# 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  
X-ray CT was initially developed by Hounsfield in the late 1960s to the early 1970s and was applied to the field of medical imaging, known as medical CT. By the end of the 1970s, researchers discovered that CT technology could effectively be used for industrial material structure detection, and the value of CT in industries and geology gradually emerged, leading to the birth of industrial CT. In the 1990s, due to significant improvements in image quality and detail resolution capabilities, industrial CT began to play an important role in the detection of cracks, porosity, material density, 3D petrophysical studies, and the 3D visualization of pore structures in natural building materials [30]. Currently, industrial CT has been widely applied in various fields, such as industrial engineering, pedology, geology, materials science, food science, archaeology, aerospace, coal, petroleum, natural gas, chemical engineering, nuclear industry, and geotechnical engineering [31], gradually becoming a common technology for inspecting geological and engineering materials. Medical CT and industrial CT are based on the same fundamental physical principles [32], but due to different application purposes, their system architectures and designs vary; this paper compiles and organizes comparative information on the differences among various CT devices, as shown in Table 1[33; 34; 35]. For medical CT, the scanning subject is a patient, with the goal being human tissue. During scanning, patients lie still and flat, and the X-ray scanner rotates rapidly around the scanning area, with the scanning table moving forward and backward to achieve volumetric scanning. However, the limitation of this scanning method is that it is difficult to ensure high resolution. With technological advancements, the increase in the \"number of rows\" of medical CT detectors and the enhancement of the data acquisition system's ability to synchronize the acquisition of image \"slices\" has led to an increased resolution (up to 600 um). This allows for more layers of images to be scanned in a shorter time during patient scanning, ensuring minimal radiation dosage and enabling more accurate diagnosis. Currently, more advanced CT technologies, such as 256-slice CT, dual-source CT, and PET-CT, have shown broad application prospects, mainly in non-invasive angiography, tumor diagnosis, and macro and micro disease diagnosis [36]. Additionally, some scholars have applied them to the fields of wood [37] and geotechnical engineering [38; 39; 40; 41]. In contrast, the scanning subjects of industrial CT are more diverse, including metals, ceramics, rocks, soil, composite materials, and synthetic materials. Samples can rotate at a certain angle on the stage or adjust along the \(Z\)-axis of the stage while the X-ray source remains fixed, typically using fan-beam flat-scanning or cone-beam scanning methods. Compared to medical CT, industrial CT usually does not focus on reducing the X-ray radiation dosage but instead uses high-intensity X-rays to pursue higher scanning resolution and accuracy [18]. Therefore, the hardware requirements for industrial CT systems are higher, and the cost is also higher. It should be noted that the higher the resolution, the more the sample size is limited. In the early stages, due to the widespread availability of medical CT and the high cost and limited acquisition of industrial CT, researchers conducted a series of geotechnical experiments using medical CT. These included studies on the damage and deformation patterns of frozen soil, the collapsible nature of loess, the evolution of rock fractures, and the monitoring and simulation of seepage in geotechnical bodies. Some important results were achieved, laying a solid foundation for research in this field. However, due to the resolution limitations of medical CT, the aforementioned studies could only be conducted