On the frontal and lateral radiographs the outline of the aorta can be traced (red dotted line) and does not clearly blend with the mass projecting towards the right of the mediastinum (blue dotted line).
When one looks at the CT, it is easy to understand why this is the case. Although the aorta (red dotted line) and aneurysm (blue dotted line) are in continuity, the x-ray beam are tangential to the posterior part of the arch or the aorta (red arrow) and the aneurysm (blue arrow) separately, both therefore casting a silhouette.