Background: Each year ascending thoracic aortic aneurysms (ATAA) are diagnosed in approximately 7,500 people in the United States and more than 15,000 people in Europe (Isselbacher 2005). Untreated aneurysms may rupture or dissect; in these emergent cases the survival rate is less than 3%. In order to prevent aneurysm rupture or dissection, pre-emptive surgery is performed to replace the aneurismal tissue with a graft. Currently there is great interest in developing patient-specific estimates of rupture susceptibility which can indicate the optimal time for surgical intervention. However, very little is known about the mechanisms that lead to rupture of ATAAs limiting our ability to develop robust models of aneurysm failure. Gathering knowledge of the local structural mechanisms involved in damage and failure of ATAAs is essential to developing patient-specific rupture risk assessments.
Methods: To understand the mechanisms underlying ATAA rupture we have combined multiphoton microscopy with bulge inflation tests. Ten ATAA specimens were obtained from patients undergoing surgery to replace their ATAA with a synthetic graft. The collection of ATAAs was done in accordance with the guidelines of the University Hospital Center of Saint-Etienne Institutional Review Board. Each ATAA was cut into a 45 mm square specimen and clamped in the bulge inflation device so that the adventitial side of the tissue faced outward. The sample was subjected to 5 inflation cycles to 10 kPa. After which it was inflated to increasing levels of pressure (0 kPa, 15 kPa, 30 kPa, 45 kPa, and 60 kPa) while images were captured using the multiphoton microscope. At each pressure 50 images were collected covering a volume of 375 x 375 x 100 micrometers cubed. From each image we calculated the fiber orientation using Fourier based image processing techniques. The result is a data set that describes the spatial organization of the collagen fiber network as a function of depth in the tissue with a resolution of 5 degrees.
Results: We focused on identifying the fiber distribution, the mean fiber angle, and a dispersion parameter kappa, which describes the level of anisotropy in the fiber distribution. In addition, we examined how the average fiber orientations vary as a function of depth from the surface of the tissue and as a function of pressure. The results show that the collagen fibers realign as increasing levels of pressure are applied. The collagen fibers are consistently organized diagonally and circumferentially in the adventitia and media, respectively.
Conclusion: Using multiphoton microscopy we were able to identify the three-dimensional distribution of collagen fibers in the ATAA as a function of pressure. It is clear that significant changes in fiber orientation take place as the pressure is increased. In the future we plan to use the collected images to estimate the strain in individual collagen fiber bundles. Using the strain data we will identify the pressure where the collagen fibers become engaged and if fiber engagement is a function of location in the ATAA.