First of all: CT scan can discover lesions in areas that are difficult to be shown by conventional chest X-ray, such as the apical part of the lung, and can also show tiny lesions that cannot be found by chest X-ray. CT helps to judge the relationship between tumor and surrounding tissues and organs, as well as to determine whether metastasis occurs and the site of metastasis, which is very important for staging of tumor. Therefore, CT has been regarded as a powerful weapon to detect lung cancer, which is beneficial to the detection of early lung cancer, and also can be used for clinical staging of lung cancer to judge the prognosis and formulate treatment strategies based on the staging.