# Heterogeneity of Ex Vivo and In Vivo Properties along the Length of the Abdominal Aortic Aneurysm
## 3 Results
### In Vivo Analysis  
The in vivo analysis was performed to obtain the CFD-based TAWSS distribution on the luminal surface of the aortic geometry and the maximum principal strain distribution derived from dynamic CT images on the outer wall surface of the artery. The in vivo analysis of maximum principal strain and wall shear stresses did not require invasive measurements and was conducted for a larger cohort of aneurysms (n = 12) including the ones selected for the ex vivo analysis. For TAWSS and maximum principal strain, a  
Figure 6: Representative images from immunostaining showing inflammatory infiltrate of CD4+, CD8+ and CD68+ cells in the neck—region RA1 (column **a**) and aneurys dilatation—region RA3 (column **b**) for one AAA patient in the population. The adventitia and media layers are in the top and bottom half of each image, respectively. All images are at 10\(\times\) magnification.  
Figure 7: Heterogeneity of cell counts along the length of the AAA for the inflammatory infiltrates (**a**) helper T-cells (CD4+) and (**b**) cytotoxic T-cells (CD8+) in the adventitia layer of the selected samples undergoing ex vivo immunohistochemical analysis. * indicates \(p\) value \(<\) 0.05, ** indicates \(p\) value \(<\) 0.01.  
region-averaged distribution was obtained in order to characterize the same regions (24 on each aorta) with an average value.  
The mean region-averaged TAWSS for the study cohort was 0.50 \(\pm\) 0.24 Pa. A significant effect of position along the length of the aorta for the TAWSS was observed (Figure 8a). The central regions across the aortic geometries--namely, regions 3, 4, 5--presented lower TAWSS resulting from the slow and highly recirculating flow patterns that characterized the aneurys dilatation, while regions upstream (aneurysm neck region 1 and 2) and downstream the dilatation (region 6) presented a more organized, high-velocity flow that corresponded to higher TAWSS values. Of note, region 1 was characterized by statistically significant higher TAWSS compared to any other region (\(p<0.001\)) while region 2 was characterized by significantly higher TAWSS compared to the more central regions in position 3 (\(p\) = 0.004) and position 4 and 5 (\(p<0.001\)). The same central regions showed significantly lower TAWSS with respect to the areas in position 6 downstream the aneurysm sac (\(p\) = 0.023 with respect to position 3, \(p<0.001\) with respect to position 4 and 5).  
The in vivo strain analysis performed on dynamic ECG-gated CT images was feasible for 11 patients in the study population, as the scan for one patient did not meet the inclusion criteria due to poor image quality. The mean region-averaged maximum principal strain for the population was 0.03 \(\pm\) 0.01. Once again, the regional analysis pointed to a significant effect of position along the length of the aorta (Figure 8b), with central patches exhibiting smaller strain compared to the aneurysm neck and downstream locations. In particular, the neck region in position 1 presented a significantly larger strain compared to any of the central regions (\(p\) = 0.02 for region 3, \(p<0.001\) for regions 4 and 5).  
Both in vivo variables of interest showed no significant effect of position along the circumference of the aorta.  
An interesting relationship was found when comparing the TAWSS for the aortic regions with collected specimens showing no discernible media layer versus discernible media layer. The aortic tissue showing highly compromised composition with no clearly discernible media layer presented statistically significant lower TAWSS (\(p<0.001\)) (Figure 9).  
Figure 8: Heterogeneity of the in vivo properties along the length of the AAA. (**a**) TAWSS. (**b**) Maximum principal strain.  
* indicates \(p\) value \(<0.05\), *** indicates \(p\) value \(<0.01\) and *** indicates \(p\) value \(<0.001\).  
Figure 9: TAWSS for specimens showing no discernible media layer versus discernible media layer. *** indicates \(p\) value \(<0.001\).