Characterization of abnormal wall shear stress using 4D flow MRI in human bicuspid aortopathy

Abstract

There exists considerable controversy surrounding the timing and extent of aortic resection for patients with BAV disease. Since abnormal wall shear stress (WSS) is potentially associated with tissue remodeling in BAV-related aortopathy, we propose a methodology that creates patient-specific 'heat maps' of abnormal WSS, based on 4D flow MRI. The heat maps were created by detecting outlier measurements from a volumetric 3D map of ensemble-averaged WSS in healthy controls. 4D flow MRI was performed in 13 BAV patients, referred for aortic resection and 10 age-matched controls. Systolic WSS was calculated from this data, and an ensemble-average and standard deviation (SD) WSS map of the controls was created. Regions of the individual WSS maps of the BAV patients that showed a higher WSS than the mean + 1.96SD of the ensemble-average control WSS map were highlighted. Elevated WSS was found on the greater ascending aorta (35% ± 15 of the surface area), which correlated significantly with peak systolic velocity (R (2) = 0.5, p = 0.01) and showed good agreement with the resected aortic regions. This novel approach to characterize regional aortic WSS may allow clinicians to gain unique insights regarding the heterogeneous expression of aortopathy and may be leveraged to guide patient-specific resection strategies for aorta repair.

Figure 1

Generation of WSS heat maps. (A) The control population-averaged mean and SD WSS maps were registered and interpolated to the aorta segmentation of the BAV patient (B). (C) The mean ± 1.96 times SD maps are created and compared with the peak systolic WSS of the BAV patients resulting in the heat maps (D).

Figure 2

Generation of WSS P-value maps. (A) The individual systolic WSS maps were registered and interpolated to the control population averaged aorta geometry (B). (C) A Wilcoxon rank sum test was performed between the individual controls and the BAV patients to create the P-value maps.

Figure 3

Peak systolic volumetric WSS velocity vectors (column 1) and WSS vectors in slices manually placed orthogonal in the ascending aorta (column 2 and 3) for (a) a BAV patient with RL fusion, (b) a control with the slice positioned similar to (a), (c) a BAV patient with RN fusion and (d) the same control subject with the slice positioned similar to (d).

Figure 4

(a) A map representing the overlap of all 10 control aortas. (b) The idealized geometry is the overlap map where the overlap is maximized over the subjects: in this case where more than 4 aortas are overlapping. The six regions where the difference in velocity before and after interpolation and the percentage of surface area with abnormal WSS was calculated is shown in (b) as well: 1) Inner curvature AAo, 2) Outer curvature AAo, 3) Inner curvature Arch, 4) Outer curvature Arch, 5) Inner curvature DAo and 6) Outer curvature DAo.

Figure 5

Right-anterior oblique views of the WSS heat maps illustrating abnormally elevated WSS (red) and depressed WSS (blue). RL indicates BAV patients with fusion of the right and left coronary cusps, RN indicates BAV patients with fusion of the right and non-coronary cusp. Lat indicates a valve without raphe that opens in lateral direction and uni indicates a functionally unicuspid valve with two raphes. R = Right, A = Anterior, H = Head.

Figure 6

(a) Right-anterior and (b) posterior-right views of the P-value map for WSS. R = Right, A = Anterior, H = Head, P = Posterior, R = Right