Research Of Theories And Methods On X-ray Differential Phase Contrast Imaging And CT

A series of X-ray phase contrast imaging methods have been developed since the 1990s. 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 refraction index distribution. The object’s refraction index is usually expressed by a complex number, whose decrement of the real part (also refer to phase-shift factor) accounts for refraction information while its imaginary part (also refer to absorption factor) is related to absorption information. Working in the hard X-ray domain, the decrement of the real part of refraction index of soft materials such as 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 last 20 years 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 dissertation, we will discuss information extraction of diffraction and GBPCI according to the theory of angular signal imaging, and some scientific questions on the phase contrast and scattering contrast combined with 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, such as phase stepping (PS) technology, interlaced phase stepping, conjugate ray analysis, Moire Analysis and energy resolved phase contrast imaging methods. Among these methods, some need high radiation dose, some will reduce the imaging resolution and some cannot extract the scattering information. In regard to these problems, this dissertation developed a theoretical framework of angular signal response imaging. Based on this theory, cosine fitting radiography is proposed, it can achieve the result which closes to that of standard method (PS) and make good in the underestimate of large refraction angle. Meanwhile, this theory build a close connection between diffraction enhanced imaging and GBPCI. The essence of these two kinds of phase contrast imaging are both angular collimator-analyzer which can detect the gradient of the phase distribution of sample. They selective pass the refracted X-ray by moving the analyzer continuously. The angle corresponding to the displacement between the collimator and analyzer is equivalent to the refraction angle of X-ray generated by the sample.In X-ray GBPCI-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. In order to solve this problem, the conjugate ray analysis is developed by our group. In this method, taking use of the conjugate characteristic of the identical attenuation and opposite refraction angle between mutual reverse projections, the conjugate ray analysis method can separate absorption and refraction information only with two images, thus achieving a faster and lower dose acquisition. This dissertation analyzes the relation between the signal-to-noise ratio (SNR) and in-plane resolution using the conjugate ray analysis method combined with CT, the result demonstrates phase tomography based on grating has a better anti-noise performance than absorption CT in high resolution imaging.Also in X-ray GBPCI-CT, the scattering image can reflect the inaccessible spatially resolved information about the distribution of micron and submicron sized structural formations, and provide more detailed of the object, thus complementing the absorption and refraction information. This dissertation reconstructs the scattering information of the object with GBPCI-CT, and analyzes the effect of particle-size selectivity and designed X-ray energy on quantitative X-ray scattering computed tomography using a grating interferometer. All of these results will helpful to distinguish the materials with similar absorption coefficient, thus extending the applications of X-ray grating interferometry in the field of material testing.