Phase-contrast magnetic resonance imaging measurements in intracranial aneurysms in vivo of flow patterns, velocity fields, and wall shear stress: comparison with computational fluid dynamics

Abstract

Evolution of intracranial aneurysms is known to be related to hemodynamic forces such as wall shear stress (WSS) and maximum shear stress (MSS). Estimation of these parameters can be performed using numerical simulations with computational fluid dynamics (CFD), but can also be directly measured with magnetic resonance imaging (MRI) using a time-dependent 3D phase-contrast sequence with encoding of each of the three components of the velocity vectors (7D-MRV). To study the accuracy of 7D-MRV in estimating these parameters in vivo, in comparison with CFD, 7D-MRV and patient-specific CFD modeling was performed for 3 patients who had intracranial aneurysms. Visual and quantitative analyses of the flow pattern and distribution of velocities, MSS, and WSS were performed using the two techniques. Spearman's coefficients of correlation between the two techniques were 0.56 for the velocity field, 0.48 for MSS, and 0.59 for WSS. Visual analysis and Bland-Altman plots showed good agreement for flow pattern and velocities but large discrepancies for MSS and WSS. These results indicate that 7D-MRV can be used in vivo to measure velocity flow fields and for estimating MSS and WSS. Currently, however, this method cannot accurately quantify the latter two parameters.

FIG. 1

Triangular meshes of the surface obtained for each patient's aneurysm. Left: patient 1, Center: patient 2, and Right patient 3.

FIG. 2

7D-MRV dataset cleaning. The 7D-MRV dataset (left) is registered with the mesh built from the CE-MRA images (green transparent surface, center). All the velocity vectors of the 7D-MRV dataset located outside the surface mesh are discarded (right).

FIG. 3

Representation of velocities for patient 1 to 3 in systolic phase. CFD predictions (left) are compared to 7D-MRV findings (right). Velocity range and global flow pattern is comparable between the two techniques.

FIG. 4

Representation of maximum shear stress (MSS) for patient 1 to 3 in systolic phase. Values obtained from CFD (left) and 7D-MRV (right) measures are shown. Cut plane position is displayed for each model in the top right corner. The spatial averaging effect is clearly seen on the 7D images, particularly in the center of the aneurysms in patient 1 and 3 leading to a lower contrast between high and low shear zones. Furthermore, the maximum shear stress values globally appear lower in 7D-MRV than in CFD as red colored areas are smaller.

FIG. 5

Representation of wall shear stress (WSS) for patients 1 to 3 in systolic phase (CFD - left, 7D-MRV - right). Despite the difference in WSS values, there is still a relatively good visual correlation between the two techniques in regard to the location of high and low wall shear stress.

FIG. 6

Bland-Altman plot representation of 7D-MRV and CFD for velocities, maximum shear stress and wall shear stress. Solid lines show the mean; dotted lines the 95% confidence interval.

FIG. 7

Scatter plot representation and regression line for 7D-MRV and CFD for velocities, maximum shear stress and wall shear stress. Slope, intercept and R-squared values are respectively 0.54, 0.2 and 0.54 for velocities, 0.26, 0.1 and 0.40 for maximum shear stress and 1.70, 3.42 and 0.33 for wall shear stress (p<0.001). All patients' values are plotted together. Triangles represent points from systolic phase, circle points from diastolic phase.

FIG. 8

Cut planes in patient 2 showing some details in the velocity field measured with 7D-MRV. Space locations of the planes are represented in the top right corners. A large vortex is visible in the recirculating area.

FIG. 9

Comparison of maximum shear stress slice profiles between CFD (black) and 7D-MRV (grey) in patient 1 at peak systolic phase at two different anatomic location (A and B). Smoothing of the curve as well as loss in contrast between high shear area (peripheral) and low shear area (middle of the lumen) clearly appear with 7D-MRV in comparison with CFD.