(ILT), which complicates AAA progression and risk of rupture. Patient-specific computational fluid dynamics modeling of 10 small human AAA was performed to investigate relations between hemodynamics and ILT progression. The patients were imaged using magnetic resonance twice in a 2-to 3-yr interval. Wall content data were obtained by a planar T1-weighted fast spin echo black-blood scan, which enabled quantification of thrombus thickness at midaneurysm location during baseline and followup. Computational simulations with patient-specific geometry and boundary conditions were performed to quantify the hemodynamic parameters of time-averaged wall shear stress (TAWSS), oscillatory shear index (OSI), and mean exposure time at baseline. Spatially resolved quantifications of the change in ILT thickness were compared with the different hemodynamic parameters. Regions of low OSI had the strongest correlation with ILT growth and demonstrated a statistically significant correlation coefficient. Prominent regions of high OSI (Ͼ0.4) and low TAWSS (Ͻ1 dyn/cm 2 ) did not appear to coincide with locations of thrombus deposition. computational fluid dynamics; hemodynamics; oscillatory shear index; thrombosis; wall shear stress ABDOMINAL AORTIC ANEURYSM (AAA) is a permanent dilation (50% greater than the normal arterial wall diameter) of the abdominal aorta. The disease is progressive and fatal if untreated. Between 11,000 and 17,000 patients die annually in the United States (18a) from AAA rupture following progressive, often asymptomatic, enlargement, and only invasive treatments have proven effective at preventing rupture and premature death (13). Male gender, age, smoking, genetic factors, hypertension, and high cholesterol are among known risk factors (10), and diabetes is a negative risk factor (47). Degradation of the aortic wall, inflammation with immune responses, and biochemical wall stress are mechanisms that promote AAA initiation, influenced by molecular genetics (1). After the irreversible process of aneurysm formation, the dilated vessel creates an environment with complex blood flow and wall shear stress distribution (4,5,9,14,22,37,38), which is thought to perpetuate aneurysm progression.The presence of intraluminal thrombus (ILT) complicates AAA growth. ILT is fibrin structure compound of platelets, blood cells, blood proteins, and cellular debris (20). ILT is more frequent in larger AAA since most patients develop thrombus as the aneurysm progresses. It has been proposed that platelets activate in regions of high shear and are subsequently advected towards the aneurysm wall where they accumulate in regions of low shear and form thrombus (8), with a possible role played by the vortical structures formed inside the aneurysm (9). It has also been shown that the flow field inside AAA may promote ILT formation by increasing residence time and trapping blood particles (7,37,38).The effect of ILT on AAA progression and rupture remains controversial (48). ILT may deprive the aneurysm wall of oxygen, which may exacerba...

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. This study aims to use magnetic resonance imaging (MRI) and computational fluid dynamics (CFD) to quantify flow stagnation and recirculation in eight AAAs by computing particle residence time (PRT). Specifically, we used gadolinium-enhanced MR angiography to obtain images of the vessel lumens, which were used to generate subject-specific models. We also used phase-contrast MRI 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 bulbous 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. We postulate that augmentation of mean infrarenal flow during exercise may reduce chronic flow stasis that may influence mural thrombus burden, degradation of the vessel wall, and aneurysm growth.

Abdominal aortic aneurysm (AAA) is a vascular disease resulting in a permanent, localized enlargement of the abdominal aorta. We previously hypothesized that the progression of AAA may be slowed by altering the hemodynamics in the abdominal aorta through exercise. To quantify the effect of exercise intensity on hemodynamic conditions in 10 AAA subjects at rest and during mild and moderate intensities of lower-limb exercise (defined as 33 ± 10% and 63 ± 18% increase above resting heart rate, respectively), we used magnetic resonance imaging and computational fluid dynamics techniques. Subject-specific models were constructed from magnetic resonance angiography data and physiologic boundary conditions were derived from measurements made during dynamic exercise. We measured the abdominal aortic blood flow at rest and during exercise, and quantified mean wall shear stress (MWSS), oscillatory shear index (OSI), and particle residence time (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.

Sn-renals were angled more inferiorly at the branch and more angulated at the stent end than F-renals due to stent placement strategies. Sn-LRAs exhibited a significant change in end-stent angle and curvature during respiration, a finding that may compromise long-term durability for parallel stent graft configurations. Further investigation is warranted to better optimize anatomic, patient, and branch vessel stent selection between fenestrated and snorkel strategies and their relationship to long-term patency.

Purpose This study was performed to quantify respiration-induced deformations of the superior mesenteric artery (SMA) and left and right renal arteries (LRA and RRA) in patients with small abdominal aortic aneurysms (AAA). Materials and Methods Sixteen men with AAAs (73±7 years) were imaged with contrast-enhanced magnetic resonance angiography during inspiratory and expiratory breath-holds. Centerline paths of the aorta and the visceral arteries were acquired by geometric modeling and segmentation techniques. Vessel translations, and changes in branching angle and curvature due to respiration were computed from the centerline paths. Results With expiration, the SMA, LRA, and RRA bifurcation points translated superiorly by 12.4±9.5, 14.5±8.8, 12.7±6.4 mm (P<0.001) and posteriorly by 2.2±2.7, 4.9±4.2, 5.6±3.9 mm (P<0.05), respectively, and the SMA translated rightward by 3.9±4.9 mm (P<0.01). With expiration, the SMA, LRA, and RRA angled upwards by 9.7±6.4°, 7.5±7.8°, and 4.9±5.3°, respectively (P<0.005). With expiration, the mean curvature increased by 0.02±0.01, 0.01±0.01, 0.01±0.01 mm−1 in the SMA, LRA, and RRA, respectively (P<0.05). For both inspiration and expiration, the RRA curvature was greater than in other vessels (P<0.025). Conclusions With expiration, the SMA, LRA, and RRA translated superiorly and posteriorly due to diaphragmatic motion, inducing upward angling of vessel branches and increase in curvature. In addition, the SMA exhibited rightward translation with expiration. The RRA was significantly more tortuous, but deformed less than the other vessels during respiration.