## 4D Flow MRI in Ascending Aortic Aneuryss: Reproducibility of Hemodynamic Parameters  
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cardiac cycle through the aorta can be analyzed using three-dimensional time-resolved phase-contrast magnetic resonance imaging (MRI), also known as four-dimensional (4D) flow MRI. From 4D flow MRI data, several relevant patient-specific hemodynamic parameters can be assessed. It has been demonstrated for patients with a tricuspid aortic valve (TAV) that the amount of flow displacement (a measure of eccentricity in the flow profile) is associated with aortic growth [7; 8]. Furthermore, TAV patients showed decreased levels of vortical flow, helical flow, and wall shear stress (WSS) compared to healthy volunteers [9; 10]. While such hemodynamic parameters could also be important for future clinical risk stratification of aneurysm patients, the majority of the studies assessing their agreements between observers and examinations only included healthy volunteers, patients with bicuspid aortic valves, or patients with mildly dilated aortas [13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. Hence, the reported outcomes are not fully translatable to TAA patients with substantially dilated aortas and a TAV. Therefore, the reproducibility of the hemodynamic parameters in such patients remains unknown [1]. Moreover, several previous studies quantified parameters based on (multiple) two-dimensional plane quantifications or ranked them qualitatively [15; 16; 17; 18; 21]. Thus, part of the spatial information is lost. In addition, the reproducibility of the WSS magnitude (WSS\({}_{\text{mag}}\)) has been assessed well, but little is known about the reproducibility of the axial or circumferential WSS component (WSS\({}_{\text{ax}}\), WSS\({}_{\text{cir}}\), respectively) and the angle between both WSS components (WSS\({}_{\text{angle}}\)). To allow quantification of hemodynamic parameters in 4D flow MRI data, a cardiac phase-specific 3D lumen segmentation is required [23; 24]. The shape of these lumen segmentations can be described by morphologic parameters such as the volume, centerline length, maximal diameter, and curvature radius. Interestingly, a recent study demonstrated that the curvature radius of the ascending aorta is associated with aortic growth [7]. In most commercial and research software tools, these lumen segmentations are constructed (semi-)automatically on the 4D flow MRI data and then manually adjusted when needed. The manual interaction may introduce observer-dependent variability in the quantitation of the hemodynamic parameters [25; 26]. This observer-dependent variability of the 3D lumen segmentation can be assessed directly by evaluating the variability in morphologic parameters. The reproducibility of these morphologic parameters from 4D flow MRI data has been primarily reported in healthy volunteers [25].