Dissertations / Theses on the topic 'Wire Tomography'
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Kumar, Santhosh Tekke Athayil, and santhosh kumar@anu edu au. "Experimental Studies of Magnetic Islands, Configurations and Plasma Confinement in the H-1NF Heliac." The Australian National University. Research School of Physical Sciences and Engineering, 2008. http://thesis.anu.edu.au./public/adt-ANU20080611.171513.
Full textBílek, Petr. "Rozvoj a využití nedestruktivních zkušebních metod z hlediska soudního inženýrství." Doctoral thesis, Vysoké učení technické v Brně. Ústav soudního inženýrství, 2019. http://www.nusl.cz/ntk/nusl-408059.
Full textPrasser, Horst-Michael. "3. Workshop "Meßtechnik für stationäre und transiente Mehrphasenströmungen", 14. Oktober 1999 in Rossendorf." Forschungszentrum Dresden, 2010. http://nbn-resolving.de/urn:nbn:de:bsz:d120-qucosa-30118.
Full textPrasser, Horst-Michael. "3. Workshop "Meßtechnik für stationäre und transiente Mehrphasenströmungen", 14. Oktober 1999 in Rossendorf." Forschungszentrum Rossendorf, 1999. https://hzdr.qucosa.de/id/qucosa%3A21838.
Full textLlamas, Juan David. "Étude expérimentale de la maldistribution des fluides dans un réacteur à lit fixe en écoulement à co-courant descendant de gaz et de liquide." Thesis, Vandoeuvre-les-Nancy, INPL, 2008. http://www.theses.fr/2008INPL006N/document.
Full textThree different measuring techniques were used to study the fluid distribution inside a trickle-bed reactor: the wire mesh tomography, the liquid collector and a set of thermistors. The liquid collector and specially the wire mesh tomography, whose first application in trickle bed reactors is described here, yielded interesting results concerning the influence of variables such as the initial liquid distribution, the loading method and the fluid flow rates on liquid maldistribution. Among the main observations, the study illustrates the importance of well defining liquid maldistribution in terms of the measured quantity and prompts to some caution when referring to some “normally accepted facts” like the advantages in terms of liquid distribution obtained when increasing the gas flow rate (which depends, according to this study, on the quality of initial liquid distribution) or when using a dense loading of the catalyst (the hypothesis according to which, compared with a sock loading, dense loading favors radial dispersion was not verified by the study). Also, a study performed under high interaction conditions showed the intimate relationship between the inlet distribution and the flow regime observed inside the reactor
Berzeg, Semin. "Drahtmarkierung der pulmonalen Rundherde." Doctoral thesis, Humboldt-Universität zu Berlin, Medizinische Fakultät - Universitätsklinikum Charité, 2006. http://dx.doi.org/10.18452/15427.
Full textWith the increasing use of multi-slice computed tomography (MSCT) pulmonary nodules are more often detected. The propose of our study was to evaluate use of the computed tomography guided spiral-wire localization before video-assisted thoracoscopic surgery (VATS). 54 patients with pulmonary nodules underwent ct-guided application of a spiralwire lung localization-set (Somatom, Berlin, Germany). The average age was 58 years, the mean distance between the nodule and the visceral pleura was 7,9 mm and the mean diameter was 13,1 mm. Following this procedure, the patients underwent a VATS resection. In all patients the placement of the spiral-wire was succesful without any dislocation. The major complications were: Asymtomatic pneumothorax, local bleeding and in 3 patients a combination of local bleeding and a minor pneumothorax. In conclusion wire localization of peripheral pulmonary nodules is a safe and effective procedure to assist VATS resection.
McCaughey, Michael. "Travel-time tomography of wide-angle seismic data." Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.389868.
Full textLester, W. Ryan. "Structure of the Chesapeake Bay Impact Crater from Wide-Angle Seismic Waveform Tomography." Thesis, Virginia Tech, 2006. http://hdl.handle.net/10919/35090.
Full textMaster of Science
Courtois, Loïc. "Monofilament entangled materials : relationship between microstructural properties and macroscopic behaviour." Thesis, Lyon, INSA, 2012. http://www.theses.fr/2012ISAL0138.
Full textPlaying with the architecture of a material is a clever way of tailoring its properties for multi-functional applications. A lot of research have been made, in the past few years, on what is now referred to as “architectured materials” (metal foams, entangled materials, steel wool, etc), mostly for their capacity to be engineered in order to present specific properties, inherent to their architecture. In this context, some studies have been carried out concerning entangled materials but only a few on monofilament entangled materials. Such a material, with no filament ends, could exhibit interesting properties for shock absorption, vibration damping and ductility. In this study, entanglements were manually produced, using different types of wire, and submitted to constrained (inside a PTFE die) in-situ compressive tests within the laboratory tomograph. This technique enabled a 3D, non destructive, microstructural characterization of the complex architecture of these materials, along with the analysis of their macroscopic mechanical properties. The stiffness of this material was found to be in a 20-200 MPa range and homogeneous samples could be obtained, while lowering their stiffness, by pre-deforming the initial wire as a spring. Damping measurements were performed using different types of entanglements (constitutive materials, volume fraction, wire diameter, wire shape) under both monotonic and dynamic loadings and directly linked to the measurements of the number of contacts. The Dynamic Mechanical Analysis underlined the great capacity of this material to absorb energy with a loss factor of about 0.25 and damping was found to decrease with the stiffness of the entanglement. The mechanical properties of this material were first modeled using an analytical “beam” model based on the experimental evolution of the mean distance between contacts and a good agreement was found with the experimental results. In parallel, a Discrete Element Method was used in order to model the compressive behaviour of Monofilament Entangled Materials. Although purely elastic properties were taken into account in the model, a very good agreement with the experimental results was obtained by adjusting the friction coefficients of the model. This tends to prove that the plasticity of these entangled materials is rather due to the structure (friction) than to the constitutive material itself
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.
Full textOliveira, Vanda Cristina Paiva Tavares de. "Cork structural characteristics and their influence on the oxygen ingress through wine stoppers." Doctoral thesis, ISA-UL, 2016. http://hdl.handle.net/10400.5/12037.
Full textCork structural characteristics and their influence on the oxygen ingress through wine stoppers were studied aiming to contribute to an increased added-value of the natural cork stoppers. The surface porosity features of cork stoppers can differentiate the three main commercial classes used nowadays: the porosity coefficient was 2.4%, 4.0% and 5.5% for premium, good and standard stoppers, respectively. Image analysis also distinguished defects in the cork structure: empty ant gallery; Coroebus undatus F. larvae gallery; and wetcork. Several predictive classification models of stoppers into quality classes were built using the results from cork stoppers surface characterization and a simplified model using the main discriminant features i.e. porosity coefficient and the RGB colour-type variables was presented. X-ray tomography was used as a non-destructive technique to study the internal structure of natural cork stoppers, allowing the visualization of some defects inside the cork stopper. After characterization, the natural cork stoppers were used as closure of bottles and oxygen diffusion measurements were made along time. The kinetics of oxygen transfer was similar and could be adjusted to logarithmic models. On average 35% of the overall oxygen ingress occurred in the first 5 days, 59% in the 1st month and 78% in the first 3 months. Microtomography images (voxel size of 50 µm) allowed the observation of lenticular channels development and geometry, and the quantification of void and high density regions (HDR) fractions. The evidence that the void fraction of lenticular channels in the innermost part of the cork stopper inserted in the bottle was strongly related to the oxygen ingress in the first month after bottling can be used for quality enhancement of natural cork stoppers with incorporation of performance requirements
Patel, Neepa. "A cone-beam computerized tomography evaluation of anterior root resorption comparing SureSmile® and conventional edgewise treatments." Oklahoma City : [s.n.], 2009.
Find full textMaciulevičius, Mindaugas. "The Light Scattering in Optical Coatings and Laser Components in a Wide Spectral Range." Doctoral thesis, Lithuanian Academic Libraries Network (LABT), 2010. http://vddb.laba.lt/obj/LT-eLABa-0001:E.02~2009~D_20100204_101017-90059.
Full textŠiame darbe yra aptariama sklaidos matavimo sistema sukurta plačioje spektro srityje reikalingiems matavimams atlikti. Joje naudojami parametriniai šviesos generatoriai bei harmonikų generatoriai, kaupinami nanosekundinės trukmės impulsais, ir apibendinami sklaidos tyrimai, atlikti įvairių tipų optinėse dangose ir netiesiniuose optiniuose kristaluose, naudojant derinamojo bangos ilgio lazerinius impulsus. Pirmą kartą buvo ištirti naujų netiesinėje optikoje perspektyvių LiInS2 ir LiInSe2 kristalų sklaidos nuostoliai infraraudonojoje srityje. Taip pat parodyta, kad koherentinės sklaidos tomografijos metodas, anksčiau taikytas puslaidininkinių kristalų tūriniams defektams tirti, yra tinkamas ir netiesinėje optikoje naudojamų kristalų kokybės tyrimams.
Razifar, Pasha. "Novel Approaches for Application of Principal Component Analysis on Dynamic PET Images for Improvement of Image Quality and Clinical Diagnosis." Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis : Univ.-bibl. [distributör], 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-6053.
Full textHoseini, Yazdi Seyed Hosein. "Spatial characteristics of the response of the human choroid to imposed defocus." Thesis, Queensland University of Technology, 2018. https://eprints.qut.edu.au/120362/1/Seyed%20Hosein_Hoseini%20Yazdi_Thesis.pdf.
Full textParisot, Amelie. "Calibrations et stratégies de commandes tomographique pour les optiques adaptatives grand champ : validations expérimentales sur le banc HOMER." Thesis, Aix-Marseille, 2012. http://www.theses.fr/2012AIXM4789/document.
Full textAdaptive Optics (AO) provides a real-time correction of the atmospheric turbulence effects. This technique is now well mastered; nonetheless it is limited by the anisoplanatism effect. Wide Field AO concepts have been developed to overcome this limitation. Turbulence is probed in several directions in order to perform a tomographic reconstruction of the turbulent volume. These complex systems raise critical challenges such as tomographic control and calibrations.My PhD work is focused on implementation and comparison of different tomographic control schemes developed in the perspective of future systems, after an optimisation of the Onera wide field AO bench. Calibration and integration of new components have been performed, and I have developed a method to identify system parameters with a twofold goal: bench alignment and control laws optimisation. Four control schemes have been studied, exploring the proposed solutions, from the simplest least-square to the optimal linear quadratic gaussian solutions including virtual deformable mirror and pseudo open loop approaches. In each case, an optimisation of tuning factors is performed and low and high noise conditions are explored, for several different fields of views. Experimental results are compared to numerical ones and control laws are analyzed in term of performance, robustness and implementation simplicity
Meléndez, i. Catalán Adrià. "Development of a New Parallel Code for 3-D Joint Refraction and Reflection Travel-Time Tomography of Wide-Angle Seismic Data. Synthetic and Real Data Applications to the Study of Subduction Zones." Doctoral thesis, Universitat de Barcelona, 2014. http://hdl.handle.net/10803/289786.
Full textAquesta tesi està dedicada a la tomografia sísmica. Concretament, he implementat una eina de modelització 3D per a la tomografia conjunta de temps de trajecte de refraccions i reflexions (TOMO3D). La raó darrere d'aquest objectiu és l'evidència de que la informació basada en dades sísmiques 2D no permet copsar la complexitat de gran part dels cossos geològics, i en particular de la zona sismogènica en marges de subducció. El desenvolupament del TOMO3D es basa en el TOMO2D, un codi d'avantguarda per a la tomografia conjunta de refraccions i reflexions en 2D. Els arxius de codi han estat reescrits, redefinint i introduint les funcions necessàries per dur a terme la inversió 3D. Els testos fets amb la versió seqüencial del codi posen de manifest la necessitat de paral·lelització ja que l'increment de la mida dels conjunts de dades així com la modelització de la dimensió espacial afegida fan que les inversions siguin computacionalment exigents. La versió paral·lelitzada del TOMO3D ha sigut aplicada a un cas sintètic complex que simula una zona de subducció. Aquesta primera aplicació 3D serveix per avaluar la correcció de la programació del codi, i com a descripció pas a pas del procediment de modelització. Els resultats demostren la capacitat del codi per recuperar acuradament la distribució de velocitat i la geometria dels dos reflectors. Finalment, el TOMO3D és aplicat a un conjunt 3D de dades de sísmica de gran angle adquirit al marge pacífic d'Equador i Colòmbia per extreure'n un model 3D de la velocitat de les plaques cavalcant i subduïda, que és comparat amb el resultat obtingut amb un codi 3D de tomografia de temps de trajecte de refraccions (FAST). La comparació indica que el TOMO3D és més acurat que el FAST però al mateix temps és computacionalment més exigent. Tot i així, la paral·lelització del TOMO3D permet utilitzar plataformes de supercomputació, a diferència del que passa amb el FAST i la majoria de codis existents.
Kirner, Stefan [Verfasser], Jochen [Akademischer Betreuer] Schein, Jochen [Gutachter] Schein, and Thomas [Gutachter] Klassen. "Tomographic Two-Color-Pyrometry of the Wire Arc Spray Process regarding Particle Temperature and in-flight Particle Oxidation / Stefan Kirner ; Gutachter: Jochen Schein, Thomas Klassen ; Akademischer Betreuer: Jochen Schein ; Universität der Bundeswehr München, Fakultät für Elektrotechnik und Informationstechnik." Neubiberg : Universitätsbibliothek der Universität der Bundeswehr München, 2018. http://d-nb.info/1175916986/34.
Full textKirner, Stefan Verfasser], Jochen [Akademischer Betreuer] [Schein, Jochen Gutachter] Schein, and Thomas [Gutachter] [Klassen. "Tomographic Two-Color-Pyrometry of the Wire Arc Spray Process regarding Particle Temperature and in-flight Particle Oxidation / Stefan Kirner ; Gutachter: Jochen Schein, Thomas Klassen ; Akademischer Betreuer: Jochen Schein ; Universität der Bundeswehr München, Fakultät für Elektrotechnik und Informationstechnik." Neubiberg : Universitätsbibliothek der Universität der Bundeswehr München, 2018. http://d-nb.info/1175916986/34.
Full textMihoubi, Abdelhafid. "Imagerie sismique de la structure profonde de la marge Algérienne orientale (secteur de Jijel) : implications en terme de potentiel pétrolier." Thesis, Brest, 2014. http://www.theses.fr/2014BRES0037/document.
Full textThis thesis has been conducted within the framework of the Algerian-French research cooperation program SPIRAL (Sismique Profonde et Investigations Régionales du Nord de l’Algérie). This project aims to study the deep structure of the Algerian margin. The area covered by this study focuses in the region of Jijel in eastern Algerian margin.The main objective of our thesis is to improve depth imaging of the Algerian margin using a combined approach of seismic techniques; wide-angle and multi- channel seismic data. The purpose of this thesis is to bring new knowledge to answer some questions about the nature of the crust, the area of continental -oceanic transition, the presence of Messinian salt, its distribution and relationship between surface sedimentary formations and crustal structures.This study presents the results of a deep seismic survey across the north Algerian margin, based on the combination of 2D multi-channel and wide-angle seismic data simultaneously recorded by 41 ocean bottom seismometers deployed along a North-South line extending 180 km off Jijel into the Algerian offshore basin, and 25 land stations deployed along a 100 km-long line, cutting through the Lesser Kabylia and the Tellian thrust-belt.In this study, our approach is a joint inversion of wide-angle seismic recordings (OBS, ocean bottom seismometers) and multi- channel seismic data (MCS). We conducted a series of first arrivals tomography, a joint inversion of reflected and refracted arrivals and gravity modelling. Since the solution of the inverse problem is not unique, two tomography programs were applied using the same data for the same study area; FAST (First Arrival Seismic Tomography) and Tomo2D. Tomography was followed by a joint inversion of reflected and refracted arrivals following an approach based on the combination of Kirchhoff prestack depth migration (PSDM) for MCS data and forward modelling of OBS. To check the consistency of the velocity model with gravity data, the free air anomaly was modeled.The final model obtained using forward modelling of the wide-angle data and pre-stack depth migration of the seismic reflection data provides an unprecedented view of the sedimentary and crustal structure of the margin. The sedimentary layers in the Algerian basin are 3.75 km thick to the north and up to 4.5 to 5 km thick at the foot of the margin. They are characterised by seismic velocities from 1.9 km/s to 3.8 km/s. Messinian salt formations are about 1 km thick in the study area, and are modelled and imaged using a velocity between 3.7 km/s to 3.8 km/s. The crust in the deep sea basin is about 4.5 km thick and of oceanic origin, presenting two distinct layers with a high gradient upper crust (4.7 km/s - 6.1 km) and a low gradient lower crust (6.2 km/s - 7.1 km/s). The upper mantle velocity is constrained to 7.9 km/s. The ocean-continent transition zone is very narrow between 15 km to 20 km wide. The continental crust reaches 25 km thickness as imaged from the most landward station and thins to 5 km over a less than 70 km distance. The continental crust presents steep and asymmetric upper and lower crustal geometry, possibly due to either asymmetric rifting of the margin, an underplated body, or flow of lower crustal material towards the ocean basin. Present-time deformation, as imaged from 3 additional seismic profiles, is characterized by an interplay of gravity-driven mobile-salt creep and active thrusting at the foot of the tectonically inverted Algerian margin
Gottschlag, Holger [Verfasser]. "Small animal positron emission tomography with multi-wire proportional counters / vorgelegt von Holger Gottschlag." 2010. http://d-nb.info/1007715235/34.
Full textKumar, Santhosh Tekke Athayil. "Experimental Studies of Magnetic Islands, Configurations and Plasma Confinement in the H-1NF Heliac." Phd thesis, 2007. http://hdl.handle.net/1885/49287.
Full textLiao, Ying-Lan, and 廖英蘭. "Radiation Dosimetry of Wide Collimation Width Computed Tomography." Thesis, 2014. http://ndltd.ncl.edu.tw/handle/19794494995134160102.
Full text國立清華大學
生醫工程與環境科學系
102
The purpose of the study was to (1) estimate the amount of underestimation of radiation dose using conventional CT dose index theory; (2) to implement the methodology of wide beam CT dosimetry suggested by report of AAPM task group 111 on a 320-detector row CT scanner. The derived values of equilibrium dose, equilibrium dose-pitch product, and equilibrium dose constant and related dose indexes using a thimble ionization chamber and 600 mm length cylindrical phantom were provided as reference tables for users who has the same CT scanner. (3) to assess the radiation dose distribution of clinical CT examinations with latest CT dose and image quality optimization techniques which included automatic tube current modulation (ATCM) techniques, adaptive iterative dose reduction 3D (AIDR3D), and dynamic wide volume scan mode. The database of our study can be an reference for improving the radiation dose measurement for wide collimation width computed tomography.
"Tomographic methods for multidimensional Born inversion with a wide-band source." Laboratory for Information and Decision Systems, Massachusetts Institute of Technology, 1985. http://hdl.handle.net/1721.1/1159.
Full textPolans, James Matthew. "Enhancing the Visualization of the Peripheral Retina with Wide Field-of-View Optical Coherence Tomography." Diss., 2016. http://hdl.handle.net/10161/12152.
Full textThe goal of my Ph.D. thesis is to enhance the visualization of the peripheral retina using wide-field optical coherence tomography (OCT) in a clinical setting.
OCT has gain widespread adoption in clinical ophthalmology due to its ability to visualize the diseases of the macula and central retina in three-dimensions, however, clinical OCT has a limited field-of-view of 300. There has been increasing interest to obtain high-resolution images outside of this narrow field-of-view, because three-dimensional imaging of the peripheral retina may prove to be important in the early detection of neurodegenerative diseases, such as Alzheimer's and dementia, and the monitoring of known ocular diseases, such as diabetic retinopathy, retinal vein occlusions, and choroid masses.
Before attempting to build a wide-field OCT system, we need to better understand the peripheral optics of the human eye. Shack-Hartmann wavefront sensors are commonly used tools for measuring the optical imperfections of the eye, but their acquisition speed is limited by their underlying camera hardware. The first aim of my thesis research is to create a fast method of ocular wavefront sensing such that we can measure the wavefront aberrations at numerous points across a wide visual field. In order to address aim one, we will develop a sparse Zernike reconstruction technique (SPARZER) that will enable Shack-Hartmann wavefront sensors to use as little as 1/10th of the data that would normally be required for an accurate wavefront reading. If less data needs to be acquired, then we can increase the speed at which wavefronts can be recorded.
For my second aim, we will create a sophisticated optical model that reproduces the measured aberrations of the human eye. If we know how the average eye's optics distort light, then we can engineer ophthalmic imaging systems that preemptively cancel inherent ocular aberrations. This invention will help the retinal imaging community to design systems that are capable of acquiring high resolution images across a wide visual field. The proposed model eye is also of interest to the field of vision science as it aids in the study of how anatomy affects visual performance in the peripheral retina.
Using the optical model from aim two, we will design and reduce to practice a clinical OCT system that is capable of imaging a large (800) field-of-view with enhanced visualization of the peripheral retina. A key aspect of this third and final aim is to make the imaging system compatible with standard clinical practices. To this end, we will incorporate sensorless adaptive optics in order to correct the inter- and intra- patient variability in ophthalmic aberrations. Sensorless adaptive optics will improve both the brightness (signal) and clarity (resolution) of features in the peripheral retina without affecting the size of the imaging system.
The proposed work should not only be a noteworthy contribution to the ophthalmic and engineering communities, but it should strengthen our existing collaborations with the Duke Eye Center by advancing their capability to diagnose pathologies of the peripheral retinal.
Dissertation
Stephenson, Andrew. "Crustal velocity structure of the Southern Nechako Basin, British Columbia, from wide-angle seismic traveltime inversion." Thesis, 2010. http://hdl.handle.net/1828/3145.
Full textTsai, Houng-Ruai, and 蔡宏睿. "The Velocity Structures of Gas Hydrate from Wide-angle Seismic Tomography in the Offshore SW Taiwan." Thesis, 2007. http://ndltd.ncl.edu.tw/handle/13182919033092806340.
Full text國立臺灣海洋大學
應用地球科學研究所
95
Offshore southwestern Taiwan is located at the junction of the South China Sea continental margin and the accretionary wedge of the Luzon subduction-collision system. Many bottom simulating reflectors ( BSRs ) have been observed in offshore southwest Taiwan. These indicate that a potential submarine gas hydrate deposits may exist in this area. In 2004, a seismic reflection survey in an area of 20 × 20 km2 in of a reported high potential gas hydrate area and some 24 Ocean Bottom Seismographs ( OBSs ) records were acquired in a semi-3D grid. A total of 4 profiles oriented in E-W and N-S direction, it is aimed with a goal to map the velocity structure of marine sediments. In this study area, the BSR has been previously identified in the multi-channel seismic reflection data. The thickness of BSR is about 200-400 meters beneath the seafloor. By considering the velocity information near the survey region, we build an initial velocity-interface model base on the time-to-depth conversion from the picked horizons of the near-offset sections. Then, using the refraction and reflection travel-time data of OBSs, it is attempted to inverse the velocity structure. By contrasting the velocity structures and that of the multi-channel seismic reflection data, we have found that most of gas hydrate ( 2.0 – 2.6 km / s ) accumulated in the passive continental margin and west of Young-An Lineament, where the distribution of the free gas ( 1.2 – 1.6 km / s ) is not affected by Young-An Lineament. In the study area across the Young-An Lineament, we estimate that the total volume of gas hydrate is 26 km3 which is equivalent to 624 km3 for a reservoir of the gas hydrate.
Sousa, Jorge Miguel Tavares Couceiro. "Simulation study of a wide axial field of view positron emission tomography system based on resistive plate chamber detectors." Doctoral thesis, 2014. http://hdl.handle.net/10316/23827.
Full textO presente trabalho teve por objectivo avaliar, através de simulações em GEANT4, os parâmetros de desempenho de um tomógrafo de Tomografia por Emissão de Positrões (PET) com 2400 mm de comprimento do Campo de Visão Axial (AFOV), baseado em detectores do tipo Câmaras de Placas Resistivas (RPC). Para estabelecer uma base de comparação para esse estudo, investigou se a dependência da sensibilidade a coincidências verdadeiras de um tomógrafo baseado em detectores de BGO em função do ângulo polar de aceitação e do comprimento do AFOV, segundo as normas NEMA NU2 1994. O tomógrafo foi definido como um anel de tungsténio de modo a obter os pontos de entrada dos fotões, tendo-se usado factores de correcção para ter em conta a fracção de empacotamento e a eficiência de detecção em função da segmentação do tomógrafo GE Advance®. Concluíu se que a sensibilidade para eventos verdadeiros é dominada pelo ângulo sólido, aumentando significativamente com o comprimento do AFOV e com o ângulo polar de aceitação, enquanto a Fracção de Radiação Dispersa (SF), se revelou quase independente da geometria, dependendo no entanto do ângulo polar de aceitação. A sensibilidade para coincidências verdadeiras obtida para um AFOV de 2400 mm e plena aceitação no ângulo polar foi cerca de 100 vezes maior que a do tomógrafo GE Advance®. Complementarmente desenvolveu se um modelo analítico simples para a sensibilidade a coincidências verdadeiras que revelou um acordo razoável com os dados de simulação. De seguida fez-se um estudo semelhante para um tomógrafo baseado em detectores de RPC, tendo o mesmo sido definido de maneira análoga. As eficiências de detecção foram obtidas simulando uma pilha de 121 placas de vidro (400 µm de espessura) separadas por 120 camadas de gás (350 µm de espessura). Verificou se que a sensibilidade para coincidências verdadeiras seguia a mesma tendência apresentada pelo tomógrafo baseado em detectores de BGO, atingindo para os 2400 mm de AFOV e plena aceitação no ângulo polar uma sensibilidade cerca de 20 (5) vezes mais elevada do que no caso do tomógrafo GE Advance® usando um ganho de TOF de 4,4 (sem ganho de TOF), e uma SF de 46,4%, excluindo a dispersão no detector. Procedeu se então a simulações detalhadas com vista à optimização do detector RPC a usar num tomógrafo PET de forma paralelepipédica definido por quatro cabeças de detecção com um AFOV de 2400 mm. Cada cabeça de detecção contém uma pilha de 20 detectores RPC, cada um com dois módulos de detecção com 5 camadas de gás (350 µm de espessura) delimitados por 6 placas de vidro. Definiram se os materiais e as espessuras para as camadas isoladoras, os eléctrodos de alta tensão e os de recolha de sinal, tendo se obtido um espessura óptima de 200 µm para as placas de vidro para detecção de fotões de 511 keV, e uma fracção de eventos mal identificados de 32% para uma distância das Linhas de Resposta (LORs) ao ponto de aniquilação igual ou inferior a 2 mm. Estudou-se também a resolução espacial do referido tomógrafo com simulações detalhadas em GEANT4. Os dados de simulação foram processados por forma a ter em conta a electrónica de leitura dos protótipos de RPCs desenvolvidos para teste. As coincidências foram efectuadas recorrendo a um classificador de coincidências de janela temporal simples, aceitando se as LORs com ângulo polar igual ou inferior a 9º. Avaliou se a resolução especial de acordo com as recomendações da norma NEMA NU2 2001, mas considerando apenas uma fonte pontual com 1 µm de diâmetro localizada no centro de uma esfera de polymetil metacrilato com 2 mm de diâmetro, que foi posicionada no plano transaxial central, desviada 100 mm do eixo segundo as duas direcções do referido plano. A resolução espacial encontrada foi de 0,9, 1,4 e 2,1 mm, respectivamente, para um segmentação do detector de 0, 1 e 2 mm nas direcções transaxial e axial, e de 3,44 mm na direcção radial. Estudou se ainda a SF, as taxas de contagem e a Taxa de Contagem Equivalente de Ruído (NECR) do tomógrafo já descrito, seguindo as normas NEMA NU2 2001. Os dados de simulação foram processados de modo a ter em conta a electrónica de leitura do detector. O melhor esquema de processamento para optimizar o NECR consistiu em efectuar as coincidências recorrendo a um classificador de coincidências do tipo janela temporal múltipla, aceitação total no ângulo polar, rejeição das LORs cujo o ponto reconstruído directamente por TOF cai fora de uma região de interesse com 2 cm de margem relativamente às dimensões do fantoma, e aceitando todos os possíveis pares de coincidências, incluindo todas as combinação possíveis retiradas das coincidências múltiplas. Considerando um tempo morto para a leitura em posição de 3.0 µs e para o fantoma NEMA NU2 2001, obteve se uma SF de 51.8% e um pico de NECR de 167 kcps a ~7.6 kBq/cm3. Para um fantoma similar mas estendido axialmente até aos1800 mm, a SF obtida foi de 53.7% e o pico de NECR de ~164 kcps a ~3.0 kBq/cm3.
(8102429), Xuedong Zhai. "MECHANICAL BEHAVIORS OF BIOMATERIALS OVER A WIDE RANGE OF LOADING RATES." Thesis, 2019.
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