Cardiovascular Biomechanics

Abdominal Aortic Aneurysm

Abdominal aortic aneurysm (AAA) is a localized dilatation of the aortic wall. The physiological processes associated with AAA development and progression are not yet fully understood. This pathologic condition has been found to affect 8.8 % of the population over the age of 65 and if left untreated it may lead to rupture. The size of the aneurysm and its rate of expansion are parameters associated with the risk of rupture. The decision for surgical intervention for patients with AAA’s is complicated by the lack of a sufficiently accurate rupture risk index. A widely used such index is the maximum transverse diameter. However ‘small’ diameter aneurysms, where ‘watchful waiting’ requiring frequent observation is preferred to surgery, are known to rupture. Therefore, the decision for surgical intervention should not be based exclusively on the maximum transverse diameter and a more reliable rupture risk index should be introduced.

Finite element analysis (FEA) has been exploited to compute the stress distribution in both simplified and anatomically correct AAA models. The hemodynamics of the AAA has been extensively investigated experimentally and computationally in both idealized and anatomically correct models in steady and time varying flow. The coupling of fluid and structure has also been studied in AAA models.

The stress distribution on the aneurysmal wall is determined by the complex intra-aneurysmal hemodynamics resulting from the complex geometric configuration of the Intralumimal thrombus modulated flow conduit. The lack of an accurate AAA rupture risk index remains an important problem in the clinical management of the disease. Accurate estimation of the patient specific AAA rupture risk requires detailed information on both the wall stress distribution and the aortic wall tissue yield stress. However, AAA wall properties and stress distribution cannot be measured or even derived with sufficient accuracy from non-invasive measurements in vivo. As an alternative, numerical approximations of the flow and wall motion equations are sought using wall constitutive models based on mean elastic properties obtained by mechanical testing of excised specimens of the aneurismal wall.

This activity aims in the development of a more accurate AAA rupture risk index thus reducing the number of unnecessary surgical interventions in low risk patients and sudden AAA rupture incidents in diagnosed patients. Fluid-structure interaction techniques have been exploited to assess the effects of wall compliance on the initiation and progression of aneurysmal disease using both idealized and patient specific AAA computational models. Furthermore, a paradigm shift in AAA growth quantification based on non-rigid registration offering localised wall surface growth assessment has been developed. 

 

Major Contributions in AAA Research

  • Highlighted the importance of fluid structure interaction in computational models used to evaluate rupture risk.

Papaharilaou, J. Ekaterinaris, E. Manousaki and A. Katsamouris. A decoupled fluid structure approach of estimating wall stress in abdominal aortic aneurysms. Journal of Biomechanics, 2007, 40, 367-377

  • Indicated aneurysm tortuosity as a marker for rupture risk stratification

E. Georgakarakos, C.V. Ioannou, Y. Kamarianakis, Y. Papaharilaou, T. Kostas, E. Manousaki, A.N. Katsamouris. The role of geometric parameters in the prediction of abdominal aortic aneurysm wall stress. European Journal of Vascular and Endovascular Surgery, 2010, 39 (1), pp. 42-48

  • Developed an inverse elastostatics method to recover the zero pressure state of image-based vascular structures.

Vavourakis, Y. Papaharilaou and J. Ekaterinaris. Coupled fluid-structure interaction hemodynamics in a stress-free state corrected arterial geometryJ. Biomechanics , 2011; 44:2453-2460

  • Developed a level set image segmentation method to extract patient specific wall thickness from CT images (CMPB; 2009)

Zohios, G. Kossioris, Y. Papaharilaou. Geometrical methods for level set based abdominal aortic aneurysm lumen, thrombus and outer wall 2D image segmentation. Computer Methods and Programs in Biomedicine, 2012; 107(2):202--17

  • Developed Non-Rigid Registration based method to quantify localized AAA wall surface growth

Metaxa, E., Iordanov, I., Maravelakis, E., Papaharilaou, Y. A novel approach for local abdominal aortic aneurysm growth quantification (2016) Medical and Biological Engineering and Computing, pp. 1-10. Article in Press.

 

In Silicone Oncology: Biomechanics of tumor growth

During the last few years significant steps have been achieved in the area of computational (inSilico) medicine and biology. The focus of this activity is the development of novel computational models and tools to support increased accuracy in diagnosis and optimization of therapy through the prediction of patient response.  Our aim is to model cancer evolution and thus support clinicians in both selecting the optimum patient specific treatment and also gaining timely information regarding the success or failure of the treatment. These are the most important parameters in optimizing the treatment results and the quality of life of the patient.

Computational modeling of tumor growth will require analysis of mechanical stress distribution in the tumor region which is an important parameter in tumor growth. It is also important to mathematically model and computationally simulate the effects of therapy.

 

Normoxic (right-top) and necrotic (right-bottom) cell distribution evolution. Evolution curve of normalized population of normoxic cells (left).

 

Related Publications

Finite element based numerical simulation of brain tumor. Konstantinos Tzirakis, John Peterson, Yannis Papaharilaou. VII European Congress on Computational Methods in Applied Sciences and Engineering. Crete, June 2016.

 

Magneto-Hemodynamics

Aim: Assess potential of exploiting a magnetic field for blood flow control 

Applications: Magnetic flow pump, magnetic devices for cell separation

In the science of magnetohydrodynamics, most of the attention is devoted to fluids that are electrical conductors, that is, fluids that feature the presence of electrical charges (positive, negative ions and free electrons). On the other hand, studies that address theoretically, numerically and experimentally the properties of polarizable and magnetizable fluids (PMFs) are scarce. Molecules of PMFs are characterized by non-vanishing electric- and magnetic-dipole moments and, therefore, their motion can be influenced by (gradients of) electromagnetic fields; the presence of (free) electric charges is not needed. Magnetohaemodynamics may have an impact in medicine, control of steady and pulsatile flow through vascular and heart valve stenosis, or control of turbulence for prevention of haemodialysis graft failure. We investigate the effects of the magnetic field in the flow that develops: in both idealized and realistic, image-based, arterial bifurcation models with our without stenosis. Arterial bifurcations are sites of significant pathophysiological interest in the vascular system due the complex 3D flow field that develops in their vicinity. The disturbed flow conditions that are associated with arterial bifurcations have been implicated in the initiation and progression of arterial wall disease leading to atheromatic stenosis of the vascular lumen and in the case of the carotid bifurcation to an increased risk of stroke. The post-stenotic flow regime which is characterized by significant flow instabilities that create structural vibrations is also of pathophysiological importance as it is believed to contribute to the development of post-stenotic dilatation of the arterial wall. Both steady state and transient flow are investigated with blood modeled as either a Newtonian or a non-Newtonian, incompressible fluid.

   

A magnetization force produced by a weak rotational magnetic field (curl B ≠ 0, |B|<0.3T) can significantly change the flow, if the field is characterized by strong spatial gradients (here |B|~1/r4)

 

Related Publications

Tzirakis, K., Botti, L., Vavourakis, V., Papaharilaou, Y. Numerical modeling of non-Newtonian biomagnetic fluid flow (2016) Computers and Fluids, 126, pp. 170-180.

 

Tzirakis, K., Papaharilaou, Y., Giordano, D., Ekaterinaris, J., Numerical investigation of biomagnetic fluids in circular ducts, (2014) International Journal for Numerical Methods in Biomedical Engineering, 30 (3), pp. 297-317