Anterior clear space: How clear? How often? How come?

PURPOSE: To compare the opacity of the retrosternal clear space with that of the retrocardiac region on normal lateral chest radiographs, to measure the clear space, and to evaluate the reasons for these findings. MATERIALS AND METHODS: Left lateral chest radiographs and computed tomographic scans of 38 patients with normal studies were evaluated and the results were correlated. RESULTS: In 19 (50%) patients, retrosternal and retrocardiac opacity were equal. In 16 (42%), the retrosternal region was more opaque than the retrocardiac region (10 of 12 women, six of 26 men). The difference between the sexes was statistically significant (P = .002). In only nine (24%) patients was the anterior margin of the ascending aorta defined on the lateral study. Mean sternum-to-aorta distance in these patients was 29 mm. CONCLUSIONS: Retrosternal opacity greater than retrocardiac opacity on lateral chest radiographs is a frequent normal finding, especially in women. Sternum-to-aorta distance may be difficult to measure, and distance greater than 2.5 cm may be normal in some persons.