where d(u, v) is unit impulse function, (u, v) is the coordinate in Fourier domain, M and W are the Fourier transform of m and Q respectively at the distance z. From Eq. (4) one can find that the recorded intensity of the image is determined by the phase shift and the absorption of the wave. Apparently, the optimal contrast depends on the spatial frequency, wavelength, and the objectdetector distance [14] . The in-line X-ray phase contrast imaging experiment was performed at X-ray imaging and biomedical application beamline (BL13W1) of Shanghai Synchrotron Radiation Facility (SSRF). SSRF is the third-generation synchrotron radiation source in China. Figure 6 shows the schematic image of the experiment setup. It consisted of two monochromator crystals, one automatic rotation sample stage and one X-ray sensitive CCD detector. The incident white synchrotron X-ray beam was first monochromatized by two Si (111) prefect crystals. The tunable energy range was from 8 to 72.5 keV, with the energy resolution of about 0.5%. In our experiment, it was adjusted to 18 keV. The theoretical spatial resolution of the system was about 1 mm. Subsequently, the highly parallel and monochromatic beam projected on the object was imaged. The CCD detector then recorded the transmitted beam at a distance of 1.2 m from the sample. During this distance, the downstream image was enhanced by Fresnel diffraction and the phase modulation was transformed via amplitude modulation. An X-ray sensitive CCD camera, which had maximum resolution of 4008 pixels62672 pixels with 13 mm613 mm each, was used as a two-dimensional detector to transform the beam into an image. During the CT data acquisition, the specimen was rotated around its cylinder axis for 180u. The number of projections was 1296, with exposure time of 80 milliseconds for each projection, and the total scanning time was 208 seconds. The surface dose was about 8 mGy for each projection. All the parameters were selected in order to obtain high quality images of the mouse lung.