Extrapleural fat sign.

Explanation: Extrapleural fat, endothoracic fascia, and the innermost layers of intercostal muscle are superficial to the parietal pleura. Extrapleural fat is usually seen as a 1mm thick low attenution line on chest CT and is almost imperceptible in normal individuals.2 It may be thicker in obese patients. Accumulation of blood in the extrapleural space results in an extrapleural hematoma (EPH), which displaces the overlying extrapleural fat centrally and is most commonly the result of injury to intercostal arteries or veins.3 EPH is an underreported complication of blunt chest trauma. Less common causes of EPH include aortic rupture, penetrating trauma, and iatrogenic injury.3–6