Research On Optical Problems Related To X-ray Phase Contrast Imaging

X-ray phase contrast imaging is a relatively recent technique that has heralded rapid advances during the last decade. From the Nobel Prize in Physics in 1953 to Frederik Zernike for his demonstration of the phase contrast method, fantastic technological and software developments forwarded phase contrast techniques at the forefront of many researches. However, X-ray phase contrast imaging methods made such huge progresses that now they allow collecting images with a contrast and a spatial resolution much superior to any conventional X-ray absorption imaging.Thanks also to the availability of synchrotron radiation sources the technological improvements are continuous and the trend will not change in the next years so that it is expected that X-ray phase contrast imaging methods will be important tools in many areas such as biology, medicine, engineering, etc.In the future, these imaging methods will be certainly necessary complements to already available imaging methods, to diagnostic tools, to scientific and technological researches. Indeed, the continuous technological developments in the material science and in particular in the nano-technology areas addressed the strategic significance of 3-D imaging methods at nanometer scale to investigate new materials and phenomena. Actually, the X-ray nano-imaging technology is a strategic research both in USA and in Europe in all third generation synchrotron radiation facilities and for the future Free Electron Laser facilities. In this work, we present a systematic overview of the imaging methods already implemented or going to be implemented at the Beijing Synchrotron Radiation Facility (BSRF). We will present and discuss also the fundamental relation between contrast and spatial resolution and all the other relevant imaging experimental parameters. Finally we will present some new digital imaging processing methods in order to enhance both resolution and contrast.The main contributions of this thesis are summarized below.1. We investigated the principles of the grating-interferometer based phase contrast imaging method and we obtained the theoretical shift-curve of this imaging method. We performed the analysis of grating imaging using the Fourier transform of theFresnel formula in this way we achieved a simpler mathematical description of the fractional Talbot effect. Moreover, we achieved for the first time the formula of the grating displacement for grating based interferometer imaging and the extraction method of the refraction angle was made straightforward i.e., we demonstrated that only two images are enough to determine all the information related to refraction and absorptionprocesses a relevant advantage respect to the previous methods based on the collection of atleast five different images. The new method is not only simpler but allows reduction of theacquisition time and as a consequence of the sample dose.2. We applied new imaging methods also to x-ray computed tomography (CT) extending the imaging method based on a grating interferometer to CT scan. To overcome the problem that the refraction angle is dependent by the path integral and as a consequence a CT reconstruction can not be directly implemented, we developed a new experimental layout and a new algorithm. Instead of collecting images on each shoulders of the rocking curve of the crystal, we collected with the new method only one set of images still allowing with a simplified procedure the reconstruction of the refraction index and its gradient, and other relevant quantities such as the absorption coefficient.3. Software for imaging treatment was developed. Original experimental data were obtained with a mouse paw showing the quality and the accuracy of the software we used. The same software may be also used to determine distortions of a crystal surface used in DEI imaging method.4. Because of the existing technological problems to fabricate gratings with small periods, we designed and tested a new multi-layered structure grating. This optical system is composed by a set of gratings with large period, reducing the technological demands and bringing the grating interferometer phase contrast imaging method into a new wider application scenario.5. We analyzed both theoretically and experimentally the partially coherent in-line phase contrast imaging technique. Effects related to spatial resolution were studied and new reliable methods were proposed to enhance the spatial resolution. Both computer simulations and experiments allowed testing the effectiveness of the methods. Although previously it was considered that high coherent sources were essential to produce good images with in-line phase contrast imaging methods we found that phase shifts are usually very small for a biological sample, so that the role of the source on the imaging quality can be minimized increasing both contrast and resolution with the algorithm we introduced.