Is the projection important in assessing for a thoracic aortic abnormality on CXR?
Yes. A supine or AP film will cause magnification of the mediastinal structures and therefore sometimes cause spurious widening of the aorta. However, in this case despite being an AP projection the aortic outline is abnormal and was present on previous films.
Whats the difference between a thoracic aortic aneurysm and ectasia (or unfolding) of the aorta?
A ectatic thoracic aorta is observed in the eldery and due to so called senile ectasia - it is not pathological in itself. An aneurysm on the other hand is a true focal dilatation of the aorta, which is pathological.
Gross dilatated aneurysmal thoracic aorta, giving rise to a highly abnormal mediastinal outline.