Accurate Dose Measurements Using Cherenkov Polarization Imaging

Purpose: Cherenkov radiation carries the potential of direct in-water dose measurements, but its precision is currently limited by a strong anisotropy. Taking advantage of polarization imaging, this work proposes a new approach for high accuracy Cherenkov dose measurements. Methods: Cherenkov produced in a 15x15x20 cm^3 water tank is imaged with a cooled CCD camera from four polarizer transmission axes [0{\deg}, 45{\deg}, 90{\deg}, 135{\deg}]. The water tank is positioned at the isocenter of a 5x5 cm^2, 6 MV photon beam. Using Malus' law, the polarized portion of the signal is extracted. Corrections are applied to the polarized signal following azimuthal and polar Cherenkov angular distributions extracted from Monte Carlo simulations. Percent depth dose and beam profiles are measured and compared with the prediction from a treatment planning system (TPS). Results: Corrected polarized signals on the central axis reduced deviations at depth from 20% to 0.8\pm1%. For the profile measurement, differences between the corrected polarized signal and the TPS calculations are 1\pm3% and 8\pm3% on the central axis and penumbra regions respectively. 29\pm1% of the Cherenkov signal was found to be polarized. Conclusions: This work proposes a novel polarization imaging approach enabling high precision water-based Cherenkov dose measurements. The method allows correction of the Cherenkov anisotropy within 3% on the beam central axis and in depth

An EPID-based method to determine mechanical deformations in a linear accelerator

Purpose: Medical linear accelerators (linac) are delivering increasingly complex treatments using modern techniques in radiation therapy. Complete and precise mechanical QA of the linac is therefore necessary to ensure that there is no unexpected deviation from the gantry's planned course. However, state-of-the-art EPID-based mechanical QA procedures often neglect some degrees of freedom (DOF) like the in-plane rotations of the gantry and imager or the source movements inside the gantry head. Therefore, the purpose of this work is to characterize a 14 DOF method for the mechanical QA of linacs. This method seeks to measure every mechanical deformation in a linac, including source movements, in addition to relevant clinical parameters like mechanical and radiation isocenters. Methods: A widely available commercial phantom and a custom-made accessory inserted in the linac's interface mount are imaged using the electronic portal imaging device (EPID) at multiple gantry angles. Then, simulated images are generated using the nominal geometry of the linac and digitized models of the phantoms. The nominal geometry used to generate these images can be modified using 14 DOF (3 rigid rotations and 3 translations for the imager and the gantry, and 2 in-plane translations of the source) and any change will modify the simulated image. The set of mechanical deformations that minimizes the differences between the simulated and measured image is found using a genetic algorithm coupled with a gradient-descent optimizer. Phantom mispositioning and gantry angular offset were subsequently calculated and extracted from the results. Simulations of the performances of the method for different levels of noise in the phantom models were performed to calculate the absolute uncertainty of the measured mechanical deformations. The measured source positions and the center of collimation were used to define the beam central axis and calculate the radiation isocenter position and radius. Results: After the simultaneous optimization of the 14 DOF, the average distance between the center of the measured and simulated ball bearings on the imager was 0.086 mm. Over the course of a full counter-clockwise gantry rotation, all mechanical deformations were measured, showing sub-millimeter translations and rotations smaller than 1° along every axis. The average absolute uncertainty of the 14 DOF (1 SD) was 0.15 mm or degree. Phantom positioning errors were determined with more than 0.1 mm precision. Errors introduced in the experimental setup like phantom positioning errors, source movements or gantry angular offsets were all successfully detected by our QA method. The mechanical deformations measured are shown to be reproducible over the course of a few weeks and are not sensitive to the experimental setup. Conclusion: This work presents of new method for an accurate mechanical QA of the linacs. It features a 14 DOF model of the mechanical deformations that is both more complete and precise than other available methods. It has demonstrated sub-millimeter accuracy through simulation and experimentation. Introduced errors were successfully detected with high precision

Multiobjective waste management optimization strategy coupling life cycle assessment and genetic algorithms: application to PET bottles

A mathematical model based on life-cycle assessment (LCA) results is developed to assess the environmental efficiency of the end-of-life management of polyethylene terephthalate (PET) bottles. For this purpose, multiobjective optimization and decision support tools are used to define optimal targets for efficient waste management. The global environmental impacts associated with the treatment of PET bottles from their cradle to their ultimate graves (incineration, landfill, recycling by mechanical, chemical or thermal processes) are computed in function of the flow of bottles in the different valorization paths. They are based on the calculation of the impacts involved in each elementary process with a LCA software tool, using the CML impact assessment method. The model takes into account the fraction λ of PET regenerated into bottles that can be further recycled, the global impacts being the cumulative impacts corresponding to each "end-of-life". A nonlinear model for the bottle waste collection stage is considered, reflecting that the more diffuse the flow of bottles is, the more difficult it is to collect and consequently, the more environmentally impacting. The resulting multiobjective problem is to find the allocation of bottles between valorization paths that minimizes the environmental impacts of bottle end-of-lives. It is solved using a genetic algorithm, and the trade-off between environmental impacts is illustrated through Pareto curves. A decision support tool then determines the best compromise among the set of solutions. The model is applied to the case of France in 2010. The variables that minimize simultaneously abiotic depletion, acidification and global warming potential are determined, in particular the number of recycling loops. The approach can be easily adapted to any specific product like bio-based plastics or organic wastes to find the optimal allocation between valorization paths

Non-negative Matrix Factorization using Partial Prior Knowledge for Radiation Dosimetry

Hyperspectral unmixing aims at decomposing a given signal into its spectral signatures and its associated fractional abundances. To improve the accuracy of this decomposition, algorithms have included different assumptions depending on the application. The goal of this study is to develop a new unmixing algorithm that can be applied for the calibration of multi-point scintillation dosimeters used in the field of radiation therapy. This new algorithm is based on a non-negative matrix factorization. It incorporates a partial prior knowledge on both the abundances and the endmembers of a given signal. It is shown herein that, following a precise calibration routine, it is possible to use partial prior information about the fractional abundances, as well as on the endmembers, in order to perform a simplified yet precise calibration of these dosimeters. Validation and characterization of this algorithm is made using both simulations and experiments. The experimental validation shows an improvement in accuracy compared to previous algorithms with a mean spectral angle distance (SAD) on the estimated endmembers of 0.0766, leading to an average error of $(0.25 \pm 0.73)$ % on dose measurements.Comment: 11 pages, 6 figure

Concepts «fondamentaux» en administration de l’éducation

Glossaire élaboré par des membres de la cellule "administration scolaire" au Département d'administration et fondements de l'éducation. Définitions retenues qui s’appliquent toutes au domaine de l’éducation. Document complémentaire aux notions véhiculées dans le cours ETA6900 : Introduction à l’administration de l’éducation

Green Edge ice camp campaigns : understanding the processes controlling the under-ice Arctic phytoplankton spring bloom

The Green Edge initiative was developed to investigate the processes controlling the primary productivity and fate of organic matter produced during the Arctic phytoplankton spring bloom (PSB) and to determine its role in the ecosystem. Two field campaigns were conducted in 2015 and 2016 at an ice camp located on landfast sea ice southeast of Qikiqtarjuaq Island in Baffin Bay (67.4797∘ N, 63.7895∘ W). During both expeditions, a large suite of physical, chemical and biological variables was measured beneath a consolidated sea-ice cover from the surface to the bottom (at 360 m depth) to better understand the factors driving the PSB. Key variables, such as conservative temperature, absolute salinity, radiance, irradiance, nutrient concentrations, chlorophyll a concentration, bacteria, phytoplankton and zooplankton abundance and taxonomy, and carbon stocks and fluxes were routinely measured at the ice camp. Meteorological and snow-relevant variables were also monitored. Here, we present the results of a joint effort to tidy and standardize the collected datasets, which will facilitate their reuse in other Arctic studies

Photorefractive gratings in optical fibres

This thesis is an investigation of the fabrication and applications of Bragg gratings produced by the transverse illumination of optical fibres with periodic patterns of ultraviolet light. A general analysis of their optical properties is first presented which considers both the three-dimensional nature of fibre gratings as well as the nonlinear response of the glass to ultraviolet irradiation. The problem of cladding-mode coupling is treated in detail.A review of the phenomena associated with the photosensitivity of germanosilicate fibres is presented, which includes original measurements of photoinduced birefringence. To explain their dynamics, a new model is proposed, based on the photo-thermal ionisation of germania-related oxygen-deficient centres. The effect of core dopants and known sensitisation techniques on the photoinduced index change are evaluated in over twenty different fibres.The fabrication of fibre Bragg gratings using a line-narrowed KrF excimer laser and a three-mirror interferometer is described in detail. Narrow, broadband and high reflectivity gratings are obtained in a variety of fibres, including several germania-free fibres. The dynamics of grating formation are measured and are accurately described by theoretical calculations. The writing process is found to be more effective at higher UV intensities, as predicted by the photo-thermal ionisation model. In an investigation of the writing of gratings using single excimer laser pulses, a new regime of grating formation is identified, which occurs above a sharply defined pulse fluence. This mechanism is used to demonstrate ~100% reflectivity single-pulse gratings. Both low and high reflectivity single pulse gratings are produced for the first time in a fibre drawing tower.The interaction of KrF excimer laser pulses with germanosilicate fibres is investigated on a nanosecond timescale. This experiment reveals the existence of broadband transient gain and absorption in the 1µm region of the spectrum, which provides supportive evidence for a proposed energy-level diagram of oxygen-deficient centres.A new type of channel-dropping filter, the grating-frustrated coupler, is analysed and demonstrated. The performance of a first prototype - 0.7 nm bandwidth, 13 dB isolation, 70% transmission - shows great promise for wavelength multiplexing and line filtering applications. Several other applications are also described, including fibre lasers, dispersion compensators and sensors