1,721,081 research outputs found
Hemodynamics and wall shear metrics in a pulmonary autograft : comparing a fluid-structure interaction and computational fluid dynamics approach
No full textObjective: In young patients, aortic valve disease is often treated by placement of a pulmonary autograft (PA) which adapts to its new environment through growth and remodeling. To better understand the hemodynamic forces acting on the highly distensible PA in the acute phase after surgery, we developed a fluid-structure interaction (FSI) framework and comprehensively compared hemodynamics and wall shear-stress (WSS) metrics with a computational fluid dynamic (CFD) simulation. Methods: The FSI framework couples a prestressed non-linear hyperelastic arterial tissue model with a fluid model using the in-house coupling code CoCoNuT. Geometry, material parameters and boundary conditions are based on in-vivo measurements. Hemodynamics, time-averaged WSS (TAWSS), oscillatory shear index (OSI) and topological shear variation index (TSVI) are evaluated qualitatively and quantitatively for 3 different sheeps. Results: Despite systolic-to-diastolic volumetric changes of the PA in the order of 20 %, the point-by-point correlation of TAWSS and OSI obtained through CFD and FSI remains high (r > 0.9, p 0.8, p < 0.01) for OSI). Instantaneous WSS divergence patterns qualitatively preserve similarities, but large deformations of the PA leads to a decrease of the correlation between FSI and CFD resolved TSVI (r < 0.7, p < 0.01). Moderate co-localization between FSI and CFD is observed for low thresholds of TAWSS and high thresholds of OSI and TSVI. Conclusion: FSI might be warranted if we were to use the TSVI as a mechano-biological driver for growth and remodeling of PA due to varying intra-vascular flow structures and near wall hemodynamics because of the large expansion of the PA
Snelle eindige-elementen simulatie gebruik makende van GPGPU technologie voor biomechanica van de weke weefsels
No full textFinite element analysis (FEA) enjoys a variety of applications in the field of biomechanics, from simulated fatigue testing of an intramedullary rod to studying stresses in a displacement field generated within a single cell. The powerful generality of the finite element method implies that often no purpose-specific method or model need be developed to study a system, but regrettably also implies a certain computational cost. FEA-supported virtual reality surgical simulation, or intraoperative surgical simulation are examples of domains whose inherent requirement of (near-)real-time computation is directly contrasted by this fact. Similarly, other, research-oriented
application areas such as optimization-driven material characterisation, rely on a large number of simulations. Here, reduced simulation time translates into more accurate characterisation or an expansion of the possible solution space. This thesis investigates the applicability of Graphics Processing Units (GPUs) to alleviate this computational burden, with particular focus on the biomechanics of soft tissues.
The solution speed requirements motivate the selection of an appropriate FE formulation that maps well onto the fine-grained GPU parallel-processing paradigm. Initially, this thesis presents a custom GPU implementation of an explicit FE code solving the governing boundary value problem in nonlinear elasticity. The implementation is analyzed, and its advantages and limitations elucidated. Subsequently, a broad study using simple materials is conducted on a plethora of devices, examining the effect of several pertinent factors, such as mesh density or the use of single/double precision floating point computation on simulation times. Aggregate results reveal an encouraging 30-250x speedup against an industry established code. However, more complex, biofidelic, fiber-reinforced materials are needed. They are next included in this thesis, with a previously unreported implementation on GPUs. The use of these materials implies more complex Gaussian quadrature schemes, whose necessity heavily impacts GPU performance, more so than the added computation due to the inherent anisotropy alone. All factors related to fast simulation using these materials are elucidated in detail, including the scaling of the problem. Results reveal an approximate 10-35% slowdown duo to the fiber addition, in contrast to the 3-8x due to the nature of integration. Finally, an important test of clinical applicability is presented using five patient-specific simulations of an Abdominal Aortic Aneurysms (AAA). In this novel application it is found that the complex morphology-imposed aortic geometry, and the histology-imposed material model,
severely impact the performance of the chosen method. Regardless, excellent overall speedup of 10-17x practically reduce solution times from approximately two days to two hours. Without recourse to simplifying assumptions and no accuracy loss, this is regarded as a significant result and further affirms the approach presented in this thesis.
Altogether, this thesis presents engineering contributions to fast FEA computation directly applicable in several areas of biomechanics, particularly in surgical training, intraoperative mitigation of critical events and research applications. While doing so, the thesis objectively demonstrates the opportunities, challenges and limitations of the use of general purpose GPU technology in this context.status: Publishe
Automisatie en evaluatie van 3D en 4D cardio CT beeldgebaseerde processen
No full textCardiovascular diseases are a major public health concern and currently the leading cause of mortality and morbidity worldwide. Medical imaging supports several cardiovascular disease applications, and recent technological advancements have increased the quality and quantity of medical images available. In turn, the additional information provided has been supporting diagnosis procedures, treatments and patient monitoring. However, to access the contained information, images need to be manipulated to a certain extent. The manipulation normally starts with segmentation or landmarking procedures that serve as an input for further processes such as patient-specific biomechanical simulation or 3D-printing. These procedures are still tedious, time-consuming, error-prone and user-dependent. The recent explosive growth in image quantity and quality has moreover increased the resources required to complete these manipulation processes.
There is, therefore, a strong need for reliable, robust and automated tools to address the limitations of the current processes. The work in this thesis addresses these limitations by automating and evaluating current image-derived workflows. This work focuses on three specific topics: segmentation, landmarking and biomechanical simulation. Given the relevance of cardiovascular diseases, and the widespread use of Computed Tomography (CT), the scope of this thesis will be limited to 3D and 4D cardiovascular CT.
Image segmentation is a process used to label the structure of interest of the cardiovascular anatomy and often represents the first step of an image processing workflow. Many technologies have attempted to simplify or to automate the process; however, manual input is still the norm. In this work, a novel automatic method for the segmentation of cardiac CT is designed, developed and validated. The method is based on an efficient combination of atlas-registration and graph-cut technology. The validation is performed with 95 cardiac CTs, and the results show comparable accuracy and unmatched computational speed in comparison with other automated tools.
The annotation of crucial anatomical structures, also known as landmarking, is a process that is often performed to morphologically assess anatomical structures. In 4D-CT, the process is especially relevant to monitor changes in the mitral annulus dimension, in order to support improvements in the treatment of patients suffering from mitral regurgitation. The annotation is however still mostly performed manually, making the process especially tedious. This work shows how deformable image registration can be used to automate landmark annotation by propagating landmarks from one phase of the image stack to all other phases. The accuracy of the method was evaluated by measuring the distance between the propagated and manually annotated landmarks on 8 4D-CTs. The results support the use of this technology as a way to automate the current landmark annotation process.
Finally, the prediction of peak stress of the aortic wall, obtained through biomechanical simulations, represents a promising tool to support aortic aneurysm risk stratification. The simulations require the use of robust and biofidelic models based on patient-specific information. Therefore, mechanical properties are a crucial input, but patient-specific information is not available in clinical cases. In this work, we evaluate the possibility to estimate the material properties \textit{in vivo} starting from 4D-CT, and as such, improve the prediction of peak wall stress. The accuracy of the method is rigorously evaluated by performing a virtual experiment. The results support the use of patient-specific material properties to perform biomechanical simulations and show that the method leads to an accurate estimation of the material properties and of the consequent peak wall stress only if the material model used provides a true representation of the material properties of the aorta. Moreover, the evaluation platform developed in the process can be easily expanded to support future developments and improvements of the method.
Altogether, this thesis presents significant engineering contributions to three image-processing workflows stemming from image acquisition: segmentation, landmarking, and biomechanical simulations. These improvements enable the extraction of information from cardiac CT more easily and efficiently. Hence, they have the potential to provide clinicians and researchers with additional resources and information by removing the need to address imaging-data processing tasks. Moreover, the additional information could provide a more comprehensive understanding about the cardiac anatomy and about the behavior of soft tissues.status: Publishe
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Biomechanica van brugvenen: naar een verbeterde experimentele karakterisatie en computationele representatie
No full textAcute subdural hematoma (ASDH) is one of the most frequent lesions inside the head observed in victims of bicycle related accidents. In nearly one third of the ASDH cases, the etiopathology directly relates to the rupture of a bridging vein (BV) which drains blood from the cerebral cortex into the superior sagittal sinus (SSS). To study the biomechanics of ASDH and other traumatic brain injuries and to assess the efficacy and improve the design of protective devices such as bicycle helmets, finite element (FE) head models are used. However, the accuracy of a FE head model depends, among other things, on the use of correct material properties for each cranial tissue which are , experimentally derived. In large contrast to other cranial tissues, e.g. the brain, the BVs is still a rather unknown complex within the head; yet it has a crucial role in the etiology of ASDH. Hence, this interdisciplinary research project conducts a thorough performance analysis of the state of the art finite element head model and its bridging vein representation. This is followed by the design and manufacturing of a novel two rail shear testing device for small soft tissue samples. Combining the experimental results with uni-axial tensile test data a fitting method is developed for a non-linear anisotropic material model. This novel material characterization approach is then applied to BVs in a pilot study.status: Publishe
Experimentele en numerieke analyse van groei en remodellering in de pulmonaire autogreffe
No full textIn several cardiac interventions, pulmonary arterial tissue is exposed to systemic conditions. One of these procedures is the Ross procedure, which replaces a diseased aortic valve with the patient's own pulmonary valve. A common complication hereby is the dilatation of this pulmonary autograft. Still, not all autografts fail and the decisive factor for autograft failure is not known. Several reinforcement strategies have been designed to counteract this dilatation, but none have proven to be consistently successful. A personalized macroporous mesh used to reinforce dilating aortic roots in Marfan patients might also bring a solution for this dilating autograft.
The goal of this thesis is therefore to investigate the mechanical and microstructural changes that occur when a pulmonary artery is placed in aortic position as an autograft, and to assess the effect of a macroporous mesh that reinforces the autograft. To this end, a combined experimental and computational approach was applied.
Two sets of experiments were conducted on sheep. Within these animal experiments, a segment of pulmonary artery was placed in aortic position. This pulmonary autograft was either reinforced with a macroporous mesh or left unreinforced in the control group. Six months after implantation, the sheep were sacrificed and the following tissues were harvested and subjected to planar biaxial testing: native aorta, native pulmonary artery, unreinforced pulmonary autograft, reinforced pulmonary autograft. The first set of animal experiments constisted of nine sheep, with two sheep serving as control group. Seventeen sheep were included in the second set of animal experiments, with eight sheep serving as control group. The mechanical behavior of the unreinforced autograft adapted to become more aorta-like in some samples whereas it retained its pulmonary artery character in other samples. Microstructurally, an increased collagen deposition, smooth muscle cell atrophy and a decrease in media thickness occured in the unreinforced pulmonary autograft. The mesh appeared to be nicely incorporated but a higher loss of smooth muscle cells was noticed in the reinforced pulmonary autograft. The follow-up MRIs, taken only in the second set of animal experiments, showed progressive dilatation of the autograft when leaving it unreinforced. The macroporous mesh around the pulmonary autograft decreased autograft dilatation, but also its compliance.
A subset of animal experiments without macroporous reinforcement was reproduced in silico using different growth and remodeling models. The ability to reproduce the experimental outcome was evaluated for two types of models: models based on kinematic growth theory and models based on constrained mixture theory. The former decompose the deformation gradient into a growth and elastic deformation gradient, with growth either in the radial or circumferential direction. The constrained mixture theory represents an artery as a mixture of constituents with their own stress-free configuration, turnover rates and material properties, but are constrained to move together. The two constituents in this case are elastin and collagen. Elastin is assumed to remain constant whereas collagen continuously degrades and is deposited, influenced by the stretch felt by the collagen fibers. The kinematic growth model with circumferential growth as well as the constrained mixture models are capable of reproducing the progressive dilatation of the autograft. However, where the experiments show an initial steep increase in diameter followed by a slower increase, the model shows a linearly increasing diameter. As opposed to the kinematic growth models, the constrained mixture models are also able to reproduce the experimentally observed changes in mechanical behaviour and collagen fraction.
In conclusion, the animal experiments showed adaptation of the mechanical behavior of the pulmonary artery when placed in aortic position. The macroporous mesh was also able to halt progressive dilatation of the autograft while retaining its microstructure. The two types of growth and remodeling models were capable of simulating the dilatation of the autograft, and the constrained mixture models were also capable of simulating changing mechanical behavior and microstructure. Nevertheless, more controlled experiments are needed to increase our understanding of autograft (mal)adaptation and to define more mechanobiologically substantiated constitutive relations.status: Publishe
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Alterations in human mitral valve mechanical properties secondary to left ventricular remodeling: a biaxial mechanical study
No full textSecondary mitral regurgitation occurs when a left ventricular problem causes leaking of the mitral valve. The altered left ventricular geometry changes the orientation of the subvalvular apparatus, thereby affecting the mechanical stress on the mitral valve. This in turn leads to active remodeling of the mitral valve, in order to compensate for the ventricular remodeling. In this study, a biomechanical analysis was performed on eight human mitral valves with secondary mitral regurgitation and ten healthy human mitral valves to better understand this pathophysiology and its effect on the mechanical properties of these tissues. Samples were obtained from the anterior and posterior leaflet and used for planar biaxial mechanical experiments. Uniaxial experiments were performed on four groups of mitral valve chords: anterior basal, anterior marginal, posterior basal and posterior marginal chords. The mechanical response of the mitral valve leaflets was fitted to the May-Newman and Yin constitutive model, whereas the material parameters of the third order Ogden model were determined for the chord samples. Next, stiffnesses calculated at low and high stress levels were statistically analyzed. Leaflet samples with secondary mitral regurgitation showed a small thickness increase and a change in anisotropy index compared to healthy control valves. Diseased leaflets were more compliant circumferentially and stiffer radially, resulting in anisotropic samples with the radial direction being stiffest. In addition, chord samples were slightly thicker and less stiff at high stress in secondary mitral regurgitation, when grouped per leaflet type and insertion region. These results confirm mechanical alterations due to the pathophysiological valvular changes caused by left ventricular remodeling. It is important that these changes in mechanical behavior are incorporated into computational models of the mitral valve.sponsorship: Fonds voor Hartchirurgie, KU Leuven|C2-ADAPT, Fonds Wetenschappelijk Onderzoek|11H5821N, Fonds Wetenschappelijk Onderzoek|SB1SA9119N, Fonds Wetenschappelijk Onderzoek|12ZC820Nstatus: Published onlin
3D-modellering en beeldverwerking voor behandeling van vaginale prolaps
No full textstatus: Publishe
- …
