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 (n511) and patients with congenital heart defects (n517). 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 midsystole, 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 (mean6 1.96 SD up to -0.2460.40N/m 2 and 0.561.38N/m 2 for 8-fold and 27-fold voxel size, respectively) and VENCdepending phase noise (mean61.96 SD up to 0.3160.12N/m 2 and 0.9460.28N/m 2 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 1.21N/m 2) 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