Comparison of 4D flow MRI and computational fluid dynamics in carotid models with different stenosis levels.

Internal carotid artery (ICA) stenoses are a major source of stroke. In addition to traditional cardiovascular risk factors, hemodynamic parameters such as wall shear stress (WSS) are recognized as critical contributors to the progression and rupture of ICA plaques. This study evaluates the ability of 4D flow MRI to predict the velocity and WSS fields in three models of carotid bifurcations representing stenosis ratios of 0% (healthy), 40% (mildly stenosed), and 80% (severely stenosed) in the ICA. The stenosis ratio significantly impacts hemodynamics, increasing peak velocity, WSS, and flow disturbances. Compared to the healthy model, peak velocity rises by 38% in severe stenosis, generating a high-velocity jet and post-stenotic recirculation. Spatially averaged WSS increases by 21.5% in the mildly stenosed model and 99% in the severely stenosed model. Additionally, flow separation and vortex formation intensify as stenosis severity increases. A robust experimental setup with carotid bifurcation phantoms has been developed to eliminate the major sources of uncertainty in the boundary conditions. This enables a detailed quantitative comparison of 4D flow MRI measurements (obtained with a 7T scanner) against results from a CFD simulation. Moreover, the CFD results were validated against results obtained from Direct Numerical Simulations (DNS). Therefore, the CFD data could be used as a reliable high-fidelity reference for the flow within the carotid models. The comparison indicates that 4D flow MRI on a 7T scanner captures the major flow patterns with reasonable accuracy, though quantitative discrepancies increase with the stenosis ratio. Specifically, the peak velocity error rises from 6.1% in healthy arteries to 11.9% in severely stenosed arteries. Similarly, the error in spatially averaged WSS magnitude in the bifurcation region increases from 18% in healthy cases to 38% in severely stenosed cases. These findings underscore the need for validated approaches to accurately analyze complex flow patterns within carotid arteries, especially as even high-quality 4D flow MRI measurements using a 7T scanner with 0.5 mm resolution show diminishing accuracy in such challenging cases.