Qualitative analysis of a microtomographic apparatus and measurement of the bone tissue density with reference to microgravity conditions

Computed Thomography is a relatively new field in the area of non destructive imaging.It allows to reconstruct the internal structure of opaque objects without destroy them. This is a great advantage compared to conventional microscopy techniques, any optical or electronic microscope, in fact, provides information on the internal structure of samples only if samples are properly processed and sectioned. Information about three-dimensional structure could be obtained by the image of a surface or a combination of several thin slices, but in both cases information cannot be certain since methods of cutting and preparation can dramatically change the structure of the sample. Microcomputed tomography, commonly referred to as µCT, like conventional computed tomography is based on the collection of projections of X rays through a specimen and the application of tomographic principles to reconstruct the 3-D structure of the specimen. Itis based on the interaction of X-rays with matter. The attenuation ofX-rays, passingthrough an object, is dependent on thedensity and atomic number of the object under investigation. This radiation is converted in a radiographic image of the object. Images obtained from different angles are analyzed by analgorithm called Filter back projection in order to reconstruct a virtual slice through the object. When different consecutive slices are reconstructed, a 3D visualizationcan be obtained. The term "micro" denotes a scanning system much higher in resolution than conventional clinical scanners. Clinical tomographic scanners may have resolutions on the order of a millimeter or less. However, high-resolution µCT scanners may have resolutions below five microns. The high resolution of this system makes it useful in the analysis of small objects such as trabecular bone samples. Trabecularbone consists of a complicated three-dimensional network of plates and rods, arranged ina lattice-like network.The architectural parameters of trabecular bone could be strongly influenced by aging or bone diseases such as osteoarthritis or osteoporosis. Until recently, information about thesestructural parameters of trabecular bone were only available by histomorphometry, adestructive procedure limited to two-dimensional analysis. Nowadays Micro-CT, because of its capability to allow three-dimensional and non destructive analysis, found largeapplications in pre-clinical bone research.The increasing incidence and prevalence of bone pathologies on the population, increases the interest of improve an accurate bone characterization by Micro-CT. Micro-CT system, object of this study is the Skyscan 1072, located at the Technology and Health Department of the Italian National Institute of Health.One of the goal of this research is set at optimizing the system for the analysis of bone samples. The first part is dedicated on determining the resolution of the system. The performance of an imaging system is usually described by the measurement of its Modulation Transfer Function or MTF whichgives a description of how much contrast at a specific spatial frequency is maintained by the imaging process.The second part of this study is focused on the process of images reconstruction, fundamental in a Micro-CT analysis. Micro-CT images are affected by several artifacts which will be widely discussed in the following chapters. One of the most difficult artifact is beam hardening. It depends on the polychromatic X-ray tube used in these systems. The X-rays beam investing the sample is composed of X-rays with a spectrum of different energies. The attenuation of an X-ray depends on its energy, the lowestX-ray energies are preferentially absorbed. Assuming that the grey level of CT images corresponds to the linear coefficientof attenuation, which is constants depending on the material, because of the beam hardening, the attenuationof a given material is not strictly proportional to its thickness. This implies visual distortions on the images and the consequent origin of quantitative problems. In order to better understand the effect of beam hardening on Micro-CT images, the filtered back projection algorithm will be implemented in LabVIEW (Version 8.2). The Skyscan 1072 allows to correct the effect of beam hardening during the process of images reconstruction by the definition of a proper parameter. In order to define the correct value of this parameter for a bone sample analysis, a comparison between the results of both the algorithm implemented and the Skyscan reconstruction software will be evaluated. After the optimization of the system for bone analysis, nineteen trabecular bone samples, extracted from femoral heads of eight patients subject to a hip arthroplasty surgery, will be analyzed. The main problem of bone analysis by micro-CT is the processing of the reconstructed cross-sections images for the sample morphometric analysis. The post-processing of the images for the morphometric characterization usually requires a process named binarization of the images which consists on the definition of a threshold value of grey-level, necessary to distinguish bone from background. The choice of this value is a crucial task since a standard method doesn’t exist. Moreover, the inhomogeneity of bone causes another problem during the binarization process. Binarization associates each pixel of the image to bone or air, not considering that each pixel can be composed by both of them. This effect is called Partial Volume Effect and it affects especially pixels at the edges of the analyzed sample. In order to avoidproblems related to the binarization, the main goal of this study is the evaluation of a new method for the histomorphometric analysis of bone sample from the direct processing of the greylevel histogram of the images. Finally, the last part of this research will be dedicated on the remodeling process of bone. The remodeling of bone is an important research topic because of its importance in the study of bone pathologies such as osteoporosis. Osteoporosis is a bone disorder characterized by an inadequate amount and faulty structure of bone, resulting in fractures from relatively minor trauma. It leads to a bone mineral density (BMD) reduction, a bone microarchitecture deterioration and an alteration of the amount and variety of proteins in bone. Aging is the main factor of osteoporosis incidence but in the last years, another factor related to long-duration spaceflight, has been considered. Because of the difficult in reproducing in-vivo space conditions, the development of numerical models is a good alternative for the remodeling process study

Assessment of structural and transport properties in fibrous C/C composite preforms as digitized by X-ray CMT. Part I : Image acquisition and geometrical properties

International audienceRaw and partially infiltrated carbon-carbon composite preforms have been scanned by high-resolution synchrotron radiation X-ray CMT. 3D high-quality images of the pore space have been produced at two distinct resolutions and have been used for the computation of geometrical quantities : porosity, internal surface area, pore sizes, and their distributions, as well as local and average fiber directions. Determination of the latter property makes use of an originalalgorithm. All quantities have been compared to experimental data, with good results. Structural models appropriate for ideal families of cylinders are shown to represent adequately the actual pore space

Contrast-enhanced micro-computed tomography and image processing integrated approach for microstructural analysis of biological soft fibrous tissues

Nel sistema muscolo-scheletrico, tendini e legamenti svolgono un ruolo importante al fine di garantire mobilità e stabilità. Questi tessuti sono composti principalmente da collagene e presentano una struttura altamente fibrosa. Evidenziare i componenti della microstruttura di legamenti e tendini in immagini tridimensionali (3D) è di fondamentale importanza per estrarre informazioni significative che posso anvere ripercussioni sulla scienza di base e sulle applicazioni ortopediche. In particolare, le proprietà meccaniche delle microstrutture fibrose sono fortemente influenzate da alcune caratteristiche geometriche, come la volume fraction, l’orientamento e il diametro; tuttavia, determinare l'orientamento e il diametro della fibra 3D è impegnativo. In questa prospettiva, questa tesi mirava ad unire tomografia microcomputerizzata (microCT) ed elaborazione delle immagini in un approccio integrato al fine di identificare e migliorare le informazioni microstrutturali sui tessuti biologici fibrosi, includendo i dati di volume e orientamento. La procedura complessiva è stata applicata per la prima volta su campioni di tendine del ginocchio umano e su legamento collaterale bovino. In una prima fase, sono state testate preparazioni specifiche del campione, inclusa una disidratazione chimica o soluzioni di acido fosfotungstico (PTA) al 2 % in acqua (H2O) o in soluzione di etanolo al 70% (EtOH), così da migliorare il contrasto dell'immagine di questi specifici tessuti. Inoltre, utilizzando i dati scansionati, è stata sviluppata una nuova tecnica di elaborazione delle immagini basata sul filtro 3D hessiano multiscala per evidenziare le strutture fibrose ed ottenere informazioni quantitative sulle fibre. È interessante notare che, per qualsiasi strategia di preparazione del campione di tendini/legamenti, l'approccio proposto è risultato adeguato per rilevare e caratterizzare le proprietà del fascicolo. I risultati del test hanno mostrato che la disposizione delle fibre è fortemente allineata lungo la direzione longitudinale principale nel tendine del tendine, più delle fibre del legamento collaterale bovino. Inoltre, questa tecnica è stata ulteriormente applicata al fine di determinare come il Legamento Crociato Anteriore (LCA) umano risponda a carichi uniassiali rispetto a valori crescenti di deformazione, considerando sia un tessuto sano che uno in condizioni patologiche, cioè acquisito da un paziente con l'artrosi. Anche in questi casi, l'approccio integrato si è rivelato valido ed affidabile nell'individuare orientamento e dimensione dei fascicoli presenti e, quindi, attraverso un modello meccanico strutturale - basato su specifiche leggi costitutive - nello stimare il modulo elastico di questi tessuti. Sono state infatti stimate le curve sforzo-deformazione, ottenendo un valore di modulo elastico di 60.8 MPa e 7.7 MPa rispettivamente per il LCA sano e patologico. In conclusione, è stato progettato e validato in via preliminare un nuovo protocollo microCT per il miglioramento del contrasto dedicato all'analisi microstrutturale dei tessuti molli biologici con caratteristiche fibrose. In una peculiare applicazione al LCA, le informazioni ottenute con il protocollo sono state utilizzate per implementare un modello meccanico dei tessuti fibrosi, stimando così il comportamento biomeccanico dei tessuti sani e patologici.ABSTRACT In the musculoskeletal system, tendons and ligaments play an important role in ensuring mobility and stability. These tissues are primarily composed of collagen and present a highly fibrous structure. Highlighting the microstructure components of ligaments and tendons in three-dimensional (3D) images is crucial for extracting meaningful information impacting basic science and orthopaedic applications. In particular, the mechanical properties of the fibrous microstructures are strongly influenced by their volume fraction, orientation, and diameter. However, determining the 3D fibre orientation and diameter is challenging. In this picture, this thesis aimed at integrating microcomputed tomography (microCT) and image processing approach to identify and enhance microstructural information about biological soft fibrous tissues, including volume and orientation. The overall procedure was first applied on human hamstring tendon and bovine collateral ligament samples. In a first phase specific sample preparations – including either a chemical dehydration, or by 2% of phosphotungstic acid (PTA) in water (H2O) or in 70% ethanol (EtOH) solution – were tested to enhance image contrast of these specific soft tissues. Further, using the scanned data, a novel image processing technique based on 3D Hessian multiscale filter for highlighting fibrous structures was developed to obtain quantitative fibre information. Interestingly, for any strategy of tendon/ligament sample preparation, the proposed approach was adequate for detecting and characterizing fascicle features. The test results showed the fibre arrangement strongly aligned along the main longitudinal direction in the human hamstring tendon more than fibres on the bovine collateral ligament. Moreover, this technique was further applied in order to determine how the human Anterior Cruciate Ligament (ACL) responds to uniaxial loads with respect to increasing values of strain, considering both a healthy tissue and a one under pathological conditions, i.e., acquired from a patient with osteoarthritis. Also in these cases, the integrated approach was valuable and reliable in identifying orientation and size of present fascicles and, thus, through a structural mechanical model - based on specific constitutive law - to estimate the elastic modulus of these tissues. In fact, stress-strain curves were estimated, obtaining a value of elastic modulus of 60.8 MPa and 7.7 MPa for the healthy and pathological ACLs, respectively. In conclusion, a novel contrast enhancement microCT protocol was designed and preliminarily validated for the microstructural analysis of biological soft fibrous tissues. In a peculiar application to ACL, the information obtained with the protocol was used to implement a mechanical model of fibrous tissues, thus estimating the biomechanical behaviour of the healthy and pathological tissues

Lattice Boltzmann simulation of liquid water transport in gas diffusion layers of proton exchange membrane fuel cells: Parametric studies on capillary hysteresis

Water management is crucial for reliable operation of Polymer Electrolyte Membrane Fuel Cells (PEMFC). Here, the gas diffusion layer (GDL) plays an essential role as it has to ensure efficient water removal from and oxygen transport to the catalyst layer. In this study water transport through porous carbon felt GDLs was simulated using a 3D Color-Gradient Lattice Boltzmann model. Simulations were carried out on microstructures of plain and impregnated fiber substrates of a Freudenberg H14. The GDL microstructures were reconstructed from high-resolution X-ray micro-computed tomography ($\mu$-CT). For the distinction of carbon fibers and polytetrafluoroethylene (PTFE) in the binarized microstructures an in-house algorithm was developed. The additive was specified heterogeneously in the GDL through-plane direction employing a PTFE loading profile as derived based on $\mu$-CT image data. In the in-plane direction the additive was furthermore defined in a realistic fashion near carbon fiber intersections. Prior to parametric studies on capillary behavior a sophisticated modeling approach for semipermeable membranes had to be developed to account for experimental boundary conditions. Capillary hysteresis was then investigated by simulation of intrusion and drainage curves and subsequent comparison to testbench data

Pore network analysis of Brae Formation sandstone, North Sea

In this work, we apply digital rock physics (DRP) to characterize the pore networks of the Brae Formation sandstones from two different wells in the Miller field area (North Sea, UK). Using X-ray micro-CT scans, we calculate the porosity and permeability and generate pore network models to assess pore shape characteristics. The porous samples are marked by macroporosities ranging from 4.9% to 15.2% with the effective porosities varying from 0 to 14.8%. The samples also contained some microporosity hosted in secondary and accessory mineral phases, varying between 2.6% and 10.7%. Pore network model results for total porosity indicate that the samples have median pore and throat radii ranging from 5.5 μm to 16.8 μm and 6.4 μm–12.9 μm, respectively. The throat length of all samples has a median value ranging between 36.3 μm and 82.4 μm. The ratio between effective porosity and total porosity (φ∗) varies with total porosity (φ) following the exponential relation φ∗ = 0.98 − e− (φ− 0.032)/0.028. Pore network connectivity is established at a porosity of 3% and full communication is achieved at porosities exceeding 10%. Permeability was found to vary with total porosity with an exponent of 3.67. Based on these observations and the results from our models, the connectivity of the pore network has important implications for predicting reservoir performance during large scale subsurface projects such as hydrocarbon production and CO2 storage

Calibrating micro-computed tomography data to permeability experiments and petrography – insights from Digital Rocks

Petrophysical measurements on core plugs integrated with petrographic information from thin-sections are established methods in reservoir quality assessment. X-ray micro-computed tomography (μCT) presents an opportunity to derive the internal structure of reservoir sandstones for digital fluid flow simulations, while si-multaneously assessing mineral distribution in 3D based on mineral densities. We compare the sin-gle-phase permeabilities obtained with fluid flow sim-ulations and experiments and discuss the anisotropic nature of the permeability tensor in both single- and two-phase flow. The results demonstrate a closer match for μCT porosity to petrophysical porosity com-pared to optical porosity, and an acceptable first order fit of the main mineralogical constituents. One-phase fluid flow simulations deliver results within 10–20 % of the laboratory measurements. Two-phase flow sim-ulations enable the assessment of relative permeabili-ties in rocks with water-sensitive minerals. However, μCT-based fluid flow simulations are computationally very demanding and time consuming due to the heter-ogeneous nature of natural sandstone samples, and require a tradeoff between resolution, representative volume, and cost. Rock composition reconstructed from μCT images can be used as a first-order approxi-mation for the composition of a sample, but is unable to confidently identify minerals that occur in minor quantities due to constraints of the chosen resolution. Thus, sandstone analyses by μCT cannot completely replace established methods