The half-moon sign. A useful roentgen sign of saccular aneurysm of the aortic arch.

OBJECTIVE

The half-moon sign is defined as a shadow seen on the lateral chest radiograph consisting of a smooth, partially demarcated opacity with the rounded portion of the half moon projecting over the aortic lumen at the level of the aortic arch, but the rest of the opacity merging with the aorta. In this study, we intended to evaluate the clinical usefulness of half-moon sign in aiding a diagnosis of saccular aneurysm of the aortic arch.

METHODS

This series consisted of 57 patients with aortic arch aneurysm and 46 patients with a variety of nonvascular intrathoracic masses that presented as soft-tissue masses adjacent to the aortic arch on the frontal chest radiographs. The half-moon sign was evaluated independently by two senior chest radiologists who had no knowledge of the final causes.

RESULTS

The half-moon sign was shown on the lateral chest radiographs in 5 of 10 patients with saccular aortic arch aneurysm but absent in 47 patients with fusiform aortic arch aneurysm. Furthermore, this roentgen sign was not seen on the lateral chest radiographs in 46 patients with nonvascular intrathoracic masses. In this selected population, the sensitivity and specificity of the half-moon sign in aiding a diagnosis of saccular aneurysm of the aortic arch were 50% and 100%, respectively.

CONCLUSION

The half-moon sign shown on the lateral chest radiograph is highly suggestive of the saccular aortic arch aneurysm.