Abdominal aortic aneurysms (AAAs) affect 5-7% of older Americans. We hypothesize that exercise may slow AAA growth by decreasing inflammatory burden, peripheral resistance, and adverse hemodynamic ...conditions such as low, oscillatory shear stress. In this study, we use magnetic resonance imaging and computational fluid dynamics to describe hemodynamics in eight AAAs during rest and exercise using patient-specific geometric models, flow waveforms, and pressures as well as appropriately resolved finite-element meshes. We report mean wall shear stress (MWSS) and oscillatory shear index (OSI) at four aortic locations (supraceliac, infrarenal, mid-aneurysm, and suprabifurcation) and turbulent kinetic energy over the entire computational domain on meshes containing more than an order of magnitude more elements than previously reported results (mean: 9.0-million elements; SD: 2.3 M; range: 5.7-12.0 M). MWSS was lowest in the aneurysm during rest 2.5 dyn/cm² (SD: 2.1; range: 0.9-6.5), and MWSS increased and OSI decreased at all four locations during exercise. Mild turbulence existed at rest, while moderate aneurysmal turbulence was present during exercise. During both rest and exercise, aortic turbulence was virtually zero superior to the AAA for seven out of eight patients. We postulate that the increased MWSS, decreased OSI, and moderate turbulence present during exercise may attenuate AAA growth.
Vessel deformations have been implicated in endoluminal device fractures, and therefore better understanding of these deformations could be valuable for device regulation, evaluation, and design. The ...purpose of this study is to describe geometric changes of the superficial femoral artery (SFA) resulting from hip and knee flexion in older subjects.
The SFAs of seven healthy subjects aged 50-70 years were imaged with magnetic resonance angiography with the legs straight and with hip and knee flexion. From geometric models constructed from these images, axial, twisting, and bending deformations were quantified.
There was greater shortening in the bottom third of the SFA than in the top two thirds (top, 5.9% +/- 3.0%; middle, 6.7% +/- 2.1%; bottom, 8.1% +/- 2.0% mean +/- SD; P < .05), significant twist in all sections (top, 1.3 degrees /cm +/- 0.8; middle, 1.8 degrees /cm +/- 1.1; bottom, 2.1 degrees /cm +/- 1.3), and greater curvature increase in the bottom third than in the top two thirds (top, 0.15 cm(-1) +/- 0.06; middle, 0.09 cm(-1) +/- 0.07; bottom, 0.41 cm(-1) +/- 0.22; P < .001).
The SFA tends to deform more in the bottom third than in the other sections, likely because of less musculoskeletal constraint distal to the adductor canal and vicinity of knee flexion. The SFAs of these older subjects curve off axis with normal joint flexion, probably resulting from known loss of arterial elasticity with age. This slackening of the vessel enables a method for noninvasive quantification of in vivo SFA strain, which may be valuable for treatment planning and device design. In addition, the spatially resolved arterial deformations quantified in this study may be useful for commercial and regulatory device evaluation.
The goal of this study was to quantify in vivo deformations of the superficial femoral artery (SFA) during maximum knee and hip flexion with use of magnetic resonance (MR) angiography to improve ...description of the complex, dynamic SFA environment.
Contrast medium-enhanced MR angiography was performed on the leg vasculature of eight healthy adults in the supine and fetal positions. The SFA was defined as the centerline path of the iliofemoral segment from the profunda femoris to the descending genicular artery. Deformations that resulted from flexion from the supine position to the fetal position were quantified with the SFA path and its branches.
Fourteen SFAs shortened from the supine position to fetal position, whereas two lengthened. Six of eight left SFAs twisted counterclockwise, and seven of eight right SFAs twisted clockwise. Straightness percentages for supine and fetal SFAs were 99.1%+/-0.4% and 98.7%+/-0.6%, respectively. From the supine position to the fetal position, the SFA shortened 13%+/-11% (P<.001) and twisted 60 degrees+/-34 degrees (P<.001). SFA arc length and percent shortening were strongly correlated (r>.8) between left and right limbs; however, no significant correlation existed for SFA twist angle.
Complex and varying vascular and muscular anatomy among study participants made SFA lengths and deformations from the supine position to the fetal position unpredictable a priori; however, there were strong symmetries between left and right SFAs in terms of arc length, length change, and direction of twist. The data show that, from the supine position to the fetal position, the SFA tended to shorten and twist substantially, suggesting these as possible fracture mechanisms and also providing important parameters for stent design.
Abstract Objective To evaluate the role of commissure orientation on downstream blood flow patterns and ascending aortic wall shear stress (WSS) in patients with bicuspid aortic valves (BAV) after ...valve-sparing aortic root replacement (V-SARR). Methods Nineteen BAV patients after V-SARR (9 Sievers' type 1/LR type 1 valve with fusion of the left and right cusps and 10 Sievers' type 0/LAT “naturally perfect”; type 0 valve without the presence of a raphe, and with the 2 commissures oriented right-anterior-to-left-posterior) were imaged using time-resolved 3-D phase contrast magnetic resonance imaging. A control group of 5 unoperated tricuspid aortic valve patients were used for comparison purposes. Wall shear stress and eccentricity of flow normalized to aortic diameter were measured in planes placed perpendicular to the axis of the ascending aorta at the level of the sinotubular junction (proximal ascending), main pulmonary artery (mid-ascending), and origin of the brachiocephalic (distal ascending). Results The ratio of WSS along the outer curvature to that along the inner curvature was greater in Sievers' type 1/LR patients compared with Sievers' type 0/LAT patients in the proximal (3.8 ± 1.6 vs 2.1 ± 0.9, P = .009) and mid- ascending aorta (4.5 ± 2.4 vs 2.4 ± 1.3, P = .027). Relative to control normal tricuspid patients, Sievers' type 1/LR BAV patients had a higher WSS ratio in the mid-ascending aorta (4.5 ± 2.4 vs 1.2 ± 1.2, P = .007). Conversely, WSS in Sievers' type 0/LAT patients was not different than in normal tricuspid patients. Conclusions After V-SARR, BAV cusp morphology has a major impact on the pattern of blood flow and WSS in the ascending aorta.
The quantification of cardiac flow and ventricular volumes is an essential goal of many congenital heart MRI examinations, often requiring acquisition of multiple 2D phase-contrast and bright-blood ...cine steady-state free precession (SSFP) planes. Scan acquisition, however, is lengthy and highly reliant on an imager who is well-versed in structural heart disease. Although it can also be lengthy, 3D time-resolved (4D) phase-contrast MRI yields global flow patterns and is simpler to perform. We therefore sought to accelerate 4D phase contrast and to determine whether equivalent flow and volume measurements could be extracted.
Four-dimensional phase contrast was modified for higher acceleration with compressed sensing. Custom software was developed to process 4D phase-contrast images. We studied 29 patients referred for congenital cardiac MRI who underwent a routine clinical protocol, including cine short-axis stack SSFP and 2D phase contrast, followed by contrast-enhanced 4D phase contrast. To compare quantitative measurements, Bland-Altman analysis, paired Student t tests, and F tests were used.
Ventricular end-diastolic, end-systolic, and stroke volumes obtained from 4D phase contrast and SSFP were well correlated (ρ = 0.91-0.95; r(2) = 0.83-0.90), with no statistically significant difference. Ejection fractions were well correlated in a subpopulation that underwent higher-resolution compressed-sensing 4D phase contrast (ρ = 0.88; r(2) = 0.77). Four-dimensional phase contrast and 2D phase contrast flow rates were also well correlated (ρ = 0.90; r(2) = 0.82). Excluding ventricles with valvular insufficiency, cardiac outputs derived from outlet valve flow and stroke volumes were more consistent by 4D phase contrast than by 2D phase contrast and SSFP.
Combined parallel imaging and compressed sensing can be applied to 4D phase contrast. With custom software, flow and ventricular volumes may be extracted with comparable accuracy to SSFP and 2D phase contrast. Furthermore, cardiac outputs were more consistent by 4D phase contrast.
To provide more complete characterization of ascending aortic blood flow, including vortex formation behind the valve cusps, in healthy subjects and patients after valve-sparing aortic root ...replacement (David reimplantation).
Time-resolved 3-dimensional magnetic resonance imaging velocity mapping was performed to analyze pulsatile blood flow by using encoded 3-directional vector fields in the thoracic aortas of 10 volunteers and 12 patients after David reimplantation using a cylindrical tube graft (T. David I) and two versions of neosinus recreation (T. David-V and T. David-V-S
mod). Aortic flow was evaluated by using 3-dimensional time-resolved particle traces and velocity vector fields reformatted onto 2-dimensional planes. Semiquantitative data were derived by using a blinded grading system (0–3: 0, none; 1, minimal; 2, medium; 3, prominent) to analyze the systolic vortex formation behind the cusps, as well as retrograde and helical flow in the ascending aorta.
Systolic vortices were seen in both coronary sinuses of all volunteers (greater in the left sinus 2.5 ± 0.5 than the right 1.8 ± 0.8) but in only 4 of 10 noncoronary sinuses (0.7 ± 0.9). Comparable coronary vortices were detected in all operated patients. Vorticity was minimal in the noncoronary cusp in T. David-I repairs (0.7 ± 0.7) but was prominent in T. David-V noncoronary graft pseudosinuses (1.5 ± 0.6;
P = .035). Retrograde flow (
P = .001) and helicity (
P = .028) were found in all patients but were not distinguishable from normal values in the T. David-V-S
mod patients.
Coronary cusp vorticity was preserved after David reimplantation, regardless of neosinus creation. Increased retrograde flow and helicity were more prominent in T. David-V patients. These novel magnetic resonance imaging methods can assess the clinical implications of altered aortic flow dynamics in patients undergoing various types of valve-sparing aortic root replacement.
Localization of atherosclerotic lesions in the abdominal aorta has been previously correlated to areas of adverse hemodynamic conditions, such as flow recirculation, low mean wall shear stress, and ...high temporal oscillations in shear. Along with its many systemic benefits, exercise is also proposed to have local benefits in the vasculature via the alteration of these regional flow patterns. In this work, subject-specific models of the human abdominal aorta were constructed from magnetic resonance angiograms of five young, healthy subjects, and computer simulations were performed under resting and exercise (50% increase in resting heart rate) pulsatile flow conditions. Velocity fields and spatial variations in mean wall shear stress (WSS) and oscillatory shear index (OSI) are presented. When averaged over all subjects, WSS increased from 4.8 +/- 0.6 to 31.6 +/- 5.7 dyn/cm2 and OSI decreased from 0.22 +/- 0.03 to 0.03 +/- 0.02 in the infrarenal aorta between rest and exercise. WSS significantly increased, whereas OSI decreased between rest and exercise at the supraceliac, infrarenal, and suprabifurcation levels, and significant differences in WSS were found between anterior and posterior sections. These results support the hypothesis that exercise provides localized benefits to the cardiovascular system through acute mechanical stimuli that trigger longer-term biological processes leading to protection against the development or progression of atherosclerosis.
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