Study Of Theory And Numeric Analysis On In-line And Differential Phase-contrast Imaging

Since the middle of 1990s, X-ray phase-contrast imaging has been attracting increasing attention because of its advantage that an extremely high sensitivity achieved for weak-absorbing material, which generates a poor contrast by conventional method based on absorbing. Phase-contrast imaging should find broad application in medicine, biological and material science et al. Several methods of phase-contrast imaging, including Zernike-type, interferometric phase-contrast, diffraction enhanced imaging (DIE), in-line phase-contrast and differential phase-contrast based on Talbot interferometer, have been developed. The requirement of monochromatic x-ray in Zernike-type is strict and fabricate of optical instrument focusing X-ray is a challenge. However the resolving power of Zernike-type can be down to submicron. The requirements of coherence, mechanical stability of instrument and precision of optical device in interferometric methods are strict. So far method of DIE only can be performed with synchrotron source, so the application is restricted. Compared with other methods, in-line phase-contrast and differential phase-contrast are easily performed in conventional laboratory and very widely used because of their moderate requirement on coherence.In-line phase-contrast imaging is a holography method which generates intensity distribution including phase information. Variations in thickness and X-ray refractive index of a sample lead to a change in the shape of an X-ray wavefront on passing through the sample. No optical instruments used in this imaging system lead to sample and compact setup, easy performance. Because of chromatic coherence of lesser important, broadband microfocus sources can give useful phase-contrast images. Taking advantage of the magnification, the requirement of detector resolution is not strict. For the weak-absorbing material, contrast is proportional to the second derivative of phase distribution when the spatial frequency is small enough. Various factors, including the distance between source and specimen R, the distance between specimen and imaging plane z, wavelength used and lateral coherence of source, can influence the contrast of image. There is a suitable z for some spatial frequency according to the variations of transfer function with z. Magnification and spatial coherence of imaging system are determined by the ratio z/R. While increasing the magnification, the spatial coherence could be decreased. So the distances R and z should be determined by the requirement of image and resolution power of detector. The image sharpness will high when shorter wavelength of X-ray is used, but contrast of image decrease. Microfocus source will benefit image.Phase object in front of interferometer cause the distorted interference fringes, which base on Talbot effect. And the phase information, which is proportional to the first derivative of phase distribution, can be obtained from the fringes. With respect to phase grating, the high efficiency of diffraction and visibility are essentially to grating imaging. The efficiency of phase grating is limited by groove depth, material, duty ratio and wavelength. Various parameters insisting of spectrum of source, imaging distance and spatial coherence can influence visibility of fringes. Their referential values were presented in this paper. The absorbing grating in front of detector decreases the requirement of resolution for detector. Methods of Fourier transform and phase-step, which be used for calculating phase information, were introduced and simulation were presented, too.