Objective: The half-moon sign is defined as a shadow seen on the later
al chest radiograph consisting of a smooth, partially demarcated opaci
ty with the rounded portion of the half moon projecting over the aorti
c lumen at the level of the aortic arch, but the rest of the opacity m
erging with the aorta. In this study, we intended to evaluate the clin
ical usefulness of half-moon sign in aiding a diagnosis of saccular an
eurysm of the aortic arch. Methods: This series consisted of 57 patien
ts with aortic arch aneurysm and 46 patients with a variety of nonvasc
ular intrathoracic masses that presented as soft-tissue masses adjacen
t to the aortic arch on the frontal chest radiographs. The half-moon s
ign was evaluated independently by two senior chest radiologists who h
ad no knowledge of the final causes. Results: The half-moon sign was s
hown on the lateral chest radiographs in 5 of 10 patients with saccula
r aortic arch aneurysm but absent in 47 patients with fusiform aortic
arch aneurysm. Furthermore, this roentgen sign was not seen on the lat
eral chest radiographs in 46 patients with nonvascular intrathoracic m
asses. 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 th
e saccular aortic arch aneurysm.