Does the CT confirm the diagnosis?
Yes, there is a saccular out-pouching of the lateral wall of the aortic arch. The arch is heavily calcified, but there is no calcification associated with the aneurysm.
Are there different types of aneurysm in this location?
Yes, aneurysms can be classified as true or false. True aneurysmal dilatation involves all layers of the vessel, e.g. abdominal aortic aneurysms. False aneurysms (pseudoaneuryms) occur when there is breech of the internal layers of the vessel and the escaping blood is contained only by the adventitia.
Which is more likely in this case?
This saccular outpouching probably represents a false aneurysm. Look at the calcified lateral margin of the aortic arch - it represents the normal position of the vessel wall and there is no vessel widening. However, at the apex of the arch (at the site of the aneurysm) there is a defect in the lateral wall resulting in aneurysm formation. This is either from prior trauma or penetrating atherosclerotic disease.
A non-contrast CT (the patient is allergic to contrast) confirms a large saccular out-pouching arising from the left side of the aortic arch. Turbulent blood flow can be seen within the aneurysm (even on the non-contrast imaging).