## 4 Discussion  
This study illustrates the usefulness of 4D-flow to quantify and assess LA stasis. The main findings of our study were: (1) LA stasis can be substantially reduced after PV ablation; (2) Standard deviation LA stasis and atrial volume significantly increase with age;  
Figure 3: Age comparison for left atrial volume and standard deviation stasis. Panel (**A**) shows the comparison of age groups for left atrial (LA) volume. Panel (**B**) indicates the comparison of age groups for standard deviation LA stasis. Figure 2: Left atrial stasis and velocity in controls, pre-ablation, and post-ablation patients. Panel (**A**) shows the comparison of all cohorts for mean left atrial (LA) stasis. Panel (**B**) shows the comparison of standard deviation LA stasis. Panels (**C**) and (**D**) show the comparison for all cohorts for mean and standard deviation LA velocity. (3) Standard deviation LA stasis was higher in patients with risk score > 2 pre-ablation and was reduced post-ablation. [PERSON]'s triad is widely accepted and defines the main factors leading to thrombogenesis [15]. These factors include abnormal stasis or reduced blood flow, endothelial/endocardial injury or dysfunction, and hypercoagulability. Time-resolved 4D-flow provides a non-invasive characterization of LA hemodynamics and has been used to assess AF patients pre-ablation [7; 8; 16]. As derived from 4D-flow, LA stasis may serve as an essential biomarker that characterizes the predisposition to atrial thrombogenesis to that of regular risk predictors (e.g., CHA2 DS2-VASc Score). Despite the comprehensive assessment obtained by 4D-flow, an accurate 3D segmentation is required. In our study, semi-manual segmentation was obtained. A careful selection of intensity threshold can facilitate the appropriate visualization of PV and LA appendage. [PERSON] et al. reported comparable mean flow velocity in the LA between paroxysmal AF patients and controls [14]. In our study, the mean LA velocity was significantly reduced in pre-ablation and post-ablation, which aligned with a more extensive study conducted by [PERSON] et al. [17]. The latter was supported by other recent studies [6; 8; 16]. Significant inverse relationships between LA mean/peak velocity and risk score were reported in previous studies [6; 17]. In our study, no significant associations were found for LA mean/peak and standard deviation velocities. Much of the attention has been given to the LA appendage as the primary source of thrombus formation in AF. A previous study from [PERSON] et al. suggested that atrial flow dynamics are disrupted in AF patients, even during coordinated activity, thus suggesting an AF component that may be unrecognized [6]. In one of our recent studies, we demonstrated that vortex formation within the LA is influenced by pulmonary vein inflow velocities and LA remodeling with a higher prevalence of stasis [8]. There may be an interplay between the functional (LA volume, pressure overload, and dilation) and hemodynamic features (stasis, vortex formation, energetics, and flow patterns). The associations between these factors are complex and may have compensatory effects. However, identifying irreversible recovery should warn for earlier detection of complications or adverse outcomes [18; 19]. Despite the lack of significant change in LA velocities between the pre-ablation and post-ablation cohort, we were able to detect an improvement via mean LA stasis. Note that LV and LA standard metrics also failed to detect a significant change pre- and post-ablation. Overall, these findings put in context new evidence characterizing paroxysmal AF complex hemodynamics. However, current guidelines rely on clinical factors and comorbidities, with the exclusion of AF burden, atrial size, flow hemodynamics, and other factors that may suggest more advanced atrial disease and this higher risk of thromboembolism events [20]. Therefore, including the above-mentioned factors may help to improve long-term LA recovery outcomes.