CT (Computed Tomography) is a major breakthrough in radiological diagnosis in the early 1970s. CT is not X-ray photography, but a reconstruction image obtained by scanning the human body with X-rays, obtaining information, and processing it by an electronic computer. It can make the organs and their lesions that are difficult to show by traditional X-ray examination show imaging, and the image is realistic and the anatomical relationship is clear, thus expanding the scope of human body examination and greatly improving the early detection rate and diagnostic accuracy of lesions. CT was initially used only for head examinations, but in 1974, whole-body CT was introduced, and in just 10 years, CT has spread all over the world, from the first generation to the fifth generation. Most of the CTs used in hospitals in major cities in China belong to the third generation. CT is best suited to identify the size, shape, number and extent of invasion of occupying lesions such as tumors, cysts, enlarged lymph nodes, hematomas, abscesses and granulomas, and it can determine the stage of cancer in certain organs and whether they can be surgically removed. In some cases, CT can also distinguish the pathological characteristics of lesions such as solid, cystic, vascular, inflammatory, calcific, fatty, etc. There are three methods of CT examination, one is plain scan, which is a general scan and is a routine examination; the second is enhanced scan, which injects water-soluble organic iodine from a vein and then scans, which can make certain lesions appear more clearly; the third is contrast scan, which first images organs or structures and then scans them, such as injecting contrast agent or air into the brain pool for brain pool imaging and then scanning, which can clearly show the brain pool and small tumors in it. Before CT examination, patient preparation is also very simple, as long as fasting is done before the examination. However, other imaging tests should not be done before the abdominal examination, especially barium should not be used to perform gastrointestinal imaging, so as to avoid the formation of artifacts from the residual contrast agent in the intestine, which may affect the quality of CT images and lead to misdiagnosis. Before the head scan, X-ray cranial plain film and tomogram should be taken. Before liver, biliary and pancreatic examinations, various laboratory tests, abdominal plain films, cholangiography and ultrasonography should be done. Before kidney examination, pyelogram and ultrasound should be done. Before the chest examination, a chest plain film and tomography should be taken. Before spinal examination, frontal and lateral and oblique photographs of the spine should be taken first. In order to choose the best scanning method and the most reasonable scanning range.