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  • 1.
    Gasser, T. Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Martufi, Giampaolo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Auer, M.
    Folkesson, M.
    Swedenborg, J.
    Micromechanical Characterization of Intra-luminal Thrombus Tissue from Abdominal Aortic Aneurysms2010In: Annals of Biomedical Engineering, ISSN 0090-6964, E-ISSN 1573-9686, Vol. 38, no 2, p. 371-379Article in journal (Refereed)
    Abstract [en]

    The reliable assessment of Abdominal Aortic Aneurysm rupture risk is critically important in reducing related mortality without unnecessarily increasing the rate of elective repair. Intra-luminal thrombus (ILT) has multiple biomechanical and biochemical impacts on the underlying aneurysm wall and thrombus failure might be linked to aneurysm rupture. Histological slices from 7 ILTs were analyzed using a sequence of automatic image processing and feature analyzing steps. Derived microstructural data was used to define Representative Volume Elements (RVE), which in turn allowed the estimation of microscopic material properties using the non-linear Finite Element Method. ILT tissue exhibited complex microstructural arrangement with larger pores in the abluminal layer than in the luminal layer. The microstructure was isotropic in the abluminal layer, whereas pores started to orient along the circumferential direction towards the luminal site. ILT's macroscopic (reversible) deformability was supported by large pores in the microstructure and the inhomogeneous structure explains in part the radially changing macroscopic constitutive properties of ILT. Its microscopic properties decreased just slightly from the luminal to the abluminal layer. The present study provided novel microstructural and micromechanical data of ILT tissue, which is critically important to further explore the role of the ILT in aneurysm rupture. Data provided in this study allow an integration of structural information from medical imaging for example, to estimate ILT's macroscopic mechanical properties.

  • 2.
    Gasser, T. Christian
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Martufi, Giampaolo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Auer, Martin
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    A constitutive model for vascular tissue that integrates fibril, fiber and continuum levels2011In: CMBE 2011: Proceedings of 2nd International Conference on Computational & Mathematical Biomedical Engineering, 2011Conference paper (Refereed)
  • 3.
    Giampaolo, Martufi
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, Thomas Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Auer, Martin
    Labruto, Fausto
    Swedenborg, Jesper
    Abdominal Aortic Aneurysm development over time: Experimental evidence and constitutive modeling2010In: Proceedings of the 6th World Congress of Biomechanics, Springer, 2010Conference paper (Refereed)
    Abstract [en]

    Abdominal Aortic Aneurysms (AAAs) are defined as a localized permanent dilatation of the infrarenal aorta at least 50 % of its normal diameter. AAAs are frequently diagnosed in the elderly male population and evaluating rupture risk is critically important as aneurysm rupture carries high mortality rates. Growth predictors might be helpful to assess AAA rupture risk and could therefore give a better graded indication for elective repair in order to reduce related mortality without unnecessarily increasing the rate of interventions. Factors associated with AAA growth are still limited but there are some evidence that higher initial AAA diameter is related to faster AAA expansion [1]. The initial dilatation is dependent on elastin degradation, but strength of the AAA is maintained by increased production of collagen. It has been suggested that rupture occurs when collagen production is insufficient to counteract load-bearing at high pressure [2].

    AAA growth quantification

    30 patients with infrarenal AAAs were included in this study. Criteria for inclusion were 1-year follow-up and availability of at least two high-resolution Computer Tomography-Angiography (CTA) scans. Consequently, 60 CT-A scans were systematically segmented, reconstructed and analyzed (A4research, VASCOPS GmbH), in order to investigate geometrical and mechanical factors likely to be correlated with AAA growth. Derived results were analyzed with an especially developed (automatic) analyzing schema (MatLab, The MathWorks), and the derived information aims at guiding the development of an analytical growth model for AAAs.

    Constitutive Modeling

    Collagen is a structural protein responsible for the mechanical strength, stiffness and toughness of biological tissues like skin, tendon, bone, cornea, lung and vasculature. In the present study we considered the enlargement of the aneurysm as a consequence of a pathological degradation and synthesis of collagen, i.e. malfunction of collagen turn-over. Consequently, the vascular wall is modeled by an (inert) matrix material representing the elastin, which is reinforced by a dynamic structure of bundles of collagen. Specifically, collagen is formed by a continuous stress-mediated process and deposited in the current configuration [3] and removed by a constant degradation rate. Finally the micro-plane concept [4] is used for the Finite Element implementation [5] of the constitutive model.

    Results and conclusions

    The quantitative description of AAA growth by examining patient follow-up data revealed novel insights into the natural history of this disease. Most interestingly not all portions of the AAA seem to enlarge, some might be stable or even shrink over time; a feature that has not yet been considered by models reported in the literature. The model proposed within this study has a

    strong biological motivation and captures saline feature of AAA growth. Besides that, the micro-plane approach allows a straight forward FE implementation and preliminary results indicate its numerical robustness.

    References

    [1]

    F.J.V. Schlösser, et al., J Vasc Surg, 47:1127–1133 2008.

    [2]

    E. Choke, et al., Eur.j.Vasc.endovasc.surg, 30(3):227-44 2005.

    [3]

    J.D.Humphrey, J Biomech Eng, 121:591–597 1999.

    [4]

    Z.P. Bazant and P.C. Prat, J Eng Mech, 113(7) 1050-1064 1987.

    [5]

    S. Federico and T.C Gasser, J R Soc Interface (in press)

  • 4.
    Martufi, Giampaolo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Biomechanics of abdominal aortic aneurysm:Experimental evidence and multiscale constitutive modeling2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The reliable assessment of Abdominal Aortic Aneurysm (AAA) rupture risk is critically important in reducing related mortality without unnecessarily increasing the rate of elective repair. A multi-disciplinary approach including vascular biomechanics and constitutive modeling is needed to better understand and more effectively treat these diseases. AAAs are formed through irreversible pathological remodeling of the vascular wall and integrating this biological process in the constitutive description could improve the current understanding of this disease as well as the predictability of biomechanical simulations.

    First in this thesis, multiple centerline-based diameter measurements between renal arteries and aortic bifurcation have been used to monitor aneurysm growth of in total 51 patients from Computer Tomography-Angiography (CT-A) data. Secondly, the thesis proposes a novel multi-scale constitutive model for the vascular wall, where collagen fibers are assembled by proteoglycan cross-linked collagen fibrils and reinforce an otherwise isotropic matrix (elastin). Collagen fibrils are dynamically formed by a continuous stretch-mediated process, deposited in the current configuration and removed by a constant degradation rate. The micro-plane concept is then used for the Finite Element (FE) implementation of the constitutive model. Finally, histological slices from intra-luminal thrombus (ILT) tissue were analyzed using a sequence of automatic image processing steps. Derived microstructural data were used to define Representative Volume Elements (RVEs), which in turn allowed the estimation of microscopic material properties using the non-linear FE.

    The thesis showed that localized spots of fast diameter growth can be detected through multiple centerline-based diameter measurements all over the AAA sac. Consequently, this information might further reinforce the quality of aneurysm surveillance programs. The novel constitutive model proposed in the thesis has a strong biological motivation and provides an interface with biochemistry. Apart from modeling the tissue’s passive response, the presented model is helpful to predict saline feature of aneurysm growth and remodeling. Finally, the thesis provided novel microstructural and micromechanical data of ILT tissue, which is critically important to further explore the role of the ILT in aneurysm rupture.

  • 5.
    Martufi, Giampaolo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Multiscale Modeling of the Normal and Aneurysmatic Abdominal Aorta2010Licentiate thesis, comprehensive summary (Other academic)
  • 6.
    Martufi, Giampaolo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Auer, Martin
    Roy, Joy
    Swedenborg, Jesper
    Sakalihasan, Natzi
    Panuccio, Giuseppe
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Multidimensional growth measurements of abdominal aortic aneurysms2013In: Journal of Vascular Surgery, ISSN 0741-5214, E-ISSN 1097-6809, Vol. 58, no 3, p. 748-755Article in journal (Refereed)
    Abstract [en]

    Background: Monitoring the expansion of abdominal aortic aneurysms (AAAs) is critical to avoid aneurysm rupture in surveillance programs, for instance. However, measuring the change of the maximum diameter over time can only provide limited information about AAA expansion. Specifically, regions of fast diameter growth may be missed, axial growth cannot be quantified, and shape changes of potential interest for decisions related to endovascular aneurysm repair cannot be captured. Methods: This study used multiple centerline-based diameter measurements between the renal arteries and the aortic bifurcation to quantify AAA growth in 51 patients from computed tomography angiography (CTA) data. Criteria for inclusion were at least 1 year of patient follow-up and the availability of at least two sufficiently high-resolution CTA scans that allowed an accurate three-dimensional reconstruction. Consequently, 124 CTA scans were systematically analyzed by using A4clinics diagnostic software (VASCOPS GmbH, Graz, Austria), and aneurysm growth was monitored at 100 cross-sections perpendicular to the centerline. Results: Monitoring diameter development over the entire aneurysm revealed the sites of the fastest diameter growth, quantified the axial growth, and showed the evolution of the neck morphology over time. Monitoring the development of an aneurysm's maximum diameter or its volume over time can assess the mean diameter growth (r = 0.69, r = 0.77) but not the maximum diameter growth (r = 0.43, r = 0.34). The diameter growth measured at the site of maximum expansion was similar to 16%/y, almost four times larger than the mean diameter expansion of 4.4%/y. The sites at which the maximum diameter growth was recorded did not coincide with the position of the maximum baseline diameter (rho = 0.12; P = .31). The overall aneurysm sac length increased from 84 to 89 mm during the follow-up (P < .001), which relates to the median longitudinal growth of 3.5%/y. The neck length shortened, on average, by 6.2% per year and was accompanied by a slight increase in neck angulation. Conclusions: Neither maximum diameter nor volume measurements over time are able to measure the fastest diameter growth of the aneurysm sac. Consequently, expansion-related wall weakening might be inappropriately reflected by this type of surveillance data. In contrast, localized spots of fast diameter growth can be detected through multiple centerline-based diameter measurements over the entire aneurysm sac. This information might further reinforce the quality of aneurysm surveillance programs.

  • 7. Martufi, Giampaolo
    et al.
    Di Martino, Elena S.
    Amon, Cristina H.
    Muluk, Satish C.
    Finol, Ender A.
    Three-Dimensional Geometrical Characterization of Abdominal Aortic Aneurysms: Image-Based Wall Thickness Distribution2009In: Journal of Biomechanical Engineering, ISSN 0148-0731, E-ISSN 1528-8951, Vol. 131, no 6, p. 061015-Article in journal (Refereed)
    Abstract [en]

    The clinical assessment of abdominal aortic aneurysm (AAA) rupture risk is based on the quantification of AAA size by measuring its maximum diameter from computed tomography (CT) images and estimating the expansion rate of the aneurysm sac over time. Recent findings have shown that geometrical shape and size, as well as local wall thickness may be related to this risk; thus, reliable noninvasive image-based methods to evaluate AAA geometry have a potential to become valuable clinical tools. Utilizing existing CT data, the three-dimensional geometry of nine unruptured human AAAs was reconstructed and characterized quantitatively. We propose and evaluate a series of 1D size, 2D shape, 3D size, 3D shape, and second-order curvature-based indices to quantify AAA geometry, as well as the geometry of a size-matched idealized fusiform aneurysm and a patient-specific normal abdominal aorta used as controls. The wall thickness estimation algorithm, validated in our previous work, is tested against discrete point measurements taken from a cadaver tissue model, yielding an average relative difference in AAA wall thickness of 7.8%. It is unlikely that any one of the proposed geometrical indices alone would be a reliable index of rupture risk or a threshold for elective repair. Rather, the complete geometry and a positive correlation of a set of indices should be considered to assess the potential for rupture. With this quantitative parameter assessment, future research can be directed toward statistical analyses correlating the numerical values of these parameters with the risk of aneurysm rupture or intervention (surgical or endovascular). While this work does not provide direct insight into the possible clinical use of the geometric parameters, we believe it provides the foundation necessary for future efforts in that direction.

  • 8.
    Martufi, Giampaolo
    et al.
    KTH, School of Technology and Health (STH). Univ Calgary, Dept Civil Engn, Calgary, AB, Canada.
    Forneris, Arianna
    Univ Calgary, Biomed Engn Grad Program, Calgary, AB, Canada..
    Nobakht, Samaneh
    Univ Calgary, Biomed Engn Grad Program, Calgary, AB, Canada..
    Rinker, Kristina D.
    Univ Calgary, Dept Chem & Petr Engn, Calgary, AB, Canada.;Univ Calgary, Ctr Bioengn Res & Educ, Calgary, AB, Canada.;Univ Calgary, Libin Cardiovasc Inst Alberta, Calgary, AB, Canada..
    Moore, Randy D.
    Univ Calgary, Dept Surg, Calgary, AB, Canada..
    Di Martino, Elena S.
    Univ Calgary, Dept Civil Engn, Calgary, AB, Canada.;Univ Calgary, Ctr Bioengn Res & Educ, Calgary, AB, Canada.;Univ Calgary, Libin Cardiovasc Inst Alberta, Calgary, AB, Canada..
    Case Study: Intra-Patient Heterogeneity of Aneurysmal Tissue Properties2018In: Frontiers in Cardiovascular Medicine, ISSN 2297-055X, Vol. 5, article id 82Article in journal (Refereed)
    Abstract [en]

    Introduction: Current recommendations for surgical treatment of abdominal aortic aneurysms (AAAs) rely on the assessment of aortic diameter as a marker for risk of rupture. The use of aortic size alone may overlook the role that vessel heterogeneity plays in aneurysmal progression and rupture risk. The aim of the current study was to investigate intra-patient heterogeneity of mechanical and fluid mechanical stresses on the aortic wall and wall tissue histopathology from tissue collected at the time of surgical repair. Methods: Finite element analysis (FEA) and computational fluid dynamics (CFD) simulations were used to predict the mechanical wall stress and the wall shear stress fields for a non-ruptured aneurysm 2 weeks prior to scheduled surgery. During open repair surgery one specimen partitioned into different regions was collected from the patient's diseased aorta according to a pre-operative map. Histological analysis and mechanical testing were performed on the aortic samples and the results were compared with the predicted stresses. Results: The preoperative simulations highlighted the presence of altered local hemodynamics particularly at the proximal segment of the left anterior area of the aneurysm. Results from the post-operative assessment on the surgical samples revealed a considerable heterogeneity throughout the aortic wall. There was a positive correlation between elastin fragmentation and collagen content in the media. The tensile tests demonstrated a good prediction of the locally varying constitutive model properties predicted using geometrical variables, i.e., wall thickness and thrombus thickness. Conclusions: The observed large regional differences highlight the local response of the tissue to both mechanical and biological factors. Aortic size alone appears to be insufficient to characterize the large degree of heterogeneity in the aneurysmal wall. Local assessment of wall vulnerability may provide better risk of rupture predictions.

  • 9.
    Martufi, Giampaolo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    A constitutive model for vascular tissue that integrates fibril, fiber andcontinuum levels2010Report (Other academic)
    Abstract [en]

    A fundamental understanding of the mechanical properties of the extracellular matrix (ECM) is critically important to quantify the amount of macroscopic stress and/or strain transmitted to the cellular level of vascular tissue. Structural constitutive models integrate histological and mechanical information, and hence, allocate stress and strain to the different micro-structural components of the vascular wall. The present work proposes a novel multi-scale structural constitutive model for passive vascular tissue, where collagen fibers are assembled by proteoglycan (PG) cross-linked collagen fibrils and reinforce an otherwise isotropic matrix material. Multiplicative kinematics account for straightening and stretching of collagen fibrils and an orientation density function captures the spatial organization of collagen fibers in the tissue. Mechanical and structural assumptions at the collagen fibril level define a piece-wise analytical stress-stretch response of collagen fibers, which in turn is integrated over the unit sphere to constitute the tissue’s macroscopic mechanical properties. The proposed model displays salient macroscopic feature of vascular tissue, and employs material and structural parameters of clear physical meaning. Model parameters were estimated from meanpopulation data of the normal and aneurysmatic aortic wall and used to predict in-vivo stress states of patient-specific vascular geometries, thought to demonstrate the robustness of the particular Finite Element (FE) implementation. The collagen fibril level of the multi-scale constitutive formulation provides an interface to integrate vascular wall biology and to account for collagen turn-over for example.

  • 10.
    Martufi, Giampaolo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    A constitutive model for vascular tissue that integrates fibril, fiber and continuum levels with application to the isotropic and passive properties of the infrarenal aorta2011In: Journal of Biomechanics, ISSN 0021-9290, E-ISSN 1873-2380, Vol. 44, no 14, p. 2544-2550Article in journal (Refereed)
    Abstract [en]

    A fundamental understanding of the mechanical properties of the extracellular matrix (ECM) is critically important to quantify the amount of macroscopic stress and/or strain transmitted to the cellular level of vascular tissue. Structural constitutive models integrate histological and mechanical information, and hence, allocate stress and strain to the different microstructural components of the vascular wall. The present work proposes a novel multi-scale structural constitutive model of passive vascular tissue, where collagen fibers are assembled by proteoglycan (PG) cross-linked collagen fibrils and reinforce an otherwise isotropic matrix material. Multiplicative kinematics account for the straightening and stretching of collagen fibrils, and an orientation density function captures the spatial organization of collagen fibers in the tissue. Mechanical and structural assumptions at the collagen fibril level define a piece-wise analytical stress-stretch response of collagen fibers, which in turn is integrated over the unit sphere to constitute the tissue's macroscopic mechanical properties. The proposed model displays the salient macroscopic features of vascular tissue, and employs the material and structural parameters of clear physical meaning. Likewise, the constitutive concept renders a highly efficient multi-scale structural approach that allows for the numerical analysis at the organ level. Model parameters were estimated from isotropic mean-population data of the normal and aneurysmatic aortic wall and used to predict in-vivo stress states of patient-specific vascular geometries, thought to demonstrate the robustness of the particular Finite Element (FE) implementation. The collagen fibril level of the multi-scale constitutive formulation provided an interface to integrate vascular wall biology and to account for collagen turnover.

  • 11.
    Martufi, Giampaolo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Remodeling of abdominal aortic aneurysm wall: A multi-scale structural approach2011In: Proceedings of 4th International Conference on the Mechanics of Biomaterials and Tissues, 2011Conference paper (Refereed)
  • 12.
    Martufi, Giampaolo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Turnover of fibrillar collagen in soft biological tissue with application to the expansion of abdominal aortic aneurysms2012In: Journal of the Royal Society Interface, ISSN 1742-5689, E-ISSN 1742-5662, Vol. 9, no 77, p. 3366-3377Article in journal (Refereed)
    Abstract [en]

    A better understanding of the inherent properties of vascular tissue to adapt to its mechanical environment is crucial to improve the predictability of biomechanical simulations. Fibrillar collagen in the vascular wall plays a central role in tissue adaptation owing to its relatively short lifetime. Pathological alterations of collagen turnover may fail to result in homeostasis and could be responsible for abdominal aortic aneurysm (AAA) growth at later stages of the disease. For this reason our previously reported multiscale constitutive framework (Martufi, G. & Gasser, T. C. 2011 J. Biomech. 44, 2544-2550 (doi:10.1016/j.jbiomech.2011.07.015)) has been enriched by a collagen turnover model. Specifically, the framework's collagen fibril level allowed a sound integration of vascular wall biology, and the impact of collagen turnover on the macroscopic properties of AAAs was studied. To this end, model parameters were taken from the literature and/or estimated from clinical follow-up data of AAAs (on average 50.7 mm-large). Likewise, the in vivo stretch of the AAA wall was set, such that 10 per cent of collagen fibres were engaged. Results showed that the stretch spectrum, at which collagen fibrils are deposed, is the most influential parameter, i.e. it determines whether the vascular geometry grows, shrinks or remains stable over time. Most importantly, collagen turnover also had a remarkable impact on the macroscopic stress field. It avoided high stress gradients across the vessel wall, thus predicted a physiologically reasonable stress field. Although the constitutive model could be successfully calibrated to match the growth of small AAAs, a rigorous validation against experimental data is crucial to further explore the model's descriptive and predictive capabilities.

  • 13.
    Martufi, Giampaolo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Auer, M.
    A multi-scale collagen turn-over model for soft biological tissues with application to abdominal aortic aneurysm growth2011In: ASME 2011 Summer Bioengineering Conference, SBC 2011, 2011, no PARTS A AND B, p. 689-690Conference paper (Refereed)
    Abstract [en]

    Collagen is a structural protein responsible for the mechanical strength, stiffness and toughness of biological tissues like skin, tendon, bone, cornea, lung and vasculature. In the present study we considered the enlargement of the aneurysm as a consequence of a pathological degradation and synthesis of collagen, i.e. malfunction of collagen turn-over. Consequently, the vascular wall is modeled by an (inert) matrix material representing the elastin, which is reinforced by a dynamic structure of bundles of collagen. Specifically, collagen is formed by a continuous stress-mediated process: deposited in the current configuration and removed by a constant degradation rate. Finally the micro-plane concept is used for the Finite Element implementation of the constitutive law. The model proposed within this study has a strong biological motivation and is able to capture both non-linear mechanics of aortic tissue and saline feature of AAA growth. Besides that, the micro-plane approach allows a straight forward FE implementation and preliminary results indicate its numerical robustness.

  • 14.
    Martufi, Giampaolo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Auer, Martin
    A Growth Model for Abdominal Aortic Aneurysms Based on Continuous Collagen Turn-over2010In: ECCM 2010: Proceedings of the IV European Conference on Computational Mechanics, 2010Conference paper (Refereed)
  • 15.
    Martufi, Giampaolo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Auer, Martin
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    A Multi-scale Collagen Turn-Over Model for Soft Biological Tissues With Application to Abdominal Aortic Aneurysms Growth2011In: SBC 2011: Proceedings of ASME 2011 Summer Bioengineering Conference, 2011Conference paper (Refereed)
  • 16.
    Martufi, Giampaolo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Auer, Martin
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Labruto, Fausto
    Swedenborg, Jesper
    Progression of Abdominal Aortic Aneurysm: Clinical evidence and Multiscale Modeling2011In: 6th International Symposium on Biomechanics in Vascular Biology and Cardiovascular Disease: Proceedings of 6th International Symposium on Biomechanics in Vascular Biology and Cardiovascular Disease, 2011Conference paper (Refereed)
  • 17.
    Martufi, Giampaolo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Folkesson, Maggie
    Swedenborg, Jesper
    Micro-structural and Micro-mechanical Analysis of Thrombus Tissue from Abdominal Aortic Aneurysms2009In: Endovascular Surgery: Bringing Basic Science into Clinical Practice, 2009Conference paper (Refereed)
  • 18.
    Martufi, Giampaolo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Folkesson, Maggie
    Swedenborg, Jesper
    Micro-structural and Micro-mechanical Characterization of Thrombus Tissue from Abdominal Aortic Aneurysms2009In: USNCCN 2010: Proceedings of 10th US National Congress on Computational Mechanics, 2009Conference paper (Refereed)
  • 19.
    Martufi, Giampaolo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Gasser, Thomas Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Review: The role of biomechanical modeling in the rupture risk assessment for abdominal aortic aneurysms2013In: Journal of Biomechanical Engineering, ISSN 0148-0731, E-ISSN 1528-8951, Vol. 135, no 2, p. 021010-Article, review/survey (Refereed)
    Abstract [en]

    AAA disease is a serious condition and a multidisciplinary approach including biomechanics is needed to better understand and more effectively treat this disease. A rupture risk assessment is central to the management of AAA patients, and biomechanical simulation is a powerful tool to assist clinical decisions. Central to such a simulation approach is a need for robust and physiologically relevant models. Vascular tissue senses and responds actively to changes in its mechanical environment, a crucial tissue property that might also improve the biomechanical AAA rupture risk assessment. Specifically, constitutive modeling should not only focus on the (passive) interaction of structural components within the vascular wall, but also how cells dynamically maintain such a structure. In this article, after specifying the objectives of an AAA rupture risk assessment, the histology and mechanical properties of AAA tissue, with emphasis on the wall, are reviewed. Then a histomechanical constitutive description of the AAA wall is introduced that specifically accounts for collagen turnover. A test case simulation clearly emphasizes the need for constitutive descriptions that remodels with respect to the mechanical loading state. Finally, remarks regarding modeling of realistic clinical problems and possible future trends conclude the article.

  • 20.
    Martufi, Giampaolo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). University of Calgary, Canada.
    Liljeqvist, Moritz Lindquist
    Sakalihasan, Natzi
    Panuccio, Giuseppe
    Hultgren, Rebecka
    Roy, Joy
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Local Diameter, Wall Stress, and Thrombus Thickness Influence the Local Growth of Abdominal Aortic Aneurysms2016In: Journal of Endovascular Therapy, ISSN 1526-6028, E-ISSN 1545-1550, Vol. 23, no 6, p. 957-966Article in journal (Refereed)
    Abstract [en]

    Purpose: To investigate the influence of the local diameter, the intraluminal thrombus (ILT) thickness, and wall stress on the local growth rate of abdominal aortic aneurysms. Methods: The infrarenal aortas of 90 asymptomatic abdominal aortic aneurysm (AAA) patients (mean age 70 years; 77 men) were retrospectively reconstructed from at least 2 computed tomography angiography scans (median follow-up of 1 year) and biomechanically analyzed with the finite element method. Each individual AAA model was automatically sliced orthogonally to the lumen centerline and represented by 100 cross sections with corresponding diameters, ILT thicknesses, and wall stresses. The data were grouped according to these parameters for comparison of differences among the variables. Results: Diameter growth was continuously distributed over the entire aneurysm sac, reaching absolute and relative median peaks of 3.06 mm/y and 7.3%/y, respectively. The local growth rate was dependent on the local baseline diameter, the local ILT thickness, and for wall segments not covered by ILT, also on the local wall stress level (all p<0.001). For wall segments that were covered by a thick ILT layer, wall stress did not affect the growth rate (p=0.08). Conclusion: Diameter is not only a strong global predictor but also a local predictor of aneurysm growth. In addition, and independent of the diameter, the ILT thickness and wall stress (for the ILT-free wall) also influence the local growth rate. The high stress sensitivity of nondilated aortic walls suggests that wall stress peaks could initiate AAA formation. In contrast, local diameters and ILT thicknesses determine AAA growth for dilated and ILT-covered aortic walls.

  • 21.
    Martufi, Giampaolo
    et al.
    Universita degli Studi di Roma Tor Vergata.
    Rodríguez, José F.
    Finol, Ender A.
    Anisotropic wall mechanics of abdominal aortic aneurysms2008In: Proceedings of the ASME Summer Bioengineering Conference, SBC2008, 2008, p. 233-234Conference paper (Refereed)
  • 22. Rodriguez, Jose F.
    et al.
    Martufi, Giampalo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Doblare, Manuel
    Finol, Ender A.
    The Effect of Material Model Formulation in the Stress Analysis of Abdominal Aortic Aneurysms2009In: Annals of Biomedical Engineering, ISSN 0090-6964, E-ISSN 1573-9686, Vol. 37, no 11, p. 2218-2221Article in journal (Refereed)
    Abstract [en]

    A reliable estimation of wall stress in Abdominal Aortic Aneurysms (AAAs), requires performing an accurate three-dimensional reconstruction of the medical image-based native geometry and modeling an appropriate constitutive law for the aneurysmal tissue material characterization. A recent study on the biaxial mechanical behavior of human AAA tissue specimens demonstrates that aneurysmal tissue behaves mechanically anisotropic. Results shown in this communication show that the peak wall stress is highly sensitive to the anisotropic model used for the stress analysis. In addition, the present investigation indicates that structural parameters (e.g., collagen fiber orientation) should be determined independently and not by means of non-linear fitting to stress-strain test data. Fiber orientation identified in this manner could lead to overestimated peak wall stresses.

  • 23. Rodríguez, José F.
    et al.
    Martufi, Giampaolo
    Universita degli Studi di Roma Tor Vergata Via del Politecnico, Italy.
    Finol, Ender A.
    The role of material anisotropy in abdominal aortic aneurysm wall mechanics2008In: WCCM8: Proceedings of the 8th World Congress on Computational Mechanics, 2008Conference paper (Refereed)
    Abstract [en]

    The prevalence of AAA is growing along with population age and according to different studies AAA rupture is the 13th most common cause of death in the U.S., causing an estimated 15,000 deaths per year. In biomechanical terms, AAA rupture is a phenomenon that occurs when the developing mechanical stresses within the aneurysm inner wall, as a result of the exerted intraluminal pressure, exceed the failure strength of the aortic tissue. To obtain a reliable estimation of wall stress, it is necessary to perform an accurate three-dimensional reconstruction of the AAA geometry and model an appropriate constitutive law for the aneurysmal tissue material characterization. In this regard, a recent study on the biaxial mechanical behavior of human AAA tissue specimens [1] demonstrates that aneurysmal arterial tissue behaves mechanically anisotropic. The objectives of the present work are to determine the effect of material anisotropy of the aneurysmal abdominal aorta on wall stress distribution and to establish a comparison of wall mechanics between ruptured and unruptured aneurysms.

  • 24. Shum, Judy
    et al.
    Di Martino, Elena S.
    Goldhammer, Adam
    Goldman, Daniel H.
    Acker, Leah C.
    Patel, Gopal
    Ng, Julie H. Y.
    Martufi, Giampaolo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Finol, Ender A.
    Semiautomatic vessel wall detection and quantification of wall thickness in computed tomography images of human abdominal aortic aneurysms2010In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 37, no 2, p. 638-648Article in journal (Refereed)
    Abstract [en]

    Purpose: Quantitative measurements of wall thickness in human abdominal aortic aneurysms (AAAs) may lead to more accurate methods for the evaluation of their biomechanical environment. Methods: The authors describe an algorithm for estimating wall thickness in AAAs based on intensity histograms and neural networks involving segmentation of contrast enhanced abdominal computed tomography images. The algorithm was applied to ten ruptured and ten unruptured AAA image data sets. Two vascular surgeons manually segmented the lumen, inner wall, and outer wall of each data set and a reference standard was defined as the average of their segmentations. Reproducibility was determined by comparing the reference standard to lumen contours generated automatically by the algorithm and a commercially available software package. Repeatability was assessed by comparing the lumen, outer wall, and inner wall contours, as well as wall thickness, made by the two surgeons using the algorithm. Results: There was high correspondence between automatic and manual measurements for the lumen area (r=0.978 and r=0.996 for ruptured and unruptured aneurysms, respectively) and between vascular surgeons (r=0.987 and r=0.992 for ruptured and unruptured aneurysms, respectively). The authors' automatic algorithm showed better results when compared to the reference with an average lumen error of 3.69%, which is less than half the error between the commercially available application Simpleware and the reference (7.53%). Wall thickness measurements also showed good agreement between vascular surgeons with average coefficients of variation of 10.59% (ruptured aneurysms) and 13.02% (unruptured aneurysms). Ruptured aneurysms exhibit significantly thicker walls (1.78±0.39 mm) than unruptured ones (1.48±0.22 mm), p=0.044. Conclusions: While further refinement is needed to fully automate the outer wall segmentation algorithm, these preliminary results demonstrate the method's adequate reproducibility and low interobserver variability.

  • 25. Shum, Judy
    et al.
    Martufi, Giampaolo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Di Martino, Elena S.
    Finol, Ender A.
    Quantitative assessment of abdominal aortic aneurysm geometry and rupture potential2009In: PROCEEDINGS OF THE ASME SUMMER BIOENGINEERING CONFERENCE - 2009, PT A AND B, 2009, p. 1303-1304Conference paper (Refereed)
    Abstract [en]

    Recent biomechanics studies have shown that the maximum transverse diameter of an abdominal aortic aneurysm (AAA) and its expansion rate are not reliable indicators of rupture potential. We hypothesize that geometrical shape and size, as well as wall thickness may be related to rupture risk and can therefore be deciding factors in the clinical management of the disease. A non-invasive, image-based evaluation of AAA size and geometry was implemented on a retrospective study of twenty subjects. The contrast enhanced, computed tomography (CT) scans of 10 patients who suffered AAA rupture within 1 month of the scan were compared to those of 10 patients who received elective repair. The images were segmented and three-dimensional models were generated. Twenty-eight geometry-based indices were calculated to characterize the size and shape of each AAA and regional variations in wall thickness were estimated. A multivariate analysis of variance was performed for all indices comparing the ruptured and non-ruptured data sets to determine which indices are statistically significant. Receiving Operating Characteristic (ROC) curves were generated to determine the indices' potential as predictors of rupture risk. In addition to maximum diameter, five other geometry-based indices were found to be statistically significant, with the minimum wall thickness being the best discriminator between ruptures and non-ruptured AAAs.

  • 26. Shum, Judy
    et al.
    Martufi, Giampaolo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Di Martino, Elena S.
    Grisafi, Joseph
    Muluk, Satish C.
    Finol, Ender A.
    Challenging the maximum diameter criterion: Quantitative assessment of abdominal aortic aneurysm shape and rupture potential2009In: Eastern Vascular Society's 23rd Annual Meeting: Proceedings of the Eastern Vascular Society's 23rd Annual Meeting, 2009Conference paper (Refereed)
  • 27. Shum, Judy
    et al.
    Martufi, Giampaolo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Di Martino, Elena S.
    Grisafi, Joseph
    Muluk, Satish C.
    Finol, Ender A.
    Quantification of abdominal aortic aneurysm shape and rupture risk2009In: Biomedical Engineering Society Annual Fall Meeting: Proceedings of the 2009 Biomedical Engineering Society Annual Fall Meeting, 2009Conference paper (Refereed)
  • 28. Shum, Judy
    et al.
    Martufi, Giampaolo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Di Martino, Elena S.
    Grisafi, Joseph
    Muluk, Satish C.
    Finol, Ender A.
    Quantitative assessment of abdominal aortic aneurysm shape and rupture potential2009In: Frontiers of Biomedical Imaging Science Conference: Proceedings of the Frontiers of Biomedical Imaging Science Conference, 2009Conference paper (Refereed)
  • 29. Shum, Judy
    et al.
    Martufi, Giampaolo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Di Martino, Elena S.
    Washington, Christopher B.
    Grisafi, Joseph
    Muluk, Satish C.
    Finol, Ender A.
    Differentiation of Abdominal Aortic Aneurysm Geometry: A Tool for Rupture Risk Assessment2010In: ISACB 2010: Proceedings of 12th Biennial Meeting of the International Society for Applied Cardiovascular Biology, 2010Conference paper (Refereed)
  • 30. Shum, Judy
    et al.
    Martufi, Giampaolo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Di Martino, Elena
    Washington, Christopher B.
    Grisafi, Joseph
    Muluk, Satish C.
    Finol, Ender A.
    Quantitative Assessment of Abdominal Aortic Aneurysm Geometry2011In: Annals of Biomedical Engineering, ISSN 0090-6964, E-ISSN 1573-9686, Vol. 39, no 1, p. 277-286Article in journal (Refereed)
    Abstract [en]

    Recent studies have shown that the maximum transverse diameter of an abdominal aortic aneurysm (AAA) and expansion rate are not entirely reliable indicators of rupture potential. We hypothesize that aneurysm morphology and wall thickness are more predictive of rupture risk and can be the deciding factors in the clinical management of the disease. A non-invasive, image-based evaluation of AAA shape was implemented on a retrospective study of 10 ruptured and 66 unruptured aneurysms. Three-dimensional models were generated from segmented, contrast-enhanced computed tomography images. Geometric indices and regional variations in wall thickness were estimated based on novel segmentation algorithms. A model was created using a J48 decision tree algorithm and its performance was assessed using ten-fold cross validation. Feature selection was performed using the chi(2)-test. The model correctly classified 65 datasets and had an average prediction accuracy of 86.6% (kappa = 0.37). The highest ranked features were sac length, sac height, volume, surface area, maximum diameter, bulge height, and intra-luminal thrombus volume. Given that individual AAAs have complex shapes with local changes in surface curvature and wall thickness, the assessment of AAA rupture risk should be based on the accurate quantification of aneurysmal sac shape and size.

1 - 30 of 30
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