Aortic flow patterns and wall shear stress maps by 4D-flow cardiovascular magnetic resonance in the assessment of aortic dilatation in bicuspid aortic valve disease

Abstract

Background: In patients with bicuspid valve (BAV), ascending aorta (AAo) dilatation may be caused by altered flow patterns and wall shear stress (WSS). These differences may explain different aortic dilatation morphotypes. Using 4D-flow cardiovascular magnetic resonance (CMR), we aimed to analyze differences in flow patterns and regional axial and circumferential WSS maps between BAV phenotypes and their correlation with ascending aorta dilatation morphotype.

Methods: One hundred and one BAV patients (aortic diameter ≤ 45 mm, no severe valvular disease) and 20 healthy subjects were studied by 4D-flow CMR. Peak velocity, flow jet angle, flow displacement, in-plane rotational flow (IRF) and systolic flow reversal ratio (SFRR) were assessed at different levels of the AAo. Peak-systolic axial and circumferential regional WSS maps were also estimated. Unadjusted and multivariable adjusted linear regression analyses were used to identify independent correlates of aortic root or ascending dilatation. Age, sex, valve morphotype, body surface area, flow derived variables and WSS components were included in the multivariable models.

Results: The AAo was non-dilated in 24 BAV patients and dilated in 77 (root morphotype in 11 and ascending in 66). BAV phenotype was right-left (RL-) in 78 patients and right-non-coronary (RN-) in 23. Both BAV phenotypes presented different outflow jet direction and velocity profiles that matched the location of maximum systolic axial WSS. RL-BAV velocity profiles and maximum axial WSS were homogeneously distributed right-anteriorly, however, RN-BAV showed higher variable profiles with a main proximal-posterior distribution shifting anteriorly at mid-distal AAo. Compared to controls, BAV patients presented similar WSS magnitude at proximal, mid and distal AAo (p = 0.764, 0.516 and 0.053, respectively) but lower axial and higher circumferential WSS components (p < 0.001 for both, at all aortic levels). Among BAV patients, RN-BAV presented higher IRF at all levels (p = 0.024 proximal, 0.046 mid and 0.002 distal AAo) and higher circumferential WSS at mid and distal AAo (p = 0.038 and 0.046, respectively) than RL-BAV. However, axial WSS was higher in RL-BAV compared to RN-BAV at proximal and mid AAo (p = 0.046, 0.019, respectively). Displacement and axial WSS were independently associated with the root-morphotype, and circumferential WSS and SFRR with the ascending-morphotype.

Conclusions: Different BAV-phenotypes present different flow patterns with an anterior distribution in RL-BAV, whereas, RN-BAV patients present a predominant posterior outflow jet at the sinotubular junction that shifts to anterior or right anterior in mid and distal AAo. Thus, RL-BAV patients present a higher axial WSS at the aortic root while RN-BAV present a higher circumferential WSS in mid and distal AAo. These results may explain different AAo dilatation morphotypes in the BAV population.

Keywords: 4D flow cardiovascular magnetic resonance (4D flow CMR); Aorta hemodynamics; Aortic dilatation; Ascending aorta; Bicuspid aortic valve; Wall shear stress.

Fig. 1

Analysis planes and parameters calculated from 4D-flow CMR (a). Eight double-oblique analysis planes were equally distributed in the ascending aorta between the sinotubular junction and the origin of the brachiocephalic trunk (b). Velocity profiles, peak velocity, flow eccentricity and in-plane rotational flow were obtained at proximal, mid and distal ascending aorta. Systolic flow reversal ratio was measured at mid and distal ascending aorta. All the analyses planes were used to calculate WSS maps (c)

Fig. 2

Systolic backward flow in a BAV patient. Red streamlines indicate forward flow in the ascending aorta, while blue streamlines indicate systolic backward flow. SFRR: systolic flow reversal ratio, V_SFF: total systolic forward flow, V_SBF: total systolic backward flow

Fig. 3

Peak-systolic center of velocities. Center of velocities at proximal, mid and distal ascending aorta (AAo) for controls and BAV. Lines show the maximum velocity path along the AAo for each patient. Wider arrow represents the mean path for each group. A: anterior, L: left, P: posterior, R: right, RL-BAV: right-left bicuspid aortic valve, RN-BAV: right-non coronary bicuspid aortic valve

Fig. 4

Through-plane velocity profiles and streamlines in BAV. a Asymmetric outflow jet to the anterior wall in a RL-BAV. b RN-BAV with a posterior outflow jet shifting to the anterior wall at mid and distal ascending aorta. Abbreviations as in Fig. 3

Fig. 5

In-plane rotational flow (IRF) and systolic backward flow (SFRR). a Mean IRF and b SFRR in controls and BAV, based on the dilatation morphotype

Fig. 6

Peak-systolic axial and circumferential WSS maps in RL- and RN-BAV. RL-BAV show maximum axial WSS in the right to right-anterior wall and lower values of circumferential WSS at all levels. In RN-, maximum axial WSS extends from the right-posterior wall proximally to right-anterior wall at mid and distal ascending aorta, with a higher distal circumferential WSS. Prox: proximal, Dist: distal, WSS: wall shear stress, other abbreviations as in Fig. 3

Fig. 7

Statistical significance maps of axial and circumferential WSS comparing BAV phenotypes. Axial WSS regions correlate well with jet direction in BAV phenotypes. Circumferential WSS is significantly higher in the distal ascending aorta for RN-BAV patients. Abbreviations as in Fig. 3

Fig. 8

Peak-systolic axial and circumferential WSS maps according to BAV aortic morphotype. Root-morphotype presents higher proximal axial WSS compared to ascending-, however, circumferential WSS is higher in ascending-morphotype at all levels. Abbreviations as in Fig. 3

Fig. 9

Statistical significance maps of axial and circumferential WSS comparing BAV morphotypes. Statistical differences in regional WSS are more pronounced in circumferential than axial WSS, and are associated with regions of systolic flow reversal for axial WSS

Fig. 10

ROC curves showing main factors associated with ascending aorta morphotypes. Composite probability shows the best AUC for prediction in both morphotypes. SFRR: systolic flow reversal ratio, WSScirc: circumferential WSS