BACKGROUND: Asymmetrical caval to pulmonary blood flow is suspected to cause complications in patients with Fontan circulation. The aim of this study was to test the feasibility of 4-D flow MRI for characterizing the relationship between 3-D blood flow distribution and vascular geometry. OBJECTIVE: We hypothesized that both flow distribution and geometry can be calculated with low interobserver variability and will detect a direct relationship between flow distribution and Fontan geometry. MATERIALS AND METHODS: Four-dimensional flow MRI was acquired in 10 Fontan patients (age: 16 +/- 4 years [mean +/- standard deviation], range: 9-21 years). The Fontan connection was isolated by 3-D segmentation to evaluate flow distribution from the inferior vena cava (IVC) and superior vena cava (SVC) to the left and right pulmonary arteries (LPA, RPA) and to characterize geometry (cross-sectional area, caval offset, vessel angle). RESULTS: Flow distribution results indicated SVC flow tended toward the RPA while IVC flow was more evenly distributed (SVC to RPA: 78% +/- 28 [9-100], IVC to LPA: 54% +/- 28 [4-98]). There was a significant relationship between pulmonary artery cross-sectional area and flow distribution (IVC to RPA: R(2)=0.50, P=0.02; SVC to LPA: R(2)=0.81, P=0.0004). Good agreement was found between observers and for flow distribution when compared to net flow values. CONCLUSION: Four-dimensional flow MRI was able to detect relationships between flow distribution and vessel geometry. Future studies are warranted to investigate the potential of patient specific hemodynamic analysis to improve diagnostic capability.