Quantification of hemodynamics and luminal wall motion in human abdominal aortic aneurysms using magnetic resonance imaging and computational fluid dynamics [electronic resource]
Keywords: Bioengineering.

An abdominal aortic aneurysm (AAA) is a localized expansion of the abdominal aorta. AAA disease affects 5--7% of Americans over age 60 and AAA rupture kills approximately 9,000 people each year. Currently, there is no effective medical therapy for AAA disease and most AAA patients undergo "watchful waiting" until their AAA reaches the threshold for surgical repair. During the course of my doctorate studies, I used magnetic resonance imaging (MRI) and computational fluid dynamics to investigate exercise as a possible medical therapy for AAA disease and to understand the baseline flow and luminal motion conditions in the aneurysmal abdominal aorta. I imaged 64 patients with small AAAs (< 5 cm) in the supine position using a 1.5T Magnetic Resonance scanner. A 3D gadolinium-enhanced magnetic resonance angiography (MRA) sequence was used to image the aorta, a cardiac-gated phase contrast sequence (PC-MRI) was used to measure blood velocity perpendicular to the aorta at the supraceliac (SC) and infrarenal (IR) levels, and a cardiac-gated cine-FIESTA sequence was used to image luminal wall motion at the SC, IR, and Mid-Aneurysm (MA) locations. This data was used to conduct the following investigations: 1. Finite Element Analysis of blood flow at rest and simulated exercise. The MRA and PC-MRI data were used in combination with finite element analysis to elucidate the hemodynamic factors by which exercise might slow AAA growth in eight patients. Results indicate that mean wall shear stress (MWSS) was lowest in the aneurysm during rest (2.5±2.1 dynes/cm2) and MWSS increased and oscillatory shear index (OSI) decreased at the SC, IR, MA, and suprabifurcation locations during exercise. Mild turbulence existed at rest, while moderate aneurysmal turbulence was present during exercise. We hypothesize that the increased MWSS, decreased OSI, and moderate turbulence present during exercise may attenuate AAA growth. 2. Allometric scaling of mean SC and IR flow. The PC-MRI data were used to derive allometric scaling equations of mean SC and IR flows versus metrics of body size in 36 patients. Results of this investigation revealed that both the SC and IR peak-aligned averaged waveforms had the biphasic shapes characteristic of older adults. Mean SC and IR flows were 51.2±10.3 ml/s and 17.5±5.44 ml/s, respectively. Linear regression of the log-log plots of mean SC and IR flows versus body size revealed that body surface area was the strongest predictor of mean SC (R2=0.29) and IR flow (R2=0.19). 3. Wall motion characterization in the aneurysmal abdominal aorta. The cine-FIESTA luminal wall motion data were used to compute average diameter, strain, elasticity, and stiffness as well as changes in wall elastic properties versus age at the SC, IR, and MA locations in 45 patients. Results revealed that aortic elasticity and stiffness varied exponentially with age, and that the average percent change in diameter was 6.69%, 5.60%, and 2.66% at the SC, IR, and MA locations, respectively. Future work entails comparing morphologic MRI data at study intake and 2- or 3- follow-up, validating our simulated velocity fields against 2D 3-component PC-MRI data, and further developing 3D phase contrast acquisition and post-processing techniques in order to compute patient-specific aortic branch vessel flow splits, as well as for improved velocity field validation.