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80 5. IMPACT OF THE FLUID-STRUCTURE INTERACTION MODELING ON THE HUMAN VESSEL HEMODYNAMICS

and stretches. Due to the pulsatile character of blood pressure, these regions of concentration show oscillating
peaks where the highest values are usually located at the bulb of the bifurcation. As for hemodynamics, the structural
problem has been usually analyzed by means of computational methods that allow studying a considerable number of
geometries, situations, and conditions, yet resulting in the possibility of performing statistical analyses. Fluid dynam-
ics and structural mechanics perspectives are not in conflict [7], as this is about changing the main focus of analysis
from fluid to solid variables. Traditionally, these two fields have been often treated separately. This is mainly due to
the high complexity problem that involves unsteady, pulsatile, turbulent, and non-Newtonian flow with anisotropic,
nonlinear, hyperelastic, and fiber-reinforced vascular tissue. Both fluid and solid parts may even change their prop-
erties due to large-scale modifications such as cardiovascular diseases. Blood flow may show a local increase in peak
velocity, an accentuation of recirculation, and a change of the resultant endothelial shear stresses. Vessel wall prop-
erties may vary in case of lipid accumulation and partial occlusion of the artery such as in the case of atherosclerosis or
a loosening of elasticity and wall thickening in the case of an aneurysm. In any case, there is a mutual interaction
between blood flow and compliant vessels because the cardiovascular flow exerts blood pressure on the walls. The
latter is accumulated as potential energy and transferred to the blood flow as kinetic energy. Under this perspective,
it is a fluid-structure interaction (FSI) problem [8]. For these reasons, a considerable number of studies have been
focused on the analysis of coupled fluid-solid problems with application to the cardiovascular field. Among others,
the aorta and the carotid artery have been frequently considered in healthy and diseased conditions [9–19]. These
works have been focused on the aorta and on the carotid artery due to their intrinsic tendency to develop cardiovas-
cular diseases such as atherosclerosis. Commercial and in-house software have been used on idealized and patient-
specific data with the aim of quantifying physical variables not evaluable in vivo. Instantaneous, average, and
oscillatory endothelial stresses are the variables mostly computed, correlated with structural variables and geometrical
factors and used as a marker for the considered pathology.
In this chapter, we present two FSI models based on medical images. The aorta and the carotid artery have been
analyzed, including the most important flow features, with the aim of showing the impact of the distensible walls on
the fluid dynamics variables. A large number of computational fluid dynamics (CFD) studies have been proposed for
hemodynamics evaluations. However, these works have the intrinsic limitation that the vessel is considered rigid.
Structural models used with computational solid mechanics (CSM) have been demonstrated to be useful for quanti-
fying and localizing peaks of stresses and strains. The latter completely neglects the effect of the blood flow that is
considered just a boundary condition through the blood pressure. The main differences that can be found using
the FSI approximation in the cardiovascular field are related to the amplitude and the locations of WSS as well as
its intensity. While the provided results are more accurately obtained with respect to the CFD or CSM computations,
the main limitation of the FSI approach is the drastic increase in computational costs that limit its applicability to
clinical daily practice, contrary to the CFD approach.

5.2 FINITE ELEMENT MODELING OF THE HUMAN BLOOD VESSELS

5.2.1 Image-Based Geometrical Reconstruction
Medical images are usually required for building the vessel geometry that corresponds to the computational
domain of the numerical model. Due to the high intervariability of the different parts of the blood vessels among sub-
jects, it is preferable to use patient-specific geometries for numerical studies. Another approach is to use average data
coming from various patients and provide a parametric model in which geometrical variations can be imposed. In all
cases, the vessel lumen (fluid domain) and the corresponding wall thickness (solid domain) are generated. Different
techniques are currently available. In the present study, for all the performed reconstructions, we have used comput-
erized tomography (CT) images that allow acquiring three-dimensional (3D) images with a spatial resolution of less
than 1 mm. The standard format for storing, transmitting, and handling medical imaging is the DICOM format (Digital
Imaging and Communication in Medicine). The images containing the thoracic slices of two different patients were
imported into the commercial software MIMICS (Materialise Software, Leuven, Belgium). Here, a manual segmenta-
tion was performed with the aim of extracting the vessel lumen. For this scope, the black cavity that represents the
lumen was manually filled in each image. As a result, a stereolithography (STL) file with the 3D model and an Initial
Graphics Exchange Specification (IGES) file of the cross-sectional slices belonging to each geometry of the two handled
patients was exported. Prior to this, the obtained data were smoothed for reducing the unavoidable noise included
in the acquisition of the images. Both formats can be easily imported and treated in commercial computer-aided
design (CAD) software. Here, the STL file was only used as a reference for the 3D models that have been created

I. BIOMECHANICS