Researchers at the Department of Bioengineering at the Politecnico di Milano in Italy have recently performed a transient analysis of an aortic aneurysm during a cardiac cycle using the fluid-structure interaction (FSI) capability in FIDAP. An aneurysm is an abnormal blood-filled dilation of the artery wall. Aneurysms are often treated surgically, but extreme care is needed to avoid rupture of the artery itself. To assess the danger prior to surgery, a novel technique has been developed which will allow doctors to evaluate the stresses on the wall in the affected area.

The technique begins with the construction of a 3D geometry from a series of 2D CT scans. The data from successive slices are consolidated using image processing software, and a 3D form is generated. Smoothing on the exterior and interior walls is done and the volumes (f luid and walls) are meshed. The 3D mesh is read into FIDAP, where the coupled motion of the blood and aortic walls is simulated using FSI. In this procedure, sets of equations are solved for the fluid, the solid, and the mesh movement. Periodic boundary conditions for the blood flow are used. The walls are assumed to be made of linear, elastic, homogeneous material with different properties in the aneurysm and healthy tissue regions.

The 3D geometry shows the lumen, or interior passageway (gold), the thrombus, or thick wall tissue (red), and the aortic wall (exterior)

A transient solution is performed for several cardiac cycles until a repeatable condition is reached. At this point, velocity, pressure, and wall displacements can be examined at key times in the cycle. Of greater interest are localized results for wall stresses in different sections of the aneurysm. These reports can be coupled with experimental data on stress limit values, and an assessment of the risk of aortic rupture can be made. This powerful diagnostic tool is now being improved to include a non-linear viscoelastic model for the aneurysm wall material and more accurate time-dependent boundary conditions for the blood flow.

Wall stress on the aortic wall

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