The causes of pleurisy mainly include tuberculous, purulent, bacterial, tumorous, traumatic, and rheumatic types, with the tuberculous type being the most common. Pleurisy is predominantly characterized by pleural effusion, which generally looks similar on a CT scan. CT scans cannot distinguish the cause of pleurisy. A small amount of pleurisy manifests as a minor amount of free effusion, appearing as an arc or crescent of uniform density along the posterior chest wall. As the effusion gradually increases to a moderate or large amount, it can compress lung tissue, leading to compressive atelectasis. In cases of large volume effusion, aside from causing atelectasis, it can also significantly push the mediastinum towards the healthy side.