Information Separation And CT In X-ray Grating-based Phase Contrast Imaging

Many X-ray phase contrast imaging methods have been developed since the1990s. Different from traditional X-ray attenuation-based imaging, they allow obtaining inner structure of an object by detecting the phase modulation derived from its internal structure. The object’s refraction index is usually expressed by a complex number, whose decrement of the real part accounts for refraction information while its imaginary part is related to absorption information. Working in the hard X-ray domain, the decrement of the real part of refraction index of soft materials like tissues is two orders of or three orders of magnitudes bigger than the imaginary part. In other words, phase contrast image has a higher sensitivity from theoretical point of view, so we can obtain higher image quality using phase contrast imaging methods.In the last20years X-ray phase contrast imaging has achieved important progress and at least five main methods have been established. However, some are unsuitable for wide practical applications due to small field of view, high requirement on mechanical stability and coherence of light source. Recently, the grating-based phase contrast imaging (GBPCI), attracting a lot of attentions in the field of X-ray imaging, is considered as a potential phase contrast imaging method for clinical applications. Compared to other phase contrast imaging methods, GBPCI offers two main advantages:low requirement on time and spatial coherence of the light source and a large field of view. In this thesis, we will discuss information extraction and computed tomography (CT) in GBPCI.Phase contrast image contains absorption, refraction and scattering signals, which relate to the object’s different physical characteristics. In order to calculate these physical quantities, many different information separation methods have been proposed. Currently, three are widely used in GBPCI:the Fourier analysis method, the phase stepping (PS) method and the reverse projection (RP) method. The last is a low dose and fast method, proposed by our group in2010. In this manuscript, we will present its generalization, the effect of the coherence, the noise properties, and compare it with the PS method. All these works accelerate and guide foreseen clinic applications of GBPCI.Using the ray with penetrating power, computed tomography is able to probe the inner3D structure of an object in a non-destructive way, and represents a common three-dimensional (3D) tool. X-ray CT (X-CT) is already widely applied in clinical medicine, non-destructive testing and materials science. In particular, the application in medicine helps to identify and locate a pathological status. However, when the sample is a soft tissue, traditional X-CT provides a poor contrast even using contrast agent. In contrast, phase contrast imaging provides high contrast two-dimensional iamge. Combination with X-CT develops into "PC-CT" method providing a superior3D image. Because of the different projection mechanisms, the reconstruction algorithms are different, e.g. different filters in the filter back-projection (FBP) algorithms are used. In this manuscript, we will present and discuss X-CT and PC-CT algorithm for parallel beam and equiangular fan beam geometry.In clinical applications, the speed and the released dose of X-ray imaging mthod are the critical issues. In X-ray grating-based PC-CT, the alternation between the translation of the grating of the PS method and the circular scan of CT determines low imaging speed and high radiation dose. On the contrary, taking use of the conjugate characteristic of the identical attenuation and opposite refraction angle between mutual reverse projections, the RP method achieves a faster and lower dose acquisition. However this approach is only compatible with parallel beam geometry, because the conjugate characteristic between mutual reverse projections is not fulfilled in non-parallel beam geomety. In this thesis, we successfully generalize the RP method to fan beam geometry by considering the single ray as the probe.The main contents are summeried in seven sections:1. Overview the different phase contrast imaging methods;2. Introduction to the GBPCI method;3. Discussion on the information seperation methods in GBPCI.4. Review of filtered back-projection algorithms in absorption and phase contrast imaging;5. Generalization of the RP method in fan beam geometry;6. The influence of the spatial coherence of the light source on the results of the RP method;7. The noise analysis of the RP method and comparison between the RP and PS methods;The main achievements presented in this research can be summarized as follows:1. A first generalization of the RP method to fan beam geometry.2. A first analysis on the effect of the spatial coherence of the light source on the results of the RP method;3. A first discuss of the noise property of the RP method using the error propagation formula;4. A first attempt to compare RP and PS methods in a quantitative way.