Wall shear stress estimation in the aorta: Impact of wall motion, spatiotemporal resolution, and phase noise

Background: Wall shear stress (WSS) presents an important parameter for assessing blood flow characteristics and evaluating flow-mediated lesions in the aorta. Purpose: To investigate the robustness of WSS and oscillatory shear index (OSI) estimation based on 4D flow MRI against vessel wall motion, spatiotemporal resolution, and velocity encoding (VENC). Study Type: Simulated and prospective. Population: Synthetic 4D flow MRI data of the aorta, simulated using the Lattice-Boltzmann method; in vivo 4D flow MRI data of the aorta from healthy volunteers (n = 11) and patients with congenital heart defects (n = 17). Field Strength/Sequence: 1.5T; 4D flow MRI with PEAK-GRAPPA acceleration and prospective electrocardiogram triggering. Assessment: Predicated upon 3D cubic B-splines interpolation of the image velocity field, WSS was estimated in mid-systole, early-diastole, and late-diastole and OSI was derived. We assessed the impact of spatiotemporal resolution and phase noise, and compared results based on tracked—using deformable registration—and static vessel wall location. Statistical Tests: Bland–Altman analysis to assess WSS/OSI differences; Hausdorff distance (HD) to assess wall motion; and Pearson's correlation coefficient (PCC) to assess correlation of HD with WSS. Results: Synthetic data results show systematic over-/underestimation of WSS when different spatial resolution (mean ± 1.96 SD up to –0.24 ± 0.40 N/m² and 0.5 ± 1.38 N/m² for 8-fold and 27-fold voxel size, respectively) and VENC-depending phase noise (mean ± 1.96 SD up to 0.31 ± 0.12 N/m² and 0.94 ± 0.28 N/m² for 2-fold and 4-fold VENC increase, respectively) are given. Neglecting wall motion when defining the vessel wall perturbs WSS estimates to a considerable extent (1.96 SD up to 1.21 N/m²) without systematic over-/underestimation (Bland–Altman mean range –0.06 to 0.05). Data Conclusion: In addition to sufficient spatial resolution and velocity to noise ratio, accurate tracking of the vessel wall is essential for reliable image-based WSS estimation and should not be neglected if wall motion is present. Level of Evidence: 2. Technical Efficacy: Stage 2. J. Magn. Reson. Imaging 2018;48:718–728.