Pleural effusion is mainly manifested as a density image of fluid in the thoracic cavity on CT. The specific manifestation will vary depending on the amount of fluid and whether the fluid is free or encapsulated. In a small amount of pleural effusion, the fluid will appear as a low-density image resting in the posterior rib diaphragmatic sinus region. A moderate or large amount of pleural effusion will appear as fluid spread out in the thoracic cavity, but more in the lower posterior part of the thorax. In encapsulated pleural effusion, because the fluid is encapsulated, it appears on CT as a smooth, flat mound-like fluid density image located in the lower posterior part of the chest with a smoother inner surface, which can be shown as a cleft sign, an image caused by the separation of the dirty layer from the wall pleura, like a cleft; in encapsulated pleural effusion, the pleural wall can also be seen as a calcified shadow.