic aneurysms, abdominal aortic aneurysms, Marfan syndrome and repaired coarctation of the aorta. A group of healthy subjects were also evaluated as controls. The methods developed in this thesis enable better assessment of the effectiveness of current treatment methods and will further improve our understanding of cardiovascular disease processes.