Imaging of the Aorta Part 1

Part 1 review of aortic imaging for radiology board exams. Download the free study guide on this episode by clicking here.

Show Notes/Study Guide

What are the leading types of aortic injury identified on imaging?

Leading types of aortic injury include aortic dissection, intramural hematoma, penetrating aortic ulcer, and in rare cases aortic transection.

What is the most common mechanism of insult causing traumatic aortic injuries?

Rapid deceleration is the most common mechanism of traumatic aortic injuries.

What are the three most common sites of injury of the thoracic aorta in the setting of trauma?

The three most common sites of traumatic aortic injury are at the three relatively fixed points of the thoracic aorta that may result in local aortic damage in the setting of rapid deceleration. These sites are the aortic root, the aortic isthmus, and the aortic hiatus. Of survivable traumatic aortic injuries, the isthmus is the most common site of injury. For injuries that are not survivable, the aortic root may be the most common site of traumatic aortic injury.

Describe which portions of the aortic wall are involved for each of the following: aortic dissection, intramural hematoma, and penetrating atherosclerotic ulcer?

Aortic dissection: involves a tear in the intima allowing infiltration of blood into the media.

Intramural hematoma: has in intact intima with a blood collection within the media, likely from injury and rupture of the vasa vasorum. 

Penetrating atherosclerotic ulcer: Intimal defect at the site of an atherosclerotic plaque that may cause a saccular aneurysm.

Note that aortic transection involves a defect in all three layers of the aorta.

What are key risk factors for each of the following: aortic dissection, intramural hematoma, and penetrating atherosclerotic ulcer?

Aortic dissection: hypertension

Intramural hematoma: either hypertension or trauma

Penetrating atherosclerotic ulcer: atherosclerosis

What are key imaging findings of traumatic aortic injury on chest radiographs?

Key imaging findings of traumatic aortic injury on a chest radiograph include a widened mediastinum due to mediastinal hematoma and mass effect such as displacement of an endotracheal tube, tracheal deviation to the right, left mainstem bronchus depression, opacity obscuring the aortopulmonary window, a left apical pleural cap, hemothorax, typically on the left, and/or widening of the paratracheal stripe and paraspinal stripe.

Note that there is some sidedness described here—tracheal deviation to the right, left mainstem bronchus depression, left apical pleural cap, and left hemothorax.

What is the so-called apical cap and what are other potential causes beyond aortic injury?

An apical cap is opacification of a lung apex on chest radiography which can involve both lung apices in certain cases. Causes include both acute and chronic pathologies. A common chronic cause of an apical cap is fibrosis of the pleura and/or pulmonary parenchyma. Acute causes include trauma with hemothorax because of aortic injury as well as infection of the pleura or lungs, and neoplasm. Acute causes of an apical cap are more likely than chronic causes to be unilateral. Key entities to keep in mind include Pancoast tumor, pulmonary tuberculosis, radiation fibrosis, fractured 1strib and upper thoracic spine injury, lymphoma, pleuroparenchymal fibroelastosis, abscess, and a large thyroid mass displacing vasculature inferiorly.

What are the most common imaging findings of traumatic aortic injury on CT?

Key imaging findings of traumatic aortic injury include detection of aortic dissection, pseudoaneurysm, and intramural hematoma. These can sometimes be seen as a spectrum of injury and grading systems exist wherein a grade I injury is an intimal tear (dissection flap), grate II is an intramural hematoma, grade III is a pseudoaneurysm, and grade IV is aortic rupture.

Aortic Dissection: This is more commonly the result of hypertension than aortic trauma. This starts as a longitudinal tear within the aortic wall that may follow a penetrating ulcer allowing blood to transect into the aortic wall medial layer through the intima, with subsequent longitudinal tracking of blood through the aortic wall forming an alternate pathway of blood flow called the false lumen. When you encounter this on a board exam, consider underlying causes of hypertension, atherosclerosis, and disease states causing structural abnormalities such as connective tissue diseases like Marfan, Ehlers-Danlos, and Loeys-Dietz syndromes, Turner syndrome, bicuspid aortic valve, aortic coarctation, and ciprofloxacin use. Remember the Stanford classification system in which type A aortic dissection involves the proximal thoracic aorta to the level of the left subclavian artery and type B aortic dissection which begins distal to the left subclavian artery.

Intramural hematoma: When acute, these appear as a high-density, crescentic-shaped area of thickening in the aortic wall, most definitive when seen on pre-contrast CT imaging. Secondary signs can include displacement of intimal calcification inwards towards the aortic lumen. No intimal flap should be present, otherwise this points towards an aortic dissection.  On a contrast-enhanced aortic phase CT series, the intramural hematoma typically is less dense than the contrast within the aortic lumen. 

Note that if calcification within the aortic wall displaces away from the aortic lumen, this is most common with an intraluminal thrombus pushing the calcification away from the lumen rather than an intramural hematoma which pushes calcification towards the lumen.

Pseudoaneurysm: This is a contained rupture of the aorta in which only the outer wall of the aortic wall and/or adventitia is intact and containing the rupture. Remember a true aneurysm has all three layers of the aortic wall intact. Rupture risk is higher in a pseudoaneurysm than a true aneurysm. Pseudoaneurysms most commonly arise from trauma and are classic with penetrating trauma such as a stab wound or along a bullet track. A common location for an aortic pseudoaneurysm is the inferior aspect of the aortic isthmus which is the portion of the aorta just beyond the left subclavian artery. These classically occur at the aortic isthmus due to tethering from the ligamentum areteriosum. The outpouching forms an acute angle with the wall of the aorta. The enhancement pattern should follow that of blood elsewhere in the aortic lumen.

What is the significance of a ductus diverticulum when evaluating acute aortic injury on CT imaging?

A ductus diverticulum is an outpouching that is also located about the aortic isthmus, but this is most common at the anteromedial aspect rather than the inferior aspect where a pseudoaneurysm is more likely to arise. Unlike an aortic pseudoaneurysm, this should form an obtuse angle with the aortic wall. Unlike an aortic pseudoaneurysm, this may be calcified, so if you see calcification at this location, consider a ductus diverticulum. This is congenital and may be seen without aortic trauma.

What are approximate normal aortic measurements in the thorax and abdomen for board exam purposes?

Approximate normal values of aortic diameter in the chest are <4 cm for the thoracic aorta and <3 cm for the abdominal aorta. An approximate 1.5x increase over this size meets threshold for aneurysm formation.