Dissertations / Theses on the topic 'Microstructural imaging'
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1
Zacharia, Nicole S. B. Massachusetts Institute of Technology. "Thermal imaging of quenched microstructural evolution in steel alloys." Thesis, Massachusetts Institute of Technology, 2001. http://hdl.handle.net/1721.1/118566.
Abstract:
Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2001.Cataloged from PDF version of thesis.
Includes bibliographical references (page 41).
A method was devised for quenching undercooled samples of Fe-12Cr-16Ni. The samples were levitated in a magnetic field as a way of containerless processing. They were dropped onto a nucleation trigger and then into a quenching bath. This process was successful in producing quenched samples , but the layer of In-Ga wetting their surface showed problematic in the analysis of the samples. Data shows that double recalescence was observed in a few cases. SEM proved inconclusive and was not sensitive enough to detect the small chemical variations expected in the dendrites. Also, across the entirety of the sample there was no significant partitioning of Cr or Ni.
by Nicole Zacharia.
S.B.
2
Fukutomi, Hikaru. "Neurite imaging reveals microstructural variations in human cerebral cortical gray matter." Kyoto University, 2020. http://hdl.handle.net/2433/253174.
3
Horne, Nikki Renee. "Microstructural white matter changes in Alzheimer's disease a diffusion tensor imaging study /." Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC campuses, 2008. http://wwwlib.umi.com/cr/ucsd/fullcit?p3296903.
Abstract:
Thesis (Ph. D.)--University of California, San Diego and San Diego State University, 2008.Title from first page of PDF file (viewed April 7, 2008). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical references (p. 127-149).
4
Nguyen, Thanh Tung. "Modeling of complex microcracking in cement based materials by combining numerical simulations based on a phase-field method and experimental 3D imaging." Thesis, Paris Est, 2015. http://www.theses.fr/2015PESC1152/document.
Abstract:
Une approche combinant simulation numérique et expérimentation est développée pour modéliser la microfissuration complexe dans des matériaux hétérogènes cimentaires. Le modèle numérique proposé a permis de prévoir précisément en 3D l'initiation et la propagation des microfissures à l'échelle de la microstructure réelle d'un échantillon soumis à un chargement de compression. Ses prévisions ont été validées par une comparaison directe avec le réseau de fissures réel caractérisé par des techniques d'imagerie 3D. Dans une première partie, nous développons et testons les outils de simulation numérique. Plus précisément, la méthode de champ de phase est appliquée pour simuler la microfissuration dans des milieux fortement hétérogènes et ses avantages pour ce type de modélisation sont discutés. Ensuite, une extension de cette méthode est proposée pour tenir compte d'un endommagement interfacial, notamment aux interfaces inclusion/matrice. Dans une deuxième partie, les méthodes expérimentales utilisées et développées au cours de cette thèse sont décrites. Les procédures utilisées pour obtenir l'évolution du réseau de fissures 3D dans les échantillons à l'aide de microtomographie aux rayons X et d'essais mécaniques in-situ sont présentées. Ensuite, les outils de traitement d'image utilisant la corrélation d'images volumiques, pour extraire les fissures des images en niveaux de gris avec une bonne précision, sont détaillés. Dans une troisième partie, les prévisions du modèle numérique sons comparées avec les données expérimentales d'un matériau modèle en billes de polystyrène expansé intégrées dans une matrice de plâtre dans un premier temps, et, dans un second temps, d'un béton léger plus complexe. Plus précisément, nous utilisons les données expérimentales pour identifier les paramètres microscopiques inconnus par une approche inverse, et utilisons les déplacements expérimentaux déterminés par corrélation d'images volumiques pour définir des conditions limites à appliquer sur les bords de sous-domaines dans l'échantillon pour les simulations. Les comparaisons directes de réseaux de microfissures 3D et de leur évolution montrent une très bonne capacité prédictive du modèle numériqueAn approach combining numerical simulations and experimental techniques is developed to model complex microcracking in heterogeneous cementitious materials. The proposed numerical model allowed us to predict accurately in 3D the initiation and the propagation of microcracks at the scale of the actual microstructure of a real sample subjected to compression. Its predictions have been validated by a direct comparison with the actual crack network characterized by 3D imaging techniques. In a first part, the numerical simulation tools are developed and tested. More specifically, the phase-field method is applied to microcracking simulations in highly heterogeneous microstructures and its advantages for such simulations are discussed. Then, the technique is extended to account for interfacial cracking, possibly occurring at inclusion/matrix interfaces. In a second part, the experimental methods used and developed in this work are described. The procedures to obtain the evolution of the 3D crack network within the samples by means of X-rays computed microtomography and in-situ mechanical testing are presented. Then, we focus on the developed image processing tools based on digital volume correlation to extract with good accuracy the cracks from the grey level images. In a third part, we compare the predictions of the numerical model with experimental results obtained, first, with a model material made of expanded polystyrene beads embedded in a plaster matrix, and second, to a more complex lightweight concrete. More precisely, we use the experimental data to identify by inverse approaches the local microstructural parameters, and use the experimental displacements measured by digital volume correlation to define boundary conditions to be applied on sub-domains within the sample for the simulations. The obtained direct comparisons of 3D microcrack networks and their evolutions demonstrate the very good predictive capability of the numerical model
5
Gong, Nanjie, and 龔南杰. "Probing tissue microstructural changes in neurodegenerative processes using non-gaussian diffusion MR imaging." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2014. http://hdl.handle.net/10722/208583.
Abstract:
Development of non-invasive imaging biomarkers sensitive to microstructural organization is crucial for deepening our understanding of mechanisms underlying neurodegenerative processes such as aging and further improving early diagnosis and monitoring of neurodegenerative disease such as Alzheimer’s disease (AD) and amnestic mild cognitive impairment (MCI). The diffusional kurtosis imaging (DKI) is an extension of conventional diffusion tensor imaging. It is hypothesized that DKI will provide complementary information to conventional diffusivity metrics in a new dimension that will more comprehensively capture microstructural changes in anisotropic white matter tracts and particularly in relatively isotropic tissues such as gray matter during neurodegenerative processing of aging, MCI and AD and probably improve the early diagnosis of the diseases.
Firstly, DKI method and a white-matter model that provided metrics of explicit neurobiological interpretations were applied on healthy participants. In white matter tracts, age-related degenerations appeared to be broadly driven by axonal loss. Demyelination may also be a major driving mechanism, although confined to the anterior brain. In terms of deep gray matter, higher mean kurtosis (MK) and fractional anisotropy (FA) in the globus pallidus, substantia nigra, and red nucleus reflected higher microstructural complexity and directionality compared with the putamen, caudate nucleus, and thalamus. In particular, unique age-related positive correlations for FA, MK, and radial kurtosis (KR) in the putamen opposite to those in other regions were observed.
Secondly, to verify the speculation that iron deposition could be one probable underlying mechanism driving changes in microstructure, another advance MRI technique of quantitative susceptibility mapping (QSM) was also used in healthy participants. Significant age-related increases of iron were observed in the putamen, red nucleus, substantia nigra, and caudate nucleus. Putamen exhibited the highest rate of iron accumulation with aging, which was nearly twice of the rates in substantia nigra and caudate nucleus. Significant positive correlations between susceptibility value and diffusion measurements were observed for FA and MK in the putamen as well as FA in the red nucleus.
Thirdly, whether DKI metrics could serve as imaging biomarkers to indicate the severity of cognitive deficiency for AD and MCI was investigated. In AD, significantly increased diffusivity and decreased kurtosis parameters were observed in both white and gray matter of the parietal and occipital lobes as compared to MCI. Significantly decreased FA was also observed in the white matter of these lobes in AD. With the exception of FA and KR, all the other five DKI metrics exhibited significant correlations with mini-mental state examination score in both white and gray matter.
Lastly, DKI metrics were compared against volumetry for diagnosis of AD and MCI. In AD vs. aMCI, although no significant difference of either FA or MD was observed in white matter tracts, it is encouraging to note that MK captured loss of microstructural complexity in the superior longitudinal fasciculus and internal capsule. MK in the putamen showed the highest power that outperformed volume of the hippocampus for discriminating AD from normal. Besides, FA in the putamen showed the second highest power for discriminating aMCI from normal.published_or_final_version
Diagnostic Radiology
Doctoral
Doctor of Philosophy
6
Qian, Wenshu, and 錢文樞. "Detecting microstructural changes in MRI normal-appearing tissues of the central nervous system by diffusion tensor and kurtosis imaging." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2013. http://hdl.handle.net/10722/193462.
Abstract:
This thesis aimed to investigate the feasibility of two diffusion imaging techniques, Diffusion Tensor Imaging (DTI) and Diffusion Kurtosis Imaging (DKI), on detecting subtle physiological or pathological microstructural changes in normal-appearing neural tissues of human central nervous system.
At first, ten patients with neuromyelitis optica (NMO) and twelve age- and gender-matched healthy subjects were recruited. DTI-derived indices including fractional anisotropy (FA), mean diffusivity (MD), axial and radial diffusivities were quantified in the lateral and dorsal columns of cervical spinal cord. Based on the regions of interest (ROIs) measurement, NMO patients showed reduced FA, increased MD and radial diffusivity compared to control subjects, while axial diffusivity did not show any significant difference. The three former DTI metrics also showed significant correlations with disability scores, and especially FA was found to be sensitive to mild NMO. Our results show that DTI-derived indices can quantitatively assess the white matter (WM) abnormalities with seemingly normal appearance in conventional MRI, and are associated with the level of clinical disability, suggesting that DTI may have great potential as a useful diagnostic tool in the clinical setting.
DKI is an extension of conventional DTI to probe the non-Gaussian diffusion property in biological tissues. Besides the four conventional DTI-derived metrics, DKI also provide three additional kurtosis metrics (mean kurtosis (MK), axial and radial kurtosis). In the second study, ROI-based analysis was used to characterize age-related microstructural changes in WM, cortical and subcortical gray matter (GM) of 27 healthy adults (21~59 yrs). Though the volumes of GM and WM were still preserved, DTI-derived metrics can detect the subtle changes in WM and GM. Meanwhile, MK and radial kurtosis significantly increased in both caudate nucleus and putamen while Thalamus showed little aging effect in the diffusivity and kurtosis metrics but significantly decreased only in FA. Our results demonstrated that DKI is sensitive to detect the age-related alterations in neural microstructures at the stage of early aging.
In addition, DKI has been applied to detect the pathological changes in the normal-appearing neural tissues of 18 patients with multiple sclerosis (MS), compared to 22 healthy controls. Diffuse WM abnormalities have been observed extensively in the brain, revealed by DKI-derived metrics. Though the volumetric and voxel-wise analysis revealed no significant changes in the volume of cortical GM, decreased FA and kurtoses with increased diffusivities in MS group were sensitive to disclose the subtle alterations in global and regional cortical GM tissues. Significant correlations have been found between FA in the global, frontal and temporal cortical GM in relapsing-remitting MS patients and their disability scores, suggesting FA as an important biomarker to monitor the disease progress in cortical GM. Moreover, elevated kurtosis indices in MS patients did not correlate with diffusivities in caudate nucleus, putamen and thalamus, suggesting these metrics may be vulnerable to different pathologic aspects of the disease.
In conclusion, DKI is sensitive to neural alterations during normal aging and in MS pathologies, and can provide complementary information to conventional MRI and DTI.published_or_final_version
Diagnostic Radiology
Doctoral
Doctor of Philosophy
7
Hillman, Timothy R. "Microstructural information beyond the resolution limit : studies in two coherent, wide-field biomedical imaging systems." University of Western Australia. School of Electrical, Electronic and Computer Engineering, 2008. http://theses.library.uwa.edu.au/adt-WU2008.0085.
8
Radlinska, Barbara. "«In vivo» imaging of microstructural and molecular neuroplasticity of fibre tracts in human subcortical stroke." Thesis, McGill University, 2013. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=119394.
Abstract:
Background Studies in acute and chronic post-stroke recovery suggest that both microstructural and molecular changes in stroke-affected fibre tracts are associated with clinical outcome. These processes can be measured noninvasily in the living human brain using Diffusion Tensor Imaging (DTI) and Positron Emission Tomography (PET), and together these neuroimaging techniques provide a detailed multimodal approach to the assessment of post-stroke neuroplasticity. With four controlled prospective studies, this thesis addresses direct and indirect microstructural and neuroinflammatory changes as they progress over time along infarct-affected fibres following subcortical ischaemic stroke of the Pyramidal tract (PT). Methods Patients with subcortical ischaemic stroke either affecting the PT (PT-group) or not (NonPT-group), as well as matched controls with Transient Ischaemic Attack (TIA), underwent both DTI and 11C-[R]-PK11195 PET at 3 weeks and 6 months post-stroke. From the DTI data, the PT (directly affected by the stroke), Callosal Motor Fibres (CMF; indirectly affected by the stroke) and Callosal Occipital Fibres (COF; not affected by stroke) were delineated using deterministic tractography. Fractional Anisotropy (FA) ratios (affected/unaffected hemisphere) were calculated for each tract. FA ratios (rFAPT; rFACMF; rFACOF) were compared within and between groups at both time-points, as well as anterograde and retrograde to the infarct. These ratios were then correlated with clinical outcome measures. From PET data, tracer uptake ratios (URs, affected/unaffected hemisphere) were determined for a set of standardised volumes of interest (VOIs) along the PT. These molecular markers of neuroinflammation were correlated with fibre tract integrity anterograde and retrograde to the infarct and with clinical outcome measures. Results DTI data analyses revealed that mean rFAPT in the PT-group was significantly lower than both NonPT and TIA groups initially and at follow-up, and correlated significantly with clinical outcome measures. PT-group rFACMF decreased over time. At follow-up, PT-group rFACMF was significantly lower than NonPT-group rFACMF and PT-group rFACOF. PT-group rFACMF at follow-up correlated with rFAPT retrograde to the infarct. PET data analyses revealed that PT-group uptake ratios were significantly increased at the level of the infarct and anterograde initially, but only anterograde to the infarct at follow-up. Anterograde uptake ratios were correlated with anterograde rFAPT at the initial time-point, whilst uptake ratios in the infarct were correlated with anterograde rFAPT only at follow-up. After controlling for PT damage, initial brainstem uptake ratios showed a positive correlation with clinical outcome, whereas follow-up uptake ratios in the infarct tended to be negatively correlated. Conclusions Overall, significant progressive changes in both microstructural and molecular neuroimaging parameters can be seen along fibres that have been either directly or indirectly affected by subcortical ischaemic stroke. Changes in fibre integrity, as modelled with DTI, are apparent both anterograde and retrograde to the ischaemic lesion, challenging the notion that decreased FA reflects Wallerian degeneration. This is underscored by the fact that decreases in FA are also apparent in regions with only indirect connections to the infarct. Molecular markers of neuroinflammation are present only in the area of the lesion itself and anterograde to the infarct and are therefore more likely to be associated with Wallerian degeneration. Taken together, DTI and PET provide a clinically meaningful assessment of neuroplasticity in the acute and chronic post-stroke phases.Contexte Les recherches dans le domaine de la récupération suite à un accident vasculaire cérébral (AVC) suggèrent que les changements morphologiques comme moléculaires sont associés à des résultats cliniques. Ces changements peuvent être mesurés de façon non invasive chez l'être humain grâce à l'utilisation de l'Imagerie par Tenseur de Diffusion (ITD) et de la Tomographique par Émission de Positrons (TEP). Au travers de quatre études prospectives contrôlées, cette thèse s'attache à décrire les changements microstructuraux et neuroinflammatoires au cours de leur progression le long des fibres du Faisceau Pyramidal (FP) affectées par un AVC sous-cortical ischémique. Méthodes Des patients présentant un AVC ischémique sous-cortical, qu'il affecte le Faisceau Pyramidal (groupe FP) ou non (groupe NonFP), ainsi que des participants contrôles ayant présenté un Accident Ischémique Transitoire (AIT) se sont vu proposer des examens de ITD et TEP11C-[R]-PK11195 3 semaines et 6 mois après leur AVC. Concernant les données issues de l'examen par ITD, le FP (affecté directement par l'AVC), les Fibres Calleuses Motrices (FCM; affectées indirectement par l'AVC) et les Fibres Calleuses Occipitales (FCO; non affectées par l'AVC) furent délimitées grâce à l'utilisation de la tractographie. Les ratios (l'hémisphère affecté/non affecté) déterminés par l'Anisotropie Fractionnelle (AF) furent calculés. Ces ratios (rAFFP;rAFFCM; rAFFCO) furent comparés à 3 semaines puis 6 mois post-AVC et également analysés selon leur orientation (antérograde ou rétrograde) par rapport à la lésion, et mis en relation avec des résultats cliniques. Concernant les données issues de l'examen de TEP, le ratio d'absorption de la dose traceuse fut déterminé pour un ensemble de zones d'intérêt standardisées le long du FP. Ces marqueurs moléculaires de neuroinflammation furent mis en relation avec l'intégrité des faisceaux, ainsi qu'avec des résultats cliniques. Résultats Les analyses de l'ITD ont révélé que le ratio moyen rAFFP dans le groupe FP était significativement moins élevé que pour les groupes NonFP comme AIT, à 3 semaines et à 6 mois de l'AVC, et que cela était corrélé à des résultats cliniques. Le ratio rAFFCM du groupe FP a décru avec le temps. 6 mois post AVC, le ratio rAFFCM du groupe FP était significativement moins élevé que celui du groupe NonFP comme celui rAFFCO du groupe FP. Le ratio rAFFCM du groupe FP à 6 mois était corrélé à celui rAFFP rétrograde de la zone lésée. Les analyses des données issues de l'examen de TEP ont révélé que les ratios d'absorption du traceurétaient significativement plus élevés au niveau de la zone lésée et dans le sens antérograde 3 semaines post-AVC pour le groupe FP, mais uniquement dans le sens antérograde à la lésion 6 mois post-AVC. Les ratios dans le sens antérograde étaient corrélés avec le ratio rAFFP antérograde 3 semaines post-AVC, alors que les ratios dans la lésion étaient corrélés avec le ratio antérograde rAFFP seulement 6 mois post-AVC. Après avoir contrôlé la présence de dommages sur le faisceau pyramidal, les ratios initiaux d'absorption au niveau du tronc cérébral présentèrent une corrélation positive avec les résultats cliniques, bien que les ratios au niveau de la lésion tendaient à être négativement corrélés. Conclusions Dans l'ensemble, des changements significatifs concernant les fibres affectées directement ou indirectement par un AVC sous-cortical ischémique peuvent être observés au niveau morphologique comme moléculaire grâce à la neuroimagerie. Des changements dans l'intégrité de la fibre, comme démontrés par l'ITD, sont mis en évidence dans les directions antrérograde comme rétrograde à la zone cérébrale lésée, contestant ainsi la notion qu'une faible Anisotropie Fractionnelle reflète une dégénération Wallérienne. Les marqueurs moléculaires de neuroinflammation sont présents uniquement dans la zone lésée elle-même, et dans les fibres antérogrades.
9
Gongvatana, Assawin. "Microstructural white matter integrity in HIV-infected individuals in the HAART era a diffusion tensor imaging study /." Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC campuses, 2008. http://wwwlib.umi.com/cr/ucsd/fullcit?p3316192.
Abstract:
Thesis (Ph. D.)--University of California, San Diego and San Diego State University, 2008.Title from first page of PDF file (viewed September 4, 2008). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical references (p. 81-94).
10
Chappell, Michael Hastings. "Developments in the use of diffusion tensor imaging data to investigate brain structure and connectivity." Thesis, University of Canterbury. Physics and Astronomy, 2007. http://hdl.handle.net/10092/1476.
Abstract:
Diffusion tensor imaging (DTI) is a specialist MRI modality that can identify microstructural changes or abnormalities in the brain. It can also be used to show fibre tract pathways. Both of these features were used in this thesis. Firstly, standard imaging analysis techniques were used to study the effects of mild, repetitive closed head injury on a group of professional boxers. Such data is extremely rare, so the findings of regions of brain abnormalities in the boxers are important, adding to the body of knowledge about more severe traumatic brain injury. The author developed a novel multivariate analysis technique which was used on the same data. This new technique proved to be more sensitive than the standard univariate methods commonly used. An important part of diagnosing and monitoring brain damage involves the use of biomarkers. A novel investigation of whether diffusion parameters obtained from DTI data could serve as bio-markers of cognitive impairment in Parkinson's disease was conducted. This also involved developing a multivariate approach, which displayed increased sensitivity compared with any of the component parameters used singly, and suggested these diffusion measures could be robust bio-markers of cognitive impairment. Fibre tract connectivity between regions of the brain is also a potentially valuable measure for diagnosis and monitoring brain integrity. The feasibility of this was investigated in a multi-modal MRI study. Functional MRI (fMRI) identifies regions of activation associated with a particular task. DTI can then find the pathway of the fibre bundles connecting these regions. The feasibility of using regional connectivity to interrogate brain integrity was investigated using a single healthy volunteer. Fibre pathways between regions activated and deactivated by a working memory paradigm were determined. Though the results are only preliminary, they suggest that this line of research should be continued.
11
Young, Bob Neill. "Experimental neuropsychological tests of feature ambiguity, attention and structural learning : associations with white matter microstructural integrity in elderly with amnesic and vascular mild cognitive impairment." Thesis, University of Canterbury. Psychology, 2014. http://hdl.handle.net/10092/9367.
Abstract:
Mild cognitive impairment (MCI) is a transition phase between normal aging and Alzheimer’s disease. Individuals with MCI show impairment in cognition as well as corresponding damage to areas of their brain. Performance on tasks such as discriminating objects with ambiguous features has been associated with damage to the perirhinal cortex, while scenes with structural (spatial) elements have been associated with damage to the hippocampus. In addition, attention is regarded as one of the first non-memory domains to decline in MCI. A relatively new MRI technique called diffusion tensor imaging (DTI) is sensitive to white matter microstructural integrity and has been associated with changes due to cognitive decline. 18 MCI (14 amnesic, 4 vascular) and 12 healthy matched controls were assessed in feature ambiguity, attention and structural learning to assess associated deficits in MCI. Associations with white matter microstructural integrity were then investigated. The MCI groups were discovered to perform worse than controls on the test of structural learning. In addition, altered attention networks were found in MCI and were associated with white matter microstructural integrity. No significant differences were found for feature ambiguity. These findings suggest there may be specific damage to the hippocampus while the perirhinal cortex may be preserved in MCI. Furthermore, dysfunction in attention was found to be associated with white matter microstructural integrity. These experimental tests may be useful in assessing dysfunction in MCI and identifying degeneration in white matter microstructural integrity. Further studies with larger sample sizes are needed to validate these findings.
12
Yang, Zheyi. "Numerical methods to estimate brain micro-structure from diffusion MRI data." Electronic Thesis or Diss., Institut polytechnique de Paris, 2023. http://www.theses.fr/2023IPPAE016.
Abstract:
L'imagerie par résonance magnétique de diffusion (IRM de diffusion) est une modalité d'imagerie non invasive couramment utilisée pour mesurer les propriétés micro-structurales des tissus biologiques au dessous de la résolution spatiale, en mesurant indirectement le déplacement de diffusion des molécules d'eau. En raison de la complexité géométrique du cerveau et du mécanisme complexe de l'IRM de diffusion, il est difficile de relier directement les signaux reçus à des paramètres biophysiques significatifs, tels que le diamètre des axones ou la densité. Ces dernières années, plusieurs modèles biophysiques ont été introduits pour répondre à ce problème de la faible interprétabilité. Ces modèles représentent les signaux d'IRM de diffusion comme un mélange de signaux analytiques sous certaines hypothèses, par exemple des membranes imperméables, de différentes géométries simples et non connectées, par exemple des sphères et des bâtonnets. Par la suite, ils visent à extraire les paramètres de ces géométries simples, qui sont corrélés avec des paramètres biophysiques, en inversant la formulation analytique. Cependant, la validité de ces hypothèses reste indéterminée dans les expériences réelles. L'objectif de cette thèse est d'améliorer la fiabilité et l'efficacité de l'estimation de la microstructure par deux moyens. Tout d'abord, pour faciliter l'étude quantitative de la domaine de validité des modèles biophysiques et de l'effet de la déformation géométrique et de la perméabilité de la membrane cellulaire par simulation, nous avons proposé deux modèles réduits dérivés de l'équation de Bloch-Torrey, respectivement. Dans le cas de membranes perméables, une nouvelle approche de simulation utilisant une base propre de Laplace imperméable est proposée. Quant à la déformation géométrique, nous utilisons une expansion asymptotique par rapport aux angles de déformation pour approximer le signal. Ces deux modèles réduits permettent de faire les calculs efficaces des signaux pour diverses valeurs de déformation/perméabilité. Des simulations numériques montrent que ces deux modèles peuvent rapidement calculer les signaux avec un niveau d'erreur raisonnable par rapport aux méthodes existantes. Plusieurs études ont été menées sur les effets de la perméabilité et de la déformation sur les signaux ou sur le coefficient de diffusion efficace (ADC en anglais), en utilisant les modèles proposés. Deuxièmement, au lieu d'inverser un modèle de géométries simplifiées, nous présentons une nouvelle approche pour associer la taille des somas dans la matière grise par des biomarqueurs intermédiaires. Des simulations numériques identifient une corrélation entre le diamètre/densité des somas et le point d'inflexion des signaux moyennés sur la direction à des amplitudes élevées (b>2500s/mm^2), offrant des perspectives pour l'estimation de la microstructure. Nous adaptons un réseau neuronal entièrement connecté en utilisant ces biomarqueurs et comparé aux modèles biophysiques, cette approche offre des résultats comparables sur les données synthétiques et in vivo et une estimation rapide car aucune inversion n'est impliquéeDiffusion magnetic resonance imaging (diffusion MRI) is a widely used non-invasive imaging modality to probe the micro-structural properties of biological tissues below the spatial resolution, by indirectly measuring the diffusion displacement of water molecules. Due to the geometrical complexity of the brain and intricate diffusion MRI mechanism, it is challenging to directly link the received signals to meaningful biophysical parameters, such as axon radii or volume fraction.In recent years, several biophysical models have been introduced to address the issue of weak interpretability. These models represent the diffusion MRI signals as a mixture of analytical signals under certain assumptions, e.g. impermeable membranes, of various disconnected simple geometries, such as spheres and sticks. Subsequently, they aim to extract the parameters of these geometries, which correlate with biophysical parameters, by inverting the analytical expression.However, the validity of these assumptions remains undetermined in actual experiments.The objective of this thesis is to improve the microstructure estimation reliability and efficiency from two perspectives. First, to facilitate the quantitative study of the valid range of biophysical models and the effect of geometrical deformation and cell membrane permeability via simulation, we proposed two reduced models derived from the Bloch-Torrey equation, respectively. For the case of the presence of permeable membranes, a new simulation approach using impermeable Laplace eigenbasis is proposed. As for the geometrical deformation, we use an asymptotic expansion with respect to the deformation angles to approximate the signal. These two reduced models enable efficient computation of signals for various values of deformation/permeability. Numerical simulations reveal that these two models can fast compute the signals within a reasonable error level compared to existing methods. Several studies have been conducted about the effects of permeability and deformation on the signals or the apparent diffusion coefficient (ADC), using the proposed models.Second, instead of inverting a simplified geometries model, we present a novel approach to associate soma size in gray matter by intermediary biomarkers. Numerical simulations identify a correlation between the volume-weighted soma radius/volume fraction and the inflection point of direction-averaged signals at high b-values (b>2500s/mm^2), offering insights for microstructure estimation. We fit a fully connected neural network using these biomarkers and compared to biophysical models, this approach offers comparable results on both synthetic and in vivo data and fast estimation since no inversion is involved
13
Schmitz, Birte [Verfasser], Xiaoqi [Akademischer Betreuer] Ding, and Karin [Akademischer Betreuer] Weißenborn. "Investigation of metabolic and microstructural alterations in human brain under physiological and pathological conditions by using magnetic resonance imaging and 1H and 31P magnetic resonance spectroscopy / Birte Schmitz ; Akademische Betreuer: Xiaoqi Ding, Karin Weißenborn ; Institut für Diagnostische und Interventionelle Neuroradiologie." Hannover : Bibliothek der Medizinischen Hochschule Hannover, 2020. http://d-nb.info/1225413656/34.
14
Zhang, Zhongping, and 张忠平. "Quantitative in vivo assessment of tissue microstructure using diffusion tensor and kurtosis imaging." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2011. http://hub.hku.hk/bib/B4694395X.
15
O'Connell, Mary Kathleen. "Understanding abdominal aortic aneurysm progression through three-dimensional microstructure imaging /." May be available electronically:, 2007. http://proquest.umi.com/login?COPT=REJTPTU1MTUmSU5UPTAmVkVSPTI=&clientId=12498.
16
Winkler, Amy. "OPTICAL METHODS FOR MOLECULAR SENSING: SUPPLEMENTING IMAGING OF TISSUE MICROSTRUCTURE WITH MOLECULAR INFORMATION." Diss., The University of Arizona, 2010. http://hdl.handle.net/10150/195176.
Abstract:
More and more researchers and clinicians are looking to molecular sensing to predict how cells will behave, seeking the answers to questions like "will these tumor cells become malignant?" or "how will these cells respond to chemotherapy?" Optical methods are attractive for answering these questions because optical radiation is safer and less expensive than alternative methods, such as CT which uses X-ray radiation, PET/SPECT which use gamma radiation, or MRI which is expensive and only available in a hospital setting. In this dissertation, three distinct optical methods are explored to detect at the molecular level: optical coherence tomography (OCT), laser-induced fluorescence (LIF), and optical polarimetry. OCT has the capability to simultaneously capture anatomical information as well as molecular information using targeted contrast agents such as gold nanoshells. LIF is less useful for capturing anatomical information, but it can achieve significantly better molecular sensitivity with the use of targeted fluorescent dyes. Optical polarimetry has potential to detect the concentration of helical molecules, such as glucose. All of these methods are noninvasive or minimally invasive.The work is organized into four specific aims. The first is the design and implementation of a fast, high resolution, endoscopic OCT system to facilitate minimally invasive mouse colon imaging. The second aim is to demonstrate the utility of this system for automatically identifying tumor lesions based on tissue microstructure. The third is to demonstrate the use of contrast agents to detect molecular expression using OCT and LIF. The last aim is to demonstrate a new method based on optical polarimetry for noninvasive glucose sensing.
17
Pan, Yi. "Terahertz time-domain spectroscopy and near-field imaging of microstructured waveguides." Thesis, University of Bath, 2013. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.607613.
Abstract:
This thesis presents studies of novel terahertz photonic devices, including photoconductive optoelectronic devices and guided-wave components, aimed at the development of next-generation terahertz systems. In chapter 2, a scalable interdigitated THz transmitter is designed to increase the output power and compared with a conventional 50 μm coplanar transmitter. In chapter 3, we compare four different receivers with different antenna geometries in terms of bandwidth and sensitivity. Then we describe a photoconductive near-field detector with a subwavelength aperture and its system integration and characterization. In chapter 4, a parallel metal plate waveguide is designed with an integrated step inside the waveguide that can couple to higher order TM modes efficiently from the TEM mode. In this chapter, we also experimentally and numerically study a 2-dimensionally tapered parallel plate waveguide, by which a free-space THz beam can be focused into a deep subwavelength-scale volume. In chapter 5, a parallel thin dielectric film waveguide is used to explore the guiding mechanism of an antiresonant optical reflection waveguide. Cylindrical silica single capillaries and a microstructured capillary, which guide in a similar way, are characterized in terms of mode profiles and attenuation. In chapter 6, we study oblique transmission through freestanding thin nickel films, which are perforated with periodic conical hole arrays. Surface modes can be supported by both metallic surfaces with different nonlinear dispersion curves, which results in spectral interferences in a near-field region when the surface modes couple out of the waveguide into free space.
18
Ma, Lin. "Multi-scale 3D imaging of the microstructure in organic-rich shales." Thesis, University of Manchester, 2016. https://www.research.manchester.ac.uk/portal/en/theses/multiscale-3d-imaging-of-the-microstructure-in-organicrich-shales(514544f2-39f8-4fe2-b8f4-fecb27380c10).html.
Abstract:
Technological advances in horizontal drilling and hydraulic fracturing have paved the way for the exploration and production of shale gas and shale oil, the fastest growing energy sector globally. The imaging and quantification of the geometry, sizes, network and distribution of extremely fine-grain minerals, organic matter and pores are a significant component for the macroscopic and microscopic characterisation of shale reservoirs but is also highly challenging. X-ray computed tomography (XCT) combined with 3D Electron Microscopy (EM) are used to address this challenge and give us information in 3D from multiple length scales over 3 orders of magnitudes: mesoscale (R1), microscale (R2), submicron-scale (R3), low-resolution nanoscale (R4) and high-resolution nanoscale (R5) with spatial resolutions of ~10micro metre, ~1micro metre, ~130 nm, ~50nm and ~5nm, respectively. The multi-scale imaging and quantification method was initially applied here to the Carboniferous Bowland Shale, the largest potential shale gas play in the UK. The appropriate length scales (both field of view and voxel size) of specified phases such as pores, organic matter, clay minerals and non-clay minerals were analysed. The low connectivity of pores and high connectivity of organic matter suggests that the 20 nm and larger pores imaged did not form connected flow paths, demonstrating that porous gas flow through this sample cannot be the main transport mechanism and diffusive transport through the organic matter and clay minerals must also be considered. Then, the variation of organic matter and pore distribution along a TOC gradient were analysis on Lublin gas-mature shale samples in Poland and Baltic oil-mature shale samples in Lithuania. The results show intergranular pores dominated in this series of samples, including organic interface pores and inter-mineral pores, which further confirm that organic matter is not the primary influencing factor for porosity, but the clay minerals. Finally, a novel multi-stage workflow of pore system is proposed relying on both image quantification and numerical modelling of geological features with studies in Jurassic Haynesville shale in the US. Three stages are divided according to pore variation, mineral variation and microfacies variation across four distinct length scales (R1-R4/R5), and permeability was simulated based on the upscaled pore system. The final computed porosity and permeability shows acceptable errors when compared with the helium porosity and press decay permeability. Beyond the lab measurements, the pore occurrence and size distribution were computed in the upscaling process. The combining of XCT and 3D-EM provides a powerful tool for the multi-scale imaging and quantification of microstructural information in shales, allowing the visualization of pores, organic matter and inorganic mineral phases over a range of scales over three orders of magnitude (~ 10 micro metre to ~ 5 nm), and the volume fraction of each phases shows a reasonable correlation to traditional physical and chemistry quantification data. The further studies, such as the variation of organic matter and pores, upscaling of porosity and permeability presented in this study, has verified the feasibility of the proposed multi-scale method and promises a bit potential for reservoir prediction and other challenges in geological studies.
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Hiremath, Pranoti. "Identifying Changes in Myocardial Microstructure via a Novel Sonographic Imaging Algorithm." Thesis, Harvard University, 2015. http://nrs.harvard.edu/urn-3:HUL.InstRepos:15821586.
Abstract:
We developed a novel ultrasound-based image analysis algorithm designed to differentiate microstructural characteristics of left ventricular (LV) myocardium. The algorithm analyzes sonographic signal distributions and produces a marker termed the signal intensity coefficient (SIC), which can serve as an enhanced surrogate measure of myocardial microstructure. We evaluated our algorithm in two disease processes that are characterized by progressive LV remodeling from microstructural to global myocardial changes: hypertensive heart disease and hypertrophic cardiomyopathy.
Results demonstrate that the SIC was significantly higher in hypertensive compared to non-hypertensive myocardium in both mice and humans, and was positively associated with increasing levels of exposure to afterload stress in humans and mice. Furthermore, in a cohort of sarcomere mutation carriers with different phenotypes of HCM, the SIC was able to distinguish between individuals with overt HCM, subclinical HCM, and healthy controls. The SIC demonstrated stronger associations with both degree of blood pressure and MRI-based ECV compared to established echocardiographic measures of adverse LV remodeling. Overall, our results demonstrate the potential of an imaging algorithm to identify the presence and extent of microstructural changes that can arise early in development of cardiac remodeling, in response to chronic exposure to afterload stress as well as genetic mutations.
20
Hudson, Kristen Kay. "Development of imaging methods to quantify the laminar microstructure in rat hearts." Thesis, Texas A&M University, 2004. http://hdl.handle.net/1969.1/1271.
Abstract:
The way in which the myocardium responds to its mechanical environment must be understood in order to develop reasonable treatments for congestive heart failure. The first step toward this understanding is to characterize and quantify the cardiac microstructure in healthy and diseased hearts. Myocardium has a laminar architecture made up of myolaminae, which are sheets of myocytes surrounded by a collagen weave. By enhancing the contrast between the myocytes and the surrounding collagen, the myocardium can be investigated and its laminar structure can be quantified. Many of the techniques that have been used to view the microstructure of the heart require the use of toxic or caustic chemicals for fixation or staining. An efficient imaging method that uses polarization microscopy and enhances the contrast between the collagen and myocytes while minimizing the use of harmful chemicals was developed in this research. Collagen is birefringent; therefore its visibility should be enhanced through polarization microscopy and image processing. The sheet angles were viewed directly by cutting slices of a rat septum perpendicular to the fiber angle. Images of different polarization combinations were taken and a region of interest was selected on the sample. Image processing techniques were used to reduce the intensity variation on the images and account for the variable gain of the camera. The contrast between the collagen and myocytes was enhanced by comparing adjusted images to the background and looking at a single image this comparison produced. Although the contrast was enhanced, the embedding media reduced the collagen signal and the enhancement was not as striking as expected.
21
Doherty, Sharon Marie. "Advanced imaging and 3D measurement of microstructure in food emulsions and foams." Thesis, University of Ulster, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.551598.
Abstract:
Standard commercially available food emulsions and foams, mayonnaise and ice cream respectively, have been examined by both ambient temperature transmission electron microscopy (TEM) and cryo-Dualbeam FIB/SEM and analysed by the 'One Stop Stereological' (OSS) method. In addition a series of specially produced mayonnaise samples with differing viscosities were examined and compared with the standard full fat and half fat mayonnaise samples. Furthermore a sub-optimally produced ice cream specimen was compared with the standard ice cream. Specimens were compared for quality of qualitative information in electron micrographs in both TEM and cryo-Dualbeam FIB/SEM following either chemical fixation for TEM of cryo preservation for SEM and FIB milling. Quantitative 3D structural geometrical data was obtained through the use of ass, on the Mercator (ExploraNova) software system. This estimates 1 st the order stereological quantities volume and surface density and star volume. In addition it gives the 2nd order covariance and cross covariance estimators. In mayonnaise oil, water and citrus fibres were measured. In ice cream air, ice and a continuous matrix were measured. The findings concluded that 1) cryo Dualbeam ™ (FIB/SEM)is superior to non-cryo TEM when measurement of structural geometry is planned; 2) stereo logical measurements are sensitive in detecting minimal changes about the structure function arrangements within food foams and emulsions; 3) the use of grazing incidence FIB planing considerably enhances the efficiency of specimen preparation; 4) a series of mayonnaise samples with different viscosities when analysed led to results that appear to be directly related to their differing physical properties; 5) comparison of ice cream, as in 4) above, with samples prepared optimally and sub-optimally detected significant differences in their 3D structure; 6) the use of 1 st and 2nd order stereological techniques are set to increase in use for analysing food emulsions and foams with respect to production control.
22
Panagiotaki, E. "Geometric models of brain white matter for microstructure imaging with diffusion MRI." Thesis, University College London (University of London), 2011. http://discovery.ucl.ac.uk/1310435/.
Abstract:
The research presented in this thesis models the diffusion-weighted MRI signal within brain white matter tissue. We are interested in deriving descriptive microstructure indices such as white matter axon diameter and density from the observed diffusion MRI signal. The motivation is to obtain non-invasive reliable biomarkers for early diagnosis and prognosis of brain development and disease. We use both analytic and numerical models to investigate which properties of the tissue and aspects of the diffusion process affect the diffusion signal we measure. First we develop a numerical method to approximate the tissue structure as closely as possible. We construct three-dimensional meshes, from a stack of confocal microscopy images using the marching cubes algorithm. The experiment demonstrates the technique using a biological phantom (asparagus). We devise an MRI protocol to acquire data from the sample. We use the mesh models as substrates in Monte-Carlo simulations to generate synthetic MRI measurements. To test the feasibility of the method we compare simulated measurements from the three-dimensional mesh with scanner measurements from the same sample and simulated measurements from an extruded mesh and much simpler parametric models. The results show that the three-dimensional mesh model matches the data better than the extruded mesh and the parametric models revealing the sensitivity of the diffusion signal to the microstructure. The second study constructs a taxonomy of analytic multi-compartment models of white matter by combining intra- and extra-axonal compartments from simple models. We devise an imaging protocol that allows diffusion sensitisation parallel and perpendicular to tissue fibres. We use the protocol to acquire data from two fixed rat brains, which allows us to fit, study and evaluate the models. We conclude that models which incorporate non-zero axon radius describe the measurements most accurately. The key observation is a departure of signals in the parallel direction from the two-compartment models, suggesting restriction, most likely from glial cells or binding of water molecules to the membranes. The addition of the third compartment can capture this departure and explain the data. The final study investigates the estimates using in vivo brain diffusion measurements. We adjust the imaging protocol to allow an in vivo MRI acquisition of a rat brain and compare and assess the taxonomy of models. We then select the models that best explain the in vivo data and compare the estimates with those from the ex vivo measurements to identify any discrepancies. The results support the addition of the third compartment model as per the ex vivo findings, however the ranking of the models favours the zero radius intra-axonal compartments.
23
Stephenson, David E. "Microstructure and Transport Properties of Porous Li-ion Electrodes." BYU ScholarsArchive, 2011. https://scholarsarchive.byu.edu/etd/2752.
Abstract:
The goal of this work is to understand the relationships between electrode microstructure and mass transport resistances. One can use this information to predict cell performance from fundamental principles. This work includes new types of particle-scale 3D models for correlating and predicting the effects of electrode microstructure on both ionic and electronic transport. The 3D models imitate the sub-micrometer-scale arrangement of active material particles, carbon, binder, and pores and use FIB/SEM images as a basis for parameterization. The 3D models are based respectively on the statistical mechanics techniques of molecular dynamics and Monte Carlo. The approach closely related to molecular dynamics, named the dynamic particle packing (DPP) model, uses aggregates of spheres to recreate electrode microstructures. The other approach, named the stochastic grid (SG) model, is closely related to Monte Carlo techniques in which a small set of fundamental interdomain and bulk energy parameters are used to generate structures.In order to predict electrode microstructures we correlated the fundamental interdomain and bulk energy parameters for the SG model to electrode mass composition and porosity. We used the revised computer program, known as predict SG, to estimate structures of which there are no experimental measurements of electrode structure. From these predicted electrode structures we obtained electronic and ionic transport properties. This allowed us to estimate the trade-offs between ionic and electronic transport for different porosities and carbon fractions. We found from experimental measurements of electrode structure that carbon and binder formed distinct agglomerates. From the 3D models we determined at commercial fractions of carbon and binder that the conductivity of these carbon agglomerates plays a large role in determining both the electronic and ionic pathways. So in order to better understand the role that these carbon/binder agglomerates play, we explored and developed several experimental methods to find the electronic and ionic conductivity of both simulated carbon domains and complete electrode films. The goal was not only to elucidate the role carbon agglomerates play, but also to develop a non-destructive method of determining overall film properties. Although we found that a non-destructive method is extremely challenging due to probe contact resistances, we did find success in determining carbon domain properties using a delamination method.
24
Razavi, Mohammad Reza. "Characterization of microstructure and internal displacement field of sand using X-ray computed tomography." Online access for everyone, 2006. http://www.dissertations.wsu.edu/Dissertations/Fall2006/M_Razavi_121206.pdf.
25
Cuevas, Assunta Mariela. "Microstructure characterization of friction-stir processed nickel-aluminum bronze through orientation imaging microscopy." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 2002. http://library.nps.navy.mil/uhtbin/hyperion-image/02sep%5FCuevas.
26
McHugh, Damien Joseph. "The effect of tumour microstructure on diffusion-weighted MRI measurements." Thesis, University of Manchester, 2015. https://www.research.manchester.ac.uk/portal/en/theses/the-effect-of-tumour-microstructure-on-diffusionweighted-mri-measurements(9821717e-df69-4dd0-baf7-51cf27a18aa2).html.
Abstract:
By sensitising the magnetic resonance signal to the diffusion of water molecules in tissue, diffusion-weighted magnetic resonance imaging provides a means of assessing tumour microstructure non-invasively. Such measurements have the potential to provide important information about tumour development and the response of tumours to treatment, but the way in which different tissue properties affect the diffusion-weighted signal remains unclear. Through simulations, in vivo studies and phantom experiments, this thesis investigates the relationship between the diffusion-weighted signal, the pulse sequence parameters used for acquisition, and microstructural properties of tumours. The use of oscillating gradient pulse sequences on a clinical scanner was investigated initially, with theoretical and practical considerations leading subsequent work to focus on pulsed gradient sequences. The forward problem of predicting the diffusion-weighted signal for given combinations of tissue properties and sequence parameters was addressed numerically through Monte Carlo simulations, focussing on how tumour cell size, intracellular volume fraction and membrane permeability affect the signal. These simulations allowed the sensitivity of the signal to changes in these tissue properties to be investigated, revealing how sensitivity depends on sequence parameters as well as the specific microstructural configuration. By repeating the simulations using the specific sequence parameters used in a clinical and preclinical study, the sensitivity of the implemented protocols was assessed, and linked to the experimental findings. The preclinical study illustrated the importance of the diffusion time in determining the sensitivity to treatment-induced changes in tumours, with larger post-treatment signal changes observed at longer diffusion times. These trends were qualitatively reflected in the sensitivity analysis derived from the simulations. Finally, the inverse problem of estimating microstructural properties from the diffusion-weighted signal was addressed using a physical phantom designed as a simple mimic of tumour tissue. By fitting a biophysical model to the diffusion data, the size and volume fraction of the approximately spherical 'cells' were estimated. The radius was slightly underestimated compared with that determined from independent measurements, the fitted volume fraction was plausible, and parameters were found to be estimated with reasonably good precision.
27
Novello, Lisa. "Towards Improving the Specificity of Human Brain Microstructure Research with Diffusion-Weighted MRI." Doctoral thesis, Università degli studi di Trento, 2022. http://hdl.handle.net/11572/342277.
Abstract:
The possibility to perform virtual, non-invasive, quantitative, in vivo histological assessments might revolutionize entire fields, among which clinical and cognitive neurosciences. Magnetic Resonance Imaging (MRI) is an ideal non-invasive imaging technique to achieve these goals. Tremendous advancements in the last decades have favored the transition of MRI scanners from “imaging devices” to “measurement devices” (Novikov, 2021), thus capable to yield measurements in physical units, which might be further combined to provide quantities describing histological properties of substrates. A central role in this community endeavor has been played by diffusion-weighted MRI (dMRI), which by measuring the dynamics of spin diffusion, allows inferences on geometrical properties of tissues. Yet, conventional dMRI methodologies suffer from poor specificity. In this thesis, techniques aiming at improving the specificity of microstructural descriptions have been explored in dMRI datasets supporting an increasing level of complexity of the dMRI signal representations. Applications in individuals with different age range, in different populations, and for different MRI scanner fields, have been considered. Firstly, tractography has been combined with Diffusion Tensor Imaging (DTI), an along-tract framework, and morphometry, in the study of the microstructure of the optic radiations in different groups of blind individuals. Secondly, DTI has been combined with Free-Water Imaging (FWI) to monitor the effect of proton-irradiation in a pediatric brain tumor case study. Thirdly, FWI and Diffusion Kurtosis Imaging (DKI) have been combined with an advanced thalamic segmentation framework to study the associations between motor performance and thalamic microstructure in a cohort of individuals affected by Parkinson’s disease. Finally, the largest contribution of this thesis is represented by the adaptation of the Correlation Tensor Imaging - a technique increasing the specificity of DKI harnessing Double Diffusion Encoding previously applied only in preclinical settings - for a clinical 3 T scanner. The ensuing investigation revealed new important insights on the sources of diffusional kurtosis, in particular of the microscopic kurtosis (μK), a component so far neglected by contemporary neuroimaging techniques, which might carry an important clinical role (Alves et al., 2022), and can now be accessed by clinical scanners. In conclusion, strategies to increase the specificity of microstructural descriptions in the brain are presented for different datasets, and their strength and limitations are discussed.
28
Yang, Xuan. "Three-dimensional Characterization of Inherent and Induced Sand Microstructure." Diss., Georgia Institute of Technology, 2005. http://hdl.handle.net/1853/7557.
Abstract:
In the last decade, a significant amount of research has been performed to characterize the microstructure of unsheared and sheared triaxial sand specimens to advance the understanding of the engineering behavior of soils. However, most of the research has been limited to two-dimensional (2-D) image analysis of section planes that resulted in loss of information regarding the skeleton of the soil (pore structure) and other attributes of the three-dimensional (3-D) microstructure. In this research, the 3-D microstructures of triaxial test specimens were, for the first time, characterized. A serial sectioning technique was developed for obtaining 3-D microstructure from 2-D sections of triaxial test specimens. The mosaic technique was used to get high-resolution large field of view images. Various 3-D characterization parameters were used to study the microstructures of the specimens.
To study the preparation method induced variation in soil microstructure, two specimens prepared with air pluviation and moist tamping methods were preserved with epoxy impregnation. A coupon was cut from the center of each specimen, and following a serial sectioning and image capture process, the 3-D structure was reconstructed. To study the evolution of structure during shearing tests, two additional specimens prepared to the same initial conditions with the same methods were subjected to axial compression loading under constant confining pressure up to an axial strain level of 14%. After shearing, the structure of these specimens were also preserved and analyzed following the same procedures as the unsheared specimens. The evolution of the pore structures was investigated accordingly.
It was found that generally, moist tamped specimens were initially less uniform but had a more isotropic structure than air pluviated specimens. The standard deviations of 2-D local void ratio and 3-D pore size in dilated regions of sheared air pluviated and moist-tamped specimens were found to be smaller than those of as-consolidated specimens at a given void ratio. Tortuosity decreased with increasing pore size. It was also evident that the soil structures evolved differently depending on the initial structure. Comparison between 2-D and 3-D results indicated that it is not sufficient to use 2-D section information for characterizing some microstructural features.
29
Lacerda, Luis Miguel Rosa Sousa Prado De. "Quantitative white matter metrics : diffusion imaging and advanced processing for detailed investigation of brain microstructure." Thesis, King's College London (University of London), 2017. https://kclpure.kcl.ac.uk/portal/en/theses/quantitative-white-matter-metrics(9058c64a-93a0-4db0-9799-c0bba7bd55fe).html.
Abstract:
Diffusion imaging is a non-invasive imaging method which has been successfully applied to study white matter. Most clinical approaches, based on Diffusion Tensor Imaging (DTI), are limited by the simple model of the underlying tissue imposed, failing to reconstruct the diffusion propagator, which fully encodes the displacement of water molecules. To do so, more comprehensive sampling schemes such as Diffusion Spectrum Imaging (DSI) have been developed. In this thesis, I have investigated the effect of different tissue configurations, sampling and processing steps in the performance of DSI. I identified specific configurations where DSI is unable to characterise diffusion without artefacts, namely aliasing caused by fast diffusion components. Furthermore, processing of the diffusion orientation distribution function (ODF) in these environments can lead to generation of spurious fibres in tractography reconstructions. To overcome this, I have applied a novel step in the processing pipeline of DSI, namely a different way of computing the ODF, which consists of restricting the range of integration to probabilities based on the physical displacement of “axonlike” diffusivities. Alternatively, it is possible to use a mathematical representation of the acquired signal, of which the Simple Harmonic Oscillator based Reconstruction and Estimation (SHORE) and Mean Apparent Propagator Magnetic Resonance Imaging (MAP-MRI) are examples. I have here used these methods and further provided optimised acquisitions based on standard propagator metrics. Finally, I have introduced new metrics that use microstructural information available at the different displacement scales, and can facilitate exploration of brain organisation even when no a-priori biophysical model is available.
30
Ran, Peipei. "Imaging and diagnostic of sub-wavelength micro-structures, from closed-form algorithms to deep learning." Electronic Thesis or Diss., université Paris-Saclay, 2020. http://www.theses.fr/2020UPASG061.
Abstract:
Le test électromagnétique d’un ensemble fini en forme de grille de tiges diélectriques cylindriques circulaires infiniment longues dont certaines manquent est investigué à partir de données fréquence simple et multiple et en régime temporel. Les distances sous-longueur d’onde entre tiges adjacentes et des diamètres de tige de sous-longueur d’onde sont considérées sur toute la bande de fréquences d’opération et cela conduit à un défi majeur en raison du besoin de super-résolution dans la microstructure, bien au-delà du critère de Rayleigh. Tout un ensemble de méthodes de résolution est étudié et des simulations numériques systématiques illustrent avantages et inconvénients, complétées par le traitement de données expérimentales en laboratoire acquises sur un prototype de micro-structure en chambre anéchoïque micro-onde. Ces méthodes, qui diffèrent selon les informations a priori prises en compte et la polyvalence qui en résulte, comprennent retournement temporel, inversions de source de contraste, binaires ou parcimonieuses, ainsi que réseaux de neurones convolutifs éventuellement combinés avec des réseaux récurrentsElectromagnetic probing of a gridlike, finite set of infinitely long circular cylindrical dielectric rods affected by missing ones is investigated from time-harmonic single and multiple frequency data. Sub-wavelength distances between adjacent rods and sub-wavelength rod diameters are assumed throughout the frequency band of operation and this leads to a severe challenge due to need of super-resolution within the present micro-structure, well beyond the Rayleigh criterion. A wealth of solution methods is investigated and comprehensive numerical simulations illustrate pros and cons, completed by processing laboratory-controlled experimental data acquired on a micro-structure prototype in a microwave anechoic chamber. These methods, which differ per a priori information accounted for and consequent versatility, include time-reversal, binary-specialized contrast-source and sparsity-constrained inversions, and convolutional neural networks possibly combined with recurrent ones
31
Gibb, Matthew Michael James. "Myocardial microstructure and its role in propagation dynamics." Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:69a1a65e-9a71-422c-86e8-c347cfabf21a.
Abstract:
Computational modelling and simulation, in close interaction with experiments, has provided invaluable insight into the biochemical, mechanical and electrophysiological function and dysfunction of the heart. However, limitations in imaging techniques and computing resources have precluded the analysis of tissue architecture near the cellular scale and the effect of this architecture on cardiac function. It is the wider aim of this thesis to develop a framework to characterise cardiac microstructure and to investigate the role of microstructure in cardiac propagation dynamics and arrhythmogenesis. An initial modelling study elucidates the effect of blood vessels in sustaining arrhythmic episodes, and how the accurate modelling of fibre direction in the vicinity of the vessels mitigates this detrimental mechanism. A mathematical model of fibre orientation in a simple geometry around blood vessels has been developed, based on information obtained from highly detailed histological and MRI datasets. A simulation regime was chosen, guided by the vasculature extracted from whole heart MRI images, to analyse ventricular wavefront propagation for different orientations and positions of blood vessels. Our results demonstrate not only that the presence of the blood vessels encourages curvature in the activation wavefront around the blood vessels, but further that vessels act to restrict and prolong phase singularities. When compared to a more simplistic implementation of fibre orientation, the model is shown to weaken wavefront curvature and reduce phase singularity anchoring. Having established the importance of microstructural detail in computational models, it seems expedient to generate accurate data in this regard. An automated registration toolchain is developed to reconstruct histological slices based on coherent block face volumes, in order to present the first 3-D sub-cellular resolution images of cardiac tissue. Although mesoscopic geometry is faithfully reproduced throughout much of the dataset, low levels of transformational noise obfuscate tissue microstructure. These distortions are all but eradicated by a novel transformational diffusion algorithm, with characteristics that outperform any previous method in the literature in this domain, with respect to robustness, conservation of geometry and extent of information transfer. Progress is made towards extracting microstructural models from the resultant histological volumes, with a view to incorporating this detail into simulations and yielding a deeper understanding of the role of microstructure in arrhythmia.
32
Chao, Chien-Wei. "An Improved Dynamic Particle Packing Model for Prediction of the Microstructure in Porous Electrodes." BYU ScholarsArchive, 2015. https://scholarsarchive.byu.edu/etd/5632.
Abstract:
The goal of this work is to develop a model to predict the microstructure of Li-ion batteries, specifically focusing on the cathode component of the batteries. This kind of model has the potential to assist researchers and battery manufacturers who are trying to optimize the capacity, cycle life, and safety of batteries. Two dynamic particle packing (DPP) microstructure models were developed in this work. The first is the DPP1 model, which simulates the final or dried electrode structure by moving spherical particles under periodic boundaries using Newton's laws of motion. The experience derived from developing DPP1 model was beneficial in making the final model, called DPP2. DPP2 is an improved version of DPP1 that includes solvent effects and is used to simulate the slurry-coating, drying, and calendering processes. Two type of properties were used to validate the DPP1 and DPP2 models in this work, although not every property was used with the DPP1 model. First are the structural properties, which include volume fraction, and electronic and ionic conductivities. Experimental structural properties were determined by analyzing 2D cross sectional images of the battery cathodes. These images were taken through focused ion beam (FIB) planarization and scanning electron microscopy (SEM). The second category are the mechanical properties, which include film elasticity and slurry viscosity. These properties were measured through experiments executed by our group. The DPP2 model was divided into two submodels : active-free and active-composite. The 2D cross sectional images of the simulated structure of the models have a similar particle arrangements as the experimental structures. The submodels show reasonable agreement with the experimental values for liquid and solid mass density, shrink ratio, and elasticity. For the viscosity, both models show shear-thinning behavior, which is a characteristic of slurries. The volume fractions of the simulated structures of the active-free and active-composite models have better agreement with the experimental values, which is also reflected in the 2D cross sectional images of the structure.
33
Li, Shiu Fai Frankie. "Imaging of Orientation and Geometry in Microstructures: Development and Applications of High Energy X-ray Diffraction Microscopy." Research Showcase @ CMU, 2011. http://repository.cmu.edu/dissertations/59.
Abstract:
Near-field High Energy X-ray Diffraction Microscopy (HEDM) is a synchrotron based imaging technique capable of resolving crystallographic orientation in a bulk, polycrystalline material non-destructively. Recent advances in data acquisition and analysis methods have led to micron-scale spatial resolution and ≤ 0.1º angular resolution of the measured volumetric orientation maps across millimeter sized samples. This is a significant improvement over the previous generation of three-dimensional X-ray techniques, which provides us with the access of statistically significant microstructure volumes. Combined with the use of state-of-the-art surface mesh generation algorithms, this markedly improved resolution results in the capability to directly measure geometrical evolution, such as grain boundary motion, and material deformation in the form of lattice rotations.
In this thesis, the algorithms and analysis methods recently developed for HEDM are discussed. This includes the descriptions of the robust geometrical extraction methods used for microstructure feature characterization. A set of validation tests for the Forward Modeling Method and the newly developed orientation reconstruction algorithm, the Stratified Monte Carlo Pruning method, is also detailed. By using HEDM to measure the annealing of high purity nickel, grain boundary motion for different boundary types are measured and presented. Moreover, the use of HEDM enabled us to observe the first ever spatially resolved lattice rotation in a high purity copper wire under uni-axial tension, thus demonstrating HEDM’s applicability to defected materials.
34
Guo, Zhen. "Insights of Taste Masking from Molecular Interactions and Microstructures of Microspheres." Thesis, University of Bradford, 2017. http://hdl.handle.net/10454/17420.
Abstract:
The effects of taste masking are determined by interactions between drug and excipients as well as the microstructures of the particulate drug delivery systems (DDS). Cyclodextrin (CD) is a widely used taste masking agent, to which the relationship between kinetic parameters (Ka and Kd) of a drug and taste masking remains unexplored, which is investigated for the first time in this study. A data base of the kinetic parameters for drug-CD was established by Surface Plasmon Resonance Imaging (SPRi) and High Performance Affinity Chromatography (HPAC). Combined with the electronic tongue, Ka and Kd based models for the taste masking effect of HP-β-CD were successfully established and applied to the prediction of taste masking effects. Paracetamol was used as a model drug for taste masking formulation optimization. As well as drug release the microstructure of solid DDS has considerable influence on drug taste. The microstructure of lipid microspheres and the molecular distribution of drug and excipients in lipid microspheres were investigated by Synchrotron radiation-based micro-computed tomography (SR-μCT) and Synchrotron radiation-based Fourier-transform infrared spectromicroscopy (SR-FTIR), respectively. The results demonstrated that the polymeric formulation components as well as shape and particle size of the drug were the key factors to taste masking of paracetamol by inhibiting bust release thereby reducing the interaction intensity of the bitterness. The FTIR absorption spectra confirmed the deposition and formation of chitosan and gelatin films on the drug microsphere surface by layer-by-layer coating. In conclusion, this research demonstrates the molecular kinetic basis of CD taste-masking as well as microstructural basis of particle systems for bitter taste masking.
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Squarcina, Letizia. "Diffusion tensor imaging and fiber tractography: new approaches to study altered microstructure and evaluation in different pathologies." Doctoral thesis, Università degli studi di Padova, 2010. http://hdl.handle.net/11577/3422176.
Abstract:
Diffusion tensor imaging is a non-invasive magnetic resonance imaging technique that allows one to infer information about the directionality of brain microstructure. In this thesis an overview on diffusion imaging is presented, as well as novel algorithms and applications, with the aim of better investigating and understanding microstructural brain charachteristics and organization in vivo. In particular, a novel tractography algorithm for the evaluation of diffusion and microstructure characteristics in white matter multiple sclerosis lesions is presented, as well as a novel technique for the study of altered thalamo-cortical connectivity in traumatic brain injury patients.L'imaging da tensore di diusione (diffusion tensor imaging) è una tecnica di risonanza magnetica (magnetic resonance imaging, MRI) non invasiva, che permette di ottenere informazioni riguardanti la direzionalità della struttura microscopica cerebrale. In questa tesi vengono presentati dei nuovi algoritmi sviluppati con lo scopo di migliorare l'analisi e la comprensione delle caratteristiche microstrutturali e dell'organizzazione del cervello in vivo. In particolare, viene esposto un nuovo algoritmo di trattografia per la valutazione delle caratteristiche microstrutturali e di diffusione delle lesioni della materia bianca causate da sclerosi multipla, e una nuova tecnica per lo studio di alterazione nella connettività talamo corticale in pazienti colpiti da traumi cranici.
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Beaujoin, Justine. "Post mortem inference of the human brain microstructure using ultra-high field magnetic resonance imaging with strong gradients." Thesis, Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLS448/document.
Abstract:
L’ambition des très hauts champs magnétiques (≥ 7T) à forts gradients (≥ 300mT/m) est de dépasser la résolution millimétrique imposée à plus bas champ pour atteindre l’échelle mésoscopique en neuroimagerie. Etudier le cerveau à cette échelle est essentiel pour comprendre le lien entre fonction et substrat anatomique. Malgré les progrès réalisés sur les aimants cliniques à 7T, il n’en est pas de même des gradients. Cette thèse vise à cartographier le cerveau humain à l’échelle mésoscopique via l’étude de pièces anatomiques post mortem. Une approche alternative a été choisie, reposant sur l'utilisation d'imageurs précliniques à très hauts champs (7T et 11.7T) et forts gradients (780mT/m). Après une première étape de préparation (extraction et fixation) opérée au CHU de Tours, une pièce anatomique complète a été scannée à 3T, avant découpe de l’hémisphère gauche en sept blocs. Un protocole d’acquisition IRM ciblant une résolution mésoscopique a ensuite été mis en place à 11.7T. Ce protocole, incluant des séquences anatomiques, relaxométriques, et de diffusion, a été validé à l’aide de deux structures clé: un hippocampe et un tronc cérébral. Les données anatomiques et de diffusion acquises à une résolution mésoscopique sur l’hippocampe ont permis de segmenter ses sous-champs, d’extraire le circuit polysynaptique et d’observer l’existence d’un gradient de connectivité et de densité neuritique positif dans la direction postéro-antérieure de l’hippocampe. L’utilisation de modèles avancés d’étude de la microstructure a également révélé l’apport de ces techniques pour la segmentation de l’hippocampe, les cartes de densité neuritique révélant les trois couches des champs ammoniens. Un tronc cérébral a ensuite été scanné, avec une résolution atteignant la centaine de micromètres. Une segmentation de 53 de ses 71 noyaux a été réalisée au sein du CHU de Tours, permettant d’établir la cartographie IRM du tronc cérébral humain la plus complète à ce jour. Les principaux faisceaux de la substance blanche ont été reconstruits, ainsi que les projections du locus coeruleus, structure connue pour être atteinte dans le maladie de Parkinson. Forts de ces résultats, la campagne d'acquisition de l'hémisphère gauche, d’une durée de 10 mois, a été initiée. Le protocole d’acquisition à 11.7T intègre des séquences anatomiques (100/150µm) ainsi que des séquences d'imagerie 3D pondérées en diffusion (b=1500/4500/8000 s/mm², 25/60/90 directions) à 200µm. Des acquisitions complémentaires réalisées à 7T comprenant des séquence d’écho de spin rapide avec inversion-récupération ont par ailleurs permis d’étudier la myéloarchitecture du cortex cérébral et d’identifier automatiquement sa structure laminaire. Un nouveau modèle de mélange de Gaussiennes a été développé, intégrant les informations myéloarchitecturales issues de la cartographie T1 et les informations cytoarchitecturales issues de l’imagerie de diffusion. Il a ainsi pu être démontré que l’utilisation conjointe de ces deux informations permettait de mettre en évidence des couches du cortex visuel, l’information myéloarchitecturale favorisant l’extraction des couches externes et la densité neuritique celle des couches plus profondes. Enfin, l’exploitation des données IRM acquises à 11.7T sur les différents blocs a nécessité la mise en place d’une chaîne de prétraitements pour corriger les artéfacts d’imagerie et reconstruire l’hémisphère entier à l’aide de stratégies de recalage difféomorphe avancées. L’objectif de ce projet est l’obtention d’un jeu de données IRM de très haute résolution spatio-angulaire de l’hémisphère gauche. Ce jeu de données anatomique et de diffusion unique permettra à terme de constituer un nouvel atlas IRM mésoscopique de la structure, de la connectivité et de la cytoarchitecture du cerveau humainThe aim of ultra-high field strength (≥7T) and ultra-strong gradient systems (≥300mT/m) is to go beyond the millimeter resolution imposed at lower field and to reach the mesoscopic scale in neuroimaging. This scale is essential to understand the link between brain structure and function. However, despite recent technological improvements of clinical UHF-MRI, gradient systems remain too limited to reach this resolution. This thesis aims at answering the need for mapping the human brain at a mesoscopic scale by the study of post mortem samples. An alternative approach has been developed, based on the use of preclinical systems equipped with ultra-high fields (7T/11.7T) and strong gradients (780mT). After its extraction and fixation at Bretonneau University Hospital (Tours), an entire human brain specimen was scanned on a 3T clinical system, before separating its two hemispheres and cutting each hemisphere into seven blocks that could fit into the small bore of an 11.7T preclinical system. An MRI acquisition protocol targeting a mesoscopic resolution was then set up at 11.7T. This protocol, including anatomical, quantitative, and diffusion-weighted sequences, was validated through the study of two key structures: the hippocampus and the brainstem. From the high resolution anatomical and diffusion dataset of the human hippocampus, it was possible to segment the hippocampal subfields, to extract the polysynaptic pathway, and to observe a positive gradient of connectivity and neuritic density in the posterior-anterior direction of the hippocampal formation. The use of advanced microstructural models (NODDI) also highlighted the potential of these techniques to reveal the laminar structure of the Ammon’s horn. A high resolution anatomical and diffusion MRI dataset was obtained from the human brainstem with an enhanced resolution of a hundred micrometers. The segmentation of 53 of its 71 nuclei was performed at the Bretonneau University Hospital, making it the most complete MR-based segmentation of the human brainstem to date. Major white matter bundles were reconstructed, as well as projections of the locus coeruleus, a structure known to be impaired in Parkinson’s disease. Buoyed by these results, a dedicated acquisition campaign targeting the entire left hemisphere was launched for total scan duration of 10 months. The acquisition protocol was performed at 11.7T and included high resolution anatomical sequences (100/150μm) as well as 3D diffusion-weighted sequences (b=1500/4500/8000 s/mm², 25/60/90 directions, 200μm). In addition, T1-weighted inversion recovery turbo spin echo scans were performed at 7T to further investigate the myeloarchitecture of the cortical ribbon at 300µm, revealing its laminar structure. A new method to automatically segment the cortical layers was developed relying on a Gaussian mixture model integrating both T1-based myeloarchitectural information and diffusion-based cytoarchitectural information. The results gave evidence that the combination of these two contrasts highlighted the layers of the visual cortex, the myeloarchitectural information favoring the extraction of the outer layers and the neuritic density favoring the extraction of the deeper layers. Finally, the analysis of the MRI dataset acquired at 11.7T on the seven blocks required the development of a preprocessing pipeline to correct artifacts and to reconstruct the entire hemisphere using advanced registration methods. The aim was to obtain an ultra-high spatio-angular resolution MRI dataset of the left hemisphere, in order to establish a new mesoscopic post mortem MRI atlas of the human brain, including key information about its structure, connectivity and microstructure
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Wang, Lin. "Binary tomography reconstruction of bone microstructures from a limited number of projections." Thesis, Lyon, 2016. http://www.theses.fr/2016LYSEI054/document.
Abstract:
La reconstruction en tomographie discrète de la microstructure de l’os joue un role très important pour le diagnostic de l’ostéoporse, une maladie des os très fréquente. Le diagnostic clinique est basé sur l’absortiométrie duale de rayons X. Avec la tomographie de rayons X, une résolution spatiale élevée avec des images reconstruites in vivo requiert une dose d’irradiation élevée et un temps de balayage long, ce qui est dangereux pour le patient. Une des méthodes pour résoudre ce problème est de limiter le nombre de projections. Cependant, avec cette méthode le problème de reconstruction devient mal posé. Deux types de régularisation par Variation Totale minimisées avec la méthode Alternate Direction of Minimization Method (ADMM) et deux schémas basés sur les méthodes de régularisation Level-set sont appliquées à deux images d’os expérimentales acquises avec un synchrotron (pixel size: 15 μm). Des images de tailles variées et avec différents niveaux de bruit Gaussien additifs ajoutés aux projections sont utlisées pour étudier l’efficacité des méthodes de régularisation. Des minima locaux sont obtenus avec ces méthodes déterministes. Une approche globale d’optimisation est nécessaire pour améliorer les résultats. Des perturbations stochastiques peuvent être un moyen très utile pour échapper aux minima locaux. Dans une première approche, une équation différentielle stochastique basée sur la régularisation level-set est étudiée. Cette méthode améliore les résultats de reconstruction mais ne modifie que les frontières entre les régions 0 et 1. Ensuite une équation aux dérivées partielles stochastique est obtenue avec la régularisation TV pour améliorer la méthode stochastique level-set. A la fin de notre travail, nous avons étendu la méthode de régularisation à des images 3D avec des données réelles. Cette algorithme a été implémenté avec RTK. Nous avons aussi étendu l’approche level-set utilisée pour la tomographie binaire au cas multi-levelDiscrete tomography reconstruction of bone microstructure is important in diagnosis of osteoporosis. One way to reduce the radiation dose and scanning time in CT imaging is to limit the number of projections. This method makes the reconstruction problem highly ill-posed. A common solution is to reconstruct only a finite number of intensity levels. In this work, we investigate only binary tomography reconstruction problem. First, we consider variational regularization methods. Two types of Total Variation (TV) regularization approaches minimized with the Alternate Direction of Minimization Method (ADMM) and two schemes based on Level-set (LS) regularization methods are applied to two experimental bone cross-section images acquired with synchrotron micro-CT. The numerical experiments have shown that good reconstruction results were obtained with TV regularization methods and that level-set regularization outperforms the TV regularization for large bone image with complex structures. Yet, for both methods, some reconstruction errors are still located on the boundaries and some regions are lost when the projection number is low. Local minima were obtained with these deterministic methods. Stochastic perturbations is a useful way to escape the local minima. As a first approach, a stochastic differential equation based on level-set regularization was studied. This method improves the reconstruction results but only modifies the boundaries between the 0 and 1 regions. Then partial stochastic differential equation obtained with the TV regularization semi-norm were studied to improve the stochastic level-set method. The random change of the boundary are performed in a new way with the gradient or wavelet decomposition of the reconstructed image. Random topological changes are included to find the lost regions in the reconstructed images. At the end of our work, we extended the TV regularization method to 3D images with real data on RTK (Reconstruction Toolkit). And we also extended the level-set to the multi-level cases
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Fick, Rutger. "Modélisation avancée du signal dMRI pour la caractérisation de la microstructure tissulaire." Thesis, Université Côte d'Azur (ComUE), 2017. http://www.theses.fr/2017AZUR4006/document.
Abstract:
Cette thèse est dédiée à améliorer la compréhension neuro-scientifique à l'aide d'imagerie par résonance magnétique de diffusion (IRMd). Nous nous concentrons sur la modélisation du signal de diffusion et l'estimation par IRMd des biomarqueurs liés à la microstructure, appelé «Microstructure Imaging». Cette thèse est organisée en trois parties. Dans partie I nous commençons par la base de l'IRMd et un aperçu de l'anisotropie en diffusion. Puis nous examinons la plupart des modèles de microstructure utilisant PGSE, en mettant l'accent sur leurs hypothèses et limites, suivi par une validation par l'histologie de la moelle épinière de leur estimation. La partie II présente nos contributions à l'imagerie en 3D et à l’estimation de microstructure. Nous proposons une régularisation laplacienne de la base fonctionnelle MAP, ce qui nous permet d'estimer de façon robuste les indices d'espace q liés au tissu. Nous appliquons cette approche aux données du Human Connectome Project, où nous l'utilisons comme prétraitement pour d'autres modèles de microstructure. Enfin, nous comparons les biomarqueurs dans une étude ex-vivo de rats Alzheimer à différents âges. La partie III présente nos contributions au représentation de l’espace qt - variant sur l'espace q 3D et le temps de diffusion. Nous présentons une approche initiale qui se concentre sur l'estimation du diamètre de l'axone depuis l'espace qt. Nous terminons avec notre approche finale, où nous proposons une nouvelle base fonctionnelle régularisée pour représenter de façon robuste le signal qt, appelé qt-IRMd. Ce qui permet l'estimation des indices d’espace q dépendants du temps, quantifiant la dépendance temporelle du signal IRMdThis thesis is dedicated to furthering neuroscientific understanding of the human brain using diffusion-sensitized Magnetic Resonance Imaging (dMRI). Within dMRI, we focus on the estimation and interpretation of microstructure-related markers, often referred to as ``Microstructure Imaging''. This thesis is organized in three parts. Part I focuses on understanding the state-of-the-art in Microstructure Imaging. We start with the basic of diffusion MRI and a brief overview of diffusion anisotropy. We then review and compare most state-of-the-art microstructure models in PGSE-based Microstructure Imaging, emphasizing model assumptions and limitations, as well as validating them using spinal cord data with registered ground truth histology. In Part II we present our contributions to 3D q-space imaging and microstructure recovery. We propose closed-form Laplacian regularization for the recent MAP functional basis, allowing robust estimation of tissue-related q-space indices. We also apply this approach to Human Connectome Project data, where we use it as a preprocessing for other microstructure models. Finally, we compare tissue biomarkers in a ex-vivo study of Alzheimer rats at different ages. In Part III, we present our contributions to representing the qt-space - varying over 3D q-space and diffusion time. We present an initial approach that focuses on 3D axon diameter estimation from the qt-space. We end with our final approach, where we propose a novel, regularized functional basis to represent the qt-signal, which we call qt-dMRI. Our approach allows for the estimation of time-dependent q-space indices, which quantify the time-dependence of the diffusion signal
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Cropper, Sherrill Lyne. "The influence of native wheat lipids on the rheological properties and microstructure of dough and bread." Diss., Kansas State University, 2015. http://hdl.handle.net/2097/18924.
Abstract:
Doctor of PhilosophyDepartment of Grain Science and Industry
Hulya Dogan
Jon Faubion
Bread quality and final crumb grain are reflective of the ability for wheat flour dough to retain and stabilize gas cells during the baking process. The visco-elastic properties of dough allow for the incorporation of air cells and expansion during fermentation and baking. The gluten-starch matrix provides the backbone support. However, following the end of proofing and during the beginning of baking, the structure weakens due to over-extension and expansion and the matrix begins to separate and eventually break down. Native wheat lipids, which are found in small quantities in wheat flour, provide a secondary support for gas cell stabilization because of their amphiphilic characteristics and ability to move to the interface and form condensed monolayers. The objectives of this research were to evaluate the influence of native wheat lipids on the rheological properties of dough and the microstructure of bread. Native wheat lipids were extracted from straight-grade flour and separated into total, free, bound, nonpolar, glycolipids, and phospholipids using solid-phase extraction (SPE) with polar and nonpolar solvents. Defatted flour was reconstituted using each lipid fraction at a range of levels between 0.2% and 2.8%. Dough and bread were made following AACC Method 10-10.03. Rheological testing of the dough and evaluation of the microstructure of the bread was conducted using small and large deformation testing, C-Cell imaging, and x-ray microtomography analysis to determine changes in visco-elastic properties and gas cell structure and distribution. Rheological assessment through small amplitude oscillatory measurements demonstrated that nonpolar, phospholipids, and glycolipid fractions had a greater interaction with both proteins and starch in the matrix, creating weaker dough. Nonpolar, phospholipids, and glycolipids, varied in their ability to stabilize gas cells as determined by strain hardening index. C-Cell imaging and x-ray microtomograpy testing found that treatments containing higher concentrations of polar lipids (glycolipids and phospholipids) had a greater effect on overall loaf volume, cell size, and distribution. This illustrates that level and type of native wheat lipids influence the visco-elastic properties of dough and gas cell size, distribution, cell wall thickness, and cell stability in bread.
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Neto, Henriques Rafael. "Advanced methods for diffusion MRI data analysis and their application to the healthy ageing brain." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/281993.
Abstract:
Diffusion of water molecules in biological tissues depends on several microstructural properties. Therefore, diffusion Magnetic Resonance Imaging (dMRI) is a useful tool to infer and study microstructural brain changes in the context of human development, ageing and neuropathology. In this thesis, the state-of-the-art of advanced dMRI techniques is explored and strategies to overcome or reduce its pitfalls are developed and validated. Firstly, it is shown that PCA denoising and Gibbs artefact suppression algorithms provide an optimal compromise between increased precision of diffusion measures and the loss of tissue's diffusion non-Gaussian information. Secondly, the spatial information provided by the diffusion kurtosis imaging (DKI) technique is explored and used to resolve crossing fibres and generalize diffusion measures to cases not limited to well-aligned white matter fibres. Thirdly, as an alternative to diffusion microstructural modelling techniques such as the neurite orientation dispersion and density imaging (NODDI), it is shown that spherical deconvolution techniques can be used to characterize fibre crossing and dispersion simultaneously. Fourthly, free water volume fraction estimates provided by the free water diffusion tensor imaging (fwDTI) are shown to be useful to detect and remove voxels corrupted by cerebrospinal fluid (CSF) partial volume effects. Finally, dMRI techniques are applied to the diffusion data from the large collaborative Cambridge Centre for Ageing and Neuroscience (CamCAN) study. From these data, the inference provided by diffusion anisotropy measures on maturation and degeneration processes is shown to be biased by age-related changes of fibre organization. Inconsistencies of previous NODDI ageing studies are also revealed to be associated with the different age ranges covered. The CamCAN data is also processed using a novel non-Gaussian diffusion characterization technique which is invariant to different fibre configurations. Results show that this technique can provide indices specific to axonal water fraction which can be linked to age-related fibre density changes.
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Mohee, Lakshana. "Collagen scaffolds for tissue engineering : the relationship between microstructure, fluid dynamics, mechanics and scaffold deformation." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/276980.
Abstract:
Collagen scaffolds are porous structures which are used in bioreactors and in a wide range of tissue engineering applications. In these contexts, the scaffolds may be subjected to conditions in which fluid is forced through the structure and the scaffold is simultaneously compressed. It is clear that fluid transport within collagen scaffolds, and the inter-relationships between permeability, scaffold structure, fluid pressure and scaffold deformation are of key importance. However, these relationships remain poorly understood. In this thesis, a series of isotropic collagen structures were produced using a freeze-drying technique from aqueous slurry concentrations 0.5, 0.75 and 1 wt%, and fully characterised using X-ray micro-tomography and compression testing. It was found that collagen wt% influenced structural parameters such as pore size, porosity, relative density and mechanical properties. Percolation theory was used to investigate the pore interconnectivity of each scaffold. Structures with lower collagen fraction resulted in larger percolation diameters, but lower mechanical stiffness. Aligned collagen scaffolds were also produced by altering the freeze-drying protocol and using different types of mould materials and designs. It was found that a polycarbonate mould with stainless base resulted in vertically aligned structures with low angular variation. When compared with isotropic scaffolds from slurry of the same concentration, aligned scaffolds had a larger percolation diameter. Tortuosity was used as a mathematical tool to characterise the interconnected pathways within each porous structure. The effect of the size of the region of interest (ROI) chosen and the size of the virtual probe particle used in the analysis on the values of tortuosity calculated were determined and an optimised calculation methodology developed. Increasing the collagen fraction within isotropic scaffolds increased the tortuosity, and aligned structures had smaller tortuosity values than their isotropic counterparts. Permeability studies were conducted using two complementary experimental rigs designed to cover a range of pressure regimes and the results were compared with predictions from mathematical models and computational simulations. At low pressures, it was found that the lower collagen fraction structures, which had more open morphologies, had higher permeabilities. Alignment of the structure also enhanced permeability. The scaffolds all experienced deformation at high pressures resulting in a restriction of fluid flow. The lower collagen fraction scaffolds experienced a sharper decrease in permeability with increased pressure and aligned structures were more responsive to deformation than their isotropic counterparts. The inter-relationships between permeability, scaffold structure, fluid pressure and deformation of collagen scaffolds were explored. For isotropic samples, permeability followed a broad $(1- \epsilon)^2$ behaviour with strain as predicted by a tetrakaidecahedral structural model, with the constant of proportionality changing with collagen fraction. In contrast, the aligned structures did not follow this behaviour with the permeability dropping much more sharply in the early stages of compression. Open-cell polyurethane (PU) foams, sometimes used as dressings in wound healing applications, are often compared with collagen scaffolds in permeability models and were used in this thesis as a comparison structure. The foam had a higher permeability than the scaffolds due to its larger pore sizes and higher interconnectivity. In the light of the effects of compression on permeability, the changes in porous structure with compression were explored in isotropic and aligned 0.75 wt% scaffolds. Unlike the fluid flow experiments, these experiments were carried out in the dry state. Deformation in simple linear compression and in step-wise compression was studied, and the stress relaxation behaviour of the scaffolds characterised. A methodology was developed to characterise the structural changes accompanying compression using X-ray micro-tomography with an in situ compression stage. The methodology accounted for the need for samples to remain unchanged during the scan collection period for stable image reconstruction. The scaffolds were studied in uniaxial compression and biaxial compression and it was found that pore size and percolation diameter decreased with increasing compressive strain, while the tortuosity increased. The aligned structure was less affected than the isotropic at low compressions, in contrast to the results from the permeability study in which the aligned structure was more responsive to strain. This suggests that the degree of hydration may affect the structural changes observed. The insights gained in this study of the inter-relationships between microstructure, fluid dynamics and deformation in collagen scaffolds are of relevance to the informed design of porous structures for medical applications.
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Röling, Christian. "Anwendungsbezogene Weiterentwicklung der abbildenden Ellipsometrie." Doctoral thesis, Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2017. http://hdl.handle.net/11858/00-1735-0000-0023-3F1F-5.
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Molina, Romero Miguel [Verfasser], Bjoern H. [Akademischer Betreuer] Menze, Marion I. [Gutachter] Menzel, and Bjoern H. [Gutachter] Menze. "Disentangling Tissue Microstructure with Magnetic Resonance Imaging / Miguel Molina Romero ; Gutachter: Marion I. Menzel, Bjoern H. Menze ; Betreuer: Bjoern H. Menze." München : Universitätsbibliothek der TU München, 2018. http://d-nb.info/117332271X/34.
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Carapella, Valentina. "Impact of tissue microstructure on a model of cardiac electromechanics based on MRI data." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:69d28c8c-832b-4ac4-aa48-3d0613708515.
Abstract:
Cardiac motion is a highly complex and integrated process of vital importance as it sustains the primary function of the heart, that is pumping blood. Cardiac tissue microstructure, in particular the alignment of myocytes (also referred to as fibre direction) and their lateral organisation into laminae (or sheets), has been shown by both experimental and computational research to play an important role in the determination of cardiac motion patterns. However, current models of cardiac electromechanics, although already embedding structural information in the models equations, are not yet able to fully reproduce the connection between structural dynamics and cardiac deformation. The aim of this thesis was to develop an electromechanical modelling framework to investigate the impact of tissue structure on cardiac motion, focussing on left ventricular contraction in rat. The computational studies carried out were complemented with a preliminary validation study based on experimental data of tissue structure rearrangement during contraction from diffusion tensor MRI.
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Coleman, Michael Thomas. "The use of backscattered electron imaging mode to assess the effect of fine dispersions on development of superplastic microstructures in Al-Mg alloys." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1993. http://handle.dtic.mil/100.2/ADA272426.
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Mirea, Iulia. "Analyse de la microstructure 3D du tissu cardiaque humain à l’aide de la micro-tomographie à rayons X par contraste de phase." Thesis, Lyon, 2017. http://www.theses.fr/2017LYSEI076/document.
Abstract:
Les pathologies cardiovasculaires restent un des problèmes majeurs de santé publique qui justifie les recherches menées pour améliorer notre compréhension de la fonction cardiaque. Celles-ci nécessitent une bonne connaissance de la microarcInstitut de Technologie de Harbin - Chineecture myocardique afin de mieux comprendre les relations entre les fonctions mécanique, hémodynamique et les changements structuraux induits par les maladies cardiaques. Pour ce faire il est nécessaire d’accéder à une connaissance précise de l’arrangement spatial des composants du tissu. Cependant, notre compréhension de l’arcInstitut de Technologie de Harbin - Chineecture du coeur est limitée par le manque de description 3D de l’organisation des structures à l’échelle microscopique. Nous proposons d’explorer la structure 3D du tissu cardiaque en utilisant l’imagerie X synchrotron par contraste de phase disponible à l’ESRF. Pour la première fois, 9 échantillons de tissu de la paroi du ventricule gauche (VG) humain sont imagés à la résolution isotrope de 3,5 μm et analysés. Cette thèse est centrée sur la description 3D d’un des constituants principal du tissu: la matrice extracellulaire (MEC). La MEC inclue: l’endomysium qui entoure et sépare les myocytes et les capillaires de façon individuelle, le perimysium qui entoure et sépare des groupes de myocytes et l’épimysium qui enveloppe le muscle cardiaque dans son ensemble. Chaque échantillon reconstruit fait environ 30 Gb, ce qui représente une quantité importante de données à traiter et à visualiser. Pour ce faire, nous avons développé un algorithme automatique de traitement d’image pour binariser chaque échantillon et isoler la MEC. Ensuite, nous avons extrait des parametres statistiques relatifs à la microarcInstitut de Technologie de Harbin - Chineecture de l’ECM, principalement l’épaisseur des plans de clivage (PC) et les distances inter-PC. Les résultats montrent que l’arrangement local des PC diffère selon l’emplacement au sein du VG (postérieur, antérieur, septal) et de leur distance à l’apex (plus complexe). L’épaisseur des PC extraite de tous les échantillons va approximativement de 24 μm à 59 μm et la distance inter-PC de 70 μm à 280 μm avec une variation locale significative de la déviation standard. Ce sont de nouveaux marqueurs quantitatifs de la MEC du tissu cardiaque humain qui sont d’un intérêt majeur pour une meilleure compréhension de la fonction cardiaqueCardiovascular diseases remain one of the most serious health problems, motivating research to deepen our understanding of the myocardial function. To succeed, there is a need to get detailed information about the spatial arrangement of the cardiac tissue components. Currently, our understanding of the cardiac microarcInstitut de Technologie de Harbin - Chineecture is limited by the lack of 3D descriptions of the cardiac tissue at the microscopic scale. This thesis investigates the 3D cardiac tissue microstructure using X-Ray µ-CT phase contrast imaging available at the ESRF. For the first time, 9 human cardiac left ventricle (LV) wall samples are imaged at an isotropic resolution (3.5 µm) and analysed. We focus on the description of the cardiac extracellular matrix (CEM) that is one of the main components of the tissue. The CEM includes: the endomysium that surrounds and separates individual myocytes and capillaries, the perimysium that surrounds groups of myocytes and the epimysium that surrounds the entire heart muscle. Each reconstructed sample is about 30 Gb which represents a large amount of data to process and display. To succeed, we developed an automatic image processing algorithm to binarise each sample by selecting the CEM. We extract statistical features of the ECM, mainly the thickness of the cleavage planes (CP) and the inter-CP distances. The results show that the local 3D arrangement of the CP differs according to their location in the LV (posterior, anterior, septal) and their distance from the apex (more complex). The thickness of the CP extracted from all the samples roughly ranges from 24 µm to 59 µm and the inter-CP distances from 70 µm to 280 µm with significant local variations of the standard deviation. Those new quantitative markers of the ECM of the human cardiac are of main interest for a better understanding of the heart function
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Yoo, Thomas. "Application of a Multimodal Polarimetric Imager to Study the Polarimetric Response of Scattering Media and Microstructures." Thesis, Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLX106/document.
Abstract:
Les travaux réalisés au cours cette thèse ont eu comme objectif l’étude de l’interaction de la lumière polarisée avec des milieux et des particules diffusants. Ces travaux s’inscrivent dans un contexte collaboratif fort entre le LPICM et différents laboratoires privés et publics. Des aspects très variées ont été traités en profondeur dont le développement instrumental, la simulation numérique avancée et la création de protocoles de mesure pour l’interprétation de donnés à caractère complexe.La partie instrumentale de la thèse a été consacrée au développement d’un instrument novateur, adapté à la prise d’images polarimétriques à différents échelles (du millimètre au micron) pouvant être rapidement reconfigurable pour offrir différents modes d’imagerie du même échantillon. Les deux aspects principaux qui caractérisent l’instrument sont i) la possibilité d’obtenir des images polarimétriques réelles de l’échantillon et des images de la distribution angulaire de lumière diffusé par une zone sur l’échantillon dont sa taille et position peuvent être sélectionnée par l’utilisateur à volonté, ii) le contrôle total de l’état de polarisation, de la taille et de la divergence des faisceaux utilisés pour l’éclairage de l’échantillon et pour la réalisation des images de celui-ci. Ces deux aspects ne se trouvent réunis sur aucun autre appareil commercial ou expérimental actuel.Le premier objet d’étude en utilisant le polarimètre imageur multimodal a été l’étude de l’effet de l’épaisseur d’un milieu diffusant sur sa réponse optique. En imagerie médicale il existe un large consensus sur les avantages de l’utilisation de différentes propriétés polarimétriques pour améliorer l’efficacité de techniques optiques de dépistage de différentes maladies. En dépit de ces avantages, l’interprétation des observables polarimétriques en termes de propriétés physiologiques des tissus se trouve souvent obscurcie par l’influence de l’épaisseur, souvent inconnue, de l’échantillon étudié.L’objectif des travaux a été donc, de mieux comprendre la dépendance des propriétés polarimétriques de différents matériaux diffusants avec l’épaisseur de ceux-ci. En conclusion, il a été possible de montrer que, de manière assez universelle, les propriétés polarimétriques des milieux diffusants varient proportionnellement au chemin optique que la lumière a parcouru à l’intérieur du milieu, tandis que le dégrée de polarisation dépend quadratiquement de ce chemin. Cette découverte a pu être ensuite utilisée pour élaborer une méthode d’analyse de données qui permet de s’affranchir de l’effet des variations d’épaisseur des tissus, rendant ainsi les mesures très robustes et liées uniquement aux propriétés intrinsèques des échantillons étudiés.Un deuxième objet d’étude a été la réponse polarimétrique de particules de taille micrométrique. La sélection des particules étudiées par analogie à la taille des cellules qui forment les tissus biologiques et qui sont responsables de la dispersion de la lumière. Grâce à des mesures polarimétriques, il a été découvert que lorsque les microparticules sont éclairées avec une incidence oblique par rapport à l’axe optique du microscope, celles-ci semblent se comporter comme si elles étaient optiquement actives. D’ailleurs, il a été trouvé que la valeur de cette activité optique apparente dépend de la forme des particules étudiées. L’explication de ce phénomène est basée sur l’apparition d’une phase topologique dans le faisceau de lumière. Cette phase topologique dépend du parcours de la lumière diffusée à l’intérieur du microscope. L’observation inédite de cette phase topologique a été possible grâce au fait que l’imageur polarimétrique multimodale permet un éclairage des échantillons à l’incidence oblique. Cette découverte peut améliorer significativement l’efficacité de méthodes optiques pour la détermination de la forme de micro-objetsThe work carried out during this thesis was aimed to study the interaction of polarized light from the scattering media and particles. This work is part of a strong collaborative context between the LPICM and various private and public laboratories. A wide variety of aspects have been treated deeply, including instrumental development, advanced numerical simulation and the creation of measurement protocols for the interpretation of complex data.The instrumental part of the thesis was devoted to the development of an innovative instrument, suitable for taking polarimetric images at different scales (from millimeters to microns) that can be quickly reconfigured to offer different imaging modes of the same sample. The two main aspects that characterize the instrument are i) the possibility of obtaining real polarimetric images of the sample and the angular distribution of light scattered by an illuminated zone whose size and position can be controlled, ii) the total control of the polarization state, size and divergence of the beams. These two aspects are not united on any other commercial or experimental apparatus today.The first object of the study using the multimodal imaging polarimeter was to study the effect of the thickness from a scattering medium on its optical response. In medical imaging, there is a broad consensus on the benefits of using different polarimetric properties to improve the effectiveness of optical screening techniques for different diseases. Despite these advantages, the interpretation of the polarimetric responses in terms of the physiological properties of tissues has been obscured by the influence of the unknown thickness of the sample.The objective of the work was, therefore, to better understand the dependence of the polarimetric properties of different scattering materials with the known thickness. In conclusion, it is possible to show that the polarimetric properties of the scattering media vary proportionally with the optical path that the light has traveled inside the medium, whereas the degree of polarization depends quadratically on the optical path. This discovery could be used to develop a method of data analysis that overcomes the effect of thickness variations, thus making the measurements very robust and related only to the intrinsic properties of the samples studied.The second object of study was to study the polarimetric responses from particles of micrometric size. The selection of the particles studied by analogy to the size of the cells that form the biological tissues, and which are responsible for the dispersion of light. By means of the polarimetric measurements, it has been discovered that when the microparticles are illuminated with an oblique incidence with respect to the optical axis of the microscope, they appear to behave as if they were optically active. Moreover, it has been found that the value of this apparent optical activity depends on the shape of the particles. The explanation of this phenomenon is based on the appearance of a topological phase of the beam. This topological phase depends on the path of the light scattered inside the microscope. The unprecedented observation of this topological phase has been done by the fact that the multimodal polarimetric imager allows illumination of the samples at the oblique incidence. This discovery can significantly improve the efficiency of optical methods for determining the shape of micro-objects
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Kerdja, Youcef. "Caractérisation 3D et modélisation multi-échelle des matériaux actifs de batteries." Thesis, Université Grenoble Alpes, 2020. http://www.theses.fr/2020GRALI033.
Abstract:
Quatre matériaux actifs de batteries Li-ion de type NMC (LiNi1/3Mn1/3Co1/3O2) ayant la même composition chimique, mais des microstructures différentes ont été synthétisés puis mis en œuvre sous forme d’électrodes composites afin de quantifier l’impact de la microstructure sur leurs performances électrochimiques. La tomographie FIB-SEM a été utilisée afin d’imager ces différentes microstructures. Les résultats 3D obtenus sur deux de nos matériaux permettent de montrer le lien entre tortuosité ionique et capacités en décharge. Des images 2D de ces microstructures ont été également extraites afin d’aller au-delà des mesures de tortuosité et de réaliser des simulations multi-physiques à l’échelle microstructurale sur des structures réelles d’électrodes. En parallèle, un modèle de simulations électrochimiques sur microstructures de matériaux d’électrodes a été développé. Ce dernier a permis dans un premier temps, via une étude paramétrique sur les propriétés physiques des matériaux, de visualiser sur une ‘microstructure modèle’, les mécanismes et les conditions par lesquels la diffusion de lithium (liquide et solide) et la cinétique électrochimique influencent la capacité en décharge et les hétérogénéités de lithiation au sein de la microstructure modèle. Les compétitions entre les différents mécanismes ont été également visualisées et quantifiées. Dans un deuxième temps, le modèle développé a été mis en œuvre sur deux des microstructures réelles (2D) extraites auparavant afin de simuler des décharges galvanostatiques. Cette démarche permet de suivre operando le courant local, ainsi que la surtension aux interfaces des particules de matériau actif au cours des décharges galvanostatiques. L’accès à ces grandeurs permet d’expliquer le biais par lequel ces deux électrodes de même composition chimique et de microstructures différentes présentent des capacités expérimentales en décharge différentesFour NMC type materials having the same chemical composition (LiNi1/3Mn1/3Co1/3O2) but different microstructures were synthesized and then used as positive electrodes to probe the impact of the microstructure over their electrochemical performances. FIB-SEM tomography was used to get 3D images of the synthesized materials, compute their ionic tortuosity and link the results to the observed electrochemical performances. 2D microscopy images were also obtained on the four materials to go beyond tortuosity computation and realize multi-physics simulations at the microstructure scale on real electrodes. To that end, an electrochemical model at the microstructure level has been developed. This model allows the visualization of the electrochemical kinetics’ as well as lithium liquid and solid diffusion’s influences over the global battery capacity and lithiation heterogeneities at the microstructure level. This study was performed, via a sensitivity analysis of the material physical properties, on a ‘template microstructure’ and allowed us to understand and quantify the different influences’ mechanism and the competition between them over the characteristics of the battery at multiple scales. After that, the developed model was used to simulate galvanostatic discharges on two of the previously extracted 2D microstructures. These simulations allowed us to get a real-time visualization of the local current density as well as of the overpotential at active material-electrolyte interface. The real-time visualization helped us to explain how two NMC type materials having the same chemical composition, but different microstructures led to different discharge capacities
49
Jomaa, Mohamed Hedi. "Elaboration, characterization and modeling of electroactive materials based on polyurethanes and grafted carbon nanotubes." Thesis, Lyon, INSA, 2015. http://www.theses.fr/2015ISAL0053/document.
Abstract:
Le besoin de sources d’énergie autonomes connaît un regain d’intérêt de plus en plus important avec la multiplication des équipements portables et le développement des réseaux de capteurs. Au-delà de l’utilisation traditionnelle des batteries, il y a un intérêt évident à générer l’énergie électrique nécessaire au cœur du système lui-même en utilisant le gisement environnemental disponible : gradients thermiques, vibrations mécaniques….Ceci est également rendu possible par la réduction importante de la consommation des composants électroniques observés ces vingt dernières années. Parmi les dispositifs susceptibles d’exploiter le gisement vibratoire, les matériaux électro-actifs occupent une place de choix. Actuellement, on recherche des matériaux légers, pouvant se déposer sur des grandes surfaces et peu coûteux à la réalisation. Ceci ouvre des perspectives séduisantes à l’utilisation de polymères électro-actifs en lieu et place des matériaux céramiques piézoélectriques. Parmi les EAP disponibles, les polyuréthanes (PU) sont des élastomères thermoplastiques d'un grand intérêt pour une vaste gamme d'applications en tant que transducteurs ou actionneurs lorsque l'on considère leur importante déformation sous champ électrique, une énergie spécifique élevée, et leur réponse rapide De plus, ces matériaux sont légers, très souples, présentent de faibles coûts de fabrication, et peuvent être facilement moulés dans n'importe quelle forme souhaitable. Des travaux récents ont montré que l'énergie récoltée peut être augmentée en incorporant des nanotubes de carbone (NTC) dans une matrice de polyuréthane. Cependant, les nanocomposites peuvent ne pas avoir été optimisées, car il est bien connu que les NTC sont difficilement dispersées dans une matrice polymère et que la force d'adhérence interfaciale est généralement médiocre. Une solution pour améliorer à la fois la dispersion et l'adhérence peut consister en greffant des chaînes de polymère sur les surfaces de la NTC. L'objectif principal de cette thèse était de développer des polymères nanocomposites à haute efficacité pour la récupération d'énergie et d'actionnement. La motivation principal était d'utiliser des NTC greffé-polymère pour améliorer la dispersion, l'adhérence interfaciale dans PU, et de comprendre comment cela peut changer les propriétés électroactifs des nanocomposites PU / NTC. En d'autres termes, ce était un projet pluridisciplinaire, y compris une optimisation du processus d'élaboration, caractérisations physiques ˗ notamment les comportements de microstructure, électriques et mécaniques dans une large gamme de fréquences et températures ˗ et la détermination des propriétés électroactifs. Il s’agissait également de développer une modélisation des lois de comportements en s’aidant de l’analyse de la microstructure par imagerieHarvesting systems capable of transforming dusty environmental energy into electrical energy have attracted considerable interest throughout the last decade. Several research efforts have focused on the transformation of the mechanical vibration into electrical energy. Most of these research activities deal with classical piezoelectric ceramic materials, but more recently, a promising new type of materials is represented by electroactive polymers (EAPs). Among the various EAPs, polyurethane (PU) elastomers are of great interest due to the significant electrical-field strains, and due to their attractive and useful properties such as flexibility, light weight, high chemical and abrasion resistance, high mechanical strength and easy processing to large area films as well as their ability to be molded into various shapes and biocompatibility with blood and tissues. In addition, it has recently been shown that the incorporation into a PU matrix of nanofillers, such as carbon nanotubes (CNTs), can greatly enhance the expected strain, or the harvested energy. However, it is well known that CNTs are hardly dispersed in a polymeric matrix, and that the interfacial adhesion strength is generally poor. An effective method to improves both dispersion and adhesion may consist in functionalizing CNTs by grafting polymer chains onto their surfaces. The main objective of this thesis was to develop high-efficiency polymers nanocomposites for harvesting energy and actuation. The key motivation was to use polymer-grafted CNTs to improve dispersion, interfacial adhesion in PU, and understand how this can change the electroactive properties of the PU/CNT nanocomposites. In other words, it was a pluridisciplinary project including an optimization of the elaboration process, physical characterizations ˗ including microstructural, electrical and mechanical behaviors in a wide range of frequencies and temperatures ˗ and the determination of the electroactive properties. A comprehensive study was then carried out first on pure PU to understand how their electroactive properties depend on their microstructure, and then on the nanocomposites to understand how the incorporation of functionalized CNT can improve the electromechanical properties
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Saur, Hugo. "Étude des microstructures par tomographie à rayons X : application aux roches clastiques à grain fin." Electronic Thesis or Diss., Pau, 2022. http://www.theses.fr/2022PAUU3005.
Abstract:
L’étude de la microstructure des roches est indispensable pour nos enjeux contemporains et futurs en matière d'énergie, d’ingénierie et de construction. D’autre part, cette étude permet de caractériser les processus de déformation géologique ayant conduit à l’état actuel des unités lithologiques. Les roches clastiques à grain fin, communément appelées "shales" en anglais, représentent environ deux-tiers de l’ensemble des roches sédimentaires. Les données 3D relatives aux grains de silt ou clastes inclus dans la matrice argileuse et poreuse de ce type de roche sont peu fréquentes. Ces données sont pourtant cruciales pour comprendre les propriétés anisotropes à l’échelle macroscopique mais aussi pour évaluer l’état de déformation de la matrice rocheuse. Mieux connaitre la microstructure de ces roches permettrait d’être prédictif quant à leurs propriétés mécaniques ou physiques indispensables pour les applications du secteur de l’énergie par exemple. La tomographie à rayons X (XCT) est une technique non destructive permettant d’obtenir une image 3D de la microstructure d'un objet. Une caractérisation géométrique directe des constituants des roches clastiques à grain fin est envisageable grâce à cette technique. Sur la base d'images XCT, cette thèse vise d’abord à développer des aspects méthodologiques pour étudier la fabrique de forme 3D des clastes ainsi que leur distribution spatiale. Ces aspects sont élaborés à partir de la méthode des moments d’inertie qui est appliquée sur les grains segmentés des images 3D numériques. Nous présentons ensuite des applications sur des roches à grain fin possédant une fabrique sédimentaire et sur des roches à grain fin déformées présentant une fabrique d’origine tectonique. Le premier volet applicatif de la thèse s’intéresse à une même unité lithologique ayant enregistrée différentes quantités de déformation. Des échantillons du bassin sud-pyrénéen et des échantillons issus d’un affleurement historique dans les Appalaches centrales ont été collectés. Nous apportons de nouvelles données sur l’évolution de la forme 3D des grains et des pores à l’échelle micrométrique et sur leur agencement dans la matrice rocheuse en fonction de la déformation. Les données obtenues permettent de discuter des mécanismes de déformation à l’échelle du grain des différentes phases minéralogiques. Cependant, la petite taille des échantillons imagés par XCT (≤ 2 mm de diamètre) soulève la question de la représentativité de ces analyses. Sur le chantier sud-pyrénéen, certains échantillons sont étudiés de manière plus approfondie pour vérifier l’homogénéité des résultats. Nous y montrons que les données XCT complètent les mesures indirectes pétrophysiques en permettant de décrire et de localiser les sous-fabriques intégrées dans une mesure globale de la fabrique. Les limites apparaissent lorsque la dimension caractéristique des structures de déformation avoisine la taille de l'échantillon imagé par XCT. Dans le second volet applicatif, des échantillons provenant de systèmes turbiditiques du bassin sud-pyrénéen sont analysés. Ces systèmes, lorsque déformés en tectonique compressive, présentent l’avantage d’enregistrer la même quantité de raccourcissement différemment exprimée dans les unités lithologiques qui les constituent. Les résultats obtenus à partir de la forme des clastes sont comparés à nos mesures magnétiques globales de la fabrique et montrent une bonne cohérence. La méthodologie présentée dans ce travail peut s’étendre à d'autres types de milieux poreux et granulaires pour une meilleure compréhension de l'influence de l'anisotropie structurale sur leurs propriétés macroscopiques et leur comportement mécaniqueThe study of the microstructure of rocks is essential for our contemporary and future challenges in energy, engineering and construction. Furthermore, this study allows us to characterize the geological deformation processes that led to the current state of geological formations. Fine-grained clastic rocks, commonly called "shales", represent about two-thirds of all sedimentary rocks. 3D data concerning silt-sized grains or clasts embedded in the porous clay-rich matrix of this type of rock are relatively scarce despite the fact that these data are crucial to understand the anisotropic properties of these rocks at the macroscale but also to evaluate the deformation state of the rock matrix. A better understanding of the microstructure of these rocks would allow us to predict their mechanical or physical properties, which are essential for applications in the energy sector, among others. X-ray computed tomography (XCT) is a non-destructive technique providing a 3D image of the microstructure of any object. A direct geometric characterization of the constituents of fine-grained clastic rocks is possible with this technique. Based on XCT images, this thesis aims first to develop methodological aspects to study the 3D shape fabric of silt particles and their spatial distribution. The moments of inertia of segmented grains from 3D digital images are used for this development. We then present applications on fine-grained rocks with a sedimentary fabric and on deformed fine-grained rocks with a tectonic fabric. The first application part of the thesis focuses on the same lithologic unit having experienced different amounts of deformation. Samples from the South Pyrenean Basin and samples from a historical outcrop in the Central Appalachians were collected. We provide new data on the evolution of the 3D shape of grains and pores at the micrometer scale and their arrangement in the rock matrix with respect to the deformation intensity. The obtained data allow discussing the deformation mechanisms at the grain scale of the different mineralogical phases. However, the limited size of the imaged samples by means of XCT (≤ 2 mm diameter) raises the question of the representativeness of these analyses. On the South Pyrenean site, some samples are studied in more detail to evaluate the homogeneity of the results. We show that the XCT data complement the indirect petrophysical measurements by providing access to localized sub-fabrics that are integrated in a bulk measurement of the rock fabric. The limits are reached when the characteristic length of the deformation structures are on the order of the sample size imaged by XCT. In the second application part, samples from turbiditic systems of the South Pyrenean basin are analyzed. These systems, when deformed in compressive tectonic settings, record the same amount of shortening differently expressed in the various siliciclastic matrices. The results obtained from the shape data of the clasts are compared to our bulk magnetic fabric measurements and show a good consistency. The methodology presented in this work can be extended to other types of porous and granular media for a better understanding of the influence of fabric anisotropy on their macroscopic properties and mechanical behavior