Hemodynamic changes quantified in abdominal aortic aneurysms with increasing exercise intensity using magnetic resonance imaging and computational fluid dynamics [electronic resource]
Keywords: Mechanical Engineering

Hemodynamic conditions are hypothesized to affect the initiation, growth, and rupture of abdominal aortic aneurysms (AAAs), a vascular disease characterized by progressive wall degradation and enlargement of the abdominal aorta. We hypothesized that the progression of AAA may be slowed by altering the hemodynamics in the abdominal aorta through exercise. The aim of the first study in this thesis was to use magnetic resonance imaging (MRI) and computational fluid dynamics (CFD) to quantify flow stagnation and recirculation in AAAs by computing particle residence time (PRT). Specifically, we used gadolinium-enhanced MR angiography (MRA) to obtain images of the vessel lumens, which were used to generate subject-specific models. Phase-contrast MRI was used to measure blood flow at supraceliac and infrarenal locations to prescribe physiologic boundary conditions. CFD was used to simulate pulsatile flow, and PRT, particle residence index, and particle half-life of PRT in the aneurysms were computed. We observed significant regional differences of PRT in the aneurysms with localized patterns that differed depending on aneurysm geometry and infrarenal flow. A saccular aneurysm with the lowest mean infrarenal flow demonstrated the slowest particle clearance. In addition, improvements in particle clearance were observed with increase of mean infrarenal flow. This result motivated the next study to quantify the effect of augmentation of mean infrarenal flow during exercise on reducing chronic flow stasis that may promote increased mural thrombus burden, degradation of the vessel wall, and aneurysm growth. In the second study, we investigated three levels of activity, rest, mild and moderate intensities of lower-limb exercise to quantify the effect of exercise on hemodynamic conditions in AAA subjects. We measured the abdominal aortic blood flow at rest and during dynamic exercise, and quantified mean wall shear stress (MWSS), oscillatory shear index (OSI), and PRT. We observed that an increase in the level of activity correlated with an increase of MWSS and a decrease of OSI at three locations in the abdominal aorta, and these changes were most significant below the renal arteries. As the level of activity increased, PRT in the aneurysm was significantly decreased: 50% of particles were cleared out of AAAs within 1.36 ± 0.43, 0.34 ± 0.10, and 0.22 ± 0.06 s at rest, mild exercise and moderate exercise levels, respectively. Most of the reduction of PRT occurred from rest to the mild exercise level suggesting that mild exercise may be sufficient to reduce flow stasis in AAAs. The third study aimed to correlate hemodynamic characteristics of AAA with its progression, and quantify morphologic changes of aneurysm from study intake to 1.5-3.5-year follow-up of subjects randomized to usual activity or exercise training cohort. We acquired MRA of 16 subjects, and mid-aneurysm wall content data of 12 subjects at each intake and follow-up visit. 3D lumen models were built based on each intake and follow-up MRAs of six subjects. We observed morphologic changes of aneurysm lumen from intake to follow-up MRA of 11 subjects which exhibited local smoothing, shrinkage or expansion. We observed thickening of thrombus burden from follow-up wall content image of seven subjects whose aneurysm lumen was eventually narrowed. Based on 3D model comparison between intake and follow-up, and the correlation of morphologic changes with wall content changes and PRT contour plots computed in the previous study, we suggest that the region of narrowed lumen induced by thrombus buildup may be consistent with the region of long PRT. As a future work, we will extend our research in conjunction with CT data to quantify the changes of aneurysm wall, and increase the number of subjects to find more conclusive results.