Pediatric bicuspid aortic valve (BAV) patients may be asymptomatic in childhood, but later go on to develop valve dysfunction and aortopathy in adolescence and adulthood. Therefore, understanding the natural history of BAV may aid in determining if and when these patients are at risk for disease progression. Time-resolved 3D phase contrast MRI with three-directional velocity encoding (4D flow MRI) can provide 3D visualization of blood flow patterns, quantification of flow velocities, and assessment of wall shear stress (WSS, an important biomarker for vessel remolding) based on a single exam. Recent 4D flow MRI studies in BAV patients showed its sensitivity to detect changes in aortic hemodynamics related to different BAV cusp fusion morphology [1, 2]. In a study with adult BAV patients, Hope at al. [3] demonstrated the potential diagnostic value of flow pattern changes to predict risk for the development of aortopathy. However, previous longitudinal studies did not investigate pediatric BAV patients and were mostly based on simple metrics (e.g. peak velocity) or qualitative data (e.g. flow patterns). The aim of this study was to employ a comprehensive evaluation of aortic hemodynamics in a cohort of pediatric BAV patients by assessing flow patterns, peak velocity and WSS changes over the course of two 4D flow MRI studies.For this retrospective IRB-approved study, we reviewed 12 pediatric patients (3 females) with BAV who received baseline (age = 14 ± 6.1 (1-21) years) and follow-up (age=15 ± 6.1 (2-22) years) 4D flow MRI studies as part of clinical cardiac MRI surveillance. All MRI scans were performed at 1.5 T (Avanto or Aera, Siemens, Germany) with spatial resolution = 1.23-3.46 x 1.13-2.5 x 1.2-3.0 mm3, temporal resolution 37.6-44 ms, TE/TR/FA = 2.3-2.8 ms/4.7-5.1 ms/15° and velocity sensitivity = 150–400 cm/s. 4D flow data were preprocessed to reduce noise and artifacts caused by velocity aliasing and phase offset errors (Maxwell terms, eddy currents). 3D phase contrast MR angiograms were computed from 4D flow data and used to obtain a 3D segmentation of the thoracic aorta (Mimics, Materialise, Belgium). Time resolved pathlines and static streamlines at peak systole were generated using the 4D flow velocity field masked by the 3D segmentation (EnSight, CEI, USA, Figure 1 A and B). Pathlines and streamlines were visually assessed, by two experienced cardiac imaging physicians in a blinded fashion. Flow pattern grading included presence and severity of helices and vortices, in the aortic root (AoR), ascending aorta (AAo), arch, descending aorta (DAo) and globally for helix flow only. The AoR was scored from 0-3, based on the size of the helix/vortex. Global helix flow was scored 0 or 1 (present or not), regional flow patterns were scored from 0-2 (degree of rotation of the helix/vortex). Peak systolic velocities [3] in the AAo, arch and DAo and mean regional WSS values [4] in the inner and outer proximal AAo (prox. inner AAo and prox. outer AAo) and the inner and outer distal AAo (dist. inner AAo and dist. outer AAo) were calculated (Figure 1 E and F). AoR z-scores were calculated for 9 subjects.The mean duration between baseline and follow-up was 19 ± 10 (7-37) months. Results from regional WSS analysis and peak systolic velocity assessment are summarized in table 1. There were no significant differences in peak velocity and mean WSS between baseline and follow-up. Figs. 2 and 3 show changes between baseline and follow-up in mean WSS and peak velocity, respectively. Results from qualitative assessment are summarized in table 2. The highest kappa coefficient, 0.42, was for grading helices in the DAo while the lowest , 0, was for vortices in the DAo. There were no significant differences in grader-averaged visualization scores between baseline and follow-up. Mean AoR z-score was 3.8 ± 1.9 at baseline and 3.8 ± 2.3 at follow-up.The stability of visual grading, WSS and peak systolic velocity between baseline and short-term follow-up studies suggest the BAV disease has progressed little during this time. The reproducibility of 4D flow peak velocity assessment and regional WSS analysis are both bolstered by the similarity of results between baseline and follow-up. The slight-to-moderate agreement among the graders highlights the difficulty and complexity of visually grading hemodynamics and suggests quantitative measures should be favored when trying to establish a patient specific baseline for monitoring BAV disease progression. This study is limited by its small cohort and short-term follow-up. By following and monitoring BAV patients over longer periods of time, future studies would be more likely to observe how and when significant hemodynamic changes occur in these patients.