Pleural effusion appears on ultrasound as a fluid collection in the pleural space, typically seen as an anechoic (transudative) or echogenic/complex (exudative or hemorrhagic) space between the visceral and parietal pleura. The lung may be seen floating or compressed within the effusion (atelectatic lung). Ultrasound is more sensitive than supine chest radiography for detecting pleural effusions and is the preferred guidance modality for thoracentesis. Estimated volume can be derived from sonographic measurements of the effusion depth.