# 4D-Flow MRI Characterization of Pulmonary Flow in Repaired Tetralogy of Fallot
###### Abstract  
Patients with Tetralogy of Fallot (TOF) have multiple surgical sequelae altering the pulmonary flow hemodynamics. Repaired TOF (rTOF) adults frequently develop pulmonary regurgitation impacting the blood flow pressure, right ventricle load, and pulmonary hemodynamics. We aimed to evaluate the pulmonary flow hemodynamics using 4D-flow magnetic resonance imaging (MRI) for characterizing altered blood flow, viscous energy loss (EL), wall shear stress (WSS), pressure drop (PD), and ventricular flow analysis (VFA) in rTOF patients. We hypothesized that 4D-flow based parameters can identify pulmonary blood flow alterations. A total of 17 rTOF patients (age: 29 \(\pm\) 10 years, 35% women) and 20 controls (age: 36 \(\pm\) 12 years, 25% women) were scanned using a dedicated cardiac MRI protocol. Peak velocity and regurgitration were significantly higher for rTOF patients (\(p<\) 0.001). WSS was consistently elevated along the PA in the rTOF (\(p\leq\) 0.05). The rTOF average circumferential WSS was higher than axial WSS at the main pulmonary artery (\(p\leq\) 0.001). PD and EL were consistently higher in the rTOF as compared with controls (\(p<\) 0.05). For VFA, delayed ejection increased and retained inflow decreased in rTOF patients (\(p<\) 0.001). To conclude, this study demonstrated that 4D-flow MRI pulmonary flow in the rTOF can exhibit altered peak velocity, valvular regurgitation, WSS, EL, PD, and VFA.  
Published: 22 February 2023  
The evaluation of cardiovascular flow is commonly performed by Doppler echocardiography. However, this imaging modality is limited by many constraints including limited acoustic window, variable velocity assessment, and operator expertise [6; 7]. In addition, the calculation of net flow and mean velocities is frequently based on assumptions (i.e., circularity and flow parabolic profile), resulting in inaccurate flow quantification in tortuous vessel geometries along with complex helical flow [6]. Thus, a more comprehensive and advanced evaluation is required for better understanding of repaired TOF (rTOF) patients.  
Cardiovascular magnetic resonance (CMR) is currently used for monitoring rTOF patients when the pulmonary artery and RV are difficult to visualize and evaluate with Doppler echocardiography due to poor acoustic windows in retrosternal position [7; 8]. The most frequently used flow sequence is two-dimensional phase-contrast (PC) CMR with through-plane velocity encoding (2D cine PC-CMR) [6]. Even though this technique is widely used for flow quantification, due to the underlying assumptions that are made, it is subject to various inaccuracies in eccentric jets and complex flow patterns. However, 4D-flow magnetic resonance imaging (MRI) measures the velocity in each spatial direction (x,y,z) throughout the cardiac cycle [9; 10]. Furthermore, 4D-flow MRI facilitates off-line measurement of hemodynamic metrics anywhere within the data volume [9]. It provides exceptional visualization of complex flow patterns, hence it is increasingly being used and beneficial to guide surgical intervention such as Fontan conduit placement [4; 10; 11]. In addition, 4D-flow MRI derivates various hemodynamic parameters, enabling the quantification of wall shear stress (WSS), viscous energy loss (EL), pressure drop (PD), and ventricular flow analysis (VFA) [1; 6; 12]. Incremental helical or vortical flow in the main pulmonary artery (MPA) is characteristic of patients with rTOF [13; 14; 15]. Hence, further higher hemodynamic parameters need to be evaluated at the MPA to better understand the abnormal hemodynamics that are presented within patients with rTOF.  
Therefore, this study aimed to investigate pulmonary hemodynamics using 4D-flow MRI for characterizing altered VFA, WSS, EL, and PD changes in rTOF patients. We hypothesize that 4D-flow-based parameters can identify pulmonary blood flow alterations.