## 2 Materials and Methods
### Image Analysis  
The image analysis consisted of three parts: 4D flow MRI lumen segmentation, aorta morphology quantification, and hemodynamic quantification. First, the aortic lumen of the patients was segmented twice by the first observer ([PERSON]) and once by the second and third observers ([PERSON] and B.K., respectively). Lumen segmentation was performed using CAAS MR Solutions v5.2.1 (Pie Medical Imaging, Maastricht, The Netherlands). The software generated initial segmentations on the peak systolic phase and two consecutive phases before and after this peak systolic phase (i.e., five systolic phases, see Figure 2 and Supplementary Materials Video S1). Next, the aortic segmentations were manually corrected for all five phases and partitioned into six consecutive segments by manually placing anatomical planes perpendicular to the aortic centerline, respectively: the aortic root (from the aortic valve to the sinotubular junction), proximal ascending aorta (from the sinotubular junction to the mid-ascending aorta, defined as the midpoint on the centerline between the sinotubular junction and brachiocephalic artery), distal ascending aorta (from the mid-ascending aorta to the brachiocephalic artery), aortic arch (from the brachiocephalic artery) until past the left subclavian artery), the proximal descending aorta (from the left subclavian artery to the mid-descending thoracic aorta, defined as the midpoint on the centerline between the left subclavian artery and the descending aorta at the level of the aortic valve), and distal descending aorta (from the mid-descending thoracic aorta to the  
Figure 1: Population selection diagram. Abbreviations: TAA—thoracic aortic aneurysm, TAV—tricuspid aortic valve, MRI—magnetic resonance imaging, and 4D—four dimensional. descending aorta at the level of the aortic valve). More details about the aortic segmentation are described by [PERSON] et al. (2020). In addition, the stroke volume and cardiac output were assessed at the level of the sinotubular junction by the first observer for characterizing the left ventricular function of the population using CAAS MR Solutions. Figure 2: 4D flow MRI visualization using CAAS MR Solutions v.5.2.1. Example of a patient (male, 45 years old) with aortic root aneurysm with a diameter of 52 mm. **(I)** 4D flow MRI reconstructions. **(II)** Image analysis including the lumen segmentation, the manual segmentation correction, anatomical partitioning visualized on the wall shear stress maps, and stroke volume assessment. **(III)** Streamline visualization of the 4D flow MRI at the 5 systolic phases (60, 90, 120, 150, and 180 ms). Abbreviations: AoR—aortic root, PAA—proximal ascending aorta, dAA—distal ascending aorta, AoA—aortic arch, pDA—proximal descending aorta, and dDA—distal descending aorta. Secondly, the morphology of the thoracic avtas was analyzed using an in-house developed Python software (Python Software Foundation, Wilmington, DE, USA). This tool was used to quantify aortic volume, centerline length, maximal diameter, and curvature radius of each anatomical segment and for each of the five systolic phases. The aortic diameter was determined by first constructing a cross-section perpendicular to the centerline at every millimeter. Next, at each cross-section, the maximal radial spike length was calculated. The curvature radius was derived by fitting a circle through the segments’ centerline as previously described (2018). The maximal aortic diameter within each segment and the radius of the fitted circle were used for statistical analysis. Thirdly, the hemodynamics of the thoracic aorta were quantified using methods described in previous studies [13; 15; 19]. These methods were implemented in an in-house developed Python software [9; 25]. Initially, the mean blood velocity over all subjects was quantified per anatomical segment. The tool was then used to quantify the normalized flow displacement, flow jet angle, vorticity norm, absolute helicity, and absolute LNH for each anatomical segment and each systolic phase.