Abdominal aortic aneurysms (AAA) are common and can be life-threatening if they progress to rupture. They have been reported in up to 8% of older men and account for over 15,000 deaths per year. Basic vessel dimensions are currently the primary imaging measurement used clinically to risk-stratify patients. But there is more to the story than dimensions. Wall stress estimated with computational biomechanical modeling may better predict growth and rupture than diameters. Furthermore, AAA growth is often not continuous, and instead marked by periods of rapid growth followed by quiescence. Small series report that unrelated surgical procedures can precipitate AAA rupture, suggesting that episodic and heterogeneous inflammatory processes in concert with adverse hemodynamics and biomechanics are important for the progression of AAA disease. The complexity of aortic disease is more fully revealed with new functional imaging techniques than with conventional anatomic analysis alone. While AAA has been extensively studied, the mechanisms of disease progression have not been fully elucidated. If better understood, they could lead to significant improvement of the management of veterans with small AAAs (< 5.5 cm). Many of these aneurysms can be followed safely with a long screening interval of 2-3 years, but some may progress to rupture. Identifying this subset would greatly streamline the surveillance imaging of veterans with AAA. On the other hand, the majority of AAAs never rupture, and identifying low risk veterans could help better manage resources and subject only those veterans at truly elevated risk to intervention. MRI uniquely offers comprehensive assessment of forces acting on the vessel wall (hemodynamics and biomechanics), as well as factors affecting wall strength (structure, morphology and inflammation). Blood flow imaging with time-resolved 3D phase-contrast MRI (4D Flow) allows quantification of key parameters including flow turbulence and wall shear stress. Dynamic contrast enhanced (DCE) MRI may offer efficient evaluation of aortic wall inflammation. Cine Displacement Encoding with Stimulated Echoes (DENSE) MRI can quantify regional stretch differences experienced by the vessel wall, and, in tandem with 3D volumetric MRI anatomic data and computational modeling can be used to calculate patient-specific mechanical wall stress.
The aim of our study is to uncover important inflammatory changes and adverse hemodynamics and biomechanics that are not addressed by current imaging, and use them to predict AAA disease progression. We also seek to optimize a short (5 minute) MRI protocol without contrast to determine if there is added value to this comprehensive assessment, as fast non-contrast MRI would be preferable and more efficiently use VA resources. Our overall goal is to meaningfully advance the assessment of risk in veterans who do not meet current intervention thresholds and thereby in the future improve outcomes by refining surveillance imaging regimens and decisions regarding early intervention for AAAs.
Abnormal blood flow patterns and inflammation have been shown to cause disease of blood vessels, but are not currently studied in the routine evaluation of patients. With innovative new imaging techniques, we aim to study how aberrant flow patterns, biomechanical stresses and transient inflammation may render veterans at increased risk for potentially life-threatening vascular disease. We will target a common vascular disease in our veteran population, abdominal aortic aneurysm, a pathologic dilation of the central artery in the belly, which effects 5% of people over 55 years old.