Voxel-by-voxel 4D flow MRI-based assessment of regional reverse flow in the aorta

Background: Complex and reverse flow in the aorta has been implicated in aneurysm development and stroke via retrograde embolization. Purpose: To evaluate global and regional differences between standard 2D plane-based and volumetric voxel-based quantification of regional forward/reverse flow, and reverse flow fraction (RFF) in the aorta. Study Type: Retrospective. Subjects: In all, 35 subjects: 10 healthy controls (age: 57 ± 7 years, nine male), nine patients without aortic valve regurgitation (AR) (age: 63 ± 10 years, seven male), six patients with mild AR (age: 66 ± 6 years, five male), and 10 with moderate or severe AR (age: 60 ± 16 years, eight male). Field Strength/Sequence: 4D flow MRI (3T and 1.5T) was employed to acquire 3D blood flow velocities with entire thoracic aorta in all subjects. Assessment: Data analysis included standard 2D plane-based quantification of forward/reverse flow, and RFF-plane. In addition, a new semiautomatic workflow based on 3D segmentation and extraction of an aorta centerline was developed for voxel-by-voxel visualization (forward/reverse flow and RFF-voxel maps) and quantification of regional voxel-by-voxel forward/reverse flow in the entire thoracic aorta. Statistical Tests: Kruskal–Wallis tests were performed to test for differences between groups. A two-sample t-test or Wilcoxon rank sum test was used to compare voxel-based and plane-based results. Results: Semiautomatic plane-based analysis showed excellent agreement with standard manual plane-based analysis for net flow and RFF-plane (RFF-plane: y = 0.99x-0.0, net flow: y = 1.00x-0.21, R > 0.99, P < 0.0001). Voxel-by-voxel maps demonstrated marked regional flow reversal in the ascending aorta in all patients and RFF-voxel was significantly increased (P < 0.001) compared to RFF-plane for all four groups, with the most pronounced differences for mild AR (18.0 ± 15.2% vs. 4.7 ± 5.4%). Voxel-based flow and RFF-voxel along the aorta showed areas with marked regional flow reversal (eg, vortex flow) compared to plane-based analysis. Data Conclusion: Voxel-based analysis demonstrated regional flow reversal that was not detected by plane-based analysis. Level of Evidence: 3. Technical Efficacy: Stage 1. J. Magn. Reson. Imaging 2018;47:1276–1286.