SCMR 22nd Annual Scientific Sessions
Background: 4D flow MRI has shown promise as a useful diagnostic tool for the quantification of altered aortic velocities or derived hemodynamic biomarkers such as 3D wall shear stress (WSS) in cardiovascular disease. Recently, a hemodynamic atlas concept was introduced which provides a cohort average distribution of normal aortic velocities and 3D WSS based on a healthy control population . Several studies have demonstrated the potential of this method to identify abnormal velocity or WSS in individual patients throughout the volume or surface of the aorta with statistical certainty [1-2]. However, gender and healthy aging will affect the normal range of values for these hemodynamic parameters. A systematic investigation of aging and sex on velocity and WSS in healthy controls is lacking.
For this prospective IRB approved and HIPAA compliant study, 100 healthy subjects (50 female) with no history of cardiovascular disease underwent 4D flow MRI (spatial resolution 1.7x1.7x2.2-3.1x3.1x3.3 mm3, temporal resolution 38-43 ms, venc 150-250 cm/s) of the thoracic aorta. The study cohort had 20 subjects per age group (18-30, 31-40, 41-50, 51-60, and 61-80 years), equally divided between male and female participants (10 per age group). Data analysis included 3D segmentation of the aorta, calculation of systolic absolute aortic velocities, and calculation of peak systolic 3D WSS along the aorta surface. Similar to a previously reported method [3-4], for each age and sex group, systolic velocity and 3D WSS atlases were generated based on a cohort-specific average 3D aorta geometry (1: rigid registration of each aorta 3D segmentation, 2: thresholding shared voxels to minimize registration error). To statistically compare velocities and 3D WSS between atlases, one cohort was rigidly registered to another, and a Wilcoxon rank-sum test was computed at each voxel (velocity) or surface point (3D WSS) with α=0.05 to generate p-value maps. For regional analyses, the atlas volumes were divided into ascending aorta (AAo), arch, and descending aorta (DAo).
Comparison between male and female velocity and 3D WSS atlases showed only minor sex differences for all age groups. Most pronounced differences were seen in the youngest cohort (18-30): males had significantly higher velocities and 3D WSS in 9% and 8% of the aortic volume or surface, respectively. In all other age groups, males and females differed in less than 5% of the aortic volume/area. In contrast, healthy aging (males and females pooled into cohorts of 20 subjects) resulted in marked reduction of systolic velocities (Figure 1) and 3D WSS (Figure 2) in large parts of the aorta. A regional analysis (AAo, arch, and DAo) confirmed that the largest changes occur in the descending aorta (Fig. 3).
Conclusion: Age-matched cohorts should be used when comparing patients to healthy atlases, as significant changes in velocity and WSS occur during aging, with only minor differences due to sex except in young (<30 years) cohorts.