A CT projection image of the mouse lung is shown in Figure 1A . Although in this image, all the tissues overlapped with each other, some bronchi, lobes and many alveoli can be discerned. Figure 1B and C are two slices of the CT reconstruction images. Compared to conventional X-ray CT, in-line X-ray phase contrast CT had a much higher resolution (about 13 mm in our experiment). Bone, bronchi and alveoli were easily discernible with clear edges, which made it possible to separate different anatomical structures using the image segmentation technique. In order to extract the lung tissue and remove background noise, we applied a threshold-based image segmentation method to the CT slices. A comparison between original CT slice and threshold-based segmentation result is presented in Figure 2 . The threshold was chosen at the valley of the histogram ( Figure 2B ). Other pixels with intensity lower than the threshold were set to zero. After gray scale transformation, the resulted image was shown in Figure 2D . From the profiles at the white line drawn in the same place of the two CT slices, the background noise in the image was considerably reduced.