Abstract
The major complication of abdominal aortic aneurysm (AAA) is sudden rupture with an associated high mortality rate. The only clinical classifier for the risk of AAA rupture is the size of the aneurysm or its maximum diameter. We reviewed the role of intraluminal thrombus (ILT) by investigating the motion of both the lumen and wall surfaces of ILT throughout the cardiac cycle. We hypothesize that the response of the thrombus to the cyclic blood pressure provides insight into the state of the thrombus and its propensity to rupture, akin to what could be obtained from a mechanical compression test. Patients were selected from two studies: patients who underwent surgical intervention and patients from a retrospective study of AAA growth. Patients were selected to provide a wide range of AAA behaviors leading to a disease progression. Multiphase computed tomographic images of 14 patients were obtained from Peter Lougheed Hospital in Calgary, Canada. The geometries of the aortic wall and lumen were segmented from the images, and thrombus volume was determined at ten evenly spaced phases of the cardiac cycle. Where possible, the results were matched to known ex vivo mechanical properties, growth, and clinical outcomes for each AAA. The relative changes of wall, lumen, and thrombus volumes throughout the cardiac cycle classified AAAs into four types: (1) type I, aneurysms with a minimal wall movement, negative lumen expansion, and positive ILT expansion, (2) type II, aneurysms whose lumen undergoes small expansion, while the expansion is accommodated by the ILT and wall, (3) type III, a transition type characterized by lumen, wall, and thrombus expansions, and (4) type IV, characterized by lumen expansion matching or exceeding wall expansion, while the thrombus exhibits very small or negative deformation. This last behavior leads to an over-pressurized thrombus, which could enable the formation of permeable channels, often observable in medical images obtained in patients experiencing aneurysm rupture. By providing insight into thrombus behavior and the likelihood of channel development in the thrombus, AAA classification can improve the assessment of clinical risk for aneurysms: type I is associated with a stiff aneurysm wall that resists thrombus deformation and may be related to the risk of dissection, type II and type III are transition types, and type IV is associated with the formation of permeable channels and thrombus cracks which may indicate possible risk of rupture.