Segmental assessment of blood flow efficiency in the total cavopulmonary connection using four-dimensional flow magnetic resonance imaging: vortical flow is associated with increased viscous energy loss rate

AIMS: To study flow-related energetics in multiple anatomical segments of the total cavopulmonary connection (TCPC) in Fontan patients from four-dimensional (4D) flow magnetic resonance imaging (MRI), and to study the relationship between adverse flow patterns and segment-specific energetics. METHODS AND RESULTS: Twenty-six extracardiac Fontan patients underwent 4D flow MRI of the TCPC. A segmentation of the TCPC was automatically divided into five anatomical segments [conduit, superior vena cava (SVC), right/left pulmonary artery (LPA), and the Fontan confluence]. The presence of vortical flow in the pulmonary arteries or Fontan confluence was qualitatively scored. Kinetic energy (KE), viscous energy loss rate, and vorticity were calculated from the 4D flow MRI velocity field and normalized for segment length and/or inflow. Energetics were compared between segments and the relationship between vortical flow and segment cross-sectional area (CSA) with segment-specific energetics was determined. Vortical flow in the LPA (n = 6) and Fontan confluence (n = 12) were associated with significantly higher vorticity (P = 0.001 and P = 0.015, respectively) and viscous energy loss rate (P = 0.046 and P = 0.04, respectively) compared to patients without vortical flow. The LPA and conduit segments showed the highest KE and viscous energy loss rate, while most favourable energetics were observed in the SVC. Conduit CSA inversely correlated with KE (r = −0.614, P = 0.019) and viscous energy loss rate (r = −0.652, P = 0.011). CONCLUSIONS: Vortical flow in the Fontan confluence and LPA associated with significantly increased viscous energy loss rate. Four-dimensional flow MRI-derived energetics may be used as a screening tool for direct, MRI-based assessment of flow efficiency in the TCPC.