MRI of Structure and Function in Assessing Hemodynamic Impact on AAA Evolution

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DESCRIPTION (provided by applicant): Abdominal Aortic Aneurysm (AAA) is a condition with potentially devastating outcomes if the aneurysm progresses to rupture. Although the evolution of AAA is a complex process that is likely mediated by an interplay of biochemical and biomechanical factors, and despite the geometrical and structural variability of the entity itself, current clinical criteria hinge on a single parameter - the maximal lumenal diameter. Under clinical guidelines, there is a large population of individuals who harbor aneurysms that have not yet reached a size (5.5 cms) where the benefit from surgical intervention (either open surgery or endovascular stent-graft placement) exceeds the risk of the procedure. These patients are then followed with watchful waiting. It is the hypothesis of this project that hemodynamic factors play an important role in determining whether a given aneurysm will progress more rapidly than would be estimated on the basis of maximal lumenal diameter alone. The project will develop advanced MRI capabilities to measure the geometric morphology of the vascular lumen and of any intralumenal thrombus that might be present. Imaging of functional characteristics such as vascular compliance and turbulent kinetic energy will also be implemented. Similarly, advanced Computational Fluid Dynamics (CFD) simulations will be performed to simulate, on a patient-specific basis, the velocity fields in these aneurysms. The simulations will include fluid-structur interactions, non-laminar effects, and non- Newtonian terms. Consistency of these methods will be checked in cross-comparison with one another, and against experimental flow models. A cohort of patients with aneurysms in the range from 3.5 to 5.0 cms will be recruited for bi-annual imaging. Interval data sets will be co-registered with each other and changes in aneurysm morphology will be measured. These changes will then be correlated with hemodynamic descriptors calculated for that specific aneurysm. Particular care will be taken to ensure that the project is formulated to provide data reporting and guidance that is directly relevant to the clinician in community practice. It is the hypothesis of this proposal that regions of reduced wall shear stress will correlate with regions of more rapid aneurysm growth. If this is indeed demonstrated to be the case, this project will provide the tools to identify patients who, despite relatively small aneurysm size, might be rapid progressors, and conversely, cases where patients with larger aneurysms might have relatively stable conditions. That information would be important in ensuring that rapid progressors are treated before they progress to rupture, and that patients with stable aneurysms can be spared premature surgeries.