## 4 Discussion  
The main findings of the current study are: (1) hemodynamic parameters (i.e., normalized flow displacement, flow jet angle, WSS\({}_{\text{mag}}\), WSS\({}_{\text{ax}}\), WSS\({}_{\text{cir}}\), WSS\({}_{\text{angle}}\), vorticity norm, absolute helicity, and volumes with increased absolute LNH) and morphological parameters (volume, centerline length, diameter and curvature radius) can be assessed with very good reproducibility in the thoracic aorta of TAA patients with a TAV, (2) the reproducibility of these parameters is slightly decreased within the aortic root, and (3) the reproducibility is affected by the observer's experience. The intra- and interobserver analyses generally demonstrated a very good reproducibility for the assessment of hemodynamic parameters in TAA patients with a TAV. Similar results have been described previously for flow displacement, flow jet angle, WSS\({}_{\text{mag}}\), WSS\({}_{\text{angle}}\), secondary flow patterns (which are reflected in vorticity and helicity), and volumes with increased absolute LNH in healthy volunteers and in patients with none-to-marginally dilated aortas [\(11\)-\(19\), \(30\)], but not yet in a clinically relevant patient group, such as patients with TAA. Moreover, the Bland-Altman plots of the current study demonstrated for most hemodynamic parameters and analyses no striking dependency of the measured differences relative to the mean. Furthermore, the intra- and interobserver analyses demonstrated very good reproducibility of the morphologic parameters in TAA patients with a TAV. Similar reproducibility results have been described previously for volume, centerline length, maximal diameter, and curvature radius in healthy volunteers [25]. However, the current study demonstrated wider LoAs for all morphologic parameters within TAA patients compared to healthy volunteers. These wider LoAs in TAA patients may potentially be associated with the presence of dilation since, for a constant flow rate, the blood velocity is inversely related to the vessel's diameter. Therefore, a larger area of reduced velocity-to-noise ratio is expected for TAA patients compared to healthy volunteers, especially close to the vessel wall. This reduced velocity-to-noise ratio potentially can have a negative influence on the reproducibility of aortic lumen segmentation. However, the velocity-to-noise ratio within these areas may be improved by 4D flow MRI sequences with multiple-velocity-encoding gradients, which consequently may also improve the reproducibility of hemodynamics parameters [\(31\),\(32\)]. When analyzing the results per anatomical segment, it was observed that the reproducibility was slightly reduced but still acceptable for most hemodynamic and morphologic parameters within the aortic root. This slight reduction in reproducibility was also seen in the proximal and distal ascending aorta for the maximal diameter. This reduced reproducibility is potentially introduced by the movement of the aortic root and ascending aorta in contrast to the relatively fixed-positioned aortic arch and descending aorta [\(33\)-\(35\)]. This effect related to the cardiac movement (contraction and relaxation) may further be amplified by the heart rate variation and breathing variation during the acquisition. Alongside, inadequate respiratory compensation or inadequate electrocardiographic gating may also perturbate the 4D flow MRI data. Moreover, within the aortic root, complex secondary flow patterns (recirculating flow) are present [\(9\),\(36\)]. Since this recirculating flow has a relatively low velocity compared to the velocity-encoding sensitivity, these recirculating areas have a reduced velocity-to-noise ratio close to the vessel wall and potentially introduce more observer-dependent variability on the aortic lumen segmentation. The observer-dependent variability of the 3D lumen segmentation was assessed directly by evaluating the variability in morphologic parameters.