Chování komponent solárních panelů po nevhodném uložení v půdách

Institute of Geochemistry, Mineralogy and Mineral ResourcesÚstav geochemie, mineralogie a nerostných zdrojůPřírodovědecká fakultaFaculty of Scienc

Recent developments in time-of-flight PET

While the first time-of-flight (TOF)-positron emission tomography (PET) systems were already built in the early 1980s, limited clinical studies were acquired on these scanners. PET was still a research tool, and the available TOF-PET systems were experimental. Due to a combination of low stopping power and limited spatial resolution (caused by limited light output of the scintillators), these systems could not compete with bismuth germanate (BGO)-based PET scanners. Developments on TOF system were limited for about a decade but started again around 2000. The combination of fast photomultipliers, scintillators with high density, modern electronics, and faster computing power for image reconstruction have made it possible to introduce this principle in clinical TOF-PET systems. This paper reviews recent developments in system design, image reconstruction, corrections, and the potential in new applications for TOF-PET. After explaining the basic principles of time-of-flight, the difficulties in detector technology and electronics to obtain a good and stable timing resolution are shortly explained. The available clinical systems and prototypes under development are described in detail. The development of this type of PET scanner also requires modified image reconstruction with accurate modeling and correction methods. The additional dimension introduced by the time difference motivates a shift from sinogram- to listmode-based reconstruction. This reconstruction is however rather slow and therefore rebinning techniques specific for TOF data have been proposed. The main motivation for TOF-PET remains the large potential for image quality improvement and more accurate quantification for a given number of counts. The gain is related to the ratio of object size and spatial extent of the TOF kernel and is therefore particularly relevant for heavy patients, where image quality degrades significantly due to increased attenuation (low counts) and high scatter fractions. The original calculations for the gain were based on analytical methods. Recent publications for iterative reconstruction have shown that it is difficult to quantify TOF gain into one factor. The gain depends on the measured distribution, the location within the object, and the count rate. In a clinical situation, the gain can be used to either increase the standardized uptake value (SUV) or reduce the image acquisition time or administered dose. The localized nature of the TOF kernel makes it possible to utilize local tomography reconstruction or to separate emission from transmission data. The introduction of TOF also improves the joint estimation of transmission and emission images from emission data only. TOF is also interesting for new applications of PET-like isotopes with low branching ratio for positron fraction. The local nature also reduces the need for fine angular sampling, which makes TOF interesting for limited angle situations like breast PET and online dose imaging in proton or hadron therapy. The aim of this review is to introduce the reader in an educational way into the topic of TOF-PET and to give an overview of the benefits and new opportunities in using this additional information

Evaluation of STIR Library Adapted for PET Scanners with Non-Cylindrical Geometry

Software for Tomographic Image Reconstruction (STIR) is an open source C++ library used to reconstruct single photon emission tomography and positron emission tomography (PET) data. STIR has an experimental scanner geometry modelling feature to accurately model detector placement. In this study, we test and improve this new feature using several types of data: Monte Carlo simulations and measured phantom data acquired from a dedicated brain PET prototype scanner. The results show that the new geometry class applied to non-cylindrical PET scanners improved spatial resolution, uniformity, and image contrast. These are directly observed in the reconstructions of small features in the test quality phantom. Overall, we conclude that the revised "BlocksOnCylindrical" class will be a valuable addition to the next STIR software release with adjustments of existing features (Single Scatter Simulation, forward projection, attenuation corrections) to "BlocksOnCylindrical"

Attenuation Correction Using Template PET Registration for Brain PET: A Proof-of-Concept Study

NeuroLF is a dedicated brain PET system with an octagonal prism shape housed in a scanner head that can be positioned around a patient's head. Because it does not have MR or CT capabilities, attenuation correction based on an estimation of the attenuation map is a crucial feature. In this article, we demonstrate this method on [18F]FDG PET brain scans performed with a low-resolution proof of concept prototype of NeuroLF called BPET. We perform an affine registration of a template PET scan to the uncorrected emission image, and then apply the resulting transform to the corresponding template attenuation map. Using a whole-body PET/CT system as reference, we quantitively show that this method yields comparable image quality (0.893 average correlation to reference scan) to using the reference µ-map as obtained from the CT scan of the imaged patient (0.908 average correlation). We conclude from this initial study that attenuation correction using template registration instead of a patient CT delivers similar results and is an option for patients undergoing brain PET. Keywords: Nifty-Reg; PET; STIR; attenuation correction; brain; image reconstruction; registration; tomography

Attenuation Correction Using Template PET Registration for Brain PET:A Proof-of-Concept Study

NeuroLF is a dedicated brain PET system with an octagonal prism shape housed in a scanner head that can be positioned around a patient’s head. Because it does not have MR or CT capabilities, attenuation correction based on an estimation of the attenuation map is a crucial feature. In this article, we demonstrate this method on [18F]FDG PET brain scans performed with a low-resolution proof of concept prototype of NeuroLF called BPET. We perform an affine registration of a template PET scan to the uncorrected emission image, and then apply the resulting transform to the corresponding template attenuation map. Using a whole-body PET/CT system as reference, we quantitively show that this method yields comparable image quality (0.893 average correlation to reference scan) to using the reference µ-map as obtained from the CT scan of the imaged patient (0.908 average correlation). We conclude from this initial study that attenuation correction using template registration instead of a patient CT delivers similar results and is an option for patients undergoing brain PET.</p

Voxel imaging pet pathfinder: a novel approach to positron emission tomography based on room temperature pixelated CdTe detector

El objetivo principal de esta investigación es la simulación y la evaluación de un nuevo concepto de escáner de tomografía por emisión de positrones (PET) basado en un detector pixelado de CdTe en el marco del proyecto “Voxel Imaging PET (VIP) Pathfinder”. El diseño se ha simulado con el programa “GEANT4-based Architecture for Medicine-Oriented Simulations” (GAMOS). El sistema se ha examinado siguiendo las prescripciones de los protocolos NEMA para la evaluación de los dispositivos PET. Varias pruebas se han realizado para evaluar la eficiencia y la calidad de imagen del escáner simulando con precisión las condiciones experimentales requeridas. Por otra parte, el escáner VIP también ha sido evaluado en condiciones pseudo-clínicas con la simulación del escaneo de un cerebro humano. Los resultados de simulación muestran que la excelente resolución de energía de los detectores de CdTe (hasta 1,6% para fotones de 511 keV a temperatura ambiente), junto con el tamaño pequeño del vóxel (1 mm x 1 mm x 2 mm), el alto poder de frenado del CdTe, y la geometría del anillo libre de huecos, dan al nuevo diseño el potencial para superar las limitaciones intrínsecas de los PETs modernos caracterizados por la relativamente pobre resolución de energía de los detectores de centelleo (~10%) y gran incertidumbre de profundidad de interacción (DOI). El escáner VIP puede conseguir una resolución de imagen de hasta ~1 mm de FWHM en todas las direcciones y recoger datos con muy bajo ruido que producen imágenes de excelente calidad en condiciones extremadamente difíciles, como el escaneo de una cabeza humana. Se pueden obtener imágenes de alta calidad con considerablemente menor número de eventos adquiridos respecto a los PETs disponibles actualmente. Esta característica puede utilizarse tanto para aumentar el rendimiento de una unidad hospitalaria de medicina nuclear, como para disminuir la dosis inyectada por paciente. Esta tesis consiste de 6 capítulos principales. Los tres primeros incluyen una revisión de los conocimientos teóricos e históricos de PET, y su estado en la medicina nuclear en la actualidad. La geometría del escáner VIP y sus características distintivas están descritos en el capítulo 5. El capítulo 6 describe la simulación del escáner VIP y el estudio de los resultados de eficiencia del nuevo diseño, mientras que el capítulo 7 está dedicado a la optimización de la técnica de reconstrucción de imagen y la evaluación de la calidad de las imágenes obtenidas con el escáner VIP. Por último, los resultados principales están resumidos en el capítulo final.The central aim of this research is the simulation and the evaluation of a novel conceptual PET scanner based on pixelated CdTe detector in the framework of the Voxel Imaging PET (VIP) Pathfinder project. The novel design is simulated with the GEANT4-based Architecture for Medicine-Oriented Simulations (GAMOS) and including the expected CdTe and electronics specifics. The system is tested mostly following the prescriptions of the NEMA protocols for the evaluation of PET devices. Several tests are performed to assess the counting and the imaging performance of the VIP with the accurate modeling of the required experimental conditions. Furthermore, the VIP scanner is also evaluated in more challenging pseudo-clinical conditions with the simulation of the screening of a real human head. The simulation results show that the excellent energy resolution of the CdTe detectors (down to 1.6% for 511 keV photons at room temperature), together with the small voxel size (1 mm x 1 mm x 2 mm), the high CdTe stopping power, and the crack-free ring geometry, give the new design the potential to overcome the intrinsic limitations of state-of-the-art crystal PETs, characterized by relatively poor energy resolution (~10%) and large depth of interaction (DOI) uncertainty. The VIP scanner can achieve an image resolution as low as ~1 mm full width at half maximum (FWHM) in all directions and collect virtually noise-free data producing excellent quality images in extremely challenging conditions such as the screening of a human head. High quality images can be obtained with significantly smaller number of collected events with respect to the currently available PETs. This characteristic can be used to, either increase the throughput of a nuclear medicine hospital unit, or to decrease the injected dose per patient. This thesis consists of 6 main chapters. The first three chapters include a review of the theoretical and historical background of PET, and its role in the nuclear medicine nowadays. The VIP scanner geometry and its distinctive features are described in the chapter number 5. Chapter 6 describes the simulation of the VIP scanner and study of the counting performance of the new design, while the chapter 7 is dedicated to optimization of the image reconstruction technique and the assessment of the quality of images obtained with the VIP scanner. Finally, the main results are summarized in the conclusive chapter