Coloured 3D computed tomography (CT) scan of the thoracic (chest) cavity of a patient with pleural effusion, or hydrothorax. The lungs (black) and airways (pink) are visible. Pleural effusion is the accumulation of fluid (yellow) in the spaces around the lungs. The fluid compresses the lungs, leading to breathing difficulties. In mild cases the lungs can return to normal on their own, but where necessary treatment can include chest drains and antibiotics. Transverse view.