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  Subjects -> ENGINEERING (Total: 2266 journals)
    - CHEMICAL ENGINEERING (190 journals)
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    - ENGINEERING (1197 journals)
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ENGINEERING (1197 journals)                  1 2 3 4 5 6 | Last

Showing 1 - 200 of 1205 Journals sorted alphabetically
3 Biotech     Open Access   (Followers: 7)
3D Research     Hybrid Journal   (Followers: 19)
AAPG Bulletin     Full-text available via subscription   (Followers: 5)
AASRI Procedia     Open Access   (Followers: 14)
Abstract and Applied Analysis     Open Access   (Followers: 3)
Aceh International Journal of Science and Technology     Open Access   (Followers: 2)
ACS Nano     Full-text available via subscription   (Followers: 207)
Acta Geotechnica     Hybrid Journal   (Followers: 6)
Acta Metallurgica Sinica (English Letters)     Hybrid Journal   (Followers: 5)
Acta Polytechnica : Journal of Advanced Engineering     Open Access   (Followers: 1)
Acta Scientiarum. Technology     Open Access   (Followers: 3)
Acta Universitatis Cibiniensis. Technical Series     Open Access  
Active and Passive Electronic Components     Open Access   (Followers: 7)
Adaptive Behavior     Hybrid Journal   (Followers: 10)
Adıyaman Üniversitesi Mühendislik Bilimleri Dergisi     Open Access  
Adsorption     Hybrid Journal   (Followers: 4)
Advanced Engineering Forum     Full-text available via subscription   (Followers: 4)
Advanced Science     Open Access   (Followers: 4)
Advanced Science Focus     Free   (Followers: 3)
Advanced Science Letters     Full-text available via subscription   (Followers: 4)
Advanced Science, Engineering and Medicine     Partially Free   (Followers: 6)
Advanced Synthesis & Catalysis     Hybrid Journal   (Followers: 17)
Advances in Artificial Neural Systems     Open Access   (Followers: 3)
Advances in Calculus of Variations     Hybrid Journal   (Followers: 2)
Advances in Catalysis     Full-text available via subscription   (Followers: 5)
Advances in Complex Systems     Hybrid Journal   (Followers: 7)
Advances in Engineering Software     Hybrid Journal   (Followers: 25)
Advances in Fuel Cells     Full-text available via subscription   (Followers: 14)
Advances in Fuzzy Systems     Open Access   (Followers: 5)
Advances in Geosciences (ADGEO)     Open Access   (Followers: 9)
Advances in Heat Transfer     Full-text available via subscription   (Followers: 18)
Advances in Human Factors/Ergonomics     Full-text available via subscription   (Followers: 22)
Advances in Magnetic and Optical Resonance     Full-text available via subscription   (Followers: 7)
Advances in Natural Sciences: Nanoscience and Nanotechnology     Open Access   (Followers: 28)
Advances in Operations Research     Open Access   (Followers: 11)
Advances in OptoElectronics     Open Access   (Followers: 5)
Advances in Physics Theories and Applications     Open Access   (Followers: 13)
Advances in Polymer Science     Hybrid Journal   (Followers: 40)
Advances in Porous Media     Full-text available via subscription   (Followers: 4)
Advances in Remote Sensing     Open Access   (Followers: 34)
Advances in Science and Research (ASR)     Open Access   (Followers: 6)
Aerobiologia     Hybrid Journal   (Followers: 1)
African Journal of Science, Technology, Innovation and Development     Hybrid Journal   (Followers: 4)
AIChE Journal     Hybrid Journal   (Followers: 28)
Ain Shams Engineering Journal     Open Access   (Followers: 5)
Akademik Platform Mühendislik ve Fen Bilimleri Dergisi     Open Access  
Alexandria Engineering Journal     Open Access  
AMB Express     Open Access   (Followers: 1)
American Journal of Applied Sciences     Open Access   (Followers: 28)
American Journal of Engineering and Applied Sciences     Open Access   (Followers: 11)
American Journal of Engineering Education     Open Access   (Followers: 9)
American Journal of Environmental Engineering     Open Access   (Followers: 16)
American Journal of Industrial and Business Management     Open Access   (Followers: 23)
Analele Universitatii Ovidius Constanta - Seria Chimie     Open Access  
Annals of Combinatorics     Hybrid Journal   (Followers: 3)
Annals of Pure and Applied Logic     Open Access   (Followers: 2)
Annals of Regional Science     Hybrid Journal   (Followers: 7)
Annals of Science     Hybrid Journal   (Followers: 7)
Applicable Algebra in Engineering, Communication and Computing     Hybrid Journal   (Followers: 2)
Applicable Analysis: An International Journal     Hybrid Journal   (Followers: 1)
Applied Catalysis A: General     Hybrid Journal   (Followers: 5)
Applied Catalysis B: Environmental     Hybrid Journal   (Followers: 6)
Applied Clay Science     Hybrid Journal   (Followers: 4)
Applied Computational Intelligence and Soft Computing     Open Access   (Followers: 12)
Applied Magnetic Resonance     Hybrid Journal   (Followers: 3)
Applied Nanoscience     Open Access   (Followers: 8)
Applied Numerical Mathematics     Hybrid Journal   (Followers: 5)
Applied Physics Research     Open Access   (Followers: 4)
Applied Sciences     Open Access   (Followers: 3)
Applied Spatial Analysis and Policy     Hybrid Journal   (Followers: 4)
Arabian Journal for Science and Engineering     Hybrid Journal   (Followers: 5)
Archives of Computational Methods in Engineering     Hybrid Journal   (Followers: 4)
Archives of Foundry Engineering     Open Access  
Archives of Thermodynamics     Open Access   (Followers: 8)
Arkiv för Matematik     Hybrid Journal   (Followers: 1)
ASEE Prism     Full-text available via subscription   (Followers: 2)
Asian Engineering Review     Open Access  
Asian Journal of Applied Science and Engineering     Open Access   (Followers: 1)
Asian Journal of Applied Sciences     Open Access   (Followers: 2)
Asian Journal of Biotechnology     Open Access   (Followers: 7)
Asian Journal of Control     Hybrid Journal  
Asian Journal of Current Engineering & Maths     Open Access  
Asian Journal of Technology Innovation     Hybrid Journal   (Followers: 9)
Assembly Automation     Hybrid Journal   (Followers: 2)
at - Automatisierungstechnik     Hybrid Journal   (Followers: 1)
ATZagenda     Hybrid Journal  
ATZextra worldwide     Hybrid Journal  
Australasian Physical & Engineering Sciences in Medicine     Hybrid Journal   (Followers: 1)
Australian Journal of Multi-Disciplinary Engineering     Full-text available via subscription   (Followers: 2)
Autonomous Mental Development, IEEE Transactions on     Hybrid Journal   (Followers: 7)
Avances en Ciencias e Ingeniería     Open Access  
Balkan Region Conference on Engineering and Business Education     Open Access   (Followers: 1)
Bangladesh Journal of Scientific and Industrial Research     Open Access  
Basin Research     Hybrid Journal   (Followers: 3)
Batteries     Open Access   (Followers: 3)
Bautechnik     Hybrid Journal   (Followers: 1)
Bell Labs Technical Journal     Hybrid Journal   (Followers: 24)
Beni-Suef University Journal of Basic and Applied Sciences     Open Access   (Followers: 3)
BER : Manufacturing Survey : Full Survey     Full-text available via subscription   (Followers: 2)
BER : Motor Trade Survey     Full-text available via subscription   (Followers: 1)
BER : Retail Sector Survey     Full-text available via subscription   (Followers: 2)
BER : Retail Survey : Full Survey     Full-text available via subscription   (Followers: 2)
BER : Survey of Business Conditions in Manufacturing : An Executive Summary     Full-text available via subscription   (Followers: 3)
BER : Survey of Business Conditions in Retail : An Executive Summary     Full-text available via subscription   (Followers: 3)
Bharatiya Vaigyanik evam Audyogik Anusandhan Patrika (BVAAP)     Open Access   (Followers: 1)
Biofuels Engineering     Open Access  
Biointerphases     Open Access   (Followers: 1)
Biomaterials Science     Full-text available via subscription   (Followers: 8)
Biomedical Engineering     Hybrid Journal   (Followers: 16)
Biomedical Engineering and Computational Biology     Open Access   (Followers: 13)
Biomedical Engineering Letters     Hybrid Journal   (Followers: 5)
Biomedical Engineering, IEEE Reviews in     Full-text available via subscription   (Followers: 16)
Biomedical Engineering, IEEE Transactions on     Hybrid Journal   (Followers: 31)
Biomedical Engineering: Applications, Basis and Communications     Hybrid Journal   (Followers: 5)
Biomedical Microdevices     Hybrid Journal   (Followers: 8)
Biomedical Science and Engineering     Open Access   (Followers: 4)
Biomedizinische Technik - Biomedical Engineering     Hybrid Journal  
Biomicrofluidics     Open Access   (Followers: 4)
BioNanoMaterials     Hybrid Journal   (Followers: 1)
Biotechnology Progress     Hybrid Journal   (Followers: 39)
Boletin Cientifico Tecnico INIMET     Open Access  
Botswana Journal of Technology     Full-text available via subscription  
Boundary Value Problems     Open Access   (Followers: 1)
Brazilian Journal of Science and Technology     Open Access   (Followers: 2)
Broadcasting, IEEE Transactions on     Hybrid Journal   (Followers: 10)
Bulletin of Canadian Petroleum Geology     Full-text available via subscription   (Followers: 14)
Bulletin of Engineering Geology and the Environment     Hybrid Journal   (Followers: 3)
Bulletin of the Crimean Astrophysical Observatory     Hybrid Journal  
Cahiers, Droit, Sciences et Technologies     Open Access  
Calphad     Hybrid Journal  
Canadian Geotechnical Journal     Full-text available via subscription   (Followers: 14)
Canadian Journal of Remote Sensing     Full-text available via subscription   (Followers: 40)
Case Studies in Engineering Failure Analysis     Open Access   (Followers: 7)
Case Studies in Thermal Engineering     Open Access   (Followers: 4)
Catalysis Communications     Hybrid Journal   (Followers: 6)
Catalysis Letters     Hybrid Journal   (Followers: 3)
Catalysis Reviews: Science and Engineering     Hybrid Journal   (Followers: 8)
Catalysis Science and Technology     Free   (Followers: 6)
Catalysis Surveys from Asia     Hybrid Journal   (Followers: 3)
Catalysis Today     Hybrid Journal   (Followers: 5)
CEAS Space Journal     Hybrid Journal  
Cellular and Molecular Neurobiology     Hybrid Journal   (Followers: 4)
Central European Journal of Engineering     Hybrid Journal   (Followers: 1)
CFD Letters     Open Access   (Followers: 6)
Chaos : An Interdisciplinary Journal of Nonlinear Science     Hybrid Journal   (Followers: 2)
Chaos, Solitons & Fractals     Hybrid Journal   (Followers: 3)
Chinese Journal of Catalysis     Full-text available via subscription   (Followers: 2)
Chinese Journal of Engineering     Open Access   (Followers: 2)
Chinese Science Bulletin     Open Access   (Followers: 1)
Ciencia e Ingenieria Neogranadina     Open Access  
Ciencia en su PC     Open Access   (Followers: 1)
Ciencias Holguin     Open Access   (Followers: 1)
CienciaUAT     Open Access  
Cientifica     Open Access  
CIRP Annals - Manufacturing Technology     Full-text available via subscription   (Followers: 10)
CIRP Journal of Manufacturing Science and Technology     Full-text available via subscription   (Followers: 13)
City, Culture and Society     Hybrid Journal   (Followers: 20)
Clay Minerals     Full-text available via subscription   (Followers: 9)
Clean Air Journal     Full-text available via subscription   (Followers: 2)
Coal Science and Technology     Full-text available via subscription   (Followers: 4)
Coastal Engineering     Hybrid Journal   (Followers: 10)
Coastal Engineering Journal     Hybrid Journal   (Followers: 3)
Coatings     Open Access   (Followers: 2)
Cogent Engineering     Open Access   (Followers: 2)
Cognitive Computation     Hybrid Journal   (Followers: 4)
Color Research & Application     Hybrid Journal   (Followers: 1)
COMBINATORICA     Hybrid Journal  
Combustion Theory and Modelling     Hybrid Journal   (Followers: 13)
Combustion, Explosion, and Shock Waves     Hybrid Journal   (Followers: 13)
Communications Engineer     Hybrid Journal   (Followers: 1)
Communications in Numerical Methods in Engineering     Hybrid Journal   (Followers: 2)
Components, Packaging and Manufacturing Technology, IEEE Transactions on     Hybrid Journal   (Followers: 23)
Composite Interfaces     Hybrid Journal   (Followers: 5)
Composite Structures     Hybrid Journal   (Followers: 241)
Composites Part A : Applied Science and Manufacturing     Hybrid Journal   (Followers: 174)
Composites Part B : Engineering     Hybrid Journal   (Followers: 215)
Composites Science and Technology     Hybrid Journal   (Followers: 160)
Comptes Rendus Mécanique     Full-text available via subscription   (Followers: 2)
Computation     Open Access  
Computational Geosciences     Hybrid Journal   (Followers: 12)
Computational Optimization and Applications     Hybrid Journal   (Followers: 7)
Computational Science and Discovery     Full-text available via subscription   (Followers: 2)
Computer Applications in Engineering Education     Hybrid Journal   (Followers: 6)
Computer Science and Engineering     Open Access   (Followers: 17)
Computers & Geosciences     Hybrid Journal   (Followers: 25)
Computers & Mathematics with Applications     Full-text available via subscription   (Followers: 5)
Computers and Electronics in Agriculture     Hybrid Journal   (Followers: 4)
Computers and Geotechnics     Hybrid Journal   (Followers: 8)
Computing and Visualization in Science     Hybrid Journal   (Followers: 6)
Computing in Science & Engineering     Full-text available via subscription   (Followers: 25)
Conciencia Tecnologica     Open Access  
Concurrent Engineering     Hybrid Journal   (Followers: 3)
Continuum Mechanics and Thermodynamics     Hybrid Journal   (Followers: 6)
Control and Dynamic Systems     Full-text available via subscription   (Followers: 7)
Control Engineering Practice     Hybrid Journal   (Followers: 40)
Control Theory and Informatics     Open Access   (Followers: 7)
Corrosion Science     Hybrid Journal   (Followers: 24)
CT&F Ciencia, Tecnologia y Futuro     Open Access  
CTheory     Open Access  
Current Applied Physics     Full-text available via subscription   (Followers: 4)

        1 2 3 4 5 6 | Last

Journal Cover Communications in Numerical Methods in Engineering
  [2 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 1069-8299 - ISSN (Online) 1099-0887
   Published by John Wiley and Sons Homepage  [1616 journals]
  • Face Shield Design against Blast-induced Head Injuries
    • Authors: Long Bin Tan; Kwong Ming Tse, Yuan Hong Tan, Mohamad Ali Bin Sapingi, Vincent Beng Chye Tan, Heow Pueh Lee
      Abstract: Blast-induced traumatic brain injury (TBI) has been on the rise in recent years due to the increasing use of improvised explosive devices (IEDs) in conflict zones. Our study investigates the response of a helmeted human head subjected to a blast of 1 atm peak overpressure, for cases with and without a standard polycarbonate (PC) face shield and for face shields comprising of composite PC and aerogel materials and with lateral edge extension. The novel introduction of aerogel into the laminate face shield is explored and its wave-structure interaction mechanics and performance in blast mitigation is analysed. Our numerical results show that the face shield prevented direct exposure of the blast wave to the face and help delays the transmission of the blast to reduce the intracranial pressures (ICPs) at the parietal lobe. However, the blast wave can diffract and enter the midface region at the bottom and side edges of the face shield, resulting in TBI. This suggests that the bottom and sides of the face shield are important regions to focus on to reduce wave ingress. The laminated PC/aerogel/PC face shield yielded higher peak positive and negative ICPs at the frontal lobe, than the original PC one. For the occipital and temporal brain regions, the laminated face shield performed better than the original. The composite face shield with extended edges reduced ICP at the temporal lobe but increases ICP significantly at the parietal lobe which suggests that a greater coverage may not lead to better mitigating effects.
      PubDate: 2017-03-22T11:17:51.724376-05:
      DOI: 10.1002/cnm.2884
       
  • Effect of Cerebrospinal Fluid Modelling on Spherically Convergent Shear
           Waves during Blunt Head Trauma
    • Authors: Amit Madhukar; Ying Chen, Martin Ostoja-Starzewski
      Abstract: The MRI-based computational model, previously validated by tagged MRI and HARP imaging analysis technique on in vivo human brain deformation, is employed to study transient wave dynamics during blunt head trauma. Three different constitutive models are used for the cerebrospinal fluid (CSF): incompressible solid elastic, viscoelastic and fluid-like elastic using an equation of state model. Three impact cases are simulated which indicate that the blunt impacts give rise not only to a fast pressure wave but also to a slow, and potentially much more damaging, shear (distortional) wave that converges spherically towards the brain center. The wave amplification due to spherical geometry is balanced by damping due to tissues’ viscoelasticity and the heterogeneous brain structure, suggesting a stochastic competition of these two opposite effects. It is observed that this convergent shear wave is dependent on the constitutive property of the CSF whereas the peak pressure is not as significantly affected.
      PubDate: 2017-03-14T03:30:46.116261-05:
      DOI: 10.1002/cnm.2881
       
  • Phase-field boundary conditions for the voxel finite cell method:
           surface-free stress analysis of CT-based bone structures
    • Authors: L. H. Nguyen; S. K. F. Stoter, T. Baum, J. S. Kirschke, M. Ruess, Z. Yosibash, D. Schillinger
      Abstract: The voxel finite cell method employs unfitted finite element meshes and voxel quadrature rules to seamlessly transfer CT data into patient-specific bone discretizations. The method, however, still requires the explicit parametrization of boundary surfaces to impose traction and displacement boundary conditions, which constitutes a potential roadblock to automation. We explore a phase-field based formulation for imposing traction and displacement constraints in a diffuse sense. Its essential component is a diffuse geometry model generated from metastable phase-field solutions of the Allen-Cahn problem that assumes the imaging data as initial condition. Phase-field approximations of the boundary and its gradient are then employed to transfer all boundary terms in the variational formulation into volumetric terms. We show that in the context of the voxel finite cell method, diffuse boundary conditions achieve the same accuracy as boundary conditions defined over explicit sharp surfaces, if the inherent length scales, i.e., the interface width of the phase-field, the voxel spacing and the mesh size, are properly related. We demonstrate the flexibility of the new method by analyzing stresses in a human femur and a vertebral body. This article is protected by copyright. All rights reserved.
      PubDate: 2017-03-11T05:25:39.273162-05:
      DOI: 10.1002/cnm.2880
       
  • Computational modeling of tracheal angioedema due to swelling of the
           submucous tissue layer
    • Authors: Kun Gou; Thomas J. Pence
      Abstract: Angioedema is a tissue-swelling pathology due to rapid change in soft tissue fluid content. Its occurrence in the trachea is predominantly localized to the soft mucous tissue that forms the innermost tracheal layer. The biomechanical consequences, such as airway constriction, are dependent upon the ensuing mechanical interactions between all of the various tissues that comprise the tracheal tube. We model the stress interactions by treating the trachea organ as a three-tissue system consisting of swellable mucous in conjunction with nonswelling cartilage and nonswelling trachealis musculature. Hyperelastic constitutive modeling is used by generalizing the standard anisotropic, incompressible soft tissue framework to incorporate the swelling effect. Finite element stress analysis then proceeds with swelling of the mucous layer providing the driving factor for the mechanical analysis. The amount of airway constriction is governed by the mechanical interaction between the three predominant tissue types. The detailed stress analysis indicates the presence of stress concentrations near the various tissue junctions. Because of the tissue's nonlinear mechanical behavior, this can lead to material stiffness fluctuations as a function of location on the trachea. Patient specific modeling is presented. The role of the modeling in the interpretation of diagnostic procedures and the assessment of therapies is discussed.This article models tracheal angioedema (swelling) using an advanced hyperelastic theory incorporated with swelling.The deformation is solely caused by internal swelling. We have various levels of modeling considering the trachea to be idealized symmetric, non-symmetric with trachealis on the back side, and patient-specific. All these different models help us understand tracheal angioedema more profoundly. The patient-specific modeling supplies a more realistic understanding of the tracheal angioedema, and assists corresponding clinical treatment.
      PubDate: 2017-03-09T05:11:02.554537-05:
      DOI: 10.1002/cnm.2861
       
  • Calculation of Cancellous Bone Elastic Properties with the
           Polarization-based FFT Iterative Scheme
    • Authors: Lucas Colabella; Ariel Alejandro Ibarra Pino, Josefina Ballarre, Piotr Kowalczyk, Adrián Pablo Cisilino
      Abstract: The FFT based method, originally introduced by Moulinec and Suquet in 1994 has gained popularity for computing homogenized properties of composites. In this work, the method is used for the computational homogenization of the elastic properties of cancellous bone. To the authors’ knowledge, this is the first study where the FFT scheme is applied to bone mechanics. The performance of the method is analyzed for artificial and natural bone samples of two species: bovine femoral heads and implanted femurs of Hokkaido rats. Model geometries are constructed using data from X-ray tomographies and the bone tissue elastic properties are measured using micro and nanoindentation tests. Computed results are in excellent agreement with those available in the literature. The study shows the suitability of the method to accurately estimate the fully anisotropic elastic response of cancellous bone. Guidelines are provided for the construction of the models and the setting of the algorithm.
      PubDate: 2017-03-07T19:25:28.518686-05:
      DOI: 10.1002/cnm.2879
       
  • Human body modeling method to simulate the biodynamic characteristics of
           spine in vivo with different sitting postures
    • Authors: Rui Chun Dong; Li Xin Guo
      Abstract: The aim of this study is to model the computational model of seated whole human body including skeleton, muscle, viscera, ligament, intervertebral disc and skin to predict effect of the factors (sitting postures, muscle and skin, buttocks, viscera, arms, gravity, and boundary conditions) on the biodynamic characteristics of spine. Two finite element (FE) models of seated whole body and a large number of FE models of different ligamentous motion segments were developed and validated. Static, modal and transient dynamic analyses were performed. The predicted vertical resonant frequency of seated body model was in the range of vertical natural frequency of 4-7Hz. Muscle, buttocks, viscera and the boundary conditions of buttocks have influence on the vertical resonant frequency of spine. Muscle played a very important role in biodynamic response of spine. Compared with the vertical posture, the posture of lean forward or backward led to an increase in stress on anterior or lateral posterior of lumbar intervertebral discs (LID). This indicated keeping correct posture could reduce the injury of vibration on LID under whole-body vibration. The driving posture not only reduced the load of spine, but also increased the resonant frequency of spine.
      PubDate: 2017-03-06T12:50:30.43009-05:0
      DOI: 10.1002/cnm.2876
       
  • Research and Primary Evaluation of an Automatic Fusion Method for
           Multi-source Tooth Crown Data
    • Authors: Ning Dai; Dawei Li, Xu Yang, Cheng Cheng, Yuchun Sun
      Abstract: BackgroundWith the development of 3D scanning technologies in dentistry, high accuracy optical scanning data from the crown and cone beam computed tomography (CBCT) data from the root can be acquired easily. In many dental fields, especially in digital orthodontics, it is useful to fuse the data from the crown and the root. However, the manual fusion method is complex and difficult. A novel automatic fusion method for two-source data from the crown and the root was researched and its accuracy was evaluated in this study.MethodsAn occlusal splint with several alumina ceramic spheres was fabricated using heat-curing resin. A multi-point (center of each sphere) alignment method was performed to achieve rapid registration of the crown data from optical scanning and the root data from CBCT. The segmentation algorithm based on heuristic search was adopted to perform extraction and segmentation of the crown from whole optical scanning data. The Level Set algorithm and the Marching Cubes algorithm were employed to reconstruct DICOM data into a 3D model. A novel multi-source data fusion algorithm, which is based on Iterative Laplacian Deformation (ILD), was researched and applied to achieve automatic fusion. Finally, the 3D errors of the method were evaluated.ResultsThe three groups of typical tooth data were automatically fused within 2 s. The mean standard deviation was less than 0.02 mm.ConclusionsThe novel method can aid the construction of a high-quality 3D model of complete teeth to enable orthodontists to safely, reliably, and visually plan tooth alignment programs.
      PubDate: 2017-03-03T18:05:29.183277-05:
      DOI: 10.1002/cnm.2878
       
  • Issue Information
    • Abstract: No abstract is available for this article.
      PubDate: 2017-03-03T10:22:24.793514-05:
      DOI: 10.1002/cnm.2870
       
  • Multiphoton microscope measurement–based biphasic multiscale analyses of
           knee joint articular cartilage and chondrocyte by using visco-anisotropic
           hyperelastic finite element method and smoothed particle hydrodynamics
           method
    • Authors: Eiji Nakamachi; Tomohiro Noma, Kaito Nakahara, Yoshihiro Tomita, Yusuke Morita
      Abstract: The articular cartilage of a knee joint has a variety of functions including dispersing stress and absorbing shock in the tissue and lubricating the surface region of cartilage. The metabolic activity of chondrocytes under the cyclic mechanical stimulations regenerates the morphology and function of tissues. Hence, the stress evaluation of the chondrocyte is a vital subject to assess the regeneration cycle in the normal walking condition and predict the injury occurrence in the accidents. Further, the threshold determination of stress for the chondrocytes activation is valuable for development of regenerative bioreactor of articular cartilage. In this study, in both macroscale and microscale analyses, the dynamic explicit finite element (FE) method was used for the solid phase and the smoothed particle hydrodynamics (SPH) method was used for the fluid phase. In the homogenization procedure, the representative volume element for the microscale finite element model was derived by using the multiphoton microscope measured 3D structure comprising 3 different layers: surface, middle, and deep layers. The layers had different anisotropic structural and rigidity characteristics because of the collagen fiber orientation. In both macroscale and microscale FE analyses, the visco-anisotropic hyperelastic constitutive law was used. Material properties were identified by experimentally determined stress-strain relationships of 3 layers. With respect to the macroscale and microscale SPH models for non-Newtonian viscous fluid, the previous observation results of interstitial fluid and proteoglycan were used to perform parameter identifications. Biphasic multiscale FE and SPH analyses were conducted under normal walking conditions. Therefore, the hydrostatic and shear stresses occurring in the chondrocytes caused by the compressive load and shear viscous flow were evaluated. These stresses will be used to design an ex-vivo bioreactor to regenerate the damaged articular cartilage, where chondrocytes are seeded in the culture chamber. To know the stress occurred on and in the chondrocytes is vitally important not only to understand the normal metabolic activity of the chondrocyte but also to develop a bioreactor of articular cartilage regeneration as the knee joint disease treatment.We developed a biphasic multiscale analysis code to evaluate the stress occurred in the chondrocyte cell of articular cartilage to elucidate the metabolic activity for regeneration and the injury. We determined RVE for microscale FE models by using MPM measured results. We evaluated stresses in the chondrocyte caused by the normal compressive loading. Our numerical code can be applied for accurate stress evaluations by using more detail experimental results for material properties identification.
      PubDate: 2017-03-03T09:25:53.42354-05:0
      DOI: 10.1002/cnm.2864
       
  • A Hybrid Computational Model to Explore the Topological Characteristics of
           Epithelial Tissues
    • Authors: Ismael González-Valverde; José Manuel García Aznar
      Abstract: Epithelial tissues show a particular topology where cells resemble a polygon-like shape, but some biological processes can alter this tissue topology. During cell proliferation, mitotic cell dilation deforms the tissue and modifies the tissue topology. Additionally, cells are reorganized in the epithelial layer and these rearrangements also alter the polygon distribution.We present here a computer-based hybrid framework focused on the simulation of epithelial layer dynamics that combines discrete and continuum numerical models. In this framework, we consider topological and mechanical aspects of the epithelial tissue. Individual cells in the tissue are simulated by an off-lattice agent-based model, which keeps the information of each cell. In addition, we model the cell-cell interaction forces and the cell cycle. Otherwise, we simulate the passive mechanical behaviour of the cell monolayer using a material that approximates the mechanical properties of the cell. This continuum approach is solved by the finite element method, which uses a dynamic mesh generated by the triangulation of cell polygons. Forces generated by cell-cell interaction in the agent-based model are also applied on the finite element mesh. Cell movement in the agent-based model is driven by the displacements obtained from the deformed finite element mesh of the continuum mechanical approach.We successfully compare the results of our simulations with some experiments about the topology of proliferating epithelial tissues in Drosophila. Our framework is able to model the emergent behaviour of the cell monolayer that is due to local cell-cell interactions, which have a direct influence on the dynamics of the epithelial tissue.
      PubDate: 2017-03-01T15:00:55.9462-05:00
      DOI: 10.1002/cnm.2877
       
  • Aerosol transport throughout inspiration and expiration in the pulmonary
           airways
    • Authors: Jessica M. Oakes; Shawn C. Shadden, Céline Grandmont, Irene E. Vignon-Clementel
      Abstract: Little is known about transport throughout the respiration cycle in the conducting airways. It is challenging to appropriately describe the time-dependent number of particles entering back into the model during exhalation. Modeling the entire lung is not feasible; therefore, multidomain methods must be used. Here, we present a new framework that is designed to simulate particles throughout the respiration cycle, incorporating realistic airway geometry and respiration. This framework is applied for a healthy rat lung exposed to  ∼ 1μm diameter particles, chosen to facilitate parameterization and validation. The flow field is calculated in the conducting airways (3D domain) by solving the incompressible Navier-Stokes equations with experimentally derived boundary conditions. Particles are tracked throughout inspiration by solving a modified Maxey-Riley equation. Next, we pass the time-dependent particle concentrations exiting the 3D model to the 1D volume conservation and advection-diffusion models (1D domain). Once the 1D models are solved, we prescribe the time-dependent number of particles entering back into the 3D airways to again solve for 3D transport. The coupled simulations highlight that about twice as many particles deposit during inhalation compared to exhalation for the entire lung. In contrast to inhalation, where most particles deposit at the bifurcation zones, particles deposit relatively uniformly on the gravitationally dependent side of the 3D airways during exhalation. Strong agreement to previously collected regional experimental data is shown, as the 1D models account for lobe-dependent morphology. This framework may be applied to investigate dosimetry in other species and pathological lungs.Little is known about transport throughout the respiration cycle in the conducting airways. It is challenging to describe the time-dependent number of particles entering back into the airways during exhalation. Modeling the full lung is not feasible; hence, multi-domain methods must be employed. Here, we present a new framework that is designed to simulate particles throughout the respiration cycle. The in silico model was parametrized following rat exposure experiments and model predictions were compared to the experimental data.
      PubDate: 2017-02-24T05:00:41.044117-05:
      DOI: 10.1002/cnm.2847
       
  • Uncertainty quantification of inflow boundary condition and proximal
           arterial stiffness–coupled effect on pulse wave propagation in a
           vascular network
    • Authors: Antoine Brault; Laurent Dumas, Didier Lucor
      Abstract: This work aims at quantifying the effect of inherent uncertainties from cardiac output on the sensitivity of a human compliant arterial network response based on stochastic simulations of a reduced-order pulse wave propagation model. A simple pulsatile output form is used to reproduce the most relevant cardiac features with a minimum number of parameters associated with left ventricle dynamics. Another source of significant uncertainty is the spatial heterogeneity of the aortic compliance, which plays a key role in the propagation and damping of pulse waves generated at each cardiac cycle. A continuous representation of the aortic stiffness in the form of a generic random field of prescribed spatial correlation is then considered. Making use of a stochastic sparse pseudospectral method, we investigate the sensitivity of the pulse pressure and waves reflection magnitude over the arterial tree with respect to the different model uncertainties. Results indicate that uncertainties related to the shape and magnitude of the prescribed inlet flow in the proximal aorta can lead to potent variation of both the mean value and standard deviation of blood flow velocity and pressure dynamics due to the interaction of different wave propagation and reflection features. Lack of accurate knowledge in the stiffness properties of the aorta, resulting in uncertainty in the pulse wave velocity in that region, strongly modifies the statistical response, with a global increase in the variability of the quantities of interest and a spatial redistribution of the regions of higher sensitivity. These results will provide some guidance in clinical data acquisition and future coupling of arterial pulse wave propagation reduced-order model with more complex beating heart models.A stochastic sparse pseudospectral polynomial approximation is deployed to quantify the effect of cardiac output uncertainties on a human compliant arterial network response based on a reduced-order pulse wave propagation model. Natural spatial variability of the aortic wall stiffness properties is modeled with a continuous random field representation. The proposed numerical method accurately predicts the sensitivity of central and peripheral pulse pressure and pressure waves reflection to the considered parametric uncertainties.
      PubDate: 2017-02-24T04:55:36.380294-05:
      DOI: 10.1002/cnm.2859
       
  • Patient-specific computational modeling of Cortical Spreading Depression
           via Diffusion Tensor Imaging
    • Authors: Julia M. Kroos; Isabella Marinelli, Ibai Diez, Jesus M. Cortes, Sebastiano Stramaglia, Luca Gerardo-Giorda
      Abstract: Cortical Spreading Depression (CSD), a depolarization wave originating in the visual cortex and traveling towards the frontal lobe, is commonly accepted as a correlate of migraine visual aura. As of today, little is known about the mechanisms that can trigger or stop such phenomenon. However, the complex and highly individual characteristics of the brain cortex suggest that the geometry might have a significant impact in supporting or contrasting the propagation of CSD. Accurate patient-specific computational models are fundamental to cope with the high variability in cortical geometries among individuals, but also with the conduction anisotropy induced in a given cortex by the complex neuronal organisation in the grey matter. In this paper we integrate a distributed model for extracellular potassium concentration with patient-specific diffusivity tensors derived locally from Diffusion Tensor Imaging data. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-22T17:05:29.312808-05:
      DOI: 10.1002/cnm.2874
       
  • An efficient multi-stage algorithm for full calibration of the hemodynamic
           model from BOLD signal responses
    • Authors: Brian Zambri; Rabia Djellouli, Meriem Laleg-Kirati
      Abstract: We propose a computational strategy that falls into the category of prediction/correction iterative-type approaches, for calibrating the hemodynamic model introduced by Friston et al. (2000). The proposed method is employed to estimate consecutively the values of the biophysiological system parameters and the external stimulus characteristics of the model. Numerical results corresponding to both synthetic and real functional Magnetic Resonance Imaging (fMRI) measurements for a single stimulus as well as for multiple stimuli are reported to highlight the capability of this computational methodology to fully calibrate the considered hemodynamic model. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-22T17:05:24.502503-05:
      DOI: 10.1002/cnm.2875
       
  • A tree-parenchyma coupled model for lung ventilation simulation
    • Authors: N. Pozin; S. Montesantos, I. Katz, M. Pichelin, I. Vignon-Clementel, C. Grandmont
      Abstract: In this article we develop a lung-ventilation model. The parenchyma is described as an elastic homogenized media. It is irrigated by a space-filling dyadic resistive pipe network, which represents the tracheo-bronchial tree. In this model the tree and the parenchyma are strongly coupled. The tree induces an extra viscous term in the system constitutive relation, which leads, in the finite element framework, to a full matrix. We consider an efficient algorithm that takes advantage of the tree structure to enable a fast matrix-vector product computation. This framework can be used to model both free and mechanically induced respiration, in health and disease. Patient-specific lung geometries acquired from CT scans are considered. Realistic Dirichlet boundary conditions can be deduced from surface registration on CT images. The model is compared to a more classical exit-compartment approach. Results illustrate the coupling between the tree and the parenchyma, at global and regional levels, and how conditions for the purely 0D model can be inferred. Different types of boundary conditions are tested, including a nonlinear Robin model of the surrounding lung structures.
      PubDate: 2017-02-22T04:20:41.251928-05:
      DOI: 10.1002/cnm.2873
       
  • Fast left ventricle tracking using localized anatomical affine optical
           flow
    • Authors: Sandro Queirós; João L. Vilaça, Pedro Morais, Jaime C. Fonseca, Jan D'hooge, Daniel Barbosa
      Abstract: In daily clinical cardiology practice, left ventricle (LV) global and regional function assessment is crucial for disease diagnosis, therapy selection and patient follow-up. Currently, this is still a time-consuming task, spending valuable human resources. In this work, a novel fast methodology for automatic LV tracking is proposed based on localized anatomically constrained affine optical flow. This novel method can be combined to previously proposed segmentation frameworks or manually delineated surfaces at an initial frame to obtain fully delineated datasets and, thus, assess both global and regional myocardial function. Its feasibility and accuracy was investigated in three distinct public databases, namely in realistically simulated 3D ultrasound (US), clinical 3D echocardiography and clinical cine cardiac magnetic resonance (CMR) images. The method showed accurate tracking results in all databases, proving its applicability and accuracy for myocardial function assessment. Moreover, when combined to previous state-of-the-art segmentation frameworks, it outperformed previous tracking strategies in both 3D US and CMR data, automatically computing relevant cardiac indices with smaller biases and narrower limits of agreement compared to reference indices. Simultaneously, the proposed localized tracking method showed to be suitable for online processing, even for 3D motion assessment. Importantly, although here evaluated for LV tracking only, this novel methodology is applicable for tracking of other target structures with minimal adaptations. This article is protected by copyright. All rights reserved.
      PubDate: 2017-02-16T17:20:25.12805-05:0
      DOI: 10.1002/cnm.2871
       
  • A monolithic 3D-0D coupled closed-loop model of the heart and the vascular
           system: Experiment-based parameter estimation for patient-specific cardiac
           mechanics
    • Authors: Marc Hirschvogel; Marina Bassilious, Lasse Jagschies, Stephen M. Wildhirt, Michael W. Gee
      Abstract: A model for patient-specific cardiac mechanics simulation is introduced, incorporating a 3-dimensional finite element model of the ventricular part of the heart, which is coupled to a reduced-order 0-dimensional closed-loop vascular system, heart valve, and atrial chamber model.The ventricles are modeled by a nonlinear orthotropic passive material law. The electrical activation is mimicked by a prescribed parameterized active stress acting along a generic muscle fiber orientation. Our activation function is constructed such that the start of ventricular contraction and relaxation as well as the active stress curve's slope are parameterized. The imaging-based patient-specific ventricular model is prestressed to low end-diastolic pressure to account for the imaged, stressed configuration. Visco-elastic Robin boundary conditions are applied to the heart base and the epicardium to account for the embedding surrounding.We treat the 3D solid-0D fluid interaction as a strongly coupled monolithic problem, which is consistently linearized with respect to 3D solid and 0D fluid model variables to allow for a Newton-type solution procedure. The resulting coupled linear system of equations is solved iteratively in every Newton step using 2  ×  2 physics-based block preconditioning.Furthermore, we present novel efficient strategies for calibrating active contractile and vascular resistance parameters to experimental left ventricular pressure and stroke volume data gained in porcine experiments. Two exemplary states of cardiovascular condition are considered, namely, after application of vasodilatory beta blockers (BETA) and after injection of vasoconstrictive phenylephrine (PHEN). The parameter calibration to the specific individual and cardiovascular state at hand is performed using a 2-stage nonlinear multilevel method that uses a low-fidelity heart model to compute a parameter correction for the high-fidelity model optimization problem. We discuss 2 different low-fidelity model choices with respect to their ability to augment the parameter optimization.Because the periodic state conditions on the model (active stress, vascular pressures, and fluxes) are a priori unknown and also dependent on the parameters to be calibrated (and vice versa), we perform parameter calibration and periodic state condition estimation simultaneously. After a couple of heart beats, the calibration algorithm converges to a settled, periodic state because of conservation of blood volume within the closed-loop circulatory system.The proposed model and multilevel calibration method are cost-efficient and allow for an efficient determination of a patient-specific in silico heart model that reproduces physiological observations very well. Such an individual and state accurate model is an important predictive tool in intervention planning, assist device engineering and other medical applications.We present a model for patient-specific cardiac mechanics, incorporating a 3D finite element ventricular model coupled to a reduced-order 0D closed-loop vascular system, heart valve, and atrial chamber model. The coupled problem is consistently linearized with respect to 3D structural and 0D vascular unknowns and iteratively solved in one monolithic Newton iteration using physics-based block preconditioning. Efficient strategies for calibrating active contractile and vascular resistance parameters to experimental data gained in porcine experiments are presented, proposing a novel 2-level nonlinear optimization procedure.
      PubDate: 2017-02-16T03:50:47.450535-05:
      DOI: 10.1002/cnm.2842
       
  • Conditions of microvessel occlusion for blood coagulation in flow
    • Authors: A. Bouchnita; T. Galochkina, P. Kurbatova, P. Nony, V. Volpert
      Abstract: Vessel occlusion is a perturbation of blood flow inside a blood vessel because of the fibrin clot formation. As a result, blood circulation in the vessel can be slowed down or even stopped. This can provoke the risk of cardiovascular events. In order to explore this phenomenon, we used a previously developed mathematical model of blood clotting to describe the concentrations of blood factors with a reaction-diffusion system of equations. The Navier-Stokes equations were used to model blood flow, and we treated the clot as a porous medium. We identify the conditions of partial or complete occlusion in a small vessel depending on various physical and physiological parameters. In particular, we were interested in the conditions on blood flow and diameter of the wounded area. The existence of a critical flow velocity separating the regimes of partial and complete occlusion was demonstrated through the mathematical investigation of a simplified model of thrombin wave propagation in Poiseuille flow. We observed different regimes of vessel occlusion depending on the model parameters both for the numerical simulations and in the theoretical study. Then, we compared the rate of clot growth in flow obtained in the simulations with experimental data. Both of them showed the existence of different regimes of clot growth depending on the velocity of blood flow.Microvessel occlusion is the perturbation of blood flow inside a vein because of the formation of a fibrin clot. Mathematical model of clot growth was developed using a system of reaction-diffusion coupled with the Navier-Stokes equations for blood flow. Conditions of microvessel occlusion were identified using numerical simulations and mathematical investigation of simplified one-dimensional model. Experimental data and numerical simulations confirmed the existence of different regimes of clot growth velocity depending on the velocity of blood flow.
      PubDate: 2017-02-16T03:35:33.981459-05:
      DOI: 10.1002/cnm.2850
       
  • Modelling mitral valvular dynamics–current trend and future
           directions
    • Authors: Hao Gao; Nan Qi, Liuyang Feng, Xingshuang Ma, Mark Danton, Colin Berry, Xiaoyu Luo
      Abstract: Dysfunction of mitral valve causes morbidity and premature mortality and remains a leading medical problem worldwide. Computational modelling aims to understand the biomechanics of human mitral valve and could lead to the development of new treatment, prevention and diagnosis of mitral valve diseases. Compared with the aortic valve, the mitral valve has been much less studied owing to its highly complex structure and strong interaction with the blood flow and the ventricles. However, the interest in mitral valve modelling is growing, and the sophistication level is increasing with the advanced development of computational technology and imaging tools. This review summarises the state-of-the-art modelling of the mitral valve, including static and dynamics models, models with fluid-structure interaction, and models with the left ventricle interaction. Challenges and future directions are also discussed.We summarize the state-of-the-art modelling of the mitral valve, including static and dynamic models, mitral valve with fluid-structure interaction, and mitral valve with the left ventricle interaction. Challenges and future directions are also discussed.
      PubDate: 2017-02-16T03:26:26.554323-05:
      DOI: 10.1002/cnm.2858
       
  • A novel modelling approach to energy transport in a respiratory system
    • Authors: Perumal Nithiarasu; Igor Sazonov
      Abstract: In this paper, energy transport in a respiratory tract is modelled using the finite element method for the first time. The upper and lower respiratory tracts are approximated as a 1-dimensional domain with varying cross-sectional and surface areas, and the radial heat conduction in the tissue is approximated using the 1-dimensional cylindrical coordinate system. The governing equations are solved using 1-dimensional linear finite elements with convective and evaporative boundary conditions on the wall. The results obtained for the exhalation temperature of the respiratory system have been compared with the available animal experiments. The study of a full breathing cycle indicates that evaporation is the main mode of heat transfer, and convection plays almost negligible role in the energy transport. This is in-line with the results obtained from animal experiments.In this paper, energy transport in a respiratory tract is modelled using the finite element method for the first time. The upper and lower respiratory tracts are approximated as a one-dimensional domain with varying cross sectional and surface areas, and the radial heat conduction in the tissue is approximated using the one dimensional cylindrical coordinate system. The governing equations are solved using one-dimensional linear finite elements with convective and evaporative boundary conditions on the wall. The results obtained for the exhalation temperature of the respiratory system have been compared with the available animal experiments. The study of a full breathing cycle indicates that evaporation is the main mode of heat transfer, and convection plays almost negligible role in the energy transport. This is inline with the results obtained from animal experiments.
      PubDate: 2017-02-16T03:15:31.101832-05:
      DOI: 10.1002/cnm.2854
       
  • Validation of a non-conforming monolithic fluid-structure interaction
           method using phase-contrast MRI
    • Authors: Andreas Hessenthaler; Oliver Röhrle, David Nordsletten
      Abstract: This paper details the validation of a non-conforming arbitrary Lagrangian-Eulerian fluid-structure interaction technique using a recently developed experimental 3D fluid-structure interaction benchmark problem. Numerical experiments for steady and transient test cases of the benchmark were conducted employing an inf-sup stable and a general Galerkin scheme. The performance of both schemes is assessed. Spatial refinement with three mesh refinement levels and fluid domain truncation with two fluid domain lengths are studied as well as employing a sequence of increasing time step sizes for steady-state cases. How quickly an approximate steady-state or periodic steady-state is reached is investigated and quantified based on error norm computations. Comparison of numerical results with experimental phase-contrast magnetic resonance imaging data shows very good overall agreement including governing of flow patterns observed in the experiment.A non-conforming arbitrary Lagrangian-Eulerian fluid-structure interaction (FSI) technique is validated using PC MRI data from a 3D FSI experiment with flow in the laminar regime. Performance of the method, spatial refinement, and time to (periodic) steady-state are studied. Very good overall agreement between numerical results and experimental data is found.
      PubDate: 2017-02-16T03:10:52.612652-05:
      DOI: 10.1002/cnm.2845
       
  • An Investigation of Dimensional Scaling Using Cervical Spine Motion
           Segment Finite Element Models
    • Authors: Dilaver Singh; Duane S. Cronin
      Abstract: The paucity of experimental data for validating computational models of different statures underscores the need for appropriate scaling methods so that models can be verified and validated using experimental data. Scaling was investigated using 50th percentile male (M50) and 5th percentile female (F05) cervical spine motion segment (C4-C5) finite element models subject to tension, flexion and extension loading. Two approaches were undertaken: geometric scaling of the models to investigate size effects (volumetric scaling) and scaling of the force-displacement or moment-angle model results (data scaling). Three sets of scale factors were considered: global (body mass), regional (neck dimensions) and local (segment tissue dimensions).Volumetric scaling of the segment models from M50 to F05, and vice-versa, produced correlations that were good or excellent in both tension and flexion (0.825-0.991); however, less agreement was found in extension (0.550-0.569). The reduced correlation in extension was attributed to variations in shape between the models leading to nonlinear effects such as different time to contact for the facet joints and posterior processes. Data scaling of the responses between the M50 and F05 models produced similar trends to volumetric scaling, with marginally greater correlations.Overall, the local tissue level and neck region level scale factors produced better correlations than the traditional global scaling. The scaling methods work well for a given subject, but are limited in applicability between subjects with different morphology, where nonlinear effects may dominate the response.
      PubDate: 2017-02-15T21:35:27.188101-05:
      DOI: 10.1002/cnm.2872
       
  • The role of the microvascular network structure on diffusion and
           consumption of anticancer drugs
    • Authors: Pietro Mascheroni; Raimondo Penta
      Abstract: We investigate the impact of microvascular geometry on the transport of drugs in solid tumors, focusing on the diffusion and consumption phenomena. We embrace recent advances in the asymptotic homogenization literature starting from a double Darcy—double advection-diffusion-reaction system of partial differential equations that is obtained exploiting the sharp length separation between the intercapillary distance and the average tumor size. The geometric information on the microvascular network is encoded into effective hydraulic conductivities and diffusivities, which are numerically computed by solving periodic cell problems on appropriate microscale representative cells. The coefficients are then injected into the macroscale equations, and these are solved for an isolated, vascularized spherical tumor. We consider the effect of vascular tortuosity on the transport of anticancer molecules, focusing on Vinblastine and Doxorubicin dynamics, which are considered as a tracer and as a highly interacting molecule, respectively. The computational model is able to quantify the treatment performance through the analysis of the interstitial drug concentration and the quantity of drug metabolized in the tumor. Our results show that both drug advection and diffusion are dramatically impaired by increasing geometrical complexity of the microvasculature, leading to nonoptimal absorption and delivery of therapeutic agents. However, this effect apparently has a minor role whenever the dynamics are mostly driven by metabolic reactions in the tumor interstitium, eg, for highly interacting molecules. In the latter case, anticancer therapies that aim at regularizing the microvasculature might not play a major role, and different strategies are to be developed.We numerically solve a double Darcy—double advection-diffusion-reaction model (derived via asymptotic homogenization) for fluid and drug transport in vascularized tumors, following a suitable algorithm to decouple microscale and macroscale spatial variations to reduce the computational cost.We consider for the first time variations of the diffusivity tensor (both in the vessel network and in the tumor interstitium) with respect to the microvascular structure, as well as its interplay with both weak and strong uptake mechanisms in the tumor and potential convective contributions across the vessels membrane.The numerical results show that geometrical tortuosity dramatically impairs diffusion and absorption of injected drugs, although the latter effect is apparently less significant for strongly interacting macromolecules.
      PubDate: 2017-02-14T02:30:33.16586-05:0
      DOI: 10.1002/cnm.2857
       
  • A velocity tracking approach for the data assimilation problem in blood
           flow simulations
    • Authors: J. Tiago; T. Guerra, A. Sequeira
      Abstract: Several advances have been made in data assimilation techniques applied to blood flow modeling. Typically, idealized boundary conditions, only verified in straight parts of the vessel, are assumed. We present a general approach, on the basis of a Dirichlet boundary control problem, that may potentially be used in different parts of the arterial system. The relevance of this method appears when computational reconstructions of the 3D domains, prone to be considered sufficiently extended, are either not possible, or desirable, because of computational costs. On the basis of taking a fully unknown velocity profile as the control, the approach uses a discretize then optimize methodology to solve the control problem numerically. The methodology is applied to a realistic 3D geometry representing a brain aneurysm. The results show that this data assimilation approach may be preferable to a pressure control strategy and that it can significantly improve the accuracy associated to typical solutions obtained using idealized velocity profiles.We present a general data assimilation approach, on the basis of a Dirichlet boundary control problem, that may potentially be used in different parts of the arterial system. The relevance of this method appears when computational reconstructions of the 3D domains, prone to be considered sufficiently extended to obtain reliable solutions, are not possible. The methodology is applied to a realistic 3D geometry representing a brain aneurysm.
      PubDate: 2017-02-14T02:25:41.478928-05:
      DOI: 10.1002/cnm.2856
       
  • Quantitative validation of anti-PTBP1 antibody for diagnostic
           neuropathology use: Image analysis approach
    • Authors: Evgin Goceri; Behiye Goksel, James B. Elder, Vinay K. Puduvalli, Jose J. Otero, Metin N. Gurcan
      Abstract: Traditional diagnostic neuropathology relies on subjective interpretation of visual data obtained from a brightfield microscopy. This approach causes high variability, unsatisfactory reproducibility, and inability for multiplexing even among experts. These problems may affect patient outcomes and confound clinical decision-making. Also, standard histological processing of pathological specimens leads to auto-fluorescence and other artifacts, a reason why fluorescent microscopy is not routinely implemented in diagnostic pathology. To overcome these problems, objective and quantitative methods are required to help neuropathologists in their clinical decision-making. Therefore, we propose a computerized image analysis method to validate anti-PTBP1 antibody for its potential use in diagnostic neuropathology. Images were obtained from standard neuropathological specimens stained with anti-PTBP1 antibody. First, the noise characteristics of the images were modeled and images are de-noised according to the noise model. Next, images are filtered with sigma-adaptive Gaussian filtering for normalization, and cell nuclei are detected and segmented with a k-means–based deterministic approach. Experiments on 29 data sets from 3 cases of brain tumor and reactive gliosis show statistically significant differences between the number of positively stained nuclei in images stained with and without anti-PTBP1 antibody. The experimental analysis of specimens from 3 different brain tumor groups and 1 reactive gliosis group indicates the feasibility of using anti-PTBP1 antibody in diagnostic neuropathology, and computerized image analysis provides a systematic and quantitative approach to explore feasibility.The experimental analysis of specimens from 3 different brain tumor groups and 1 reactive gliosis group indicates the feasibility of using anti-PTBP1 antibody in diagnostic neuropathology, and computerized image analysis provides a systematic and quantitative approach to explore feasibility.
      PubDate: 2017-02-10T06:11:08.256298-05:
      DOI: 10.1002/cnm.2862
       
  • How coagulation zone size is underestimated in computer modeling of RF
           ablation by ignoring the cooling phase just after RF power is switched off
           
    • Authors: Ramiro M. Irastorza; Macarena Trujillo, Enrique Berjano
      Abstract: All the numerical models developed for radiofrequency (RF) ablation so far have ignored the possible effect of the cooling phase (just after RF power is switched off) on the dimensions of the coagulation zone. Our objective was thus to quantify the differences in the minor radius of the coagulation zone computed by including and ignoring the cooling phase. We built models of RF tumor ablation with two needle-like electrodes: a dry electrode (5 mm long and 17G in diameter) with a constant temperature protocol (70 °C) and a cooled electrode (30 mm long and 17G in diameter) with a protocol of impedance control. We observed that the computed coagulation zone dimensions were always underestimated when the cooling phase was ignored. The mean values of the differences computed along the electrode axis were always lower than 0.15 mm for the dry electrode and 1.5 mm for the cooled electrode, which implied a value lower than 5% of the minor radius of the coagulation zone (which was 3 mm for the dry electrode, and 30 mm for the cooled electrode). The underestimation was found to be dependent on the tissue characteristics: being more marked for higher values of specific heat and blood perfusion and less marked for higher values of thermal conductivity.
      PubDate: 2017-02-01T10:40:58.299103-05:
      DOI: 10.1002/cnm.2869
       
  • Experiment for validation of fluid-structure interaction models and
           algorithms
    • Authors: A. Hessenthaler; N. R. Gaddum, O. Holub, R. Sinkus, O. Röhrle, D. Nordsletten
      Abstract: In this paper a fluid-structure interaction (FSI) experiment is presented. The aim of this experiment is to provide a challenging yet easy-to-setup FSI test case that addresses the need for rigorous testing of FSI algorithms and modeling frameworks. Steady-state and periodic steady-state test cases with constant and periodic inflow were established. Focus of the experiment is on biomedical engineering applications with flow being in the laminar regime with Reynolds numbers 1283 and 651. Flow and solid domains were defined using computer-aided design (CAD) tools. The experimental design aimed at providing a straightforward boundary condition definition. Material parameters and mechanical response of a moderately viscous Newtonian fluid and a nonlinear incompressible solid were experimentally determined. A comprehensive data set was acquired by using magnetic resonance imaging to record the interaction between the fluid and the solid, quantifying flow and solid motion.A fluid-structure interaction (FSI) experiment is presented with the aim to provide a challenging yet easy-to-setup FSI test case that addresses the need for rigorous testing of FSI algorithms and modeling frameworks. Focus of the experiment is on biomedical engineering applications. A comprehensive data set was acquired by employing magnetic resonance imaging to record the interaction between the fluid and the solid, quantifying flow and solid motion for steady-state and periodic steady-state test cases.
      PubDate: 2017-01-27T03:31:43.428518-05:
      DOI: 10.1002/cnm.2848
       
  • Large Eddy Simulations for blood dynamics in realistic stenotic carotids
    • Authors: Rocco Michele Lancellotti; Christian Vergara, Lorenzo Valdettaro, Sanjeeb Bose, Alfio Quarteroni
      Abstract: In this paper, we consider Large Eddy Simulations (LES) for human stenotic carotids in presence of atheromasic plaque, a pathological condition where transitional effects to turbulence may occur, with relevant clinical implications such as plaque rupture. We provide a reference numerical solution obtained at high resolution without any subgrid scale model, to be used to assess the accuracy of LES simulations. In the context we are considering, i.e. hemodynamics, we cannot refer to a statistically homogeneous, isotropic and stationary turbulent regime, hence the classical Kolmogorov theory cannot be used. For this reason, a mesh size and a time step are deemed fine enough if they allow to capture all the features of the velocity field in the shear layers developed after the bifurcation. To assess these requirements, we consider a simplified model of the evolution of a 2D shear layer, a relevant process in the formation of transitional effects in our case. Then, we compare the results of LES σ model (both static and dynamic) and of mixed LES models (where also a similarity contribution is considered). In particular, we consider a realistic scenario of a human carotid and we use the reference solution as gold standard. The results highlight the accuracy of the LES σ models, especially for the static model. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-26T06:20:23.882565-05:
      DOI: 10.1002/cnm.2868
       
  • Three-dimensional assessment of impingement risk in geometrically
           parameterised hips compared with clinical measures
    • Authors: Robert J. Cooper; Marlène Mengoni, Dawn Groves, Sophie Williams, Marcus J. K. Bankes, Philip Robinson, Alison C. Jones
      Abstract: Abnormal bony morphology is a factor implicated in hip joint soft tissue damage and an increased lifetime risk of osteoarthritis. Standard two-dimensional radiographic measurements for diagnosis of hip deformities, such as cam deformities on the femoral neck, do not capture the full joint geometry and are not indicative of symptomatic damage.In this study, a three-dimensional geometric parameterisation system was developed to capture key variations in the femur and acetabulum of subjects with clinically diagnosed cam deformity. The parameterisation was performed for Computed Tomography scans of 20 patients (10 female, 10 male). Novel quantitative measures of cam deformity were taken and used to assess differences in morphological deformities between males and females.The parametric surfaces matched the more detailed, segmented hip bone geometry with low fitting error. The quantitative severity measures captured both the size and position of cams, and distinguished between cam and control femurs. The precision of the measures was sufficient to identify differences between subjects that could not be seen with the sole use of two-dimensional imaging. In particular, cams were found to be more superiorly located in males than in females.As well as providing a means to distinguish between subjects more clearly, the new geometric hip parameterisation facilitates the flexible and rapid generation of a range of realistic hip geometries including cams. When combined with material property models, these stratified cam shapes can be used for further assessment of the effect of the geometric variation under impingement conditions.
      PubDate: 2017-01-23T11:00:24.744388-05:
      DOI: 10.1002/cnm.2867
       
  • Method for the unique identification of hyperelastic material properties
           using full field measures. Application to the passive myocardium material
           response
    • Authors: Luigi E. Perotti; Aditya V. Ponnaluri, Shankarjee Krishnamoorthi, Daniel Balzani, Daniel B. Ennis, William S. Klug
      Abstract: Quantitative measurement of the material properties (e.g., stiffness) of biological tissues is poised to become a powerful diagnostic tool. There are currently several methods in the literature to estimating material stiffness and we extend this work by formulating a framework that leads to uniquely identified material properties. We design an approach to work with full field displacement data — i.e., we assume the displacement field due to the applied forces is known both on the boundaries and also within the interior of the body of interest — and seek stiffness parameters that lead to balanced internal and external forces in a model. For in vivo applications, the displacement data can be acquired clinically using magnetic resonance imaging while the forces may be computed from pressure measurements, e.g., through catheterization. We outline a set of conditions under which the least-square force error objective function is convex, yielding uniquely identified material properties. An important component of our framework is a new numerical strategy to formulate polyconvex material energy laws that are linear in the material properties and provide one optimal description of the available experimental data. An outcome of our approach is the analysis of the reliability of the identified material properties, even for material laws that do not admit unique property identification. Lastly, we evaluate our approach using passive myocardium experimental data at the material point and show its application to identifying myocardial stiffness with an in silico experiment modeling the passive filling of the left ventricle. This article is protected by copyright. All rights reserved.
      PubDate: 2017-01-18T07:25:31.597019-05:
      DOI: 10.1002/cnm.2866
       
  • Direct numerical simulation of transitional hydrodynamics of the
           cerebrospinal fluid in Chiari I malformation: The role of cranio-vertebral
           junction
    • Authors: Kartik Jain; Geir Ringstad, Per-Kristian Eide, Kent-André Mardal
      Abstract: Obstruction to the cerebrospinal fluid (CSF) outflow caused by the herniation of cerebellar tonsils as a result of Chiari malformation type I leads to altered CSF hydrodynamics. This contribution explores the minutest characteristics of the CSF hydrodynamics in cervical subarachnoid space (SAS) of a healthy subject and 2 Chiari patients by performing highly resolved direct numerical simulation. The lattice Boltzmann method is used for the simulations because of its scalability on modern supercomputers that allow us to simulate up to approximately 109 cells while resolving the Kolmogorov microscales. The results depict that whereas the complex CSF flow remains largely laminar in the SAS of a healthy subject, constriction of the cranio-vertebral junction in Chiari I patients causes manifold fluctuations in the hydrodynamics of the CSF. These fluctuations resemble a flow that is in a transitional regime rather than laminar or fully developed turbulence. The fluctuations confine near the cranio-vertebral junction and are triggered due to the tonsillar herniation, which perturbs the flow as a result of altered anatomy of the SAS.Chiari malformation type I obstructs the outflow of the cerebrospinal fluid near the foramen magnum. We conducted direct numerical simulations with meshes containing up to 1 billion cells on case specific subarachnoid spaces of one control subject and 2 Chiari patients on a modern supercomputer. We found the onset of transitional-like hydrodynamics of CSF in 2 Chiari patients whereas the flow remained laminar in the control subject.
      PubDate: 2017-01-13T04:35:34.389861-05:
      DOI: 10.1002/cnm.2853
       
  • Assessment of reduced-order unscented Kalman filter for parameter
           
    • Authors: A. Caiazzo; Federica Caforio, Gino Montecinos, Lucas O. Muller, Pablo J. Blanco, Eluterio F. Toro
      Abstract: This work presents a detailed investigation of a parameter estimation approach on the basis of the reduced-order unscented Kalman filter (ROUKF) in the context of 1-dimensional blood flow models. In particular, the main aims of this study are (1) to investigate the effects of using real measurements versus synthetic data for the estimation procedure (i.e., numerical results of the same in silico model, perturbed with noise) and (2) to identify potential difficulties and limitations of the approach in clinically realistic applications to assess the applicability of the filter to such setups. For these purposes, the present numerical study is based on a recently published in vitro model of the arterial network, for which experimental flow and pressure measurements are available at few selected locations. To mimic clinically relevant situations, we focus on the estimation of terminal resistances and arterial wall parameters related to vessel mechanics (Young's modulus and wall thickness) using few experimental observations (at most a single pressure or flow measurement per vessel). In all cases, we first perform a theoretical identifiability analysis on the basis of the generalized sensitivity function, comparing then the results owith the ROUKF, using either synthetic or experimental data, to results obtained using reference parameters and to available measurements.This work considers a parameter estimation approach on the basis of the reduced-order unscented Kalman filter in the context of one-dimensional blood flow models, investigating the effects of using real measurements versus synthetic data for the estimation procedure. The filter is assessed considering the results of an in vitro model of the human arterial network and the available experimental measurements, comparing the estimation results with an identifiability analysis on the basis of the generalized sensitivity function and considering flow and pressure observations.
      PubDate: 2017-01-13T04:10:37.92513-05:0
      DOI: 10.1002/cnm.2843
       
  • Multiphase fluid-solid coupled analysis of shock-bubble-stone interaction
           in shockwave lithotripsy
    • Authors: Kevin G. Wang
      Abstract: A novel multiphase fluid-solid–coupled computational framework is applied to investigate the interaction of a kidney stone immersed in liquid with a lithotripsy shock wave (LSW) and a gas bubble near the stone. The main objective is to elucidate the effects of a bubble in the shock path to the elastic and fracture behaviors of the stone. The computational framework couples a finite volume 2-phase computational fluid dynamics solver with a finite element computational solid dynamics solver. The surface of the stone is represented as a dynamic embedded boundary in the computational fluid dynamics solver. The evolution of the bubble surface is captured by solving the level set equation. The interface conditions at the surfaces of the stone and the bubble are enforced through the construction and solution of local fluid-solid and 2-fluid Riemann problems. This computational framework is first verified for 3 example problems including a 1D multimaterial Riemann problem, a 3D shock-stone interaction problem, and a 3D shock-bubble interaction problem. Next, a series of shock-bubble-stone–coupled simulations are presented. This study suggests that the dynamic response of a bubble to LSW varies dramatically depending on its initial size. Bubbles with an initial radius smaller than a threshold collapse within 1 μs after the passage of LSW, whereas larger bubbles do not. For a typical LSW generated by an electrohydraulic lithotripter (pmax = 35.0MPa, pmin =− 10.1MPa), this threshold is approximately 0.12mm. Moreover, this study suggests that a noncollapsing bubble imposes a negative effect on stone fracture as it shields part of the LSW from the stone. On the other hand, a collapsing bubble may promote fracture on the proximal surface of the stone, yet hinder fracture from stone interior.A 3D computational fluid dynamics (CFD)-computational solid dynamics (CSD) coupled computational framework is applied to investigate the interaction of model kidney stones immersed in liquid with a lithotripsy shock wave (LSW) and a gas bubble near the stone. The simulation results suggest that bubbles smaller than a certain threshold may collapse violently during the process, thereby promoting fracture on stone surface, yet hindering fracture in the interior.
      PubDate: 2017-01-13T04:05:46.50725-05:0
      DOI: 10.1002/cnm.2855
       
  • High Resolution Data Assimilation of Cardiac Mechanics Applied to a
           Dyssynchronous Ventricle
    • Authors: Gabriel Balaban; Henrik Finsberg, Hans Henrik Odland, Marie Rognes, Stian Ross, Joakim Sundnes, Samuel Wall
      Abstract: Computational models of cardiac mechanics, personalized to a patient, offer access to mechanical information above and beyond direct medical imaging. Additionally, such models can be used to optimize and plan therapies in-silico, thereby reducing risks and improving patient outcome. Model personalization has traditionally been achieved by data assimilation, which is the tuning or optimization of model parameters to match patient observations. Current data assimilation procedures for cardiac mechanics are limited in their ability to efficiently handle high dimensional parameters. This restricts parameter spatial resolution, and thereby the ability of a personalized model to account for heterogeneities that are often present in a diseased or injured heart. In this paper we address this limitation by proposing an adjoint-gradient based data assimilation method that can efficiently handle high-dimensional parameters. We test this procedure on a synthetic data set, and provide a clinical example with a dyssynchronous left ventricle with highly irregular motion. Our results show that the method efficiently handles a high dimensional optimization parameter, and produces an excellent agreement for personalized models to both synthetic and clinical data. This article is protected by copyright. All rights reserved.
      PubDate: 2016-12-31T02:06:17.85015-05:0
      DOI: 10.1002/cnm.2863
       
  • Numerical simulation of volume-controlled mechanical ventilated
           respiratory system with 2 different lungs
    • Authors: Yan Shi; Bolun Zhang, Maolin Cai, Xiaohua Douglas Zhang
      Abstract: Mechanical ventilation is a key therapy for patients who cannot breathe adequately by themselves, and dynamics of mechanical ventilation system is of great significance for life support of patients. Recently, models of mechanical ventilated respiratory system with 1 lung are used to simulate the respiratory system of patients. However, humans have 2 lungs. When the respiratory characteristics of 2 lungs are different, a single-lung model cannot reflect real respiratory system. In this paper, to illustrate dynamic characteristics of mechanical ventilated respiratory system with 2 different lungs, we propose a mathematical model of mechanical ventilated respiratory system with 2 different lungs and conduct experiments to verify the model. Furthermore, we study the dynamics of mechanical ventilated respiratory system with 2 different lungs. This research study can be used for improving the efficiency and safety of volume-controlled mechanical ventilation system.Because coupling effects of 2 lungs has a significant influence on safety and efficiency of mechanical ventilation, a pneumatic model with 2 lungs has been built in this paper to study the coupling effects of 2 lungs in volume-controlled ventilation. It can be concluded that a change of compliance or air resistance of one lung can affect both lungs and an unbalance of 2 lungs may result in overly high pressure in the trachea and overventilation.
      PubDate: 2016-12-29T10:30:32.441507-05:
      DOI: 10.1002/cnm.2852
       
  • A novel approach to the quantification of aortic root in vivo structural
           mechanics
    • Authors: E. Votta; M. Presicce, A. Della Corte, S. Dellegrottaglie, C. Bancone, F. Sturla, A. Redaelli
      Abstract: Understanding aortic root in vivo biomechanics can help in elucidating key mechanisms involved in aortic root pathologies and in the outcome of their surgical treatment. Numerical models can provide useful quantitative information. For this to be reliable, detailed aortic root anatomy should be captured. Also, since the aortic root is never unloaded throughout the cardiac cycle, the modeled geometry should be consistent with the in vivo loads acting on it. Achieving such consistency is still a challenge, which was tackled only by few numerical studies.Here we propose and describe in detail a new approach to the finite element modeling of aortic root in vivo structural mechanics. Our approach exploits the anatomical information yielded by magnetic resonance imaging by reconstructing the 3-dimensional end-diastolic geometry of the aortic root and makes the reconstructed geometry consistent with end-diastolic loading conditions through the estimation of the corresponding prestresses field.We implemented our approach through a semiautomated modeling pipeline, and we applied it to quantify aortic root biomechanics in 4 healthy participants. Computed results highlighted that including prestresses into the model allowed for pressurizing the aortic root to the end-diastolic pressure while matching the image-based ground truth data. Aortic root dynamics, tissues strains, and stresses computed at relevant time points through the cardiac cycle were consistent with a broad set of data from previous computational and in vivo studies, strongly suggesting the potential of the method. Also, results highlighted the major role played by the anatomy in driving aortic root biomechanics.We simulated aortic root (AR) dynamics combining subject-specific anatomical reconstructions, based on in vivo medical imaging, with a realistic modeling of AR loading conditions. This novel approach aimed for the consistency between AR geometry and pressure loads loading it, and hence at a reliable computation of tissues strains and stresses. The proposed strategy was successfully applied to 4 subject-specific AR models; simulations highlighted the key role of both geometrical features and tissues prestresses and suggested the potential of our method.
      PubDate: 2016-12-28T09:40:46.544795-05:
      DOI: 10.1002/cnm.2849
       
  • Improved hybrid/GPU algorithm for solving cardiac electrophysiology
           problems on Purkinje networks
    • Authors: M. Lange; S. Palamara, T. Lassila, C. Vergara, A. Quarteroni, A. F. Frangi
      Abstract: Cardiac Purkinje fibers provide an important pathway to the coordinated contraction of the heart. We present a numerical algorithm for the solution of electrophysiology problems across the Purkinje network that is efficient enough to be used in in silico studies on realistic Purkinje networks with physiologically detailed models of ion exchange at the cell membrane. The algorithm is on the basis of operator splitting and is provided with 3 different implementations: pure CPU, hybrid CPU/GPU, and pure GPU. Compared to our previous work, we modify the explicit gap junction term at network bifurcations to improve its mathematical consistency. Due to this improved consistency of the model, we are able to perform an empirical convergence study against analytical solutions. The study verified that all 3 implementations produce equivalent convergence rates, and shows that the algorithm produces equivalent result across different hardware platforms. Finally, we compare the efficiency of all 3 implementations on Purkinje networks of increasing spatial resolution using membrane models of increasing complexity. Both hybrid and pure GPU implementations outperform the pure CPU implementation, but their relative performance difference depends on the size of the Purkinje network and the complexity of the membrane model used.We present an improved numerical algorithm to solve the cardiac electrophysiology problem on Purkinje fiber networks. It is efficient enough to be used on realistic networks with physiologically detailed membrane models and is mathematically and physiologically consistent. The algorithm was implemented on 3 different hardware configurations, including on graphic processing units. All implementations are verified to produce equivalent convergence rates using analytical solutions. Finally, we compare the efficiency of the different implementations on problems of varying size and model complexity.
      PubDate: 2016-12-01T23:45:43.044452-05:
      DOI: 10.1002/cnm.2835
       
  • Automated femoral landmark extraction for optimal prosthesis placement in
           total hip arthroplasty
    • Authors: Diogo F. Almeida; Rui B. Ruben, João Folgado, Paulo R. Fernandes, João Gamelas, Benedict Verhegghe, Matthieu De Beule
      Abstract: The automated extraction of anatomical reference landmarks in the femoral volume may improve speed, precision, and accuracy of surgical procedures, such as total hip arthroplasty. These landmarks are often hard to achieve, even via surgical incision. In addition, it provides a presurgical guidance for prosthesis sizing and placement. This study presents an automated workflow for femoral orientation and landmark extraction from a 3D surface mesh. The extraction of parameters such as the femoral neck axis, the femoral middle diaphysis axis, both trochanters and the center of the femoral head will allow the surgeon to establish the correct position of bony cuts to restore leg length and femoral offset. The definition of the medullary canal endosteal wall is used to position the prosthesis' stem. Furthermore, prosthesis alignment and sizing methods were implemented to provide the surgeon with presurgical information about performance of each of the patient-specific femur-implant couplings. The workflow considers different commercially available hip stems and has the potential to help the preoperative planning of a total hip arthroplasty in an accurate, repeatable, and reliable way. The positional and orientation errors are significantly reduced, and therefore, the risk of implant failure and subsequent revision surgery are also reduced.The presented method provides an automated presurgical guidance for prosthesis sizing and placement. In a first stage, it is able to accurately locate key anchor points in the 3D femoral mesh volume. Moreover, optimal prosthesis alignment and sizing estimation can be achieved based on the patient-specific femoral and medullary canal dimensions in an accurate, repeatable, and reliable way. The positional and orientation errors are significantly reduced, and therefore, the risk of implant failure and consequent revision surgery are minimized.
      PubDate: 2016-11-25T06:00:33.338561-05:
      DOI: 10.1002/cnm.2844
       
  • An atlas- and data-driven approach to initializing reaction-diffusion
           systems in computer cardiac electrophysiology
    • Authors: Corné Hoogendoorn; Rafael Sebastian, José Félix Rodriguez, Karim Lekadir, Alejandro F. Frangi
      Abstract: The cardiac electrophysiology (EP) problem is governed by a nonlinear anisotropic reaction-diffusion system with a very rapidly varying reaction term associated with the transmembrane cell current. The nonlinearity associated with the cell models requires a stabilization process before any simulation is performed. More importantly, when used in a 3-dimensional (3D) anatomy, it is not sufficient to perform this stabilization on the basis of isolated cells only, since the coupling of the different cells through the tissue greatly modulates the dynamics of the system. Therefore, stabilization of the system must be performed on the entire 3D model. This work develops a novel procedure for the initialization of reaction-diffusion systems for numerical simulations of cardiac EP from steady-state conditions. We exploit surface point correspondence to establish volumetric point correspondence. Upon introduction of a new 3D anatomy with surface point correspondence, a prediction of the cell model steady states is derived from the set of earlier biophysical simulations. We show that the prediction error is typically less than 10% for all model variables, with most variables showing even greater accuracy. When initializing simulations with the predicted model states, it is demonstrated that simulation times can be cut by at least two-thirds and potentially more, which saves hours or days of high-performance computing. Overall, these results increase the clinical applicability of detailed computational EP studies on personalized anatomies.In cardiac electrophysiology modeling and simulation, the nonlinearity associated with the cell models requires a stabilization process in the entire 3D domain. This work develops a novel procedure for the initialization of reaction-diffusion systems for simulations of cardiac electrophysiology from steady-state conditions. Obtained prediction error was typically less than 10% for all model variables. Simulation times could be cut by at least two-thirds and potentially more, which saves hours or days of high-performance computing.
      PubDate: 2016-11-23T02:13:26.103421-05:
      DOI: 10.1002/cnm.2846
       
  • Adaptive Unified Continuum FEM Modeling of a 3D FSI Benchmark Problem
    • Authors: Johan Jansson; Niyazi Cem Degirmenci, Johan Hoffman
      Abstract: In this paper we address a 3D fluid-structure interaction (FSI) benchmark problem that represents important characteristics of biomedical modeling. We present a goal-oriented adaptive finite element methodology for incompressible FSI based on a streamline-diffusion type stabilization of the balance equations for mass and momentum for the entire continuum in the domain, which is implemented in the Unicorn/FEniCS software framework. A phase marker function and its corresponding transport equation is introduced to select the constitutive law, where the mesh tracks the discontinous fluid-structure interface. This results in a unified simulation method for fluids and structures. We present detailed results for the benchmark problem compared to experiments, together with a mesh convergence study.
      PubDate: 2016-11-10T13:45:23.372305-05:
      DOI: 10.1002/cnm.2851
       
  • An implicit solver for 1D arterial network models
    • Authors: Jason Carson; Raoul Van Loon
      Abstract: In this study, the 1D blood flow equations are solved using a newly proposed enhanced trapezoidal rule method (ETM), which is an extension to the simplified trapezoidal rule method. At vessel junctions, the conservation of mass and conservation of total pressure are held as system constraints using Lagrange multipliers that can be physically interpreted as external flow rates. The ETM scheme is compared with published arterial network benchmark problems and a dam break problem. Strengths of the ETM scheme include being simple to implement, intuitive connection to lumped parameter models, and no restrictive stability criteria such as the Courant-Friedrichs-Lewy (CFL) number. The ETM scheme does not require the use of characteristics at vessel junctions, or for inlet and outlet boundary conditions. The ETM forms an implicit system of equations, which requires only one global solve per time step for pressure, followed by flow rate update on the elemental system of equations; thus, no iterations are required per time step. Consistent results are found for all benchmark cases, and for a 56-vessel arterial network problem, it gives very satisfactory solutions at a spatial and time discretization that results in a maximum CFL of 3, taking 4.44 seconds per cardiac cycle. By increasing the time step and element size to produce a maximum CFL number of 15, the method takes only 0.39 second per cardiac cycle with only a small compromise on accuracy.In this study, the 1D blood flow equations are solved using the enhanced trapezoidal rule method (ETM). The ETM is an implicit scheme and uses Lagrange multipliers, which are physically interpreted as external flow rates, to impose conservation of total pressure and conservation of mass at vessel junctions. The method is compared with benchmark problems and a shock problem.
      PubDate: 2016-11-10T06:45:37.377709-05:
      DOI: 10.1002/cnm.2837
       
  • Non-Newtonian versus numerical rheology: Practical impact of
           shear-thinning on the prediction of stable and unstable flows in
           intracranial aneurysms
    • Authors: M. O. Khan; D. A. Steinman, K. Valen-Sendstad
      Abstract: Computational fluid dynamics (CFD) shows promise for informing treatment planning and rupture risk assessment for intracranial aneurysms. Much attention has been paid to the impact on predicted hemodynamics of various modelling assumptions and uncertainties, including the need for modelling the non-Newtonian, shear-thinning rheology of blood, with equivocal results. Our study clarifies this issue by contextualizing the impact of rheology model against the recently demonstrated impact of CFD solution strategy on the prediction of aneurysm flow instabilities. Three aneurysm cases were considered, spanning a range of stable to unstable flows. Simulations were performed using a high-resolution/accuracy solution strategy with Newtonian and modified-Cross rheology models and compared against results from a so-called normal-resolution strategy. Time-averaged and instantaneous wall shear stress (WSS) distributions, as well as frequency content of flow instabilities and dome-averaged WSS metrics, were minimally affected by the rheology model, whereas numerical solution strategy had a demonstrably more marked impact when the rheology model was fixed. We show that point-wise normalization of non-Newtonian by Newtonian WSS values tended to artificially amplify small differences in WSS of questionable physiological relevance in already-low WSS regions, which might help to explain the disparity of opinions in the aneurysm CFD literature regarding the impact of non-Newtonian rheology. Toward the goal of more patient-specific aneurysm CFD, we conclude that attention seems better spent on solution strategy and other likely “first-order” effects (eg, lumen segmentation and choice of flow rates), as opposed to “second-order” effects such as rheology.Computational fluid dynamics shows promise for rupture risk assessment of intracranial aneurysms, and the need for modelling the non-Newtonian behavior of blood has been questioned; with equivocal results. We sought to clarify this issue by contextualizing the impact of rheology (HR-NN vs HR-N) against impact of computational fluid dynamics solution strategy (NR-N). Based on Figure  (showing commonly computed hemodynamic indices), we conclude that attention seems better spent on solution strategy, as opposed to “second-order” effects such as rheology.
      PubDate: 2016-11-09T15:17:24.990996-05:
      DOI: 10.1002/cnm.2836
       
  • Modeling the mechanics of axonal fiber tracts using the embedded finite
           element method
    • Authors: Harsha T. Garimella; Reuben H. Kraft
      Abstract: A subject-specific human head finite element model with embedded axonal fiber tractography obtained from diffusion tensor imaging was developed. The axonal fiber tractography finite element model was coupled with the volumetric elements in the head model using the embedded element method. This technique enables the calculation of axonal strains and real-time tracking of the mechanical response of the axonal fiber tracts. The coupled model was then verified using pressure and relative displacement-based (between skull and brain) experimental studies and was employed to analyze a head impact, demonstrating the applicability of this method in studying axonal injury. Following this, a comparison study of different injury criteria was performed. This model was used to determine the influence of impact direction on the extent of the axonal injury. The results suggested that the lateral impact loading is more dangerous compared to loading in the sagittal plane, a finding in agreement with previous studies. Through this analysis, we demonstrated the viability of the embedded element method as an alternative numerical approach for studying axonal injury in patient-specific human head models.A human head finite element model with embedded axonal tractography (developed using DTI) was developed using the “embedded element method.” The model was verified and was used to analyze a concussive head impact, demonstrating the applicability of this method in studying axonal injury. Effects of different loading directions and the importance of axonal orientation were studied, and the results show that the axonal tract arrangement could be one of the reasons for differences in axonal damage under different loading directions.
      PubDate: 2016-11-08T15:16:10.632292-05:
      DOI: 10.1002/cnm.2823
       
  • A quasi-3D wire approach to model pulmonary airflow in human airways
    • Authors: Ravishekar Kannan; Z. J. Chen, Narender Singh, Andrzej Przekwas, Renishkumar Delvadia, Geng Tian, Ross Walenga
      Abstract: The models used for modeling the airflow in the human airways are either 0-dimensional compartmental or full 3-dimensional (3D) computational fluid dynamics (CFD) models. In the former, airways are treated as compartments, and the computations are performed with several assumptions, thereby generating a low-fidelity solution. The CFD method displays extremely high fidelity since the solution is obtained by solving the conservation equations in a physiologically consistent geometry. However, CFD models (1) require millions of degrees of freedom to accurately describe the geometry and to reduce the discretization errors, (2) have convergence problems, and (3) require several days to simulate a few breathing cycles. In this paper, we present a novel, fast-running, and robust quasi-3D wire model for modeling the airflow in the human lung airway. The wire mesh is obtained by contracting the high-fidelity lung airway surface mesh to a system of connected wires, with well-defined radii. The conservation equations are then solved in each wire. These wire meshes have around O(1000) degrees of freedom and hence are 3000 to 25 000 times faster than their CFD counterparts. The 3D spatial nature is also preserved since these wires are contracted out of the actual lung STL surface. The pressure readings between the 2 approaches showed minor difference (maximum error = 15%). In general, this formulation is fast and robust, allows geometric changes, and delivers high-fidelity solutions. Hence, this approach has great potential for more complicated problems including modeling of constricted/diseased lung sections and for calibrating the lung flow resistances through parameter inversion.Schematic showing the CFD to Q3D conversion, pressure distribution for a sample simulation. Q3D is 3000–27000 times faster than the CFD approach.
      PubDate: 2016-11-04T10:05:40.929872-05:
      DOI: 10.1002/cnm.2838
       
  • A review of personalized blood glucose prediction strategies for T1DM
           patients
    • Authors: Silvia Oviedo; Josep Vehí, Remei Calm, Joaquim Armengol
      Abstract: This paper presents a methodological review of models for predicting blood glucose (BG) concentration, risks and BG events. The surveyed models are classified into three categories, and they are presented in summary tables containing the most relevant data regarding the experimental setup for fitting and testing each model as well as the input signals and the performance metrics. Each category exhibits trends that are presented and discussed. This document aims to be a compact guide to determine the modeling options that are currently being exploited for personalized BG prediction.This paper presents a methodological review of models for predicting blood glucose (BG) concentration, risks, and BG events. The surveyed models are classified into three categories, and they are presented in summary tables containing the most relevant data regarding the experimental setup for fitting and testing each model as well as the input signals and the performance metrics. Each category exhibits trends that are presented and discussed. This document aims to be a compact guide to determine the modeling options that are currently being exploited for personalized BG prediction.
      PubDate: 2016-10-28T16:05:29.778773-05:
      DOI: 10.1002/cnm.2833
       
  • Modeling of light propagation in the human neck for diagnoses of thyroid
           cancers by diffuse optical tomography
    • Authors: H. Fujii; Y. Yamada, K. Kobayashi, M. Watanabe, Y. Hoshi
      Abstract: Diffuse optical tomography using near-infrared light in a wavelength range from 700 to 1000 nm has the potential to enable non-invasive diagnoses of thyroid cancers; some of which are difficult to detect by conventional methods such as ultrasound tomography. Diffuse optical tomography needs to be based on a physically accurate model of light propagation in the neck, because it reconstructs tomographic images of the optical properties in the human neck by inverse analysis. Our objective here was to investigate the effects of three factors on light propagation in the neck using the 2D time-dependent radiative transfer equation: (1) the presence of the trachea, (2) the refractive-index mismatch at the trachea-tissue interface, and (3) the effect of neck organs other than the trachea (spine, spinal cord, and blood vessels). There was a significant influence of reflection and refraction at the trachea-tissue interface on the light intensities in the region between the trachea and the front of the neck surface. Organs other than the trachea showed little effect on the light intensities measured at the front of the neck surface although these organs affected the light intensities locally. These results indicated the necessity of modeling the refractive-index mismatch at the trachea-tissue interface and the possibility of modeling other neck organs simply as a homogeneous medium when the source and detectors were far from large blood vessels.A significant influence of reflection and refraction at the trachea (void region)-tissue interface on the light propagation. Only little influence of other organs (spine, spinal cord, and blood vessels) than the trachea on the light detected at the front surface of the neck in a case of source and detector locations far from the organs.
      PubDate: 2016-10-27T07:40:36.54234-05:0
      DOI: 10.1002/cnm.2826
       
  • Computational design and engineering of polymeric orthodontic aligners
    • Authors: S. Barone; A. Paoli, A. V. Razionale, R. Savignano
      Abstract: Transparent and removable aligners represent an effective solution to correct various orthodontic malocclusions through minimally invasive procedures. An aligner-based treatment requires patients to sequentially wear dentition-mating shells obtained by thermoforming polymeric disks on reference dental models. An aligner is shaped introducing a geometrical mismatch with respect to the actual tooth positions to induce a loading system, which moves the target teeth toward the correct positions. The common practice is based on selecting the aligner features (material, thickness, and auxiliary elements) by only considering clinician's subjective assessments.In this article, a computational design and engineering methodology has been developed to reconstruct anatomical tissues, to model parametric aligner shapes, to simulate orthodontic movements, and to enhance the aligner design. The proposed approach integrates computer-aided technologies, from tomographic imaging to optical scanning, from parametric modeling to finite element analyses, within a 3-dimensional digital framework.The anatomical modeling provides anatomies, including teeth (roots and crowns), jaw bones, and periodontal ligaments, which are the references for the down streaming parametric aligner shaping. The biomechanical interactions between anatomical models and aligner geometries are virtually reproduced using a finite element analysis software. The methodology allows numerical simulations of patient-specific conditions and the comparative analyses of different aligner configurations.In this article, the digital framework has been used to study the influence of various auxiliary elements on the loading system delivered to a maxillary and a mandibular central incisor during an orthodontic tipping movement. Numerical simulations have shown a high dependency of the orthodontic tooth movement on the auxiliary element configuration, which should then be accurately selected to maximize the aligner's effectiveness.This article presents a digital methodology to study patient-specific orthodontic treatments based on the use of polymeric aligners. The approach integrates computer-aided technologies, from tomographic imaging to optical scanning, from parametric modeling to finite element analyses. Numerical analyses proved to be a powerful tool to study different aligner's configurations evidencing how auxiliary elements features might affect the loading system delivered by the aligner. For instance, results pointed out that the loads elicited by divot geometries are higher than those provided by using attachment geometries.
      PubDate: 2016-10-21T07:55:38.530121-05:
      DOI: 10.1002/cnm.2839
       
  • Calibration of parameters for cardiovascular models with application to
           arterial growth
    • Authors: Sebastian Kehl; Michael W. Gee
      Abstract: We present a computational framework for the calibration of parameters describing cardiovascular models with a focus on the application of growth of abdominal aortic aneurysms (AAA). The growth rate in this sort of pathology is considered a critical parameter in the risk management and is an essential indicator for the assessment of surveillance intervals. Parameters describing growth of AAAs are not measurable directly and need to be estimated from available data often given by medical imaging technologies. Registration procedures often applied in standard workflows of parameter identification to extract the image encoded information are a source of significant systematic error. The concept of surface currents provides means to effectively avoid this source of errors by establishing a mathematical framework to compare surface information, directly accessible from image data. By utilizing this concept it is possible to inversely estimate growth parameters using sophisticated numerical models of AAAs from measurements available as surface information. In this work we present a framework to obtain spatial distributions of parameters governing growth of arterial tissue, and we show how the use of surface currents can significantly improve the results. We further present the application to patient specific follow-up data resulting in a spatial map of volumetric growth rates enabling, for the first time, prediction of further AAA expansion.We present a parameter identification framework for the estimation of growth parameters of cardiovascular tissue with application to growth of abdominal aortic aneurysms (AAA). We demonstrate on a synthetic example how the application of surface currents in combination with a flexible total variation regularization allows for the accurate determination of spatial distributions of parameter estimates. Finally we present the application to patient specific follow-up data enabling prediction of further AAA expansion.
      PubDate: 2016-10-21T07:45:32.074739-05:
      DOI: 10.1002/cnm.2822
       
  • A methodology for constraining power in finite element modeling of
           radiofrequency ablation
    • Authors: Yansheng Jiang; Ricardo Possebon, Stefaan Mulier, Chong Wang, Feng Chen, Yuanbo Feng, Qian Xia, Yewei Liu, Ting Yin, Raymond Oyen, Yicheng Ni
      Abstract: Radiofrequency ablation (RFA) is a minimally invasive thermal therapy for the treatment of cancer, hyperopia, and cardiac tachyarrhythmia. In RFA, the power delivered to the tissue is a key parameter. The objective of this study was to establish a methodology for the finite element modeling of RFA with constant power. Because of changes in the electric conductivity of tissue with temperature, a nonconventional boundary value problem arises in the mathematic modeling of RFA: neither the voltage (Dirichlet condition) nor the current (Neumann condition), but the power, that is, the product of voltage and current was prescribed on part of boundary. We solved the problem using Lagrange multiplier: the product of the voltage and current on the electrode surface is constrained to be equal to the Joule heating. We theoretically proved the equality between the product of the voltage and current on the surface of the electrode and the Joule heating in the domain. We also proved the well-posedness of the problem of solving the Laplace equation for the electric potential under a constant power constraint prescribed on the electrode surface. The Pennes bioheat transfer equation and the Laplace equation for electric potential augmented with the constraint of constant power were solved simultaneously using the Newton-Raphson algorithm. Three problems for validation were solved. Numerical results were compared either with an analytical solution deduced in this study or with results obtained by ANSYS or experiments. This work provides the finite element modeling of constant power RFA with a firm mathematical basis and opens pathway for achieving the optimal RFA power.It is shown that λ(P − Q) = 0, where λ is Lagrange multiplier, P denotes the delivered power of RFA, and Q is the Joule heating, which cannot be used as the power constraint equation. Instead, we used λ(P − Qs) = 0 as the power constraint equation, where Qs denotes the total power flux across boundary, solved together with Laplace equation for electric potential.The correctness proof of our method is demonstrated through solutions of test problems.
      PubDate: 2016-10-14T00:40:39.670398-05:
      DOI: 10.1002/cnm.2834
       
  • 3D physiological model of the aortic valve incorporating small coronary
           arteries
    • Authors: Hossein Mohammadi; Raymond Cartier, Rosaire Mongrain
      Abstract: The diseases of the coronary arteries and the aortic root are still the leading causes of mortality and morbidity worldwide. In this study, a 3D global fluid-structure interaction of the aortic root with inclusion of anatomically inspired small coronary arteries using the finite element method is presented. This innovative model allows to study the impact and interaction of root biomechanics on coronary hemodynamics and brings a new understanding to small coronary vessels hemodynamics. For the first time, the velocity profiles and shear stresses are reported in distal coronary arteries as a result of the aortic flow conditions in a global fluid-structure interaction model.This paper presents an innovative 3D global fluid-structure interaction of the aortic valve incorporating small coronary arteries using the finite element method. Inclusion of the small coronaries and incorporating realistic material properties introduced the notion of globality into the model. This global model was assessed with the results of echocardiography studies. The findings proved that decoupling the aorta and coronary structures brings non-obvious problems on how to manage the dynamic mechanical conditions at the interface during the flow cycle.
      PubDate: 2016-10-13T02:36:45.106975-05:
      DOI: 10.1002/cnm.2829
       
  • Design of a numerical model of lung by means of a special boundary
           condition in the truncated branches
    • Authors: Ana F. Tena; Joaquín Fernández, Eduardo Álvarez, Pere Casan, D. Keith Walters
      Abstract: BackgroundThe need for a better understanding of pulmonary diseases has led to increased interest in the development of realistic computational models of the human lung.MethodsTo minimize computational cost, a reduced geometry model is used for a model lung airway geometry up to generation 16. Truncated airway branches require physiologically realistic boundary conditions to accurately represent the effect of the removed airway sections. A user-defined function has been developed, which applies velocities mapped from similar locations in fully resolved airway sections. The methodology can be applied in any general purpose computational fluid dynamics code, with the only limitation that the lung model must be symmetrical in each truncated branch.ResultsUnsteady simulations have been performed to verify the operation of the model. The test case simulates a spirometry because the lung is obliged to rapidly perform both inspiration and expiration. Once the simulation was completed, the obtained pressure in the lower level of the lung was used as a boundary condition. The output velocity, which is a numerical spirometry, was compared with the experimental spirometry for validation purposes.ConclusionsThis model can be applied for a wide range of patient-specific resolution levels. If the upper airway generations have been constructed from a computed tomography scan, it would be possible to quickly obtain a complete reconstruction of the lung specific to a specific person, which would allow individualized therapies.A reduced symmetrical model is used in a model lung airway geometry up to generation 16 and can be applied in any general model.A user-defined function has been developed, which applies velocities mapped from similar locations in fully resolved airway sections.If the upper airway generations have been constructed from a computed tomography scan, it would be possible to quickly obtain a complete reconstruction of the lung particularized for each person, which would allow individualized therapies.
      PubDate: 2016-10-07T02:01:17.72731-05:0
      DOI: 10.1002/cnm.2830
       
  • Hemodynamics-driven deposition of intraluminal thrombus in abdominal
           aortic aneurysms
    • Authors: P. Di Achille; G. Tellides, J. D. Humphrey
      Abstract: Accumulating evidence suggests that intraluminal thrombus plays many roles in the natural history of abdominal aortic aneurysms. There is, therefore, a pressing need for computational models that can describe and predict the initiation and progression of thrombus in aneurysms. In this paper, we introduce a phenomenological metric for thrombus deposition potential and use hemodynamic simulations based on medical images from 6 patients to identify best-fit values of the 2 key model parameters. We then introduce a shape optimization method to predict the associated radial growth of the thrombus into the lumen based on the expectation that thrombus initiation will create a thrombogenic surface, which in turn will promote growth until increasing hemodynamically induced frictional forces prevent any further cell or protein deposition. Comparisons between predicted and actual intraluminal thrombus in the 6 patient-specific aneurysms suggest that this phenomenological description provides a good first estimate of thrombus deposition. We submit further that, because the biologically active region of the thrombus appears to be confined to a thin luminal layer, predictions of morphology alone may be sufficient to inform fluid-solid–growth models of aneurysmal growth and remodeling.Accumulating evidence suggests that intraluminal thrombus plays multiple, detrimental roles in the natural history of abdominal aortic aneurysms. Relying on a phenomenological metric for thrombus deposition potential, we introduce an optimization method to determine the growth of the thrombus into the lumen. This optimization is based on the expectation that thrombus growth will continue until increasing hemodynamically induced frictional forces prevent any further cell or protein deposition. Comparisons between predicted and actual intraluminal thrombus in 6 patient-specific aneurysms suggest that this phenomenological description provides a good first estimate of thrombus deposition
      PubDate: 2016-10-07T01:37:43.535533-05:
      DOI: 10.1002/cnm.2828
       
  • Machine learning–based 3-D geometry reconstruction and modeling of
           aortic valve deformation using 3-D computed tomography images
    • Authors: Liang Liang; Fanwei Kong, Caitlin Martin, Thuy Pham, Qian Wang, James Duncan, Wei Sun
      Abstract: To conduct a patient-specific computational modeling of the aortic valve, 3-D aortic valve anatomic geometries of an individual patient need to be reconstructed from clinical 3-D cardiac images. Currently, most of computational studies involve manual heart valve geometry reconstruction and manual finite element (FE) model generation, which is both time-consuming and prone to human errors. A seamless computational modeling framework, which can automate this process based on machine learning algorithms, is desirable, as it can not only eliminate human errors and ensure the consistency of the modeling results but also allow fast feedback to clinicians and permits a future population-based probabilistic analysis of large patient cohorts. In this study, we developed a novel computational modeling method to automatically reconstruct the 3-D geometries of the aortic valve from computed tomographic images. The reconstructed valve geometries have built-in mesh correspondence, which bridges harmonically for the consequent FE modeling. The proposed method was evaluated by comparing the reconstructed geometries from 10 patients with those manually created by human experts, and a mean discrepancy of 0.69 mm was obtained. Based on these reconstructed geometries, FE models of valve leaflets were developed, and aortic valve closure from end systole to middiastole was simulated for 7 patients and validated by comparing the deformed geometries with those manually created by human experts, and a mean discrepancy of 1.57 mm was obtained. The proposed method offers great potential to streamline the computational modeling process and enables the development of a preoperative planning system for aortic valve disease diagnosis and treatment.We present a novel computational modeling method to automatically reconstruct the 3-D geometries of the aortic valve from computed tomographic images and build finite element models with mesh correspondence. The method was validated on clinical computed tomographic images, and the results showed good agreement with the human experts: a mean discrepancy of 0.69 mm in geometry reconstruction and a mean discrepancy of 1.57 mm in deformed geometries from finite element simulation of the aortic valve closure.
      PubDate: 2016-10-07T01:22:03.662758-05:
      DOI: 10.1002/cnm.2827
       
  • Computational virtual evaluation of the effect of annuloplasty ring shape
    • Authors: Ahnryul Choi; David D. McPherson, Hyunggun Kim
      Abstract: Mitral regurgitation (MR) is a result of mitral valve (MV) pathology. Its etiology can be categorized as degenerative or functional MR. Ring annuloplasty aims to reconfigure a dilated mitral annulus to its normal size and shape. We investigated the effect of annuloplasty ring shape on MR outcome using our established 3-dimensional (3-D) echocardiography-based computational MV evaluation protocols. Virtual patient MV models were created from 3-D transesophageal echocardiographic data in patients with MR because of mitral annular dilation. Two distinct annuloplasty rings (Physio II and GeoForm) were designed and virtually implanted to the patient MVs. Dynamic finite element simulations of MV function were performed for each MV after virtual ring annuloplasty of either ring, and physiologic and biomechanical characteristics of MV function were compared. Excessive stress values appeared primarily in the midanterior and midposterior regions, and lack of leaflet coaptation was found in pre-annuloplasty patient MVs. Both rings demonstrated marked reduction of stresses and efficient leaflet coaptation. The Physio II ring demonstrated more evenly distributed stress reduction across the leaflets and annulus compared with the GeoForm ring. Conversely, the highly nonplanar curvature of the GeoForm ring more effectively increased leaflet coaptation compared with the Physio II ring. This indicates that the shape of annuloplasty ring affects post-annuloplasty physiologic and biomechanical conditions, which can lead to tissue alteration over a longer period after ring annuloplasty. This virtual ring annuloplasty simulation strategy provides detailed physiologic and biomechanical information and may help better plan the optimal ring selection and improved patient-specific MV repairs.We investigated the effect of annuloplasty ring shape on MV repair outcome using our established 3-D echocardiography-based computational MV evaluation protocols. The Physio II ring demonstrated more evenly distributed stress reduction across the leaflets compared with the GeoForm ring. Conversely, the highly nonplanar curvature of the GeoForm ring more effectively increased leaflet coaptation. This virtual ring annuloplasty strategy provides detailed physiologic and biomechanical information and may help better plan the optimal ring selection and improved patient-specific MV repairs.
      PubDate: 2016-10-05T05:17:05.606832-05:
      DOI: 10.1002/cnm.2831
       
  • Effects of walking in deep venous thrombosis: a new integrated solid and
           fluid mechanics model
    • Authors: Josep M. López; Gerard Fortuny, Dolors Puigjaner, Joan Herrero, Francesc Marimon, Josep Garcia-Bennett
      Abstract: Deep venous thrombosis (DVT) is a common disease. Large thrombi in venous vessels cause bad blood circulation and pain; and when a blood clot detaches from a vein wall, it causes an embolism whose consequences range from mild to fatal. Walking is recommended to DVT patients as a therapeutical complement. In this study the mechanical effects of walking on a specific patient of DVT were simulated by means of an unprecedented integration of 3 elements: a real geometry, a biomechanical model of body tissues, and a computational fluid dynamics study. A set of computed tomography images of a patient's leg with a thrombus in the popliteal vein was employed to reconstruct a geometry model. Then a biomechanical model was used to compute the new deformed geometry of the vein as a function of the fiber stretch level of the semimembranosus muscle. Finally, a computational fluid dynamics study was performed to compute the blood flow and the wall shear stress (WSS) at the vein and thrombus walls. Calculations showed that either a lengthening or shortening of the semimembranosus muscle led to a decrease of WSS levels up to 10%. Notwithstanding, changes in blood viscosity properties or blood flow rate may easily have a greater impact in WSS.The mechanical effects of walking on a specific patient of deep venous thrombosis was simulated by using an unprecedented integration of three elements: a real geometry, a biomechanical model of body tissues, and a computational fluid dynamics study. Computed tomography images of a patient's leg with a thrombus in the popliteal vein were employed to reconstruct a geometrical model. Then a biomechanical model was used to compute the new deformed geometry as a function of the compression state of the semimembranosus muscle. Finally, the blood flow was computed, and the wall shear stress field at the vein and thrombus walls was determined.
      PubDate: 2016-09-27T06:12:42.868596-05:
      DOI: 10.1002/cnm.2819
       
  • Dental implant customization using numerical optimization design and
           3-dimensional printing fabrication of zirconia ceramic
    • Authors: Yung-Chang Cheng; Deng-Huei Lin, Cho-Pei Jiang, Yuan-Min Lin
      Abstract: This study proposes a new methodology for dental implant customization consisting of numerical geometric optimization and 3-dimensional printing fabrication of zirconia ceramic. In the numerical modeling, exogenous factors for implant shape include the thread pitch, thread depth, maximal diameter of implant neck, and body size. Endogenous factors are bone density, cortical bone thickness, and non-osseointegration. An integration procedure, including uniform design method, Kriging interpolation and genetic algorithm, is applied to optimize the geometry of dental implants. The threshold of minimal micromotion for optimization evaluation was 100 μm. The optimized model is imported to the 3-dimensional slurry printer to fabricate the zirconia green body (powder is bonded by polymer weakly) of the implant. The sintered implant is obtained using a 2-stage sintering process. Twelve models are constructed according to uniform design method and simulated the micromotion behavior using finite element modeling. The result of uniform design models yields a set of exogenous factors that can provide the minimal micromotion (30.61 μm), as a suitable model. Kriging interpolation and genetic algorithm modified the exogenous factor of the suitable model, resulting in 27.11 μm as an optimization model. Experimental results show that the 3-dimensional slurry printer successfully fabricated the green body of the optimization model, but the accuracy of sintered part still needs to be improved. In addition, the scanning electron microscopy morphology is a stabilized t-phase microstructure, and the average compressive strength of the sintered part is 632.1 MPa.A new methodology for dental implant customization is proposed. It consists of numerical modeling for optimized dental shape, 3-dimensional slurry printing for fabricating the zirconia green body, and sintering to obtain high-density zirconia dental implant. Numerical results present that the Kriging interpolation and genetic algorithm can modify the initial model to obtain the optimized model. Experimental results show that the 3-dimensional slurry printer successfully fabricated the green body of the optimization model. In addition, the scanning electron microscopy morphology is a stabilized t-phase microstructure, and the average compressive strength of the sintered part is 632.1 MPa.
      PubDate: 2016-09-23T03:55:45.05661-05:0
      DOI: 10.1002/cnm.2820
       
  • Data assimilation for identification of cardiovascular network
           characteristics
    • Authors: Rajnesh Lal; Bijan Mohammadi, Franck Nicoud
      Abstract: A method to estimate the hemodynamics parameters of a network of vessels using an Ensemble Kalman filter is presented. The elastic moduli (Young's modulus) of blood vessels and the terminal boundary parameters are estimated as the solution of an inverse problem. Two synthetic test cases and a configuration where experimental data are available are presented. The sensitivity analysis confirms that the proposed method is quite robust even with a few numbers of observations. The simulations with the estimated parameters recovers target pressure or flow rate waveforms at given specific locations, improving the state-of-the-art predictions available in the literature. This shows the effectiveness and efficiency of both the parameter estimation algorithm and the blood flow model.An ensemble Kalman filter is use to identify the hemodynamics parameters such as Young's modulus and the terminal boundary parameters as the solution of inverse problems. The sensitivity analysis confirms that the proposed method is quite robust even with a few numbers of observations. The paper demonstrates that joint use of data assimilation and flow solution by a computational fluid dynamics code greatly improves available results in the literature for a realistic human arterial model with an available experimental reference.
      PubDate: 2016-09-21T07:10:33.912164-05:
      DOI: 10.1002/cnm.2824
       
  • Creation of an idealized nasopharynx geometry for accurate computational
           fluid dynamics simulations of nasal airflow in patient-specific models
           lacking the nasopharynx anatomy
    • Authors: Azadeh A.T. Borojeni; Dennis O. Frank-Ito, Julia S. Kimbell, John S. Rhee, Guilherme J.M. Garcia
      Abstract: Virtual surgery planning based on computational fluid dynamics (CFD) simulations has the potential to improve surgical outcomes for nasal airway obstruction patients, but the benefits of virtual surgery planning must outweigh the risks of radiation exposure. Cone beam computed tomography (CT) scans represent an attractive imaging modality for virtual surgery planning due to lower costs and lower radiation exposures compared with conventional CT scans. However, to minimize the radiation exposure, the cone beam CT sinusitis protocol sometimes images only the nasal cavity, excluding the nasopharynx. The goal of this study was to develop an idealized nasopharynx geometry for accurate representation of outlet boundary conditions when the nasopharynx geometry is unavailable. Anatomically accurate models of the nasopharynx created from 30 CT scans were intersected with planes rotated at different angles to obtain an average geometry. Cross sections of the idealized nasopharynx were approximated as ellipses with cross-sectional areas and aspect ratios equal to the average in the actual patient-specific models. CFD simulations were performed to investigate whether nasal airflow patterns were affected when the CT-based nasopharynx was replaced by the idealized nasopharynx in 10 nasal airway obstruction patients. Despite the simple form of the idealized geometry, all biophysical variables (nasal resistance, airflow rate, and heat fluxes) were very similar in the idealized vs patient-specific models. The results confirmed the expectation that the nasopharynx geometry has a minimal effect in the nasal airflow patterns during inspiration. The idealized nasopharynx geometry will be useful in future CFD studies of nasal airflow based on medical images that exclude the nasopharynx.An idealized nasopharynx geometry was developed for accurate representation of outlet boundary conditions in nasal airflow simulations when the nasopharynx anatomy is not available. We demonstrated that the nasopharynx geometry has a negligible effect on nasal airflow patterns during inspiration, which means that the anatomically accurate nasopharynx can be replaced by an idealized geometry without significantly affecting airflow estimates in the main nasal cavity.
      PubDate: 2016-09-21T07:00:46.000702-05:
      DOI: 10.1002/cnm.2825
       
  • A comparison of hemodynamic metrics and intraluminal thrombus burden in a
           common iliac artery aneurysm
    • Authors: Lachlan J. Kelsey; Janet T. Powell, Paul E. Norman, Karol Miller, Barry J. Doyle
      Abstract: Aneurysms of the common iliac artery (CIAA) are typically found in association with an abdominal aortic aneurysm (AAA). Isolated CIAAs, in the absence of an AAA, are uncommon. Similar to AAAs, CIAA may develop intraluminal thrombus (ILT). As isolated CIAAs have a contralateral common iliac artery for comparison, they provide an opportunity to study the hemodynamic mechanisms behind ILT formation.In this study, we compared a large isolated CIAA and the contralateral iliac artery using computational fluid dynamics to determine if hemodynamic metrics correlate with the location of ILT. We performed a comprehensive computational fluid dynamics study and investigated the residence time of platelets and monocytes, velocity fields, time-averaged wall shear stress, oscillatory shear index, and endothelial cell activation potential. We then correlated these data to ILT burden determined with computed tomography.We found that high cell residence times, low time-averaged wall shear stress, high oscillatory shear index, and high endothelial cell activation potential all correlate with regions of ILT development. Our results show agreement with previous hypotheses of thrombus formation in AAA and provide insights into the computational hemodynamics of iliac artery aneurysms.In this study, we compared a large isolated aneurysm of the common iliac artery and the contralateral iliac artery using computational fluid dynamics to determine if hemodynamic metrics correlate with the location of ILT. We investigated the residence behavior of particles, the velocity fields, time-averaged wall shear stress, oscillatory shear index, and endothelial cell activation potential. Our results show agreement with previous hypotheses of thrombus formation in abdominal aortic aneurysm and provide insights into the computational hemodynamics of iliac artery aneurysms.
      PubDate: 2016-09-08T06:50:37.004257-05:
      DOI: 10.1002/cnm.2821
       
  • Vessel segmentation from abdominal magnetic resonance images: adaptive and
           reconstructive approach
    • Authors: Evgin Goceri; Zarine K. Shah, Metin N. Gurcan
      Abstract: The liver vessels, which have low signal and run next to brighter bile ducts, are difficult to segment from MR images. This study presents a fully automated and adaptive method to segment portal and hepatic veins on magnetic resonance images. In the proposed approach, segmentation of these vessels is achieved in four stages: (i) initial segmentation, (ii) refinement, (iii) reconstruction, and (iv) post-processing. In the initial segmentation stage, k-means clustering is used, the results of which are refined iteratively with linear contrast stretching algorithm in the next stage, generating a mask image. In the reconstruction stage, vessel regions are reconstructed with the marker image from the first stage and the mask image from the second stage. Experimental data sets include slices that show fat tissues, which have the same gray level values with vessels, outside the margin of the liver. These structures are removed in the last stage. Results show that the proposed approach is more efficient than other thresholding-based methods. Copyright © 2016 John Wiley & Sons, Ltd.In this paper, we present a fully automated and adaptive method to segment portal and hepatic veins on magnetic resonance images. In the proposed approach, segmentation of these vessels is achieved in four stages: (i) initial segmentation, (ii) refinement, (iii) reconstruction, and (iv) post-processing. The proposed approach can detect boundaries of vessels by estimating potential edge pixels that belong to vessels. The proposed method has the capability to detect not only the central, larger liver vasculature but also small, peripheral vessels.
      PubDate: 2016-08-02T05:50:45.811524-05:
      DOI: 10.1002/cnm.2811
       
  • Fluid–structure interaction and structural analyses using a
           comprehensive mitral valve model with 3D chordal structure
    • Authors: Milan Toma; Daniel R. Einstein, Charles H. Bloodworth, Richard P. Cochran, Ajit P. Yoganathan, Karyn S. Kunzelman
      Abstract: Over the years, three-dimensional models of the mitral valve have generally been organized around a simplified anatomy. Leaflets have been typically modeled as membranes, tethered to discrete chordae typically modeled as one-dimensional, non-linear cables. Yet, recent, high-resolution medical images have revealed that there is no clear boundary between the chordae and the leaflets. In fact, the mitral valve has been revealed to be more of a webbed structure whose architecture is continuous with the chordae and their extensions into the leaflets. Such detailed images can serve as the basis of anatomically accurate, subject-specific models, wherein the entire valve is modeled with solid elements that more faithfully represent the chordae, the leaflets, and the transition between the two. These models have the potential to enhance our understanding of mitral valve mechanics and to re-examine the role of the mitral valve chordae, which heretofore have been considered to be ‘invisible’ to the fluid and to be of secondary importance to the leaflets. However, these new models also require a rethinking of modeling assumptions. In this study, we examine the conventional practice of loading the leaflets only and not the chordae in order to study the structural response of the mitral valve apparatus. Specifically, we demonstrate that fully resolved 3D models of the mitral valve require a fluid–structure interaction analysis to correctly load the valve even in the case of quasi-static mechanics. While a fluid–structure interaction mode is still more computationally expensive than a structural-only model, we also show that advances in GPU computing have made such models tractable. Copyright © 2016 John Wiley & Sons, Ltd.Over the years, 3D models of the mitral valve have generally been organized around a simplified anatomy. In this study, we examine the conventional practice of loading the leaflets only and not the chordae in order to study the structural response of the mitral valve apparatus. Specifically, we demonstrate that fully resolved 3D models of the mitral valve require a fluid–structure interaction analysis to correctly load the valve even in the case of quasi-static mechanics.
      PubDate: 2016-07-28T06:30:34.816973-05:
      DOI: 10.1002/cnm.2815
       
  • Third-order harmonic expansion of the magnetoencephalography forward and
           inverse problems in an ellipsoidal brain model
    • Authors: Mauricio Alcocer-Sosa; David Gutiérrez
      Abstract: We present a forward modeling solution in the form of an array response kernel for magnetoencephalography. We consider the case when the brain's anatomy is approximated by an ellipsoid and an equivalent current dipole model is used to approximate brain sources. The proposed solution includes the contributions up to the third-order ellipsoidal harmonic terms; hence, we compare this new approximation against the previously available one that only considered up to second-order harmonics. We evaluated the proposed solution when used in the inverse problem of estimating physiologically feasible visual evoked responses from magnetoencephalography data. Our results showed that the contribution of the third-order harmonic terms provides a more realistic representation of the magnetic fields (closer to those generated with a numerical approximation based on the boundary element method) and, subsecuently, the estimated equivalent current dipoles are a better fit to those observed in practice (e.g., in visual evoked potentials). Copyright © 2016 John Wiley & Sons, Ltd.The solution of the forward problem in magnetoencephalography, for the case when the brain's geometry is approximated by an ellipsoid, previuosly considered only up to the second-order harmonics. However, we show that the forward solution is improved by adding the contribution of third-order ellipsoidal harmonics. Furthermore, such improvement in the generated magnetic fields also has a positive impact in the inverse problem of estimating evoked brain responses.
      PubDate: 2016-07-27T06:25:52.376519-05:
      DOI: 10.1002/cnm.2810
       
  • Transversally enriched pipe element method (TEPEM): An effective numerical
           approach for blood flow modeling
    • Authors: Luis Mansilla Alvarez; Pablo Blanco, Carlos Bulant, Enzo Dari, Alessandro Veneziani, Raúl Feijóo
      Abstract: In this work, we present a novel approach tailored to approximate the Navier–Stokes equations to simulate fluid flow in three-dimensional tubular domains of arbitrary cross-sectional shape. The proposed methodology is aimed at filling the gap between (cheap) one-dimensional and (expensive) three-dimensional models, featuring descriptive capabilities comparable with the full and accurate 3D description of the problem at a low computational cost. In addition, this methodology can easily be tuned or even adapted to address local features demanding more accuracy. The numerical strategy employs finite (pipe-type) elements that take advantage of the pipe structure of the spatial domain under analysis. While low order approximation is used for the longitudinal description of the physical fields, transverse approximation is enriched using high order polynomials. Although our application of interest is computational hemodynamics and its relevance to pathological dynamics like atherosclerosis, the approach is quite general and can be applied in any internal fluid dynamics problem in pipe-like domains. Numerical examples covering academic cases as well as patient-specific coronary arterial geometries demonstrate the potentialities of the developed methodology and its performance when compared against traditional finite element methods. Copyright © 2016 John Wiley & Sons, Ltd.We present a numerical strategy tailored to simulate fluid flow in 3D tubular domains. The methodology features descriptive capabilities comparable to 3D models, but at a low computational cost. The domain of analysis is split into pipe elements (slabs). While low order approximation for the longitudinal description of the physical fields is employed, transverse approximation is enriched using high order polynomials. Several numerical examples ranging from academic cases to patient-specific coronary arterial geometries demonstrate the potentialities proposed methodology.
      PubDate: 2016-07-27T06:16:23.273082-05:
      DOI: 10.1002/cnm.2808
       
  • Simulated transcatheter aortic valve deformation: A parametric study on
           the impact of leaflet geometry on valve peak stress
    • Authors: Kewei Li; Wei Sun
      Abstract: In this study, we developed a computational framework to investigate the impact of leaflet geometry of a transcatheter aortic valve (TAV) on the leaflet stress distribution, aiming at optimizing TAV leaflet design to reduce its peak stress. Utilizing a generic TAV model developed previously [Li and Sun, Annals of Biomedical Engineering, 2010. 38(8): 2690–2701], we first parameterized the 2D leaflet geometry by mathematical equations, then by perturbing the parameters of the equations, we could automatically generate a new leaflet design, remesh the 2D leaflet model and build a 3D leaflet model from the 2D design via a Python script. Approximately 500 different leaflet designs were investigated by simulating TAV closure under the nominal circular deployment and physiological loading conditions. From the simulation results, we identified a new leaflet design that could reduce the previously reported valve peak stress by about 5%. The parametric analysis also revealed that increasing the free edge width had the highest overall impact on decreasing the peak stress. A similar computational analysis was further performed for a TAV deployed in an abnormal, asymmetric elliptical configuration. We found that a minimal free edge height of 0.46 mm should be adopted to prevent central backflow leakage. This increase of the free edge height resulted in an increase of the leaflet peak stress. Furthermore, the parametric study revealed a complex response surface for the impact of the leaflet geometric parameters on the peak stress, underscoring the importance of performing a numerical optimization to obtain the optimal TAV leaflet design. Copyright © 2016 John Wiley & Sons, Ltd.We presented a computational framework to investigate the impact of leaflet geometry of a transcatheter aortic valve (TAV) on the leaflet stress distribution aiming at optimizing TAV leaflet design to reduce its peak stresses. Computational simulations of TAV closure were performed for approximately 500 possible 2D leaflet designs. The parametric analysis resulted in an optimal leaflet design that could reduce the valve peak stress by about 5%.
      PubDate: 2016-07-26T05:55:41.335527-05:
      DOI: 10.1002/cnm.2814
       
  • Finite element analysis of the effect of medullary contact on fracture
           healing and remodeling in the intramedullary interlocking nail-fixed tibia
           fracture
    • Authors: Haosen Wang; Zhixiu Hao, Shizhu Wen
      Abstract: Intramedullary interlocking nail is an effective treatment for tibial diaphyseal fracture. The contact between medullary rod and diaphyseal cortex is able to enhance fracture stability. However, how and to what degree the contact affects fracture healing and subsequent bone remodeling is still unclear. To investigate this, fracture healing and remodeling algorithms were combined, improved, and used to simulate the healing and remodeling processes in a transverse tibial diaphyseal fracture fixed with an intramedullary interlocking nail device. Two different diaphyseal fracture statuses, three different initial loading levels, and two nail materials were considered. The results showed that the medullary contact could significantly enhance the fixation stability; the strain reduction was up to 80% in the initial granulation callus. However, low initial loading level was found to be a more potential risk factor for the insufficient loading-induced nonunion other than medullary contact and stiffer nail material. Furthermore, the stabilizing effect of medullary contact diminished when stiff bone tissue formed in the callus; thus, the remodeling in the long-term was not affected by medullary contact. Copyright © 2016 John Wiley & Sons, Ltd.A numerical model composed of fracture healing, and remodeling model was developed and used to simulate the effect of medullary contact between intramedullary rod and cortical bone. The contact was found to be beneficial for fracture stability. Although medullary contact increased the risk of insufficient stimulation-induced nonunion, the main cause to this complication was found to be the low initial loading. The medullary contact did not compromise the final remodeling of the fracture callus, as compared with non-contact group.
      PubDate: 2016-07-26T05:40:31.884607-05:
      DOI: 10.1002/cnm.2816
       
  • Coupling schemes for the FSI forward prediction challenge: Comparative
           study and validation
    • Authors: Mikel Landajuela; Marina Vidrascu, Dominique Chapelle, Miguel A. Fernández
      Abstract: This paper presents a numerical study in which several partitioned solution procedures for incompressible fluid-structure interaction are compared and validated against the results of an experimental fluid-structure interaction benchmark. The numerical methods discussed cover the three main families of coupling schemes: strongly coupled, semi-implicit, and loosely coupled. Very good agreement is observed between the numerical and experimental results. The comparisons confirm that strong coupling can be efficiently avoided, via semi-implicit and loosely coupled schemes, without compromising stability and accuracy. Copyright © 2016 John Wiley & Sons, Ltd.Several state-of-the-art partitioned solution procedures for incompressible fluid-structure interaction are thoroughly compared and validated against experimental data. The considered methods are parameter free and cover the three main categories of coupling schemes: strongly coupled, semi-implicit and loosely coupled. Previous studies in this direction used synthetic data as reference or were limited to a single method. Very good agreement is observed between the numerical and experimental results.
      PubDate: 2016-07-21T05:40:36.405277-05:
      DOI: 10.1002/cnm.2813
       
  • Cortical bone fracture analysis using XFEM – case study
    • Authors: Ashraf Idkaidek; Iwona Jasiuk
      Abstract: We aim to achieve an accurate simulation of human cortical bone fracture using the extended finite element method within a commercial finite element software abaqus. A two-dimensional unit cell model of cortical bone is built based on a microscopy image of the mid-diaphysis of tibia of a 70-year-old human male donor. Each phase of this model, an interstitial bone, a cement line, and an osteon, are considered linear elastic and isotropic with material properties obtained by nanoindentation, taken from literature.The effect of using fracture analysis methods (cohesive segment approach versus linear elastic fracture mechanics approach), finite element type, and boundary conditions (traction, displacement, and mixed) on cortical bone crack initiation and propagation are studied. In this study cohesive segment damage evolution for a traction separation law based on energy and displacement is used. In addition, effects of the increment size and mesh density on analysis results are investigated.We find that both cohesive segment and linear elastic fracture mechanics approaches within the extended finite element method can effectively simulate cortical bone fracture. Mesh density and simulation increment size can influence analysis results when employing either approach, and using finer mesh and/or smaller increment size does not always provide more accurate results. Both approaches provide close but not identical results, and crack propagation speed is found to be slower when using the cohesive segment approach. Also, using reduced integration elements along with the cohesive segment approach decreases crack propagation speed compared with using full integration elements. Copyright © 2016 John Wiley & Sons, Ltd.We find that cohesive segment and linear elastic fracture mechanics approaches within the extended finite element method can both effectively simulate cortical bone fracture. However, results obtained from both approaches are sensitive to finite element model mesh density and to analysis increment size, where using finer mesh and/or smaller analysis increment size does not always provide more accurate results. Also the linear elastic fracture mechanics approach showed faster cracks propagation with more realistic behavior compared with the cohesive segment approach.
      PubDate: 2016-07-12T06:40:40.987749-05:
      DOI: 10.1002/cnm.2809
       
  • Personalized blood flow computations: A hierarchical parameter estimation
           framework for tuning boundary conditions
    • Authors: Lucian Itu; Puneet Sharma, Constantin Suciu, Florin Moldoveanu, Dorin Comaniciu
      Abstract: We propose a hierarchical parameter estimation framework for performing patient-specific hemodynamic computations in arterial models, which use structured tree boundary conditions. A calibration problem is formulated at each stage of the hierarchical framework, which seeks the fixed point solution of a nonlinear system of equations. Common hemodynamic properties, like resistance and compliance, are estimated at the first stage in order to match the objectives given by clinical measurements of pressure and/or flow rate. The second stage estimates the parameters of the structured trees so as to match the values of the hemodynamic properties determined at the first stage. A key feature of the proposed method is that to ensure a large range of variation, two different structured tree parameters are personalized for each hemodynamic property.First, the second stage of the parameter estimation framework is evaluated based on the properties of the outlet boundary conditions in a full body arterial model: the calibration method converges for all structured trees in less than 10 iterations. Next, the proposed framework is successfully evaluated on a patient-specific aortic model with coarctation: only six iterations are required for the computational model to be in close agreement with the clinical measurements used as objectives, and overall, there is a good agreement between the measured and computed quantities. Copyright © 2016 John Wiley & Sons, Ltd.We propose a hierarchical parameter estimation framework for performing patient-specific hemodynamic computations in arterial models, which use structured tree boundary conditions. The calibration algorithms estimate several structured tree parameters so as to ensure a large range of variation for each hemodynamic property of interest (resistance and compliance). The framework is successfully evaluated on a patient-specific aortic model with coarctation: only six iterations are required for the computational model to be in close agreement with the clinical measurements.
      PubDate: 2016-07-12T04:25:29.854546-05:
      DOI: 10.1002/cnm.2803
       
  • Fluid-structure interaction including volumetric coupling with homogenised
           subdomains for modeling respiratory mechanics
    • Authors: Lena Yoshihara; Christian J. Roth, Wolfgang A. Wall
      Abstract: In this article, a novel approach is presented for combining standard fluid-structure interaction with additional volumetric constraints to model fluid flow into and from homogenised solid domains. The proposed algorithm is particularly interesting for investigations in the field of respiratory mechanics as it enables the mutual coupling of airflow in the conducting part and local tissue deformation in the respiratory part of the lung by means of a volume constraint. In combination with a classical monolithic fluid-structure interaction approach, a comprehensive model of the human lung can be established that will be useful to gain new insights into respiratory mechanics in health and disease. To illustrate the validity and versatility of the novel approach, three numerical examples including a patient-specific lung model are presented. The proposed algorithm proves its capability of computing clinically relevant airflow distribution and tissue strain data at a level of detail that is not yet achievable, neither with current imaging techniques nor with existing computational models. Copyright © 2016 John Wiley & Sons, Ltd.In this work, a standard monolithic fluid–structure interaction approach is extended by an additional constraint that couples fluid flow and regional parenchymal inflation. Higher airway tree generations are homogenised within the solid domains and provide physiologically correct boundary conditions to airflow into the single regions of the lung. Monolithic treatment enables an efficient and stable coupling in real patient-specific models.
      PubDate: 2016-07-08T03:16:00.048821-05:
      DOI: 10.1002/cnm.2812
       
  • Contributions of prestrains, hyperelasticity, and muscle fiber activation
           on mitral valve systolic performance
    • Authors: Victorien Prot; Bjorn Skallerud
      Abstract: The present study addresses the contributions of prestrains and muscle fiber activation to the global response of the mitral valve during systole. A finite element model of a porcine mitral valve is created using anatomical measurements and 3D echocardiographic recordings. The passive behavior of the leaflets is modeled using a transversely isotropic hyperelastic constitutive model, and we assume orthotropic muscle activations in the anterior leaflet. A simple approach to incorporate prestrains in the mitral valve apparatus is used by expanding the mitral annulus before applying the ventricular pressure to the mitral leaflets. Several finite element analyses are run with or without muscle activation and with or without prestrains. The analysis results are compared at peak systole with the echocardiograpic recordings. The case where prestrains and activation are accounted for simultaneously is the most efficient to approach the physiological flat shape of the closed valve observed in the echocardiograpic measurements. These results suggest that the active components present in the mitral leaflets and the presence of prestrains contribute to the physiological deformations of the mitral valve at peak systole and that material models based on in vitro mechanical testing are not sufficient for numerical studies of the mitral apparatus. Copyright © 2016 John Wiley & Sons, Ltd.The present study addresses the contributions of prestrains and muscle fiber activation to the global response of the mitral valve during systole using the finite element method. The analysis where prestrains and muscle activation are accounted for simultaneously is the most efficient to approach the physiological flat shape observed in the echocardiography. This study suggests that the active components present in the mitral leaflets and the presence of prestrains contribute to the physiological deformations of the valve.
      PubDate: 2016-07-05T09:00:47.771441-05:
      DOI: 10.1002/cnm.2806
       
  • The effects of injection modes on instantaneous particle deposition in a
           realistic human nasal cavity
    • Authors: Jian Wen; Xin Gu, Mengmeng Wang, Guanping Jian, Simin Wang, Guoxi Zheng
      Abstract: To understand the instantaneous particle deposition in nasal cavity, effects of two injection models on particle deposition characteristic were discussed in this paper. Based on a realistic human nasal cavity geometry obtained from CT scans, a comparison of deposition pattern in the nasal cavity between single injection and continuous injection was investigated through the Lagrangian approach. The instantaneous airflow field was simulated with the tidal volume of 159 and 318 mL by two sine wave curves at inlet. For the case of single injection, particles have finished deposition in the first half of inhalation, and a negative correlation between the tidal volumes and deposition can be observed when the particle diameter was larger than 10 µm. Moreover, particles were mainly deposited in the turbinate area that was beneficial for aerosol therapy. The inertial parameter was not suitable to predict the particle deposition in the case of single injection. With respect to continuous injection, a reduction in total deposition caused by the deceleration process of inhalation can be observed after 1.5 s. The deposition was closely associated with the time-varying flow field, and particles were mainly deposited in the anterior region and turbinate area. Besides, the particle deposition increased with the inertial parameter for continuous injection. The results indicated that the injection modes had an influence on both the total deposition and local deposition pattern in the nasal cavity. Copyright © 2016 John Wiley & Sons, Ltd.The instantaneous particle deposition characteristic in the nasal cavity was investigated with both single and continuous injection modes during inhalation. For single injection mode, higher flow rates may result in the lower particle deposition under the influence of gravity and deposition time. For continuous injection mode, the particle deposition was strongly dependent on the airflow rate. The inertial parameter I was not suitable to predict the particle deposition for single injection, but the opposite was true for continuous injection.
      PubDate: 2016-07-01T07:45:36.295148-05:
      DOI: 10.1002/cnm.2802
       
  • Mathematical-based modeling and prediction of the effect of external
           stimuli on human gait
    • Authors: Hamidreza Namazi; Vladimir V. Kulish
      Abstract: Human gait is defined as human locomotion that is achieved through the movement of limbs. Different limb movement patterns result in different gait patterns. Different internal and external stimuli can affect the human gait. During the years, scientists have worked on the analysis of the effect of external stimuli on human gait, but no work has been reported yet that suggests a mathematical model for analysis of this effect by linking to the nervous system. Considering the diffusion of external stimuli to the nervous system on one side and fractality of human gait on another side, in this research, for the first time, we develop a model for prediction of human gait using fractional diffusion equation. Using this model, we will predict the effect of auditory stimuli on human gait. The model developed in this research is useful not only for analysis of the effect of auditory stimuli on human gait but can be used also for analysis of different types of stimuli on subjects with healthy conditions or having some types of diseases. Copyright © 2016 John Wiley & Sons, Ltd.Considering the diffusion of external stimuli to the nervous system on one side and fractality of human gait on another side, in this research for the first time, authors developed a model for prediction of human gait using fractional diffusion equation (FDE). Using this model, they predicted the effect of auditory stimuli on human gait.
      PubDate: 2016-07-01T07:35:27.614074-05:
      DOI: 10.1002/cnm.2805
       
  • A computational framework to characterize and compare the geometry of
           coronary networks
    • Authors: C. A. Bulant; P. J. Blanco, T. P. Lima, A.N. Assunção , G. Liberato, J. R. Parga, L. F. R. Ávila, A. C. Pereira, R. A. Feijóo, P. A. Lemos
      Abstract: This work presents a computational framework to perform a systematic and comprehensive assessment of the morphometry of coronary arteries from in vivo medical images. The methodology embraces image segmentation, arterial vessel representation, characterization and comparison, data storage, and finally analysis. Validation is performed using a sample of 48 patients. Data mining of morphometric information of several coronary arteries is presented. Results agree to medical reports in terms of basic geometric and anatomical variables. Concerning geometric descriptors, inter-artery and intra-artery correlations are studied. Data reported here can be useful for the construction and setup of blood flow models of the coronary circulation. Finally, as an application example, similarity criterion to assess vasculature likelihood based on geometric features is presented and used to test geometric similarity among sibling patients. Results indicate that likelihood, measured through geometric descriptors, is stronger between siblings compared with non-relative patients. Copyright © 2016 John Wiley & Sons, Ltd.We describe a computational framework to perform a systematic and comprehensive assessment of the morphometry of coronary arteries from in vivo images. Data mining of morphometric information of several arteries is presented for a sample of 48 patients. Concerning geometric descriptors, inter-artery and intra-artery correlations are studied. Similarity criterion to assess vasculature likelihood based on geometric features is presented and used to test similarity among sibling patients. Results indicate that likelihood, as defined here, is stronger between siblings compared with non-relatives.
      PubDate: 2016-06-24T16:10:31.238194-05:
      DOI: 10.1002/cnm.2800
       
  • Computational comparison of regional stress and deformation
           characteristics in tricuspid and bicuspid aortic valve leaflets
    • Authors: K. Cao; P. Sucosky
      Abstract: The bicuspid aortic valve (BAV) is the most common congenital valvular defect and a major risk factor for secondary calcific aortic valve disease. While hemodynamics is presumed to be a potential contributor to this complication, the validation of this theory has been hampered by the limited knowledge of the mechanical stress abnormalities experienced by BAV leaflets and their dependence on the heterogeneous BAV fusion patterns. The objective of this study was to compare computationally the regional and temporal fluid wall shear stress (WSS) and structural deformation characteristics in tricuspid aortic valve (TAV), type-0, and type-I BAV leaflets. Arbitrary Lagrangian–Eulerian fluid-structure interaction models were designed to simulate the flow and leaflet dynamics in idealized TAV, type-0, and type-I BAV geometries subjected to physiologic transvalvular pressure. The regional leaflet mechanics was quantified in terms of temporal shear magnitude (TSM), oscillatory shear index (OSI), temporal shear gradient (TSG), and stretch. The simulations identified regions of WSS overloads and increased WSS bidirectionality (174% increase in temporal shear magnitude, 0.10 increase in OSI on type-0 leaflets) in BAV leaflets relative to TAV leaflets. BAV leaflets also experienced larger radial deformations than TAV leaflets (4% increase in type-0 BAV leaflets). Type-I BAV leaflets exhibited contrasted WSS environments marked by WSS overloads on the non-coronary leaflet and sub-physiologic WSS levels on the fused leaflet. This study provides important insights into the mechanical characteristics of BAV leaflets, which may further our understanding of the role played by hemodynamic forces in BAV disease. Copyright © 2016 John Wiley & Sons, Ltd.Fluid-structure interaction models were designed to compare the regional stress and deformation characteristics of tricuspid aortic valve and bicuspid aortic valve (BAV) leaflets. BAV hemodynamic abnormalities were found to be morphotype-dependent and leaflet-dependent. BAV leaflets experience larger radial deformations, higher wall shear stress (WSS) levels and more pronounced WSS bidirectionality than TAV leaflets. Type-I BAV leaflets exhibit contrasted hemodynamics marked by WSS overloads on the non-coronary leaflet and sub-physiologic WSS levels on the fused leaflet.
      PubDate: 2016-06-24T16:00:42.491712-05:
      DOI: 10.1002/cnm.2798
       
  • Modeling tensional homeostasis in multicellular clusters
    • Authors: Sze Nok Tam; Michael L. Smith, Dimitrije Stamenović
      Abstract: Homeostasis of mechanical stress in cells, or tensional homeostasis, is essential for normal physiological function of tissues and organs and is protective against disease progression, including atherosclerosis and cancer. Recent experimental studies have shown that isolated cells are not capable of maintaining tensional homeostasis, whereas multicellular clusters are, with stability increasing with the size of the clusters. Here, we proposed simple mathematical models to interpret experimental results and to obtain insight into factors that determine homeostasis. Multicellular clusters were modeled as one-dimensional arrays of linearly elastic blocks that were either jointed or disjointed. Fluctuating forces that mimicked experimentally measured cell–substrate tractions were obtained from Monte Carlo simulations. These forces were applied to the cluster models, and the corresponding stress field in the cluster was calculated by solving the equilibrium equation. It was found that temporal fluctuations of the cluster stress field became attenuated with increasing cluster size, indicating that the cluster approached tensional homeostasis. These results were consistent with previously reported experimental data. Furthermore, the models revealed that key determinants of tensional homeostasis in multicellular clusters included the cluster size, the distribution of traction forces, and mechanical coupling between adjacent cells. Based on these findings, we concluded that tensional homeostasis was a multicellular phenomenon. Copyright © 2016 John Wiley & Sons, Ltd.Experimental measurements of cellular traction forces have shown that isolated cells are not capable of maintaining their cytoskeletal stress at a stable, set point value (known as tensional homeostasis), whereas multicellular clusters are, with stability increasing with the cluster size. Here, we proposed mathematical models of cell clusters to interpret the experimental results. The models revealed that key determinants of tensional homeostasis included the cluster size, the distribution of traction forces applied to the cluster, and cell–cell mechanical coupling.
      PubDate: 2016-06-14T20:00:33.927622-05:
      DOI: 10.1002/cnm.2801
       
  • Patient-specific parameter estimation in single-ventricle lumped
           circulation models under uncertainty
    • Authors: Daniele E. Schiavazzi; Alessia Baretta, Giancarlo Pennati, Tain-Yen Hsia, Alison L. Marsden
      Abstract: Computational models of cardiovascular physiology can inform clinical decision-making, providing a physically consistent framework to assess vascular pressures and flow distributions, and aiding in treatment planning. In particular, lumped parameter network (LPN) models that make an analogy to electrical circuits offer a fast and surprisingly realistic method to reproduce the circulatory physiology. The complexity of LPN models can vary significantly to account, for example, for cardiac and valve function, respiration, autoregulation, and time-dependent hemodynamics. More complex models provide insight into detailed physiological mechanisms, but their utility is maximized if one can quickly identify patient specific parameters. The clinical utility of LPN models with many parameters will be greatly enhanced by automated parameter identification, particularly if parameter tuning can match non-invasively obtained clinical data. We present a framework for automated tuning of 0D lumped model parameters to match clinical data. We demonstrate the utility of this framework through application to single ventricle pediatric patients with Norwood physiology. Through a combination of local identifiability, Bayesian estimation and maximum a posteriori simplex optimization, we show the ability to automatically determine physiologically consistent point estimates of the parameters and to quantify uncertainty induced by errors and assumptions in the collected clinical data. We show that multi-level estimation, that is, updating the parameter prior information through sub-model analysis, can lead to a significant reduction in the parameter marginal posterior variance. We first consider virtual patient conditions, with clinical targets generated through model solutions, and second application to a cohort of four single-ventricle patients with Norwood physiology. Copyright © 2016 John Wiley & Sons, Ltd.Lumped parameter models can simulate complex hemodynamic phenomena with surprising realism, but their utility is maximized if one can robustly identify patient specific parameters and predict clinically relevant indicators that are otherwise difficult to measure. We investigate parameter identifiability using both local and global metrics on simplified and full circulation models under virtual and real patient data. A multi-level estimation approach based on adaptive multi-chain Monte Carlo is also proposed, leading to improved parameter learning in patients with Norwood physiology.
      PubDate: 2016-06-08T04:42:31.661534-05:
      DOI: 10.1002/cnm.2799
       
 
 
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