# Exploring Frontiers of 4D X-ray Tomography
## 3 High-Speed X-ray CT  
### With Sample Rotation  
The X-ray CT scanners commonly used in hospitals generally take several tens of seconds to several tens of minutes to take a 3D image. In the case of medical X-ray CT, an X-ray source and X-ray image detector are rotated around a target object in order to capture projection images from many directions. The speed of these rotations places a limit on the measurement time. Sub-second scan times have been achieved in state-of-the-art devices [26], but now, the intensity of the X-rays is an issue. A low-brilliance X-ray source based on bombarding a target material with accelerated electrons is commonly used in hospitals, and the melting of the target determines the intensity limit. Therefore, it is necessary for high-speed measurements to sacrifice the spatial resolution of the X-ray image detector to increase the number of counts for each pixel. Time scales that can be perceived by the human eye have been the mainstream for not only X-ray CT but also other methods of non-destructive three-dimensional observation of the interior of objects opaque to visible light. Figure 4 outlines the temporal and spatial resolutions of various 3D visualization methods for samples opaque to visible light, including X-ray CT. It can be seen that the domain of millisecond- and micrometer-order spatio-temporal resolution introduced in this paper has never been explored. The pink zone in the figure shows, as an example, the spatial and temporal resolutions suitable for observing the movement of animals. This zone roughly represents kinetic speed determined by metabolic rate \(\times\) biological time. The zone corresponding to the mechanical motion of artifacts (green zone) is one or two orders of magnitude higher than this. Many of the phenomena we come across in our daily lives occur in these zones, making them extremely important subjects of research in science, technology, and industry. Various applications are possible for measurement methods that can access these temporal and spatial scales, from academic research, such as observing phenomena inside living animals (e.g., insects), the failure process of materials, and the behavior of liquids, to industrial applications, such as developing intelligent materials, and dynamic biomimetics applications. The trade-off between temporal and spatial resolutions (dashed line) is also shown in Figure 4. To increase the temporal resolution, it is generally sufficient to sacrifice the spatial resolution. In fact, temporal resolution on the order of milliseconds has been achieved for a spatial resolution reduced to the order of millimeters. It is worth noting, however, that the direction of this trade-off relationship is close to orthogonal to the pink and green zones. In other words, the smaller the observation object, the larger the demand for high-speed viewing, but improving both spatial and temporal resolutions is often technically challenging. Thus, how can we realize high-speed X-ray CT that far exceeds the limits of what the human eye can perceive? It is impossible to rotate the X-ray source and the X-ray image detector of a hospital X-ray CT scanner at the speed that is needed. Additionally, the intensity of X-ray sources used at the laboratory level is insufficient. Highly intense synchrotron radiation X-rays can solve this problem [27; 28; 29; 30; 31]. The authors succeeded in X-ray CT with an imaging time on the order of milliseconds by using a third-generation synchrotron radiation source and rotating samples at high speed [27; 28; 29]. The experiments were performed at the experimental station BL28B2 at SPring-8, a large synchrotron radiation facility. Third-generation synchrotron radiation sources such as SPring-8 are capable of producing much more brilliant X-rays (i.e., a high-intensity X-ray beam from a small light source) than laboratory X-ray sources. The small size of the light source allows us to use high-spatial-coherence X-rays, which is suitable for X-ray phase-contrast imaging.