# Whole-Heart Assessment of Turbulent Kinetic Energy in the Repaired Tetralogy of Fallot
## 4 Discussion
It is reasonable to consider that these basic metrics can support decision making, but there could be other factors besides RV volume and deformation rate that can contribute to an adverse outcome. Flow-derived metrics, as TKE, could provide a major understanding on 3D intra-cardiac hemodynamics and local alterations within the blood flow. It is important to remark that 3D hemodynamics are not fully characterized using standard ultrasound or MRI. Furthermore, studies also demonstrated the alteration of flow patterns by overserving higher retrograde flow and pathological vortices within the right side of the heart in patients with rTOF compared to controls [41, 42]. The observation of retrograde flow and vortices may explain the elevated TKE measured in the PA and RV throughout the cardiac cycle, as an expenditure of energy occurs during vortex formation and dissipation. Right-sided vortices may represent energy loss and poor efficient circulation, which can potentially be harmful for the RV in TOF with impaired contractile capacity and could indicate an early intervention. However, in the current study, the vortex formation was not evaluated or investigated in association with TKE. Moreover, this study also evaluated the comparison between TKE with standard clinical measurements including LVEF, RVEF, LVEDVi, LVESVi, RVEDVi, and RVESVi. As no significant or strong correlation was observed between TKE and standard clinical measurements evaluated, this suggests that TKE is an independent local measurement providing further insight into the abnormal flow seen in this patient cohort. A previous study performed by [PERSON] et al. also demonstrated similar results. This study also found that the total TKE was not related to global flow patterns that are evaluated by magnetic-resonance-measured velocity fields [30]. As the current work was an exploratory pilot study, the recruitment of more patients and healthy controls is highly recommended in further understanding how TKE may play a role in various vessels within this patient cohort. Moreover, the evaluation of other irregular flow parameters should be conducted and correlated for each vessel for a better understanding of hemodynamic differences between patients and controls. In the current study, we developed simple reporting templates for the visual assessment of individuals and cohorts. The latter could simplify the TKE interpretation and could be used for a visual follow-up assessment. However, not all templates respected current clinical guidelines. In particular, ventricular templates could use the American Heart Association's (AHA) standard reporting procedures when reporting to clinicians. Our simplified approach aimed to provide a quick snapshot of TKE. The latter highlights the need for well-defined reporting for TKE and 4D-flow measurements, which can be substantially complex given the large amount of data. This aspect explains why a single time point is frequently reported (e.g., peak systole) or simplified using time-average values or maximum intensity projections. Furthermore, in future, TKE measurements could be indexed to the volume of each vessel for both controls and patients, which may provide further insight into the difference observed between the right and left chambers. The association of TKE with the onset of any rTOF complications must be investigated to understand the TKE role in adverse outcomes. Lastly, intra-observer and inter-observer variabilities should be evaluated to validate this technique. Our study only considered rTOF patients for the investigation of TKE. However, other congenital diseases could benefit from the characterization of abnormal TKE. A recent numerical study demonstrated that lower vortex formation and lower TKE can identify deteriorated intra-cardiac performance in Fontan patients, while the ejection fraction did not, whereas the latter still needs to be demonstrated in vivo [43]. The use of computational fluid dynamics (CFD) to complement and/or validate 4D-flow-derived calculations is not new.