There are several potential limitations that should be acknowledged. First, the spatial and temporal resolution of 4D flow CMR in this study was relatively low. In the setting of spatial and temporal resolution, there is a trade-off between scan time and the accuracy of parameters such as the flow rate, WSS, and oscillatory shear index estimation [
39]. Decreasing the spatial and temporal resolution in order to shorten the scan time affects the accuracy of flow quantification and visualization adversely [
40] and leads to underestimation of WSS [
11,
39,
40]. As the voxel size increases, the accuracy of WSS estimates decreases [
41]. Since we favored the accuracy of WSS, we used an optimized, small-size in-plane spatial resolution (1.6 × 1.8 mm
2). To minimize the influence of anisotropic voxels, we set the slice gap to 2–2.5 mm. In our preliminary study with young healthy volunteers (n = 5), the scan time was much longer for 20 phases (12.3 ± 1.9 min) than for 12 phases (6.6 ± 0.6 min). In elderly patients with severe heart disease, longer scan duration was impractical, and we decided to reduce the temporal resolution. The lower temporal resolution might imply the loss of possible information contained in other systolic phases [
42]. However, in this study, all acquisitions were made with the same imaging parameters and analyzed with the same methodology both pre- and post-TAVR. Since our study focused on changes in parameters between pre- and post-TAVR, underestimation of WSS might be compensated. Compressed sensing (CS), which exploits the inherent compressibility of CMR data, has been combined with parallel imaging to achieve even higher acceleration rates, enabling 4D flow CMR scan times to become clinically feasible [
43,
44]. Ma et al. applied CS to 4D flow imaging of the thoracic aorta and achieved a scan time of 2 min [
45]. However, CS 4D flow CMR has been reported to underestimate peak flow and velocity. Further investigation using CS-based protocols is needed. Second, since our CMR unit did not allow multiple VENC settings, we set a single VENC value according to the velocity in the head/foot direction. However, this setting could measure the majority of high velocity compared to anterior/posterior or right/left directions. Third, the Reynolds number could provide more information on the impact of turbulent flow in the aorta from the stenotic aortic valve and how it affects the calculation of the parameters using 4D flow CMR. However, we were unable to actually calculate the Reynolds number due to the inability to measure aortic inlet diameter by 4D flow CMR and to accurately evaluate flow velocity after TAVR due to signal loss caused by metal artifacts of the implanted valve. Therefore, we calculated the substitute Reynolds number for each case using the diameter of the AAo on computed tomography, mean aortic velocity on echocardiography, and blood kinematic viscosity ν = 3.3 × 10
−6. Substitute Reynolds number decreased significantly from 9741 [8992–12,574) before TAVR to 4358 [3688–4790] after TAVR (P < 0.001). From these numbers, because the WSS calculation was done in a turbulent state in both pre- and post-TAVR, we think that the comparison of WSS is valid. Furthermore, because the WSS is calculated by directly deriving the blood flow velocity gradient from the flow velocity near the wall in the software used for this study, the calculation is applicable in both laminar and turbulent flows. Generally, in 4D flow CMR, turbulent flow decreases the signal and causes an underestimation of the flow velocity [
46]. Due to the limitations of the spatial resolution of 4D flow CMR, the reliability of the absolute value of WSS is controversial, however, we believe that it is meaningful in comparing relative changes before and after the intervention. We recognize that the failure to assess the actual effect of the Reynolds number on the changes of blood flow dynamics in 4D flow CMR is a major limitation of the present study. Fourth, although the blood flow patterns were evaluated after thorough confirmation of the evaluation methods by two radiologists and one cardiologist, these evaluations may have differed among them. This is considered to be a limitation of the visual semi-quantitative method. In this study, those assessments were reproducible (but not yet well standardized) and susceptible to observer bias. Fifth, we did not have sufficient sample size for a subgroup analysis to determine the differences in blood flow dynamics between balloon- and self-expanding bioprostheses. Although our findings showed that the self-expanding bioprostheses might reduce the WSS and EL in the AAo more than the balloon-expanding bioprostheses, further studies are warranted to confirm these findings. Finally, the number of adverse events and the sample size were too small to evaluate changes in the LV function or remodeling. Therefore, we were unable to confirm the relationship between impaired blood flow dynamics (obtained from 4D flow CMR) and worse clinical outcomes. Accordingly, further larger-scale studies are necessary to confirm the prognostic significance of the changes in blood flow dynamics in patients who undergo TAVR.