| Grating-based x-ray phase-contrast images have better imaging results on cartilage in joints than absorption images,and have potential application value for cartilage-related diseases such as arthritis.Grating-based dark-field images have obtained good experimental results in detecting emphysema,pulmonary fibrosis,lung tumors,and breast microcalcifications.X-ray phase-contrast imaging and dark-field imaging can be performed by the dual-phase grating interferometer and the incoherent imaging system in addition to the Talbot-Lau interferometer.Compared with the Talbot-Lau interferometer,the advantages of the dual-phase grating interferometer are that it does not need an analyser grating,which is conducive to remove the restriction of the imaging field of view by the analyser grating.For incoherent imaging systems,the length of the system can be kept within a small range when the x-ray energy is high.The work of this thesis mainly includes the research of dual-phase grating interferometer and the optimization of incoherent imaging system.The main contents are:1.Using Fresnel diffraction,we analyzed the correlation between the main diffraction orders of x-rays after passing through the phase grating,and obtained the fringe period and condition of maximum contrast of the single-phase grating.This analysis method was applied to phase gratings with other duty cycles and phase shifts to obtained fringes periods and the conditions of maximum contrast of the fringes.Based on the above analysis,the fringe period,the condition of the maximum contrast,and the period of the source grating in the dual-phase grating interferometer were studied in this thesis.The thesis showed that the dual ?-phase grating interferometer and the dual ?2/-phase grating interferometer had different fringes periods,conditions of maximum contrast,and periods of source gratings.2.This thesis has compared the similarities and differences between the dual-phase grating interferometer and the Talbot-Lau interferometer in terms of imaging formulas,fringe periods and the conditions of maximum contrast.The similar aspect was that the imaging formula and fringe period of the dual-phase grating interferometer included that of theTalbot-Lau interferometer.The conditions of the maximum contrast of the dual ?-phase grating interferometer also included that of the Talbot-Lau interferometer,but the two ?2/-phase grating interferometer didn’t.At the same time,the parameters of the symmetrical dual Talbot-Lau interferometers just met the conditions of the maximum contrast of the symmetrical dual-phase grating interferometer,which provided a theoretical guidance for simplifying the parameters design of the dual-phase grating interferometer.3.By analyzing the function of the analyser grating in the Talbot-Lau interferometer,we showed that when the phase shift method was used to solve the object information,the function of analyser grating was to select the integral area of the fringe instead of forming the moire fringe.Based on this,a double-pixel readout method was proposed in this thesis,that was,the fringe period was twice larger than the pixel size of the detector.For inverse geometric Talbot-Lau interferometers and dual-phase grating interferometers,the method’s requirement for the pixel size of the detector could be reduced from one-quarter of the original fringe period to one-half,which was beneficial to expanding the imaging field of view.At the same time,the sensitivity of the incoherent x-ray imaging system was improved by using the double-pixel readout method,so that both the phase contrast signal and the dark field signal were improved.4.The hypothesis that x-ray scattering in an object could be regarded as x-ray refraction multiple times inside the object was proposed,and this hypothesis was used to simplify the theory of the impact of the grating position and period on incoherent x-ray dark-field imaging system.The hypothesis was verified by experiments.The results of imaging the breast model showed that,the dark-field signals of the microcalcifications are higher than the absorption signals in the incoherent x-ray imaging system.5.Aiming at reducing the artifacts due to spectral aliasing in the Fourier transform,a method of dual-picture Fourier transform was proposed.This method combined with the three-pixel readout method could effectively remove artifacts while retaining as much of the object’s high-frequency information as possible. |
