Abnormal aortic hemodynamics at predilection sites for dissection in patients with Marfan Syndrome: a 4D flow study

Funding Acknowledgements Type of funding sources: Private grant(s) and/or Sponsorship. Main funding source(s): AMC Foundation Horstingstuit Foundation Introduction Patients with Marfan syndrome (MFS) may develop aneurysmatic dilatation and dissection of the aorta with a consequence of sudden death at relatively young age. We performed an aortic 4D flow MRI analysis, providing a comprehensive quantification and visualization of abnormal aortic velocity and wall shear stress (WSS) magnitude and direction with recently developed techniques (1,2). We hypothesize that abnormal hemodynamics are found at predilection sites for aortic dissection in MFS patients. Methods This prospective study included 56 MFS patients and 25 healthy subjects as controls. Aortic 4D flow MRI was performed on a 3T Philips Ingenia system (Best, Netherlands). The aorta was manually segmented on time-averaged phase contrast MR angiogram images (phase contrast images multiplied by absolute velocity) by thresholding, watershed, and manual voxel in-/exclusion. The segmentations were used to mask the velocities, calculate WSS, and co-registration for quantification of abnormal hemodynamics (3). Abnormally elevated velocity and WSS were defined as higher than the three-dimensional 95% confidence interval as determined in the control group. Abnormally directed velocity and WSS were defined as vector angle differences higher than 120°. The aorta was subdivided in six regions of interest (ROIs) for total multiple linear regression with age, aortic diameter, and blood pressure characteristics. Independent predictors were defined as characteristics that were significant in the total model. Significance was defined as p < 0.05 with Bonferonni correction. The 3D-maps with abnormal hemodynamics were co-registered and added to create 3D-maps that show the incidence of abnormal hemodynamics. Results Figure 1 shows examples of maps with abnormal velocity and WSS magnitude and direction respectively. Ascending elevated velocity was associated with age, aortic diameter and blood pressure characteristics, whereas elevated WSS was associated with blood pressure characteristics only. No independent predictors were found for abnormally directed hemodynamics. Figure 2 shows the incidence maps for abnormally elevated velocity and abnormally directed WSS in two patients. The maximum incidence for elevated velocity and WSS were 32% and 20%, respectively, and found in the ascending aorta. The maxima for abnormally directed velocity and WSS were 18% and 39%, respectively, and found in the inner proximal descending aorta. Conclusion Altered aortic geometry and wall properties in MFS patients cause detectable hemodynamic effects in 30% of our cohort at known predilection sites for aortic dissection in MFS patients: the ascending aorta and proximal descending aorta. Independent measures of altered hemodynamics could possibly indicate individual patients at risk for aortic dissection.

Fully quantitative mapping of abnormal aortic velocity and wall shear stress direction in patients with bicuspid aortic valves and repaired coarctation using 4D flow cardiovascular magnetic resonance

Background Helices and vortices in thoracic aortic blood flow measured with 4D flow cardiovascular magnetic resonance (CMR) have been associated with aortic dilation and aneurysms. Current approaches are semi-quantitative or when fully quantitative based on 2D plane placement. In this study, we present a fully quantitative and three-dimensional approach to map and quantify abnormal velocity and wall shear stress (WSS) at peak systole in patients with a bicuspid aortic valve (BAV) of which 52% had a repaired coarctation. Methods 4D flow CMR was performed in 48 patients with BAV and in 25 healthy subjects at a spatiotemporal resolution of 2.5 × 2.5 × 2.5mm3/ ~ 42 ms and TE/TR/FA of 2.1 ms/3.4 ms/8° with k-t Principal Component Analysis factor R = 8. A 3D average of velocity and WSS direction was created for the normal subjects. Comparing BAV patient data with the 3D average map and selecting voxels deviating between 60° and 120° and > 120° yielded 3D maps and volume (in cm3) and surface (in cm2) quantification of abnormally directed velocity and WSS, respectively. Linear regression with Bonferroni corrected significance of P < 0.0125 was used to compare abnormally directed velocity volume and WSS surface in the ascending aorta with qualitative helicity and vorticity scores, with local normalized helicity (LNH) and quantitative vorticity and with patient characteristics. Results The velocity volumes > 120° correlated moderately with the vorticity scores (R ~ 0.50, P < 0.001 for both observers). For WSS surface these results were similar. The velocity volumes between 60° and 120° correlated moderately with LNH (R = 0.66) but the velocity volumes > 120° did not correlate with quantitative vorticity. For abnormal velocity and WSS deviating between 60° and 120°, moderate correlations were found with aortic diameters (R = 0.50–0.70). For abnormal velocity and WSS deviating > 120°, additional moderate correlations were found with age and with peak velocity (stenosis severity) and a weak correlation with gender. Ensemble maps showed that more than 60% of the patients had abnormally directed velocity and WSS. Additionally, abnormally directed velocity and WSS was higher in the proximal descending aorta in the patients with repaired coarctation than in the patients where coarctation was never present. Conclusion The possibility to reveal directional abnormalities of velocity and WSS in 3D provides a new tool for hemodynamic characterization in BAV disease.

Levy Swimmer In An Asymmetric Channel

In this article, we numerically study the dynamics of a model Levy walker, moving in a 2 dimensional medium confined by the walls of an asymmetric channel. We show that, as a result of both asymmetric potential due to the channel and also the power law step size distribution, the Levy walker will achieve a net directed velocity in the direction preferred by the channel. Other statistical properties of the walker such as mean first passage time and the mean square displacements are also examined

On the drag on an object immersed in a flowing plasma: the control surface approach

A theory of the drag force acting on an object immersed in a flowing plasma is presented. A control surface method is developed following the approach used in fluid mechanics rather than the linear dielectric response approach found in the literature. The situation examined is one in which the directed velocity of the ions dominates their thermal velocities; a cold ion model is employed.

Leibniz's critique of Cartesian principles of motion

Velocity of a moving body is a paradigmatic case of vector. In this paper it is argued that this conception of velocity was originally introduced by Leibniz in the context of his critique of Cartesian principles of motion. In Cartesian metaphysics velocity of a moving body is a strictly positive scalar, independent from its direction. As a result of an impact, a body can change its direction and preserve its velocity; such a change would immediately falsify the principle of continuity. In order to save the principle of continuity, Leibniz revised the notion of velocity and reduced the direction of movement to a class of newly conceived directed velocity. It is further shown that negative properties are not allowed in Leibnizian metaphysics. To reconcile the principle of continuity with apparent leaps, Leibniz was forced to deny the existence of atoms: he claimed that the monads, or the true elements of things, had to be indestructible and immaterial.