The extrapleural fat in empyema: CT appearance.

Pleural empyema can be accompanied by changes in the adjacent chest wall. We examined the chest wall on computed tomographic scans in 24 patients with pleural effusions. Eighteen patients had pleural empyema and six had transudative effusions. Of the 18 empyema patients, 13 had abnormally high attenuation in the extrapleural tissues. In 12 of these 13, the high attenuation was probably caused by reaction to the pleural infection. In two, it was probably caused by haematoma (one patient had haematoma and empyema). In five patients there was either no clear abnormality in the extrapleural space or an insufficient amount of fat to permit detection of an abnormality, or the parietal pleura could not be distinguished from the pleural fluid because intravenous contrast medium was not given. Of the six patients with proven transudative pleural effusions, all had extrapleural fat which appeared normal. Abnormally high attenuation in the extrapleural tissues can be expected to accompany exudative pleural effusion, particularly empyema, but not transudative effusion.