# State-of-the-Art Research on Loess Microstructure Based on X-ray Computer Tomography
## 2 The Development History of CT Technology
### Development of CT Equipment
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**CT Category** & **Resolution** & **Common Current** & **Common Voltage** & **Equipment Schematic** & **Scanning Time** \\ \hline Medical CT & 600 um & 10–600 mA & 120–140 KV & 1–10 s \\ \hline Industrial CT & 50–500 um & 10–1000 mA & 225–600 KV & 5–60 min or longer \\ \hline Micro-CT & 0.5–50 um & 5–100 mA & 30–160 KV & 15–60 min or longer \\ \hline Nano-CT & 50–0.5 um & 30 uA–50 mA & 8–150 KV & 1–60 min or longer \\ \hline \hline \end{tabular} Note: The current, voltage, and scanning time used during CT scanning are contingent upon the scanning conditions.  
\end{table} 
Table 1: Differences among various CT devices. at the microscale, observing only millimeter- or sub-millimeter-scale changes in pores and fractures. Moreover, they were mostly based on two-dimensional CT slices and numbers to represent various hydraulic parameters. For geotechnical bodies, their microstructural characteristics and changes are fundamental to determining macroscopic properties, and the anisotropy at the three-dimensional scale is a key and challenging point in revealing the true hydraulic properties. With the evolution and iteration of CT technology, from the first generation to the fifth generation CT, and from planar scanning to cone-beam scanning, the development of high-energy, high-precision, and high-safety CT systems, such as micro-focus CT, nano-CT, and synchrotron radiation micro-CT, along with the advancement of three-dimensional reconstruction and visualization techniques, has greatly promoted the extensive application and popularization of industrial CT in various fields [36]. As for the definitions of micro-focus CT and nano-focus CT or the differences between them, they are currently somewhat ambiguous, lacking clear industry standards. Typically, the industry classifies devices with a resolution lower than 0.5 um as micro-focus CT, while those with a resolution higher than 0.5 um are referred to as nano-focus CT [34; 35], as shown in Table 1. In the field of geotechnical materials, a multitude of significant research and application achievements have emerged, such as three-dimensional reconstruction and the quantitative characterization of soil structure [3; 18; 19; 42; 43; 44; 45; 46]; the identification of soil macropores and the study of macropore flow and migration mechanisms [47; 48]; the characterization, extension patterns, and seepage simulation of fractures and pores in granite, sandstone, oil shale, and coal rock bodies [49; 50; 51; 52; 53; 54]; the mechanism of gas percolation in coal seams [55]; and the identification of paleosol pores and fractures in loess and the study of their hydrological significance for slope stability [43; 44; 55; 56; 57]. Loess is pervasively characterized by the development of macropores, and while there is no universal size standard for these macropores, summaries, and classifications by [PERSON] et al. [58] suggest that the pore diameter of macropores ranges from 32 um to 500 um. The resolution of medical CT (600 um) is clearly inadequate for such a detailed investigation. Nowadays, the most advanced forms of micro-CT and nano-CT can achieve a resolution of 0.5 um or higher values. Therefore, these technologies can be used to describe the microstructure of loess more accurately. However, researchers must keep in mind that the limitation of the micro-CT and nano-CT, i.e., to achieve higher resolution of a smaller sample size, is required according to the principles of CT technology [34; 35]. For instance, to attain a resolution of 0.5 um, the sample diameter must be less than 1 mm. Consequently, the study on the microstructure of loess has primarily benefited from the rapid development of advanced precision instruments and testing technology levels. In addition, three-dimensional reconstruction and visualization have provided robust technical support for the detailed characterization of the microstructure of loess.