Performance Research Of X-ray Phase Contrast CT Device Based On Geometric Projection

Since X-ray was discovered by Rontgen in 1895,X-ray has been widely used in medical diagnosis,nondestructive detection,and other fields.Traditional X-ray absorption imaging method cannot provide high-quality imaging for weakly absorbed structures,such as biological soft tissues mainly consist of light elements.Different from the traditional X-ray imaging based on absorption contrast,phase contrast imaging is performed by detecting the phase shift of X-ray.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.Theoretically,phase contrast image has a higher sensitivity,so phase-contrast imaging can provide better imaging contrast for weakly absorbent materials.In the past two decades,X-ray phase contrast imaging technology has developed rapidly,and grating-based phase contrast imaging(GBPCI)has attracted a lot of attention.Compared with other phase contrast imaging methods,GBPCI can use a high-power conventional CT tube as the light source and has a larger imaging field of view,so it is widely considered as a potential phase contrast imaging method for clinical applications.In this thesis,we will discuss the information extraction and reconstruction algorithm in GBPCI,introduce the phase contrast CT device constructed by National Synchrotron Radiation Laboratory(NSRL),develope the optical detection method of grating,and analyze the effects of optical elements on imaging performance.The main contents of this thesis are summarized as follows:1.Based on the reverse projection(RP)method,an improved information extraction method is proposed,and the inverse function is used to replace the linear approximation near the half slope of the shifting curve.Considering that the signal of the refraction is related to the distance from the pixel of the sample to the analysis grating(G2),an improved reconstruction algorithm is proposed to improve the image reconstruction accuracy of the large-size object.2.According to the projection-based imaging method,the phase contrast CT device constructed by NSRL,and phase contrast CT was realized by using a medical CT bulb and detector for the first time.The phase signal is carried in the geometric moire fringe introduced by absorbing gratings with large period in these GBPCI setups.Because the limitation of grating period is relaxed,the requirement of grating fabrication technique is relaxed sharply.Combining the characteristics of high power and high energy of medical CT tube and large size and high frame rate of CT detector,fast and high energy phase contrast imaging of large field of view is realized.Moreover,we explored the experimental technology and data processing and provided guidance for the clinical application of GBPCI.3.Because the fabrication of high aspect ratio absorption grating is very difficult,the processed gratings are usually not ideal,and grating is easily deformed,such as distortion,tilt,and collapse.At the same time,the focal spot of the CT tube will drift during operation.Aiming at these defects of imaging system components,this thesis quantitatively analyzes the effects of grating and light source defects on the imaging performance of the system.By simulating the effect of different shapes of the grating,the size and the drift of the focal spot on the imaging performance,the factors that have a decisive influence on the imaging performance of the system is obtained.This will guide the processing of the grating and point out the direction of the improvement of the imaging system.4.In view of the structural characteristics of the absorption grating used in the phase-contrast CT device,an optical detection method for high aspect ratio grating is proposed.The simulation and experimental results verify the feasibility of this method and solve the problem of detecting high aspect ratio grating.Based on the modulation of incident light by grating structure,the method can determine the change of grating structure and realize fast and nondestructive detection of high aspect ratio gratings by observing the distribution of reflected light.