It is normal for the pleural cavity to contain a small amount of fluid. The pleural cavity is a physiologic space between the dirty pleura and the wall pleura. The normal pleural cavity contains 5-15 ml of fluid, which plays a lubricating role in the respiratory process, and the maintenance of the normal volume of fluid in the pleural cavity relies on the formation of a balance between the pleural fluid exudation and fluid absorption. Excessive production of fluid in the pleural cavity or decreased reabsorption due to any reason will cause pleural effusion. Clinically, pleural effusions are categorized into leaky pleural effusions and exudative pleural effusions depending on the mechanism of formation. In the process of imaging examination, ordinary chest radiographs or chest CT generally can not find a small amount of physiological pleural effusion, but if chest radiographs and CT examination suggests that there is a small amount of pleural effusion, it often suggests that the amount of fluid in the pleural cavity has been greater than the physiological 5-15 ml, so a small amount of pleural effusion found in daily life imaging examination must be highly valued.