Atlas-free Brain Tissue Segmentation Using a Single T1-weighted MRI Acquisition

Many studies investigating the aging brain or disease-induced brain alterations rely on accurate and reproducible brain tissue segmentation. Being a preliminary processing step prior to the segmentation, reliableskull-stripping the removal ofnon-brain tissue is also crucial for all later image assessment. Typically, segmentation algorithms rely on an atlas i.e. pre-segmented template data. Brain morphology, however, differs considerably depending on age, sex and race. In addition, diseased brains may deviate significantly from the atlas information typically gained from healthy volunteers. The imposed prior atlas information can thus lead to degradation of segmentation results. The recently introduced MP2RAGE sequence provides a bias-free T1 contrast with heavily reduced T2*- and PD-weighting compared to the standard MP-RAGE [1]. To this end, it acquires two image volumes at different inversion times in one acquisition, combining them to a uniform, i.e. homogenous image. In this work, we exploit the advantageous contrast properties of the MP2RAGE and combine it with a Dixon (i.e. fat-water separation) approach. The information gained by the additional fat image of the head considerably improves the skull-stripping outcome [2]. In conjunction with the pure T1 contrast of the MP2RAGE uniform image, we achieve robust skull-stripping and brain tissue segmentation without the use of an atlas

Zone design of specific sizes using adaptive additively weighted voronoi diagrams

Territory or zone design processes entail partitioning a geographic space, organized as a set of areal units, into different regions or zones according to a specific set of criteria that are dependent on the application context. In most cases, the aim is to create zones of approximately equal sizes (zones with equal numbers of inhabitants, same average sales, etc.). However, some of the new applications that have emerged, particularly in the context of sustainable development policies, are aimed at defining zones of a predetermined, though not necessarily similar, size. In addition, the zones should be built around a given set of seeds. This type of partitioning has not been sufficiently researched; therefore, there are no known approaches for automated zone delimitation. This study proposes a new method based on a discrete version of the adaptive additively weighted Voronoi diagram that makes it possible to partition a two-dimensional space into zones of specific sizes, taking both the position and the weight of each seed into account. The method consists of repeatedly solving a traditional additively weighted Voronoi diagram, so that each seed?s weight is updated at every iteration. The zones are geographically connected using a metric based on the shortest path. Tests conducted on the extensive farming system of three municipalities in Castile-La Mancha (Spain) have established that the proposed heuristic procedure is valid for solving this type of partitioning problem. Nevertheless, these tests confirmed that the given seed position determines the spatial configuration the method must solve and this may have a great impact on the resulting partition.

Brain tissue segmentation on Diffusion Weighted Magnetic Resonance data

Introduction Diffusion weighted Imaging (DWI) techniques are able to measure, in vivo and non-invasively, the diffusivity of water molecules inside the human brain. DWI has been applied on cerebral ischemia, brain maturation, epilepsy, multiple sclerosis, etc. [1]. Nowadays, there is a very high availability of these images. DWI allows the identification of brain tissues, so its accurate segmentation is a common initial step for the referred applications. Materials and Methods We present a validation study on automated segmentation of DWI based on the Gaussian mixture and hidden Markov random field models. This methodology is widely solved with iterative conditional modes algorithm, but some studies suggest [2] that graph-cuts (GC) algorithms improve the results when initialization is not close to the final solution. We implemented a segmentation tool integrating ITK with a GC algorithm [3], and a validation software using fuzzy overlap measures [4]. Results Segmentation accuracy of each tool is tested against a gold-standard segmentation obtained from a T1 MPRAGE magnetic resonance image of the same subject, registered to the DWI space. The proposed software shows meaningful improvements by using the GC energy minimization approach on DTI and DSI (Diffusion Spectrum Imaging) data. Conclusions The brain tissues segmentation on DWI is a fundamental step on many applications. Accuracy and robustness improvements are achieved with the proposed software, with high impact on the application’s final result.

A general framework to testing the effect of transport policy measures to achieve a modal shift: a sequental hybrid model

In order to achieve to minimize car-based trips, transport planners have been particularly interested in understanding the factors that explain modal choices. In the transport modelling literature there has been an increasing awareness that socioeconomic attributes and quantitative variables are not sufficient to characterize travelers and forecast their travel behavior. Recent studies have also recognized that users? social interactions and land use patterns influence travel behavior, especially when changes to transport systems are introduced, but links between international and Spanish perspectives are rarely deal. In this paper, factorial and path analyses through a Multiple-Indicator Multiple-Cause (MIMIC) model are used to understand and describe the relationship between the different psychological and environmental constructs with social influence and socioeconomic variables. The MIMIC model generates Latent Variables (LVs) to be incorporated sequentially into Discrete Choice Models (DCM) where the levels of service and cost attributes of travel modes are also included directly to measure the effect of the transport policies that have been introduced in Madrid during the last three years in the context of the economic crisis. The data used for this paper are collected from a two panel smartphone-based survey (n=255 and 190 respondents, respectively) of Madrid.

Geometric Study of the Weak Equilibrium in a Weighted Case for a Two-Dimensional Competition Game

In this work, an improvement of the results presented by [1] Abellanas et al. (Weak Equilibrium in a Spatial Model. International Journal of Game Theory, 40(3), 449-459) is discussed. Concretely, this paper investigates an abstract game of competition between two players that want to earn the maximum number of points from a finite set of points in the plane. It is assumed that the distribution of these points is not uniform, so an appropriate weight to each position is assigned. A definition of equilibrium which is weaker than the classical one is included in order to avoid the uniqueness of the equilibrium position typical of the Nash equilibrium in these kinds of games. The existence of this approximated equilibrium in the game is analyzed by means of computational geometry techniques.

On the Use of Matrix-Free Shift-Invert Strategies For Global Flow Instability Analysis

A novel time-stepping shift-invert algorithm for linear stability analysis of laminar flows in complex geometries is presented. This method, based on a Krylov subspace iteration, enables the solution of complex non-symmetric eigenvalue problems in a matrix-free framework. Validations and comparisons to the classical exponential method have been performed in three different cases: (i) stenotic flow, (ii) backward-facing step and (iii) lid-driven swirling flow. Results show that this new approach speeds up the required Krylov subspace iterations and has the capability of converging to specific parts of the global spectrum. It is shown that, although the exponential method remains the method of choice if leading eigenvalues are sought, the performance of the present method could be dramatically improved with the use of a preconditioner. In addition, as opposed to other methods, this strategy can be directly applied to any time-stepper, regardless of the temporal or spatial discretization of the latter.

Securing implementations of feedback-shift-register-based ciphers using compiler optimizations and co-processors

Los algoritmos basados en registros de desplazamiento con realimentación (en inglés FSR) se han utilizado como generadores de flujos pseudoaleatorios en aplicaciones con recursos limitados como los sistemas de apertura sin llave. Se considera canal primario a aquel que se utiliza para realizar una transmisión de información. La aparición de los ataques de canal auxiliar (en inglés SCA), que explotan información filtrada inintencionadamente a través de canales laterales como el consumo, las emisiones electromagnéticas o el tiempo empleado, supone una grave amenaza para estas aplicaciones, dado que los dispositivos son accesibles por un atacante. El objetivo de esta tesis es proporcionar un conjunto de protecciones que se puedan aplicar de forma automática y que utilicen recursos ya disponibles, evitando un incremento sustancial en los costes y alargando la vida útil de aplicaciones que puedan estar desplegadas. Explotamos el paralelismo existente en algoritmos FSR, ya que sólo hay 1 bit de diferencia entre estados de rondas consecutivas. Realizamos aportaciones en tres niveles: a nivel de sistema, utilizando un coprocesador reconfigurable, a través del compilador y a nivel de bit, aprovechando los recursos disponibles en el procesador. Proponemos un marco de trabajo que nos permite evaluar implementaciones de un algoritmo incluyendo los efectos introducidos por el compilador considerando que el atacante es experto. En el campo de los ataques, hemos propuesto un nuevo ataque diferencial que se adapta mejor a las condiciones de las implementaciones software de FSR, en las que el consumo entre rondas es muy similar. SORU2 es un co-procesador vectorial reconfigurable propuesto para reducir el consumo energético en aplicaciones con paralelismo y basadas en el uso de bucles. Proponemos el uso de SORU2, además, para ejecutar algoritmos basados en FSR de forma segura. Al ser reconfigurable, no supone un sobrecoste en recursos, ya que no está dedicado en exclusiva al algoritmo de cifrado. Proponemos una configuración que ejecuta múltiples algoritmos de cifrado similares de forma simultánea, con distintas implementaciones y claves. A partir de una implementación sin protecciones, que demostramos que es completamente vulnerable ante SCA, obtenemos una implementación segura a los ataques que hemos realizado. A nivel de compilador, proponemos un mecanismo para evaluar los efectos de las secuencias de optimización del compilador sobre una implementación. El número de posibles secuencias de optimizaciones de compilador es extremadamente alto. El marco de trabajo propuesto incluye un algoritmo para la selección de las secuencias de optimización a considerar. Debido a que las optimizaciones del compilador transforman las implementaciones, se pueden generar automáticamente implementaciones diferentes combinamos para incrementar la seguridad ante SCA. Proponemos 2 mecanismos de aplicación de estas contramedidas, que aumentan la seguridad de la implementación original sin poder considerarse seguras. Finalmente hemos propuesto la ejecución paralela a nivel de bit del algoritmo en un procesador. Utilizamos la forma algebraica normal del algoritmo, que automáticamente se paraleliza. La implementación sobre el algoritmo evaluado mejora en rendimiento y evita que se filtre información por una ejecución dependiente de datos. Sin embargo, es más vulnerable ante ataques diferenciales que la implementación original. Proponemos una modificación del algoritmo para obtener una implementación segura, descartando parcialmente ejecuciones del algoritmo, de forma aleatoria. Esta implementación no introduce una sobrecarga en rendimiento comparada con las implementaciones originales. En definitiva, hemos propuesto varios mecanismos originales a distintos niveles para introducir aleatoridad en implementaciones de algoritmos FSR sin incrementar sustancialmente los recursos necesarios. ABSTRACT Feedback Shift Registers (FSR) have been traditionally used to implement pseudorandom sequence generators. These generators are used in Stream ciphers in systems with tight resource constraints, such as Remote Keyless Entry. When communicating electronic devices, the primary channel is the one used to transmit the information. Side-Channel Attack (SCA) use additional information leaking from the actual implementation, including power consumption, electromagnetic emissions or timing information. Side-Channel Attacks (SCA) are a serious threat to FSR-based applications, as an attacker usually has physical access to the devices. The main objective of this Ph.D. thesis is to provide a set of countermeasures that can be applied automatically using the available resources, avoiding a significant cost overhead and extending the useful life of deployed systems. If possible, we propose to take advantage of the inherent parallelism of FSR-based algorithms, as the state of a FSR differs from previous values only in 1-bit. We have contributed in three different levels: architecture (using a reconfigurable co-processor), using compiler optimizations, and at bit level, making the most of the resources available at the processor. We have developed a framework to evaluate implementations of an algorithm including the effects introduced by the compiler. We consider the presence of an expert attacker with great knowledge on the application and the device. Regarding SCA, we have presented a new differential SCA that performs better than traditional SCA on software FSR-based algorithms, where the leaked values are similar between rounds. SORU2 is a reconfigurable vector co-processor. It has been developed to reduce energy consumption in loop-based applications with parallelism. In addition, we propose its use for secure implementations of FSR-based algorithms. The cost overhead is discarded as the co-processor is not exclusively dedicated to the encryption algorithm. We present a co-processor configuration that executes multiple simultaneous encryptions, using different implementations and keys. From a basic implementation, which is proved to be vulnerable to SCA, we obtain an implementation where the SCA applied were unsuccessful. At compiler level, we use the framework to evaluate the effect of sequences of compiler optimization passes on a software implementation. There are many optimization passes available. The optimization sequences are combinations of the available passes. The amount of sequences is extremely high. The framework includes an algorithm for the selection of interesting sequences that require detailed evaluation. As existing compiler optimizations transform the software implementation, using different optimization sequences we can automatically generate different implementations. We propose to randomly switch between the generated implementations to increase the resistance against SCA.We propose two countermeasures. The results show that, although they increase the resistance against SCA, the resulting implementations are not secure. At bit level, we propose to exploit bit level parallelism of FSR-based implementations using pseudo bitslice implementation in a wireless node processor. The bitslice implementation is automatically obtained from the Algebraic Normal Form of the algorithm. The results show a performance improvement, avoiding timing information leakage, but increasing the vulnerability against differential SCA.We provide a secure version of the algorithm by randomly discarding part of the data obtained. The overhead in performance is negligible when compared to the original implementations. To summarize, we have proposed a set of original countermeasures at different levels that introduce randomness in FSR-based algorithms avoiding a heavy overhead on the resources required.

A Wireless Charging System Applying Phase-Shift and Amplitude Control to Maximize Efficiency and Extractable Power

Wireless power transfer (WPT) is an emerging technology with an increasing number of potential applications to transfer power from a transmitter to a mobile receiver over a relatively large air gap. However, its widespread application is hampered due to the relatively low efficiency of current Wireless power transfer (WPT) systems. This study presents a concept to maximize the efficiency as well as to increase the amount of extractable power of a WPT system operating in nonresonant operation. The proposed method is based on actively modifying the equivalent secondary-side load impedance by controlling the phase-shift of the active rectifier and its output voltage level. The presented hardware prototype represents a complete wireless charging system, including a dc-dc converter which is used to charge a battery at the output of the system. Experimental results are shown for the proposed concept in comparison to a conventional synchronous rectification approach. The presented optimization method clearly outperforms state-of-the-art solutions in terms of efficiency and extractable power.