Current ACC/AHA guidelines recommend surgical treatment of aortopathy based on a diameter threshold. 1 However, controversy remains regarding the choice of surgery as well as the extent of aortic tissue to resect. 2 Several studies based on 4D flow MRI have suggested that abnormal aortic blood flow hemodynamics at the wall are associated with aortopathy. 3-6 In addition, a recent study of patients undergoing aortic surgery demonstrated that elastin and extracellular matrix function were altered in resected regions exhibiting abnormally high wall shear stress (WSS) by pre-operative 4D flow MRI. ⁷ The purpose of the present study was to further investigate the evolution of such ‘at-risk’ aortic tissue after surgery using 4D flow MRI.Twenty-three patients (51±15 years; 5 women) who underwent aortic valve and/or ascending aorta (AA) surgery with pre- and post-operative MRI, were included. MRI data were acquired either at 1.5T or 3T (Siemens Healthcare, Erlangen, Germany). It included 4D flow acquisition in a sagittal volume encompassing the entire thoracic aorta, which was performed using prospective ECG and respiratory gating and the following parameters: TE/TR/FA = 2.2–2.8 ms/4.5–5.4 ms/7–15°, spatial resolution = 2.2–3.8 x 1.7–2.7 x 2.2–3.0 mm³, temporal resolution = 36-43 ms and encoding velocity = 150-400 cm/s depending on aortic valve stenosis severity. After pre-processing for noise reduction, anti-aliasing and Eddy current correction using a in-house software, ⁸ the aortic volume was segmented using Mimics (Materialise, Leuven, Belgium). WSS throughout the aortic surface was automatically estimated using 4D flow MRI data both before and after surgery (Figure 1.A). ⁹ ‘Heat maps’ were subsequently created using physiologically normal WSS atlases previously established in age-matched control volunteers (Figure 1.B and C). ¹⁰ Aortic at-risk tissue was defined as regions with WSS above the 95% confidence interval (CI) of normal volunteers. Finally, the 3D area of at-risk tissue was measured in three segments (Figure 1.C): 1) AA (from the aortic valve to the first supraaortic branch), 2) arch (from the first to the last branches) and 3) proximal descending aorta (DA) (from the last branch to the DA, at the level of the aortic valve). Area was further expressed in percentage of the total area of the corresponding segment. Of note, pre-surgical areas excluded aortic tissue to be resected, as assessed retrospectively using CE-MR Angiography (CE-MRA) images, and post-surgical areas excluded the implanted graft. In addition, sinus of Valsalva (SOV) and mid-ascending aortic (MAA) diameters were measured using CE-MRA.One patient had a unicuspid aortic valve, 3 had a tricuspid aortic valve while the 19 remaining patients had a bicuspid aortic valve. At baseline, 5 patients had a severe aortic valve stenosis and 3 had severe aortic regurgitation. Baseline SOV and MAA diameters were 44.2±5.2 mm and 44.6±6.2 mm, respectively. Mean follow-up duration between the 2 MRI exams was 159±238 [7-977] days. Five patients underwent aortic valve replacement (AVR) alone, 9 underwent root and AA replacement (including 1 valve-sparing and 1 Ross procedure), and 9 underwent root and AA replacement along with hemiarch repair. Patient characteristics according to the surgery group are described in Figure 2. Figure 3 provides heatmaps for each patient sub-group, along with absolute areas of at-risk tissue, outside of resected tissue or graft location. Of note, the illustrated 2D heatmaps do not always represent the whole area, which was calculated throughout the 3D aortic surface. Percentage of pre- and post-surgical at-risk tissue areas in the AA, arch and DA are reported in Figure 4. Overall, a post-surgical decrease in aortic at-risk tissue area was observed in patients undergoing AVR without aortic resection, while it tended to increase outside of the graft in patients who underwent AA replacement.These preliminary results indicate unique patient-specific hemodynamics, and suggest the potential value of a patient-centered imaging to identify the most suitable type of surgery to perform. However, care must be taken when interpreting the results, as they were variable and taken from a relatively small sample, as reflected by the high standard deviations within groups.The post-surgical decrease in aortic at-risk tissue area in patients who underwent AVR might indicate the normalization of aortic hemodynamics after intervention. The slight increase in patients who underwent aortic replacement may reflect the effect of the graft on local hemodynamics, due to the alterations in thoracic aortic compliance. However, larger studies are warranted to confirm these findings and shed light on the impact of the type of surgery, size of aortic valve prosthesis, etc. on aortic hemodynamics.