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

Abstract

Objectives To study flow-related energetics in multiple anatomical segments of the total cavopulmonary connection (TCPC) in Fontan patients from 4D flow MRI, and to study the relationship between adverse flow patterns and segment-specific energetics. Methods Twenty-six extracardiac Fontan patients underwent 4D flow MRI of the TCPC. A segmentation of the TCPC was automatically divided into 5 anatomical segments (conduit, superior vena cava, right/left pulmonary artery (PA) and the Fontan confluence). The presence of vortical flow in the PAs or Fontan confluence was qualitatively scored. Kinetic energy, viscous energy loss 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. Results 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 kinetic energy and viscous energy loss rate, while most favorable energetics were observed in the superior vena cava. Conduit CSA inversely correlated with kinetic energy (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. 4D flow MRI derived energetics may be used as a screening tool for direct, MRI-based assessment of flow efficiency in the TCPC.

4D flow cardiovascular magnetic resonance derived energetics in the Fontan circulation correlate with exercise capacity and CMR-derived liver fibrosis/congestion

“…The region of interest covered the area between the Fontan tunnel (above entry of the hepatic veins), superior vena cava (SVC) (below the brachiocephalic vein) and the right- and left pulmonary arteries (PAs) up to the levels of the segmental branches. The right upper lobe branch was excluded from the segmentation since the resolution of the 4D flow CMR for this small vessel is insufficient [ 19 ].…”

“…In patients in whom the entire TCPC was available for analysis, energetics were normalized for inflow (SVC + Fontan tunnel flow, in L/min); KE norm_flow in mJ per L/min and EL norm_flow in mW per L/min [ 25 ].…”

“…To further explore and compare the energetics in the specific components of the TCPC, a sub analysis was performed to determine segment-specific energetics (Fontan tunnel, central Fontan confluence, SVC, left PA (LPA) and right PA (RPA), using in-house developed software as previously described [ 25 ]. Additional benefit of this approach is that in patients with a fenestration closure device or PA stent in situ, in whom the entire TCPC cannot be segmented because of device-related flow artefacts, energetics in the TCPC segments without device-related artefacts can still be studied.…”

“…Segments with a length < 1.5 cm were excluded from the analysis to ensure sufficient voxels for energetic analysis. [ 25 ].…”

“…In summary, the TCPC was automatically divided into 5 segments. Energetics within each segment were normalized for segment-specific inflow (Fontan confluence) or inflow + length (Fontan tunnel, SVC, LPA, RPA; KE norm_flow+length and EL norm_flow+length ) [ 25 ]. The additional normalization for length in these segments is required as these segments are not always completely available for analysis (e.g.…”