Study On Information Extraction And CT Reconstruction In Analyzer-Based Imaging

Until now, the X-ray phase-contrast imaging technology is existed as a hot research in the imaging field. Compared with the conventional absorption radiography, it can generate an image with higher contrast and obtain the clearer details of internal microstructure in sample (especially in the sample made up primarily of low-Z elements). It can be widely applied in many fields such as medicine, biology and material sciences. Hence, the research on phase-constrast imaging is very valuable. The analyzer-based imaging is a very important phase-contrast imaging in which the refraction angle, ultra-small angle scattering and attenuation image can be yielded using two or more than two raw images measured at different analyzer positions. In this paper, a few problems present in the information extraction and CT reconstruction of analyzer-based imaging were fully studied, and the solution were proposed, the main works and research results are summarized as follows:Firstly, the extracting effect of information (refraction angle, ultra-small angle scattering and attenuation) from each of three existed algorithms of diffraction-enhanced imaging, generalized diffraction-enhanced imaging and multiple-image radiography was discussed using the different raw image-collecting strategies. The findings show that under the different raw image-collecting strategies, the extraction results of information have obvious difference for each of the above three algorithms. By means of the comparison of results, the prior choose to the strategy of image collection is recommended using the characteristic of each algorithm. This study will be helpful for reasonably choosing the image-collecting strategies in actual experiment, and help the information-extracting algorithms reach their full potential of work.Secondly, compared with the two other algorithms, the multiple image radiography can gives the more accurate values of refraction angle, but the calculated values still underestimate the actual ones, particularly when the refraction angles are very high. Therefore, for acquiring the more accurate refraction angles, an improved method of multiple image radiography was effectively proposed and developed on the basis of the feature of information extraction in original algorithm. With the same condition of collecting images, the accuracy of refraction angles is significantly increased using the improved method. Based on the principle of improvement to the extraction of refraction information, an appropriate improvement to extract the ultrasmall angle scattering information is implemented as well. When the improved method is used, the extraction results of ultrasmall angle scattering information become better at some extent. In addition, compared to the original algorithm, the improved algorithm shows the stronger ability to suppress the disturbance from scattering information, and the similar ability to resist noise. Besides,when the information is extracted using the original and improved algorithms, the difference of computation time between them is very small, and can be very negligible due to the similar complexity of calculation. The improved algorithm may be useful in practical application on the X-ray tube source.Lastly, the parallel-beam and fan-beam computed tomography (CT) applying the algebraic reconstruction technique (ART) iterative algorithm are implemented to reconstruct the distribution of the decrement of refraction index 8 and the two components of its gradient (?)δ/(?)x and (?)δ/(?)y.The results of CT reconstruction by the use of projections of refraction angles from the algorithm of multiple image radiography and its improved algorithm have been compared in this work. The results of computer simulation experiment suggest that compared to the original algorithm, the projections calculated with the improved method cause the more accurate results of parallel-beam or fan-beam CT reconstruction, and have strong ability to tolerate noise like the original algorithm. The fan-beam projections were weighted to ensure that the ART algorithm used to parallel beam projections can be successfully implemented in fan-beam CT reconstruction, and the weighting process is accomplished through a valid method of coordinate transformation. The X-ray wavelengths at different positions of fan angle have significant difference (polychromatic X-rays) due to the variant incident angles on monochromatic crystal. As a consequence, the difference of refraction angle projections between the fan beam and parallel beam (monochromatic X-ray)is great, and have a severe impact on the accuracy of reconstructed images. In order to solve this problem, a method is proposed to modify the projections of fan beam. With the modified projections, the more accurate CT images are given. It is proved that the proposed method modifying the projections of fan beam is effective.