## 8 Phase-Contrast X-ray Imaging in Various Thermal Environments  
X-ray CT by means of absorption-contrast X-ray imaging has been used to image gas-filled pores within bio tissues and inorganic materials. However, there is a great demand for visualizing water and its phase changes in these materials. For instance, freezing food is one of the most effective methods for preserving food, but the formation of large ice crystals greatly damages the tissues of the food. Understanding the freeze-thaw process of water in materials, such as fuel cells and construction cement is crucial to facilitating their effective use. Unfortunately, the quality of the X-ray CT imaging dataset is not sufficient to visualize multi-phase materials without a contrast agent because of their low density and the small density differences, as shown in Table 6. A major difficulty is the detection of changes in the water phase using non-destructive observation techniques. This section introduces non-destructive 3D observations of various materials at low temperatures, which enables the observation of icy materials using CXI. Figure 11: Representative studies using CXI images acquired from the Kyoto collection. CT planes and reconstructed images were shown. (**a**): Coronal plane of the face and reconstruction of the secondary palate immediately before fusion (Carnegie stage (CS) 23) [50]. (**b**): Sagittal plane of the eyeball and reconstruction of the eyeball using the optic nerve (CS17 and CS19) [51]. (**c**): Transverse plane of the temporal bone and reconstruction of the inner ear with an enlarged brain and inner ear (CS21) [52]. (**d**): Sagittal plane of the thigh and reconstruction of the femur (CS19, CS21) [53]. (**e**): Transverse plane of the thorax, reconstruction, and median line of the bronchial tree (CS22) [54]. (**f**): Transverse plane of the thorax, reconstruction of the entire embryo and thoracic cage, and analysis of the reconstructed image (CS22) [55]. (**g**): Transverse plane of the thorax and abdomen and reconstruction of the thoracic and abdominal organs (CS21) [56]. In this sample, the liver is defective, and the positions of the stomach, pancreas, and heart have been changed. The thermal effect in a crystal-based XI is not negligible because the thermal drift of the deformations of crystal wafers in a crystal-based XI reduces the sensitivity of CXI. The cryochamber (Figure 12a), including the X-ray window, was insulated with a vacuum layer to maintain the temperature of the crystal-based XI. The X-ray windows were made of aluminum foil and were 60 \(\times\) 30 mm. They were placed at each end of the sample chamber for CXI measurement so that the object and reference X-ray beams could pass through the crystal-based XI and recombine (Figure 12b). The temperature inside the cryochamber was controlled by blowing temperature-controlled dry N\({}_{2}\) gas, supplied by a liquid N\({}_{2}\) container, continuously for more than 24 h. During X-ray measurement, a sample is introduced into a container consisting of two X-ray windows 25 \(\times\) 35 mm (Figure 12b): cylindrical samples with sizes up to a maximum diameter of 15 mm and 20 mm in length can be measured. The container is filled with a buffer liquid to keep the density difference between the sample and the surrounding area within the dynamic range, and to maintain the sample temperature between 190 and 370 K [57]. Two sheet heaters attached to the container maintain the temperature of the liquid within \(\pm\)1 K. Accordingly, the liquid controls the sample temperature and prevents undesirable outline contrasts from the outer surface of the sample. For measurement of icy materials, methyl acetate was used as the buffer liquid to reduce the density differences between the sample and background area because the density of methyl acetate is 0.928 g/cm\({}^{3}\) at 298 K, which is close to that of hexagonal ice (0.917 g/cm\({}^{3}\) at 273 K), and methyl acetate remains in the liquid state even at low temperatures (>175 K).