Secure Transmission in Cooperative Relaying Networks with Multiple Antennas

We investigate the secrecy performance of dualhop amplify-and-forward (AF) multi-antenna relaying systems over Rayleigh fading channels, by taking into account the direct link between the source and destination. In order to exploit the available direct link and the multiple antennas for secrecy improvement, different linear processing schemes at the relay and different diversity combining techniques at the destination are proposed, namely, 1) Zero-forcing/Maximal ratio combining (ZF/MRC), 2) ZF/Selection combining (ZF/SC), 3) Maximal ratio transmission/MRC (MRT/MRC) and 4) MRT/Selection combining (MRT/SC). For all these schemes, we present new closed-form approximations for the secrecy outage probability. Moreover, we investigate a benchmark scheme, i.e., cooperative jamming/ZF (CJ/ZF), where the secrecy outage probability is obtained in exact closed-form. In addition, we present asymptotic secrecy outage expressions for all the proposed schemes in the high signal-to-noise ratio (SNR) regime, in order to characterize key design parameters, such as secrecy diversity order and secrecy array gain. The outcomes of this paper can be summarized as follows: a) MRT/MRC and MRT/SC achieve a full diversity order of M + 1, ZF/MRC and ZF/SC achieve a diversity order of M, while CJ/ZF only achieves unit diversity order, where M is the number of antennas at the relay. b) ZF/MRC (ZF/SC) outperforms the corresponding MRT/MRC (MRT/SC) in the low SNR regime, while becomes inferior to the corresponding MRT/MRC (MRT/SC) in the high SNR. c) All of the proposed schemes tend to outperform the CJ/ZF with moderate number of antennas, and linear processing schemes with MRC attain better performance than those with SC.

Ground Plane Slot Structures for Isolation of Cosited Microstrip Antennas

Ground plane slot structures have been shown to reduce coupling between cosited antennas. Although some such structures have already been reported, no analytical model exists to describe their behavior and there are no design guidelines. In this work, the behavior of reported ground plane structures is used as a clue to obtain generalizable information about such structures' behavior. The structures' scalability and excitation behavior is investigated. Next a circuit model is derived that describes the interaction of microstrip patch antennas with a ground plane slot structure based on mutual admittances between the ground plane slots and the effective slots at the antennas' radiating edges. The circuit model leads to design guidelines for the ground plane slot structure and an approximate relationship between mutual admittances which must be satisfied in order to isolate the antennas. Finally, we present a novel ground plane slot structure that mitigates some of the disadvantages of earlier designs.

Growth and application of planar III-V semiconductor nanowires grown with MOCVD

The semiconductor nanowire has been widely studied over the past decade and identified as a promising nanotechnology building block with application in photonics and electronics. The flexible bottom-up approach to nanowire growth allows for straightforward fabrication of complex 1D nanostructures with interesting optical, electrical, and mechanical properties. III-V nanowires in particular are useful because of their direct bandgap, high carrier mobility, and ability to form heterojunctions and have been used to make devices such as light-emitting diodes, lasers, and field-effect transistors. However, crystal defects are widely reported for III-V nanowires when grown in the common out-of-plane <111>B direction. Furthermore, commercialization of nanowires has been limited by the difficulty of assembling nanowires with predetermined position and alignment on a wafer-scale. In this thesis, planar III-V nanowires are introduced as a low-defect and integratable nanotechnology building block grown with metalorganic chemical vapor deposition. Planar GaAs nanowires grown with gold seed particles self-align along the <110> direction on the (001) GaAs substrate. Transmission electron microscopy reveals that planar GaAs nanowires are nearly free of crystal defects and grow laterally and epitaxially on the substrate surface. The nanowire morphology is shown to be primarily controlled through growth temperature and an ideal growth window of 470 +\- 10 °C is identified for planar GaAs nanowires. Extension of the planar growth mode to other materials is demonstrated through growth of planar InAs nanowires. Using a sacrificial layer, the transfer of planar GaAs nanowires onto silicon substrates with control over the alignment and position is presented. A metal-semiconductor field-effect transistor fabricated with a planar GaAs nanowire shows bulk-like low-field electron transport characteristics with high mobility. The aligned planar geometry and excellent material quality of planar III-V nanowires may lead to highly integrated III-V nanophotonics and nanoelectronics.

Components and circuits for tunneling diode based high frequency sources

Terahertz (THz) technology has been generating a lot of interest because of the potential applications for systems working in this frequency range. However, to fully achieve this potential, effective and efficient ways of generating controlled signals in the terahertz range are required. Devices that exhibit negative differential resistance (NDR) in a region of their current-voltage (I-V ) characteristics have been used in circuits for the generation of radio frequency signals. Of all of these NDR devices, resonant tunneling diode (RTD) oscillators, with their ability to oscillate in the THz range are considered as one of the most promising solid-state sources for terahertz signal generation at room temperature. There are however limitations and challenges with these devices, from inherent low output power usually in the range of micro-watts (uW) for RTD oscillators when milli-watts (mW) are desired. At device level, parasitic oscillations caused by the biasing line inductance when the device is biased in the NDR region prevent accurate device characterisation, which in turn prevents device modelling for computer simulations. This thesis describes work on I-V characterisation of tunnel diode (TD) and RTD (fabricated by Dr. Jue Wang) devices, and the radio frequency (RF) characterisation and small signal modelling of RTDs. The thesis also describes the design and measurement of hybrid TD oscillators for higher output power and the design and measurement of a planar Yagi antenna (fabricated by Khalid Alharbi) for THz applications. To enable oscillation free current-voltage characterisation of tunnel diodes, a commonly employed method is the use of a suitable resistor connected across the device to make the total differential resistance in the NDR region positive. However, this approach is not without problems as the value of the resistor has to satisfy certain conditions or else bias oscillations would still be present in the NDR region of the measured I-V characteristics. This method is difficult to use for RTDs which are fabricated on wafer due to the discrepancies in designed and actual resistance values of fabricated resistors using thin film technology. In this work, using pulsed DC rather than static DC measurements during device characterisation were shown to give accurate characteristics in the NDR region without the need for a stabilisation resistor. This approach allows for direct oscillation free characterisation for devices. Experimental results show that the I-V characterisation of tunnel diodes and RTD devices free of bias oscillations in the NDR region can be made. In this work, a new power-combining topology to address the limitations of low output power of TD and RTD oscillators is presented. The design employs the use of two oscillators biased separately, but with the combined output power from both collected at a single load. Compared to previous approaches, this method keeps the frequency of oscillation of the combined oscillators the same as for one of the oscillators. Experimental results with a hybrid circuit using two tunnel diode oscillators compared with a single oscillator design with similar values shows that the coupled oscillators produce double the output RF power of the single oscillator. This topology can be scaled for higher (up to terahertz) frequencies in the future by using RTD oscillators. Finally, a broadband Yagi antenna suitable for wireless communication at terahertz frequencies is presented in this thesis. The return loss of the antenna showed that the bandwidth is larger than the measured range (140-220 GHz). A new method was used to characterise the radiation pattern of the antenna in the E-plane. This was carried out on-wafer and the measured radiation pattern showed good agreement with the simulated pattern. In summary, this work makes important contributions to the accurate characterisation and modelling of TDs and RTDs, circuit-based techniques for power combining of high frequency TD or RTD oscillators, and to antennas suitable for on chip integration with high frequency oscillators.

The development of planar high-K/III-V p-channel MOSFETs for post-silicon CMOS

Conventional Si complementary-metal-oxide-semiconductor (CMOS) scaling is fast approaching its limits. The extension of the logic device roadmap for future enhancements in transistor performance requires non-Si materials and new device architectures. III-V materials, due to their superior electron transport properties, are well poised to replace Si as the channel material beyond the 10nm technology node to mitigate the performance loss of Si transistors from further reductions in supply voltage to minimise power dissipation in logic circuits. However several key challenges, including a high quality dielectric/III-V gate stack, a low-resistance source/drain (S/D) technology, heterointegration onto a Si platform and a viable III-V p-metal-oxide-semiconductor field-effect-transistor (MOSFET), need to be addressed before III-Vs can be employed in CMOS. This Thesis specifically addressed the development and demonstration of planar III-V p-MOSFETs, to complement the n-MOSFET, thereby enabling an all III-V CMOS technology to be realised. This work explored the application of InGaAs and InGaSb material systems as the channel, in conjunction with Al2O3/metal gate stacks, for p-MOSFET development based on the buried-channel flatband device architecture. The body of work undertaken comprised material development, process module development and integration into a robust fabrication flow for the demonstration of p-channel devices. The parameter space in the design of the device layer structure, based around the III-V channel/barrier material options of In_x≥0.53Ga_1-xAs/In_0.52Al_0.48As and In_x≥0.1Ga_1-xSb/AlSb, was systematically examined to improve hole channel transport. A mobility of 433 cm²/Vs, the highest room temperature hole mobility of any InGaAs quantum-well channel reported to date, was obtained for the In_0.85Ga_0.15As (2.1% strain) structure. S/D ohmic contacts were developed based on thermally annealed Au/Zn/Au metallisation and validated using transmission line model test structures. The effects of metallisation thickness, diffusion barriers and de-oxidation conditions were examined. Contacts to InGaSb-channel structures were found to be sensitive to de-oxidation conditions. A fabrication process, based on a lithographically-aligned double ohmic patterning approach, was realised for deep submicron gate-to-source/drain gap (L_side) scaling to minimise the access resistance, thereby mitigating the effects of parasitic S/D series resistance on transistor performance. The developed process yielded gaps as small as 20nm. For high-k integration on GaSb, ex-situ ammonium sulphide ((NH₄)₂S) treatments, in the range 1%-22%, for 10min at 295K were systematically explored for improving the electrical properties of the Al₂O₃/GaSb interface. Electrical and physical characterisation indicated the 1% treatment to be most effective with interface trap densities in the range of 4 - 10×10¹²cm^-2eV^-1 in the lower half of the bandgap. An extended study, comprising additional immersion times at each sulphide concentration, was further undertaken to determine the surface roughness and the etching nature of the treatments on GaSb. A number of p-MOSFETs based on III-V-channels with the most promising hole transport and integration of the developed process modules were successfully demonstrated in this work. Although the non-inverted InGaAs-channel devices showed good current modulation and switch-off characteristics, several aspects of performance were non-ideal; depletion-mode operation, modest drive current (I_d,sat=1.14mA/mm), double peaked transconductance (g_m=1.06mS/mm), high subthreshold swing (SS=301mV/dec) and high on-resistance (R_on=845kΩ.μm). Despite demonstrating substantial improvement in the on-state metrics of I_d,sat (11×), g_m (5.5×) and R_on (5.6×), inverted devices did not switch-off. Scaling gate-to-source/drain gap (L_side) from 1μm down to 70nm improved I_d,sat (72.4mA/mm) by a factor of 3.6 and gm (25.8mS/mm) by a factor of 4.1 in inverted InGaAs-channel devices. Well-controlled current modulation and good saturation behaviour was observed for InGaSb-channel devices. In the on-state In_0.3Ga_0.7Sb-channel (I_d,sat=49.4mA/mm, g_m=12.3mS/mm, R_on=31.7kΩ.μm) and In_0.4Ga_0.6Sb-channel (I_d,sat=38mA/mm, g_m=11.9mS/mm, R_on=73.5kΩ.μm) devices outperformed the InGaAs-channel devices. However the devices could not be switched off. These findings indicate that III-V p-MOSFETs based on InGaSb as opposed to InGaAs channels are more suited as the p-channel option for post-Si CMOS.

Bull's eye leaky wave antenna for terrestrial and space applications

This work presents the study of Bull's eye antenna designs, a type of leaky wave antenna (LWA), operating in the 60 GHz band. This band emerged as a new standard for specific terrestrial and space applications because the radio spectrumbecomes more congested up to the millimetre-wave band, starting at 30 GHz. Built on existing Bull's eye antenna designs, novel structures were simulated, fabricated and measured, so as to provide more exibility in the implementation of wireless solutions at this frequency. Firstly, the study of a 60 GHz Bull's eye antenna for straightforward integration onto a CubeSat is presented. An investigation of the design is carried out, from the description of the radiation mechanism supported by simulation results, to the radiation pattern measurement of a prototype which provides a gain of 19.1 dBi at boresight. Another design, based on a modified feed structure, uses a microstrip to waveguide transition to provide easier and inexpensive integration of a Bull's eye antenna onto a planar circuit. Secondly, the design of Bull's eye antennas capable of creating beam deflection and multi-beam is presented. In particular, a detail study of the deflection mechanism is proposed, followed by the demonstration of a Bull's eye antenna generating two separate beams at ±16° away from the boresight. In addition, a novel mechanically steerable Bull's eye antenna, based on the division of the corrugated area in paired sectors is presented. A prototype was fabricated and measured. It generated double beams at ±8° and ±15° from the boresight, and a single boresight beam. Thirdly, a Bull's eye antenna capable of generating two simultaneous orbital angular momentum (OAM) modes l = 3 is proposed. The design is based on a circular travelling wave resonator and would allow channel capacity increase through OAM multiplexing. An improved design based on two stacked OAM Bull's eye antennas capable of producing four orthogonal OAM modes l = (±3,±13) simultaneously is presented. A novel receiving scheme based on discretely sampled partial aperture receivers (DSPAR) is then introduced. This solution could provide a lower windage and a lower cost of implementation than current whole or partial continuous aperture.

Planar Odometry from a Radial Laser Scanner. A Range Flow-based Approach

In this paper we present a fast and precise method to estimate the planar motion of a lidar from consecutive range scans. For every scanned point we formulate the range flow constraint equation in terms of the sensor velocity, and minimize a robust function of the resulting geometric constraints to obtain the motion estimate. Conversely to traditional approaches, this method does not search for correspondences but performs dense scan alignment based on the scan gradients, in the fashion of dense 3D visual odometry. The minimization problem is solved in a coarse-to-fine scheme to cope with large displacements, and a smooth filter based on the covariance of the estimate is employed to handle uncertainty in unconstraint scenarios (e.g. corridors). Simulated and real experiments have been performed to compare our approach with two prominent scan matchers and with wheel odometry. Quantitative and qualitative results demonstrate the superior performance of our approach which, along with its very low computational cost (0.9 milliseconds on a single CPU core), makes it suitable for those robotic applications that require planar odometry. For this purpose, we also provide the code so that the robotics community can benefit from it.

Neuronal networks with gap junctions: A study of piece-wise linear planar neuron models

The presence of gap junction coupling among neurons of the central nervous systems has been appreciated for some time now. In recent years there has been an upsurge of interest from the mathematical community in understanding the contribution of these direct electrical connections between cells to large-scale brain rhythms. Here we analyze a class of exactly soluble single neuron models, capable of producing realistic action potential shapes, that can be used as the basis for understanding dynamics at the network level. This work focuses on planar piece-wise linear models that can mimic the firing response of several different cell types. Under constant current injection the periodic response and phase response curve (PRC) is calculated in closed form. A simple formula for the stability of a periodic orbit is found using Floquet theory. From the calculated PRC and the periodic orbit a phase interaction function is constructed that allows the investigation of phase-locked network states using the theory of weakly coupled oscillators. For large networks with global gap junction connectivity we develop a theory of strong coupling instabilities of the homogeneous, synchronous and splay state. For a piece-wise linear caricature of the Morris-Lecar model, with oscillations arising from a homoclinic bifurcation, we show that large amplitude oscillations in the mean membrane potential are organized around such unstable orbits.

Modelling and analysis of planar cell polarity

Planar cell polarity (PCP) occurs in the epithelia of many animals and can lead to the alignment of hairs, bristles and feathers; physiologically, it can organise ciliary beating. Here we present two approaches to modelling this phenomenon. The aim is to discover the basic mechanisms that drive PCP, while keeping the models mathematically tractable. We present a feedback and diffusion model, in which adjacent cell sides of neighbouring cells are coupled by a negative feedback loop and diffusion acts within the cell. This approach can give rise to polarity, but also to period two patterns. Polarisation arises via an instability provided a sufficiently strong feedback and sufficiently weak diffusion. Moreover, we discuss a conservative model in which proteins within a cell are redistributed depending on the amount of proteins in the neighbouring cells, coupled with intracellular diffusion. In this case polarity can arise from weakly polarised initial conditions or via a wave provided the diffusion is weak enough. Both models can overcome small anomalies in the initial conditions. Furthermore, the range of the effects of groups of cells with different properties than the surrounding cells depends on the strength of the initial global cue and the intracellular diffusion.

Graph-based structural analysis of planar mechanisms.

Kinematic structure of planar mechanisms addresses the study of attributes determined exclusively by the joining pattern among the links forming a mechanism. The system group classification is central to the kinematic structure and consists of determining a sequence of kinematically and statically independent-simple chains which represent a modular basis for the kinematics and force analysis of the mechanism. This article presents a novel graph-based algorithm for structural analysis of planar mechanisms with closed-loop kinematic structure which determines a sequence of modules (Assur groups) representing the topology of the mechanism. The computational complexity analysis and proof of correctness of the implemented algorithm are provided. A case study is presented to illustrate the results of the devised method.

Implementation of an enhanced planar processing protocol in clinical practice

Introdução: A cintigrafia óssea é um dos exames mais frequentes em Medicina Nuclear. Esta modalidade de imagem médica requere um balanço apropriado entre a qualidade de imagem e a dose de radiação, ou seja, as imagens obtidas devem conter o número mínimo de contagem necessárias, para que apresentem qualidade considerada suficiente para fins diagnósticos. Objetivo: Este estudo tem como principal objetivo, a aplicação do software Enhanced Planar Processing (EPP), nos exames de cintigrafia óssea em doentes com carcinoma da mama e próstata que apresentam metástases ósseas. Desta forma, pretende-se avaliar a performance do algoritmo EPP na prática clínica em termos de qualidade e confiança diagnóstica quando o tempo de aquisição é reduzido em 50%. Material e Métodos: Esta investigação teve lugar no departamento de Radiologia e Medicina Nuclear do Radboud University Nijmegen Medical Centre. Cinquenta e um doentes com suspeita de metástases ósseas foram administrados com 500MBq de metilenodifosfonato marcado com tecnécio-99m. Cada doente foi submetido a duas aquisições de imagem, sendo que na primeira foi seguido o protocolo standard do departamento (scan speed=8 cm/min) e na segunda, o tempo de aquisição foi reduzido para metade (scan speed=16 cm/min). As imagens adquiridas com o segundo protocolo foram processadas com o algoritmo EPP. Todas as imagens foram submetidas a uma avaliação objetiva e subjetiva. Relativamente à análise subjetiva, três médicos especialistas em Medicina Nuclear avaliaram as imagens em termos da detetabilidade das lesões, qualidade de imagem, aceitabilidade diagnóstica, localização das lesões e confiança diagnóstica. No que respeita à avaliação objetiva, foram selecionadas duas regiões de interesse, uma localizada no terço médio do fémur e outra localizada nos tecidos moles adjacentes, de modo a obter os valores de relação sinal-ruído, relação contraste-ruído e coeficiente de variação. Resultados: Os resultados obtidos evidenciam que as imagens processadas com o software EPP oferecem aos médicos suficiente informação diagnóstica na deteção de metástases, uma vez que não foram encontradas diferenças estatisticamente significativas (p>0.05). Para além disso, a concordância entre os observadores, comparando essas imagens e as imagens adquiridas com o protocolo standard foi de 95% (k=0.88). Por outro lado, no que respeita à qualidade de imagem, foram encontradas diferenças estatisticamente significativas quando se compararam as modalidades de imagem entre si (p≤0.05). Relativamente à aceitabilidade diagnóstica, não foram encontradas diferenças estatisticamente significativas entre as imagens adquiridas com o protocolo standard e as imagens processadas com o EPP software (p>0.05), verificando-se uma concordância entre os observadores de 70.6%. Todavia, foram encontradas diferenças estatisticamente significativas entre as imagens adquiridas com o protocolo standard e as imagens adquiridas com o segundo protocolo e não processadas com o software EPP (p≤0.05). Para além disso, não foram encontradas diferenças estatisticamente significativas (p>0.05) em termos de relação sinal-ruído, relação contraste-ruído e coeficiente de variação entre as imagens adquiridas com o protocolo standard e as imagens processadas com o EPP. Conclusão: Com os resultados obtidos através deste estudo, é possível concluir que o algoritmo EPP, desenvolvido pela Siemens, oferece a possibilidade de reduzir o tempo de aquisição em 50%, mantendo ao mesmo tempo uma qualidade de imagem considerada suficiente para fins de diagnóstico. A utilização desta tecnologia, para além de aumentar a satisfação por parte dos doentes, é bastante vantajosa no que respeita ao workflow do departamento.

Assessment of the performance of an enhanced planar processing algorithm for whole-body bone scintigraphy : a phantom study

A cintigrafia óssea de corpo inteiro representa um dos exames imagiológicos mais frequentes realizados em medicina nuclear. Para além de outras aplicações, este procedimento é capaz de fornecer o diagnóstico de metástases ósseas. Em doentes oncológicos, a presença de metástases ósseas representa um forte indicador prognóstico da longevidade do doente. Para além disso, a presença ou ausência de metástases ósseas irá influenciar o planeamento do tratamento, requerendo para isso uma interpretação precisa dos resultados imagiológicos. Problema: Tendo em conta que a metastização óssea é considerada uma complicação severa relacionada com aumento da morbilidade e diminuição de sobrevivência dos doentes, o conceito de patient care torna-se ainda mais imperativo nestas situações. Assim, devem ser implementadas as melhores práticas imagiológicas de forma a obter o melhor resultado possível do procedimento efetuado, associado ao desconforto mínimo do doente. Uma técnica provável para atingir este objetivo no caso específico da cintigrafia óssea de corpo inteiro é a redução do tempo de aquisição, contudo, as imagens obtidas por si só teriam qualidade de tal forma reduzida que os resultados poderiam ser enviesados. Atualmente, surgiram novas técnicas, nomeadamente relativas a processamento de imagem, através das quais é possível gerar imagens cintigráficas com contagem reduzida de qualidade comparável àquela obtida com o protocolo considerado como standard. Ainda assim, alguns desses métodos continuam associados a algumas incertezas, particularmente no que respeita a sustentação da confiança diagnóstica após a modificação dos protocolos de rotina. Objetivos: O presente trabalho pretende avaliar a performance do algoritmo Pixon para processamento de imagem por meio de um estudo com fantoma. O objetivo será comparar a qualidade de imagem e a detetabilidade fornecidas por imagens não processadas com aquelas submetidas à referida técnica de processamento. Para além disso, pretende-se também avaliar o efeito deste algoritmo na redução do tempo de aquisição. De forma a atingir este objetivo, irá ser feita uma comparação entre as imagens obtidas com o protocolo standard e aquelas adquiridas usando protocolos mais rápidos, posteriormente submetidas ao método de processamento referido. Material e Métodos: Esta investigação for realizada no departamento de Radiologia e Medicina Nuclear do Radboud University Nijmegen Medical Centre, situado na Holanda. Foi utilizado um fantoma cilíndrico contendo um conjunto de seis esferas de diferentes tamanhos, adequado à técnica de imagem planar. O fantoma foi preparado com diferentes rácios de atividade entre as esferas e o background (4:1, 8:1, 17:1, 22:1, 32:1 e 71:1). Posteriormente, para cada teste experimental, o fantoma foi submetido a vários protocolos de aquisição de imagem, nomeadamente com diferentes velocidades de aquisição: 8 cm/min, 12 cm/min, 16 cm/min e 20 cm/min. Todas as imagens foram adquiridas na mesma câmara gama - e.cam Signature Dual Detector System (Siemens Medical Solutions USA, Inc.) - utilizando os mesmos parâmetros técnicos de aquisição, à exceção da velocidade. Foram adquiridas 24 imagens, todas elas submetidas a pós-processamento com recurso a um software da Siemens (Siemens Medical Solutions USA, Inc.) que inclui a ferramenta necessária ao processamento de imagens cintigráficas de corpo inteiro. Os parâmetros de reconstrução utilizados foram os mesmos para cada série de imagens, estando estabelecidos em modo automático. A análise da informação recolhida foi realizada com recurso a uma avaliação objetiva (utilizando parâmetros físicos de qualidade de imagem) e outra subjetiva (através de dois observadores). A análise estatística foi efetuada recorrendo ao software SPSS versão 22 para Windows. Resultados: Através da análise subjetiva de cada rácio de atividade foi demonstrado que, no geral, a detetabilidade das esferas aumentou após as imagens serem processadas. A concordância entre observadores para a distribuição desta análise foi substancial, tanto para imagens não processadas como imagens processadas. Foi igualmente demonstrado que os parâmetros físicos de qualidade de imagem progrediram depois de o algoritmo de processamento ter sido aplicado. Para além disso, observou-se ao comparar as imagens standard (adquiridas com 8 cm/min) e aquelas processadas e adquiridas com protocolos mais rápidos que: imagens adquiridas com uma velocidade de aquisição de 12 cm/min podem fornecer resultados melhorados, com parâmetros de qualidade de imagem e detetabilidade superiores; imagens adquiridas com uma velocidade de 16 cm/min fornecem resultados comparáveis aos standard, com valores aproximados de qualidade de imagem e detetabilidade; e imagens adquiridas com uma velocidade de 20 cm/min resultam em valores diminuídos de qualidade de imagem, bem como redução a nível da detetabilidade. Discussão: Os resultados obtidos foram igualmente estabelecidos por meio de um estudo clínico numa investigação independente, no mesmo departamento. Foram incluídos cinquenta e um doentes referidos com carcinomas da mama e da próstata, com o objetivo de estudar o impacto desta técnica na prática clínica. Os doentes foram, assim, submetidos ao protocolo standard e posteriormente a uma aquisição adicional com uma velocidade de aquisição de 16 cm/min. Depois de as imagens terem sido cegamente avaliadas por três médicos especialistas, concluiu-se que a qualidade de imagem bem como a detetabilidade entre imagens era comparável, corroborando os resultados desta investigação. Conclusão: Com o objetivo de reduzir o tempo de aquisição aplicando um algoritmo de processamento de imagem, foi demonstrado que o protocolo com 16 cm/min de velocidade de aquisição será o limite para o aumento dessa mesma velocidade. Após processar a informação, este protocolo fornece os resultados mais equivalentes àqueles obtidos com o protocolo standard. Tendo em conta que esta técnica foi estabelecida com sucesso na prática clínica, pode-se concluir que, pelo menos em doentes referidos com carcinomas da mama e da próstata, o tempo de aquisição pode ser reduzido para metade, duplicando a velocidade de aquisição de 8 para 16 cm/min.

Packaging and Deployment of Large Planar Spacecraft Structures

This thesis presents a set of novel methods to biaxially package planar structures by folding and wrapping. The structure is divided into strips connected by folds that can slip during wrapping to accommodate material thickness. These packaging schemes are highly efficient, with theoretical packaging efficiencies approaching 100%. Packaging tests on meter-scale physical models have demonstrated packaging efficiencies of up to 83%. These methods avoid permanent deformation of the structure, allowing an initially flat structure to be deployed to a flat state.

Also presented are structural architectures and deployment schemes that are compatible with these packaging methods. These structural architectures use either in-plane pretension -- suitable for membrane structures -- or out-of-plane bending stiffness to resist loading. Physical models are constructed to realize these structural architectures. The deployment of these types of structures is shown to be controllable and repeatable by conducting experiments on lab-scale models.

These packaging methods, structural architectures, and deployment schemes are applicable to a variety of spacecraft structures such as solar power arrays, solar sails, antenna arrays, and drag sails; they have the potential to enable larger variants of these structures while reducing the packaging volume required. In this thesis, these methods are applied to the preliminary structural design of a space solar power satellite. This deployable spacecraft, measuring 60 m x 60 m, can be packaged into a cylinder measuring 1.5 m in height and 1 m in diameter. It can be deployed to a flat configuration, where it acts as a stiff lightweight support framework for multifunctional tiles that collect sunlight, generate electric power, and transmit it to a ground station on Earth.