Phase contrast imaging (PCI) is a relatively new imaging technique, which can provide high contrast images by using phase shift of the X-ray. It is well known that X-ray is a form of electromagnetic wave. When it propagates through an object, both the amplitude and the phase of the wave are modified. It can be described by a complex reflective index [11] , given by n = 12d2ib. The imaginary part b is related to the attenuation based on absorption of the object, which we use in the conventional X-ray imaging. Unfortunately, the variations in Xray absorption between different biological soft tissues are quite small; this technique may lead to very low contrast and poor spatial resolution. The real part d is responsible for the X-ray phase shift. For biological soft tissues, the phase shift is almost one thousand times greater than the absorption term. Therefore, the aim of phase contrast imaging is to use the information of X-ray phase shift and convert it into image contrast [12] . This technique can greatly improve the image quality of soft tissues, particularly at the interface of tissues where the refractive index changes significantly. During recent years, it has been widely used by researchers for imaging of small animals. Among all the main PCI methods, in-line X-ray phase contrast imaging (IL-XPCI) is the simplest and the most straightforward, making it more suitable for clinical applications than other methods [13] . IL-XPCI has previously been used to study soft tissues of both human and small animals such as mice, rats, and rabbits. The results are satisfactory: high resolution images were obtained [14] [15] [16] . It may be an alternative method for lung observation and it is totally noninvasive.