## 4 Discussion
Therefore, 4D flow MRI velocity patterns exhibit smaller secondary flow velocities compared to that of the CFD models, i.e., less swirling more parallel flows, and less transitional flows in the recirculation regions at the carotid sinus and bifurcation [22]. For clinicians, secondary flow patterns are of great importance in evaluating vascular disease [27]. Hence, mapping these behaviors with an acceptable resolution in 4D flow MRI is still not optimal. The velocity fields from the 4D flow MRI and CFD showed large differences in the ECA velocity streamline maps. This may be due to differences in the geometry generated from the two methods in the axis CCA-ECA. The geometrical axis of CCA-ECA created by the 4D flow MRI segmentation appeared to be more vertical. As a result, the visualization of the ECA blood flow velocity magnitude in the 4D flow MRI simulation represented a greater value than in the CFD model in almost all cases. The time it takes to calculate the CFD model depends on the computer system's ability, the number of mesh, and the calculation conditions. The calculation time may vary from 5-8 h. Generally, 4D flow MRI measurements are acquired in 15-20 min for MR examination and in less than 1 h for processing. The 4D flow MRI was therefore thought to be a simple and easy instrument to measure and analyze carotid artery hemodynamics. As shown in this study, the results from the 4D flow MRI were not the same as those obtained by CFD, but in 3D flow patterns, there was a moderate to high degree of correlation. Our study found that the 4D flow MRI and CFD are complementary methods. Both techniques represent that the blood flow at the outer wall of the carotid sinus and carotid bifurcation is characterized by low flow, where the flow recirculation occurs. Despite the poorer resolution of the 4D flow MRI in the visualization of the recirculation regions, 4D flow MR images of secondary flow directions moderately coincided with the CFD visualization. This could provide additional insight into the pathophysiology and the prediction of the localization of high-risk atherosclerotic plaque at the carotid bifurcation. Our study had several limitations. First of all, the approach of using 4D flow MRI as boundary conditions of CFD model introduces a bias for the subsequent comparison between the two techniques. However, the choice of the comparison plane at the carotid bifurcation, has some distances from the inlet and the outlet boundary conditions, may be helpful to reduce this bias. On the other hand, the purpose of this study was to merely assess the ability of the 3D specific model generation by the both techniques. With subject-specific measurements, our methods improve the feasibility in clinical practice. Other limitations were due to simplifications and uncertainties associated with our model. For example, blood viscosity can be associated with a variety of factors that researchers may not be available to identify with certainty. The rigidity of the wall assumed in this paper is another uncertainty associated with the simulation. Besides, the spatial and temporal resolutions used in the MRI scans are another limitation. With an acquisition time of 20 min in the conventional time-resolved 3D phase-contrast sequence, it was not possible to obtain better than 1.1-mm\({}^{3}\) spatial resolution and 50-msec temporal resolution. A longer scan time would allow this resolution to be improved but it also increases the likelihood of motion artifacts due to patient movement, which can significantly degrade the accuracy of the results. Another limitation is the lack of evaluation of the 4D flow MRI technique's interpatient reproducibility. A determination of this method's statistical significance across multiple subjects will require a study with a larger number of patients. While the reproducibility of 4D flow MRI was high in healthy volunteers, further study is needed to determine whether it is also reliable for future applications in patients with carotid atherosclerotic disease.