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