# 4D-Flow MRI Characterization of Pulmonary Flow in Repaired Tetralogy of Fallot
## 4 Discussion  
Our main findings derived from the 4D-flow MRI assessment and showed that: (1) peak velocity and regurgitant fraction are elevated in the rTOF with respect to the controls; (2) Axial and circumferential WSS demonstrated better discrimination between the rTOF and the controls than the WSS magnitude; (3) PD and EL were significantly higher in the rTOF in comparison to the controls; (4) VFA demonstrated a reduced delay ejection and an elevated retained flow on the rTOF as compared with the controls; and (6) PA diameter, pulmonary peak velocity, and right ventricular function were associated, in diverse degrees, to 4D-flow derived metrics. This study demonstrated that rTOF patients exhibited altered hemodynamics in the right heart as characterized by peak velocity, regurgitant fraction, WSS, EL, PD, and VFA using 4D-flow MRI. The RV functional assessment can be challenging in the rTOF due to the dyssynchronous contraction patterns and anatomic complexity present in these patients. As expected, RV function was found to be slightly impaired in the rTOF cohort. In the present study, total volume, peak velocity, and regurgitant fraction succeeded in identifying local differences along the PA. A study performed on rTOF by [PERSON] et al. showed that 4D-flow plane measurements are more accurate than single direction 2D phase contrast for valvular flow and assessment of RV diastolic function [18]. Our results showed that 4D-flow can retrospectively assess standard flow metrics at any heart location. The latter has also been highlighted by the current recommendations for congenital heart diseases using 4D-flow MRI [10]. An important aspect of standard flow measurements at valve locations is valve tracking [19]. In the present study, we did not use valve tracking at the valve locations. However, recent studies have demonstrated that 4D-flow valve plane retrospective tracking can make the flow assessment more accurate by reducing errors in through-plane motion and flow angulation [10; 19; 20]. It is common in patients with rTOF to observe varying degrees of pulmonary regurgitation, pulmonary valve stenosis, PA branch stenosis, and PA dilation. Therefore, PA flow is expected to show some level of abnormalities. Previous work investigated the development of abnormal blood flow patterns in the RV and PA in the rTOF [4; 14; 21]. They reported helical flow and vortex formation in the RV and in the PA of the rTOF. [PERSON] et al. showed counter-rotating helical streams along the MPA and its association at peak systole with helical flow observed at the bifurcations [22]. The development of helical flow is not well understood. It has been suggested that it may be a response for driving the blood flow efficiently in the presence of stenosis in the vessel [23]. However, it can also be a physical adaptation due to characteristics of stenosis. We did not assess or quantify blood flow helicity or vorticity. Instead, we concentrated on understanding WSS, EL, and PD effects. There are few studies investigating the role of WSS in the rTOF. Our findings supported new evidence demonstrating that WSS magnitude was able to discriminate abnormal WSS along the PA. Our findings aligned with the results reported by [PERSON] et al. [1]. Of particular interest, WSS vector decomposition (axial and circumferential WSS) demonstrated the capacity to detect abnormalities. Axial and circumferential WSS would be useful to explain the wall interactions with helical flow and vortex formation in future studies. In addition, WSS measurement showed varying degrees of correlation with PA diameter, pulmonary valve peak velocity, and regurgitant fraction supporting its potential clinical usefulness. PD is currently calculated using 2D phase-contrast measurements by applying the simplified Bernoulli's equation. [PERSON] et al. showed that the simplified Bernoulli method underestimated PD throughout all cardiac phases and it is necessary to consider the entire velocity field and unsteady conditions [24].