The purpose of this study was to evaluate the changes in blood flow and kinetic energy distribution between inspiration and expiration plateaus in TCPC patients using 4D flow MRI. Altered hemodynamics in total cavopulmonary connection (TCPC), a palliation of single ventricle defects, results in long-term complications, such as decreased exercise capacity, arrhythmia, and ventricular failure¹. Non-invasive hemodynamic evaluation of TCPC has been an important clinical challenge. Several studies have tried to understand and predict specific flow features using a combination of patient-specific MRI data and computational tools to develop more realistic numerical and physical models. Most numerical studies have based their analyses of TCPC efficiency on energy loss calculations, but assumptions such as rigid walls, unchanged flow between respiration phases and idealized flow conditions might affect accuracy and hinder clinical applicability². 4D flow MRI using radial projections (PC-VIPR³) allows for flexible retrospectively sorting of data due to intrinsic oversampling of central k-space and pseudo-random sampling trajectories. The purpose of this study was to evaluate changes in blood flow and kinetic energy distribution between inspiration and expiration plateaus in TCPC and single ventricle patients for assessing efficiency of the system using 4D flow MRI. In this IRB-approved and HIPAA-compliant study six (6) patients with TCPC (2M/4F, 24 ± 5.9 years old, 59.8 ± 7 kg) were imaged on a 3T system (Discovery MR750, GE Healthcare) using PC-VIPR prescribed over a large chest imaging volume (FOV = 32 x 32 x 32 cm3, TR/TE = 5.5/2.3 ms, α = 15°, Venc = 150 cm/s, projection number ≈ 22000, 16 reconstructed cardiac time frames, scan time: 11 minutes 30 seconds). In recent work, we developed a scheme that allows for double gating to the ECG and respiratory cycles based on the bellows signal to provide a cardiac series of flow data for separate respiratory phases^4,5. After a moving average filter was applied to the respiratory waveform to subdivide data into two sets: above (inspiration) and below (expiration) the moving average bellows signal, a 40% acceptance threshold above and below the moving average for each respiratory cycle was applied to mitigate potential motion during active respiration. This window was chosen to mimic our standard prospective expiration respiratory gating in which data is only acquired during the lower 40-50% of the bellows signal. The two image sets were exported to an advanced visualization software package (EnSight, CEI). Metrics of total flow (mL/min) and peak kinetic energy (KE=mv² [mJ]) were computed from the velocity vectors at each plane for both respiration plateaus at the inferior (IVC) and superior (SVC) venae cava as well as the right (RPA) and left (LPA) pulmonary arteries (Fig1). Percentage change in flow and kinetic energy was calculated as ((Expiration–Inspiration)/Expiration)*100. Between-plateau differences at each measurement location were assessed using Student’s paired t-tests and were considered significant at the 5% level (p < 0.05). High-quality angiograms as well as flow distribution visualizations were achieved in all patients (Fig2). Across all patients, no significant differences in kinetic energy or flow were found between inspiration and expiration in any of the vessels. Figures 3 and 4 show the flow and kinetic energy results respectively for the six patients. In these figures the lines connect the measurements at inspiration (left) and expiration (right). Percentage change in blood flow and kinetic energy between expiration and inspiration are shown in Table 1. Though no statistical differences in blood flow or kinetic energy were found in any of the vessels in the TCPC, there were large patient specific variations and high variability in the respiratory effects in blood flow and kinetic energy. Interestingly, the changes in kinetic energy were more marked than those in flow, suggesting that even though the intrathoracic pressure changes in the different respiratory phases do not largely influence the blood flow, it changes the acceleration of the fluid within the vessels. These results exhibit greater respiratory-induced flow changes within a single subject than previous work has shown in the same analysis performed on healthy controls, suggesting that respiration plays a larger role in regulating flow in these patients⁵. Future work will focus on direct comparisons between these subject groups Recently published data using real-time 2D PC2 suggested significant changes in blood flow in the IVC and SVC in the TCPC patients. We expect the addition of more TCPC subjects into this data to strengthen these results. In conclusion, future studies using 4D flow MRI and computational fluid dynamics simulations in TCPC subjects must take into account patient specific respiratory variations when determining the boundary conditions.