Influence of normal aging on structural and mechanical properties of the total aorta assessed by CMR.

Elastin and collagen fibres define the elastic properties of the aorta. Alterations of these fibres during aging lead to mechanical changes, associated with aortic disease such as aneurysm and dissection. Regional variations in fiber orientation cause these regions to respond differently to hemodynamic effects. This study investigated the influence of hemodynamic parameters on structural and mechanical properties of the aorta in a healthy population, as assessed by CMR.

Methods:

166 healthy volunteers (48% male) distributed in five age groups (1: 18-29, 2: 30-39, 3: 40-49, 4: 50-59, 5: 60-70 years) were included in this study and underwent CMR imaging of the entire aorta including 4D flow MRI sequences. The aorta was divided into three anatomical regions: ascending aorta [AAo], descending aorta [DAo], and infra-renal aorta [IRA]. Aortic wall thickness (AWT) was determined at the level of the pulmonary trunk in the AAo (AWT_AAo) and the DAo (AWT_DAo), and in the infra-renal aorta (AWTIRA), on two 2D black-blood dual-inversion recovery MRI acquisitions (Figure 1). PWV was assessed from 2D phase contrast MRI. Regional PWV was assessed by calculating a weighted average for all three regions. Flow displacement, peak systolic WSS (magnitude, axial, and circumferential), and WSS angle were also determined in all three regions.

Results:

PWV_AAo, PWV_DAo, and PWV_IRA increase with age, with significant differences between age groups (P< 0.001), but not between sexes. Post-hoc analysis demonstrated that for age group 5 the PWV in all regions were significantly higher compared to all other groups (P< 0.001).

AWT_AAo, AWT_DAo, and AWT_IRA increases with age, with significant differences between age groups (P< 0.001), but not between sexes. Post-hoc analysis demonstrated that for age group 5 the AWT_AAo was significantly higher compared to age groups 1-3 (P< 0.001). For age group 5 the AWT_DAo was significantly higher compared to all other groups (P< 0.001). AWT_IRA was significantly higher for age group 5 compared to age groups 1 and 2 (P=0.001).

 WSS angle is positively correlated with AWT in AAo for age groups 2 (r=0.42, P=0.009), 3 (r=0.40, P=0.015), and 5 (r=0.41, P=0.022). In DAo for age groups 1 (r=0.48, P=0.007), 3-5 (r=0.40, 0.49, 0.57, respectively) and IRA for age groups 3 (r=0.59, P< 0.001) and 5 (r=0.55, P=0.04). Maximum FD was only negatively correlated with AWT_AAo for age group 2 (r=-0.39, P< 0.018). We did not find significant correlations between PWV and WSS angle, and between PWV and FD.

In multivariable analysis, WSS angle was the only parameter independently associated with increase in AWT_AAo (ß=0.34, P< 0.001), in AWT_DAo (ß=0.36, P< 0.001), and in AWT_IRA (ß=0.29, P< 0.001).

Conclusion:

This study demonstrates that wall shear stress angle is associated with age related growth of aortic wall thickness, in a healthy population.