A presente tese demonstra, a partir da avaliação convencional pelo método das Funções de Transferência de sistemas de imagem radiológica, que é necessário obter imagens de fenda em diversas orientações no campo para que essa análise tenha um significado mais real no caso de sistemas não isotrópicos. Isso provém da não linearidade na variação entre as FTMs obtidas para diversas direções 0 e 90&#176C relativas ao eixo do tubo de raios-X. Essa verificação, entretanto, representa um sério problema prático, pois indica um aumento no grau de complexidade de um método que, embora considerado o mais preciso pela maioria dos pesquisadores, tem sido utilizado apenas por laboratórios muito bem equipados. Assim, visando solucionar esse problema, esta tese propõe um novo método de simulação por computador que calcula a FEL e a FTM devidas ao ponto focal, dispensando, portanto, todo o complexo aparato experimental convencionalmente utilizado, o que contribui para tornar acessível à avaliação pelas funções de transferência a qualquer unidade radiológica. Por fim, faz parte desse trabalho também uma investigação do significado físico das variações registradas entre as FTMs e um estudo formal desenvolvido acerca dos conceitos da característica de campo e da magnificação lateral. / From the conventional evaluation by the radiological systems Transfer Functions, this work shows that it is necessary to obtain slit images at several field orientations so that this annalysis has a more real significance for non-isotropic systems. This is achieved from the non-linearity on the variations among the MTFs obtained in several directions between 0 and 90&#176C relative to the X-ray tube axis. This notification, however, represents a serious practical matter, because it shows an increase on the complexity of a method which has been used just by well structured laboratories, although many researchers have considered it the most accurate. Hence, in order to solve this problem, we present a new computer simulation method which calculates the LSF and the MTF due to the focal spot, without all the conventional complex experimental apparatus. This makes the evaluation by the transfer functions suitable to any radiological unit. Finally, it is also part of this work an investigation of the physical meaning of the variations among the MTFs and a formal study about the field characteristics and the lateral magnification concepts.

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We develop a theory of vectorial wave coupling in cubic photorefractive crystals placed in an alternating ac-field to enhance the nonlinear response. First we analytically and numerically investigate the dependences of the first Fourier harmonics of the space-charge field, induced in an AC-biased sillenite crystal by a light-interference pattern, on the light contrast m. The data obtained was used to extend the vectorial beam-coupling theory on the whole contrast region. In particular, we proved in the general case that despite of essential differences between thediffusion and AC nonlocal responses the later keeps the light interference fringes straight during the interaction. This fundamental feature allows, under certain restrictions, to reduce the nonlinear problem of vectorial coupling to the known linear problem of vectorial Bragg diffraction from a spatially uniform grating, which admits an exact solution. As a result, the nonlinear vectorial problem can be effectively solved for a number of practically important cases.The developed theory was applied to describe the transformation of a momentary phase changes of one of the input beams into the output intensity modulation (so-called grating translation technique). In contrast to the previous studies, we take into account the change of the space-charge field amplitude across the crystal (the coupling effects). The theory developed is employed to optimize the conditions for the linear signal detection under polarization filtering for the transverse and longitudinal optical configurations. We also analyze the possibility of the linear detection without polarization filtering.Illumination of AC-biased photorefractive BTO crystals with a coherent light beam results in development of strong nonlinear scattering. We investigate the angular and polarization characteristics of the scattered light for the diagonal optical configuration and different polarization states of the pump.