Normal  thoracic  aorta—sagittal reconstructed CT image of thoracic aorta showing the various segments of thoracic aorta and the arch branch vessels. R: root; AS: ascending aorta; AR: arch; I: isthmus; DS: descending thoracic aorta. BR: brachiocephalic artery; LCA: left common carotid artery; LSCA: left subclavian artery.