In conventional radiography, it is impossible to conduct alveoli imaging without contrast agent. Although contrast agents are generally safe, adverse effects do sometimes occur [25] . In PCI experiment, some researchers instilled saline into the nasal airway of the mouse in order to see the alveoli [17] . Actually, the alveoli in human lungs had been visualized by conventional radiography. Some researchers used micro-CT to image human lungs [20] . In this case, an autopsy lung from a dead person was used and silver nitrate was needed as the contrast agent. However, in practice, when small pathological changes happen, it is impossible for doctors to open a live patient's chest or stain his/her alveoli to see where the lesion is. Therefore, non-invasive and non-staining technique is preferable.