On the Richardson-Lucy Algorithm with A Varying Point Spread Function along The Iterations

Abstract: This work analyses the soundness of two algorithms, Fishsint and Almexp to improve images using the Richardson-Lucy (RL) algorithm under a varying Point Spread Function (PSF) along the iterations. A plethora of methods based on Richardson-Lucy has been published, but no further proposal involving such an alternative has been published. Whereas the unnamed predecessor of Fishsint addressed only small size synthetic images under a blind fashion procedure, Fishsint and Almexp employs an algorithm Wdet to determine the initial PSF and all subsequent values after each iteration, respectively. Fishsint performs a loop, where the last determined PSF improves the previously obtained image and vice versa. Its original unnamed version has been modified in the present work by entering a previously determined initial PSF to accelerate the convergence. The algorithm Almexp, as well, uses the algorithm Wdet to determine the PSF of the last obtained image to deconvolve itself. Therefore, whereas the Fishsint unnamed predecessor used an initial guess PSF - chosen by the customer - Almexp determines the PSF always through the algorithm Wdet. Fishsint and Almexp furnish final images which outperform those obtained with the original Richardson-Lucy approach working under a constant PSF along the iterations. Hence, in order to carry out a comparison between their performances, all the algorithms have been embedded into an ad hoc written Fortran 90 program. The results corroborate the soundness of a varying PSF along the iterations with the Richardson Lucy algorithm. Keywords: Richardson-Lucy, varying PSF along iterations, image improvement, neutron radiograph

Development of an emulator for tomographic images generated by radiation transmission

A computer program to emulate tomographic images generated by radiation transmission has been developed. The algorithm uses a deterministic approach, to generate the projections which are the supplied to an existing image reconstruction software. A dispersion related to the counting statistics, is also incorporated to the algorithm to simulate the influence of the detector efficiency and counting time on the quality of the final image. The detector resolution is as well included in the algorithm, by assuming a Gaussian shape for its line spread function – LSF. The program deals with cylindrical samples containing any desired number of cylindrical rods inside them, requiring their positions, dimensions and attenuation properties as input data. Some images taken with a thermal neutron tomograph equipped with a position sensitive detector have been compared with those emulated by the developed program

An enhanced shanks transformation applied to gift solutions

Um algoritmo incorporando uma transformação de Shanks modificada foi desenvolvido visando a eliminação de instabilidades que ocorrem na aplicação de sua forma original à alguns tipos de séries. A ideia básica dessa transformação é a geração de somas adicionais parciais através de uma interpolação com um polinômio de grau M-1 definido pelas M somas parciais originais. Apesar do seu caráter virtual, essas somas melhoram o resultado final reduzindo a faixa de magnitude das instabilidades. Uma vez que a posição das instabilidades remanescentes depende do número de somas virtuais utilizadas, os resultados obtidos sob vários números delas- onde os “outliers” foram eliminados- são combinados para compor a solução final. O critério de eliminação utilizado é o valor máximo aceitável para a 2ª derivada. Quando esse critério é violado o potencial e zerado. Esse valor é elevado gradativamente até que a solução final não apresente mais nenhum vazio. Esse algoritmo foi aplicado à um problema específico envolvendo a dispersão de poluentes na atmosfera resolvido pela Técnica da Transformada Integral Generalizada – GITT. Os resultados mostraram uma melhora substancial em termos de tempo de processamento computacional graças à eficácia da própria Transformação de Shanks, bem como às modificações nela introduzidas, o que tornou possível limitar a ordem truncamento dos autovalores à um nível razoável, mantendo no entanto a solução final livre de instabilidades

Evaluation of the Divergence of a Thermal Neutron Beam using a Position Sensitive Detector Brazilian

The divergence of the thermal neutron beam available at the main outlet channel of the Argonauta Reactor in the Instituto de Engenharia Nuclear has been measured by using a Position Sensitive Detector. The measurement process addressing a proper beam characterization has been conducted under several geometric arrangements, including attempts to reduce the beam divergence by using a honeycomb-type gadolinium collimator. Since the detector presents finite resolution, each measured position spectrum is a convolution of the true Rocking Curve expressing the beam divergence with the function that describes the detector response. Therefore, those spectra have been submitted to an unfolding process to recover the concealed Rocking Curve. Prior to undertake this task however, the detector Line Spread Function -LSF was determined as required by the unfolding process. This was done by taking the response of the detector to a line source simulated by a narrow slit-collimator placed at several distances from the detector window. After a Lorentzian fitting to the acquired position spectra, the related FWHM were plotted against the source-detector distance. An extrapolation to zero furnishes then the width of the unknown LSF expressed as a normalized Lorentz function, which was then incorporated to a computer program developed to perform the unfolding. The final Rocking Curve emerges then as a normalized Lorentzian function properly defined by its width and a zero offset, showing that the beam diverges about 2.2 cm in 1 meter. The honeycomb collimator could not reduce this divergence due to its low aspect ratio

Tomografia computadorizada no reator Argonauta - parte I - fundamentos teóricos

A linha de pesquisa em imagens radiográficas utilizando nêutrons provenientes do reator Argonauta do Instituto de Engenharia Nuclear tem apresentado resultados promissores, tanto em neutrongrafia como em tomografia. O desenvolvimento de tomógrafos de diferentes gerações, incluindo o desenvolvimento de detetores especiais para nêutrons, está possibilitando cada vez mais a utilização dessa técnica na resolução de problemas em diversas áreas onde a inspeção com nêutrons é complementar a outras radiações, como raios-X e raios-γ. Este trabalho é o primeiro de uma série que tem como objetivo relatar os estágios da pesquisa, especialmente em tomografia. Ele tem um enfoque teórico apresentando, de forma resumida, a técnica de tomografia, as diferenças na utilização do nêutron e do raio-X como radiação, os parâmetros que influenciam na qualidade da imagem fornecida por um sistema tomográfico e as gerações de tomógrafos

Improvement of a Gaseous Position Sensitive Detector for use in Thermal Neutron Tomographic Systems

The utilization of a position sensitive detector in tomographic systems is an attractive possibility because it is capable of furnishing the position where the ionizing event occurs. This feature can reduce significantly the image acquiring time, since a sample translation is no longer required. In this work the performance of a gaseous position sensitive detector equipping a thermal neutron tomographic system has been improved by a stepwise increase of the filling-gas ( 3 He-enriched helium) pressure from 3 to 6 atm. Important quantitative detector parameters such as resolution, linearity and homogeneity have been measured for that pressure range, and compared with the tomographic images of test-samples. Several test-samples have been studied, all of them constituted by an aluminum cylinder containing inserts of different materials. Besides that, the modulation transfer function-MTF for the system has been experimentally obtained and compared with the expected theoretical curve. An improvement of both detector efficiency and resolution has been observed, as theoretically expected from an increase of the filling-gas pressure.