Application of novel technologies for the development of next generation MR compatible PET inserts

Multimodal imaging integrating Positron Emission Tomography and Magnetic Resonance Imaging (PET/MRI) has professed advantages as compared to other available combinations, allowing both functional and structural information to be acquired with very high precision and repeatability. However, it has yet to be adopted as the standard for experimental and clinical applications, due to a variety of reasons mainly related to system cost and flexibility. A hopeful existing approach of silicon photodetector-based MR compatible PET inserts comprised by very thin PET devices that can be inserted in the MRI bore, has been pioneered, without disrupting the market as expected. Technological solutions that exist and can make this type of inserts lighter, cost-effective and more adaptable to the application need to be researched further. In this context, we expand the study of sub-surface laser engraving (SSLE) for scintillators used for PET. Through acquiring, measuring and calibrating the use of a SSLE setting we study the effect of different engraving configurations on detection characteristics of the scintillation light by the photosensors. We demonstrate that apart from cost-effectiveness and ease of application, SSLE treated scintillators have similar spatial resolution and superior sensitivity and packing fraction as compared to standard pixelated arrays, allowing for shorter crystals to be used. Flexibility of design is benchmarked and adoption of honeycomb architecture due to geometrical advantages is proposed. Furthermore, a variety of depth-of-interaction (DoI) designs are engraved and studied, greatly enhancing applicability in small field-of-view tomographs, such as the intended inserts. To adapt to this need, a novel approach for multi-layer DoI characterization has been developed and is demonstrated. Apart from crystal treatment, considerations on signal transmission and processing are addressed. A double time-over-threshold (ToT) method is proposed, using the statistics of noise in order to enhance precision. This method is tested and linearity results demonstrate applicability for multiplexed readout designs. A study on analog optical wireless communication (aOWC) techniques is also performed and proof of concept results presented. Finally, a ToT readout firmware architecture, intended for low-cost FPGAs, has been developed and is described. By addressing the potential development, applicability and merits of a range of transdisciplinary solutions, we demonstrate that with these techniques it is possible to construct lighter, smaller, lower consumption, cost-effective MRI compatible PET inserts. Those designs can make PET/MRI multimodality the dominant clinical and experimental imaging approach, enhancing researcher and physician insight to the mysteries of life.La combinación multimodal de Tomografía por Emisión de Positrones con la Imagen de Resonancia Magnética (PET/MRI, de sus siglas en inglés) tiene clara ventajas en comparación con otras técnicas multimodales actualmente disponibles, dada su capacidad para registrar información funcional e información estructural con mucha precisión y repetibilidad. Sin embargo, esta técnica no acaba de penetrar en la práctica clínica debido en gran parte a alto coste. Las investigaciones que persiguen mejorar el desarrollo de insertos de PET basados en fotodetectores de silicio y compatibles con MRI, aunque han sido intensas y han generado soluciones ingeniosas, todavía no han conseguido encontrar las soluciones que necesita la industria. Sin embargo, existen opciones todavía sin explorar que podrían ayudar a evolucionar este tipo de insertos consiguiendo dispositivos más ligeros, baratos y con mejores prestaciones. Esta tesis profundiza en el estudio de grabación sub-superficie con láser (SSLE) para el diseño de los cristales centelladores usados en los sistemas PET. Para ello hemos caracterizado, medido y calibrado un procedimiento SSLE, y a continuación hemos estudiado el efecto que tienen sobre las especificaciones del detector las diferentes configuraciones del grabado. Demostramos que además de la rentabilidad y facilidad de uso de esta técnica, los centelladores SSLE tienen resolución espacial equivalente y sensibilidad y fracción de empaquetamiento superiores a las matrices de centelleo convencionales, lo que posibilita utilizar cristales más cortos para conseguir la misma sensibilidad. Estos diseños también permiten medir la profundidad de la interacción (DoI), lo que facilita el uso de estos diseños en tomógrafos de radio pequeño, como pueden ser los sistemas preclínicos, los dedicados (cabeza o mama) o los insertos para MRI. Además de trabajar en el tratamiento de cristal de centelleo, hemos considerado nuevas aproximaciones al procesamiento y transmisión de la señal. Proponemos un método innovador de doble medida de tiempo sobre el umbral (ToT) que integra una evaluación de la estadística del ruido con el propósito de mejorar la precisión. El método se ha validado y los resultados demuestran su viabilidad de uso incluso en conjuntos de señales multiplexadas. Un estudio de las técnicas de comunicación óptica analógica e inalámbrica (aOWC) ha permitido el desarrollo de una nueva propuesta para comunicar las señales del detector PET insertado en el gantry a un el procesador de señal externo, técnica que se ha validado en un demostrador. Finalmente, se ha propuesto y demostrado una nueva arquitectura de análisis de señal ToT implementada en firmware en FPGAs de bajo coste. La concepción y desarrollo de estas ideas, así como la evaluación de los méritos de las diferentes soluciones propuestas, demuestran que con estas técnicas es posible construir insertos de PET compatibles con sistemas MRI, que serán más ligeros y compactos, con un reducido consumo y menor coste. De esta forma se contribuye a que la técnica multimodal PET/MRI pueda penetrar en la clínica, mejorando la comprensión que médicos e investigadores puedan alcanzar en su estudio de los misterios de la vida.Programa Oficial de Doctorado en Ingeniería Eléctrica, Electrónica y AutomáticaPresidente: Andrés Santos Lleó.- Secretario: Luis Hernández Corporales.- Vocal: Giancarlo Sportell

Landau Level Physics in a Quantum Well: new singular features in Magnetization and violations of de Haas - van Alphen periodicities

Analytical calculations based on a Landau Level (LL) picture are reported for an interface (with a finite-width Quantum Well (QW)) and for a fully three-dimensional charged quantum electronic system in an external magnetic field. They lead to a sequence of previously unnoticed singular features in global magnetization and magnetic susceptibility that lead to nontrivial corrections to the standard de Haas - van Alphen periods. Additional features due to Zeeman splitting are also reported (such as new energy minima that originate from the interplay of QW, Zeeman and LL Physics) that are possibly useful for the design of quantum devices. A corresponding calculation in a Composite Fermion picture leads to new predictions on magnetic response properties of a fully-interacting electron liquid in a finite-width interface.Comment: 5 pages, 9 figures, brief report submitted for the Proceedings of the ICMM-2010 Conference (Kolkata, October 2010

Thickness-induced violation of de Haas-van Alphen effect through exact analytical solutions at a one-electron and a one-Composite Fermion level

A systematic study of the energetics of electrons in an interface in a magnetic field is reported with exact analytical calculations based on a Landau Level (LL) picture, by serious consideration of the finite thickness of the Quantum Well (QW). The approach is physically transparent and subtly different in its line of reasoning from standard methods avoiding any semi-classical approximation. We find "internal" phase transitions (at partial LL filling) for magnetisation and susceptibility that are not captured by other approaches and that give rise to nontrivial violations of the standard de Haas-van Alphen periods, in a manner that reproduces the exact quantal astrophysical behaviours in the limit of full three-dimensional (3D) space. Upon inclusion of Zeeman splitting, additional features are also found, such as global energy minima originating from the interplay of QW, Zeeman and LL Physics, while a corresponding calculation in a Composite Fermion picture with {\Lambda}-Levels, leads to new predictions on magnetic properties of an interacting electron liquid. By pursuing the same line of reasoning for a topologically nontrivial system with a relativistic spectrum, we find evidence that similar effects might be operative in the dimensionality crossover of 3D strong topological insulators to 2D topological insulator quantum wells.Comment: 59 pages, many figures. An extensive treatment of the not-so-harmless role of thickness (but mostly at a one-particle level). Based on arXiv:1209.5102. Accepted for publication in European Physical Journal B. arXiv admin note: substantial text overlap with arXiv:1209.510

TNFS Resistant Families of Pairing-Friendly Elliptic Curves

Recently there has been a significant progress on the tower number field sieve (TNFS) method, reducing the complexity of the discrete logarithm problem (DLP) in finite field extensions of composite degree. These new variants of the TNFS attacks have a major impact on pairing-based cryptography and particularly on the selection of the underlying elliptic curve groups and extension fields. In this paper we revise the criteria for selecting pairing-friendly elliptic curves considering these new TNFS attacks in finite extensions of composite embedding degree. Additionally we update the criteria for finite extensions of prime degree in order to meet today’s security requirements