Helical and retrograde secondary flow patterns in the aortic arch studied by three-directional magnetic resonance velocity mapping.

162

138

Purpose:To assess the distribution and regional differences of flow and vessel wall parameters such as wall shear stress (WSS) and oscillatory shear index (OSI) in the entire thoracic aorta. Materials and Methods:Thirty-one healthy volunteers (mean age ϭ 23.7 Ϯ 3.3 years) were examined by flow-sensitive four-dimensional (4D)-MRI at 3T. For eight retrospectively positioned 2D analysis planes distributed along the thoracic aorta, flow parameters and vectorial WSS and OSI were assessed in 12 segments along the vascular circumference. Conclusion:The normal distribution of vectorial WSS and OSI in the entire thoracic aorta derived from flow-sensitive 4D-MRI data provides a reference constituting an important perquisite for the examination of patients with aortic disease. Marked regional differences in absolute WSS and OSI may help explaining why atherosclerotic lesions predominantly develop and progress at specific locations in the aorta. COMPLEX VASCULAR GEOMETRY AND PULSATILE FLOW in the human arterial system lead to regionally different flow characteristics and thus spatial and temporal changes in shear forces acting on the vessel wall. These forces can be characterized by wall shear stress (WSS) or oscillatory shear index (OSI) that play an important role in flow-mediated atherogenesis and arterial remodeling (1-3). While WSS values reported in the literature typically reflect the time-averaged shear forces acting on the vessel wall, OSI describes the existence and magnitude of WSS changes over the cardiac cycle. Recent reports stressed the importance of WSS and OSI with respect to the formation and stability of atherosclerotic plaques (4). A number of studies have shown that low WSS and high OSI represent sensitive markers for formation of plaques in the aorta, carotid, or coronary arteries (5,6). Particularly, the assessment of both WSS and OSI can help to determine the complexity of the lesions. A recent study with animal models and deliberately altered flow characteristics in the carotid arteries demonstrated the close correlation of low WSS with the development of vulnerable high-risk plaques whereas high OSI induce stable lesions (4). In addition, the effects of selected pathologies on regionally-varying WSS and OSI values have been reported (7,8).Among other methods, MRI is a feasible and extensively validated technique to derive quantitative flow information from arterial vessels (9 -12). Due to its intrinsic sensitivity to flow and the possibility to acquire true time-resolved three-dimensional (3D) data, in vivo analyses of blood-flow and derived vessel wall parameters are promising. However, earlier reports on MRbased analysis of aortic hemodynamics were either based on incomplete vascular coverage and separately acquired 2D slices (13-17), a combination of MR mea-

Section: Discussionmentioning

confidence: 88%

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confidence: 99%

Three‐dimensional analysis of segmental wall shear stress in the aorta by flow‐sensitive four‐dimensional‐MRI

Frydrychowicz

Stalder

Russe

et al. 2009

162

138

“…Previous studies (10,12,14) have described a normal aortic flow pattern including right-handed helical laminar flow oc-cupying the entire aortic lumen in systole, retrograde flow along the inner wall of the distal ascending aorta occurring from the end systole to early diastole, and vortical flow in the sinuses of Valsalva after peak systole and persisting until diastole. Using time-resolved 3D MR velocity mapping, Markl et al (15) and Kvitting et al (16) reported that the vortices in the coronary sinuses were preserved in patients after aortic valve-sparing surgery.…”

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confidence: 99%

Breathhold time‐resolved three‐directional MR velocity mapping of aortic flow in patients after aortic valve‐sparing surgery

Liu

Weale

Reiter

et al. 2009

Purpose:To evaluate the utility of breathhold time-resolved three-directional MR velocity mapping for quantifying the restoration of normal flow patterns in patients after aortic valve-sparing surgery. Materials and Methods:Breathhold time-resolved threedirectional MR velocity mapping was performed on 13 patients with aortic valve-sparing surgery. Ten healthy volunteers and 12 patients with ascending aortic aneurysm underwent the same MR examination for comparison. Aortic laminar flow, turbulent flow, and the presence of vortical flow in the sinuses of Valsalva were semiquantitatively assessed and statistically compared between the three groups of subjects. Results:The average score of laminar flow in the ascending aorta for patients with surgery was not significantly different from that of volunteers (P ϭ 0.210), but was significantly greater than that of patients with aneurysm (P Ͻ 0.01). The average score of turbulent flow in patients with surgery was significantly smaller than that of patients with aneurysm (P Ͻ 0.01). The presence of systolic vortical flow in the sinuses of Valsalva for patients with surgery was not significantly different from that of healthy volunteers (P ϭ 0.405) and patients with aneurysm (P ϭ 0.238). Conclusion:Breathhold time-resolved three-directional MR velocity mapping allows for quantifying flow patterns in the aortic root and ascending aorta.Normal laminar flow in the ascending aorta and vortical flow in the sinuses of Valsalva can be restored in patients after aortic valve-sparing surgery.

“…Using MR phase-velocity mapping (PVM) flow through a vessel can be measured accurately (15)(16)(17)(18). PVM has been used to measure the mitral regurgitant volume in an indirect way, by subtracting the aortic outflow mea sured in systole from the mitral inflow measured in di astole (19).…”

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confidence: 99%

Quantification of mitral regurgitation with MR phase‐velocity mapping using a control volume method

Chatzimavroudis

Oshinski

Pettigrew

et al. 1998

Reliable diagnosis and quantification of mitral regurgitation are important for patient management and for optimizing the time for surgery. Previous methods have often provided suboptimal results. The aim of this in vitro study was to evaluate MR phase‐velocity mapping in quantifying the mitral regurgitant volume (MRV) using a control volume (CV) method. A number of contiguous slices were acquired with all three velocity components measured. A CV was then selected, encompassing the regurgitant orifice. Mass conservation dictates that the net inflow into the CV should be equal to the regurgitant flow. Results showed that a CV, the boundary voxels of which excluded the region of flow acceleration and aliasing at the orifice, provided accurate measurements of the regurgitant flow. A smaller CV provided erroneous results because of flow acceleration and velocity aliasing close to the orifice. A large CV generally provided inaccurate results because of reduced velocity sensitivity far from the orifice. Aortic outflow, orifice shape, and valve geometry did not affect the accuracy of the CV measurements. The CV method is a promising approach to the problem of quantification of the MRV.