Quantitative Phase Retrieval In X-ray Phase Contrast Imaging

In recent years, an increasing attention has been devoted to X-ray phase contrast imaging, since using phase variations, it may provide high-contrast images. Among the different existing techniques, the X-ray Zernike phase contrast microscopy is one of the most popular phase-sensitive imaging techniques capable to investigate the fine structure of a sample at high spatial resolution. And the X-ray phase contrast imaging based on Talbot interferometry has been widely used in biological research, material science and so on. In the X-ray phase contrast imaging, the contrast is indeed a mixture of absorption, phase contrast and scattering. Therefore, the technique provides only qualitative information of the object, which makes difficult the interpretation of the image. As a consequence, the interpretation of the image and the quantitative tomographic reconstruction require a quantitative retrieval of the phase information from intensity measurements.The dissertation presents a systematic overview of X-ray phase contrast imaging and a deep investigation of the quantitative retrieval of the phase information from intensity measurements.The main contributions of the thesis are summarized as follows:1、In this section, we propose a novel approach for quantitative phase retrieval in the X-ray Zernike phase contrast microscopy. By shifting the phase of the direct light by π/2and3π/2, two images of the same object are measured in sequence. The phase information of the object can then be quantitatively retrieved by a proper combination of these images. Numerical experiments were carried out and results confirm the feasibility of the method. A wide number of possible applications in biology and materials science can be foreseen with this new approach.2、In this section, we discuss the behavior of this method at different photon energies. Intensity images of PMMA samples are simulated at2.5keV,6.2keV and8keV, respectively, and phase retrieval is performed using the proposed method. The results demonstrated that the proposed phase retrieval method is applicable in a wide energy range. For weakly absorbing features, the optimal photon energy is2.5keV, from the point of view of image contrast and accuracy of phase retrieval. On the other hand, in the case of strong absorption objects, a higher photon energy is preferred to reduce the error of phase retrieval. These results can be used as guidelines to perform quantitative phase retrieval in Zernike phase contrast X-ray microscopy with the proposed method.3、In this section, we propose a novel single exposure X-ray phase contrast imaging based on wavelet transform to obtain a high-resolution phase map. By using wavelet transform, the phase information of the object can be retrieved from the Moire pattern. Compared to the phase stepping technique, the advantages of the proposed method are increased imaging speed and reduced exposure time. With respect to the Fourier transform method, the novel wavelet transform technique has a much improved resolution. Therefore, we expect the new method will find widespread applications in many fields, such as high-speed phase contrast imaging, time-resolved studies.