Computed Tomography Imaging of the Acute Aorta - Pictorial Essay
- 1. Department of Radiology, McGill University Health Center, Montreal, Canada
Abstract
This is a pictorial essay reviewing typical computed tomography findings of acute aortic syndrome, traumatic aortic injury, aortitis and some common diagnostic pitfalls.
Citation
Semionov A, Kosiuk J (2017) Computed Tomography Imaging of the Acute Aorta - Pictorial Essay. JSM Atheroscler 2(2): 1024.
Keywords
• Acute aortic syndrome
• Acute aortic traumatic injury
• Aortitis
ABBREVIATIONS
AAS: Acute Aortic Syndrome; ATAI: Acute Traumatic Aortic Injury; CT: Computed Tomography; ECG: Electrocardiogram; PAU: Penetrating Atherosclerotic Ulcer; TA: Takayasu’s Arteritis
INTRODUCTION
Acute aortic syndrome (AAS) refers to several conditions that result in acute disruption of the aortic wall, which in turn can lead to exsanguination or various organs hypo-perfusion or ischemia. These conditions comprise aortic dissection, aortic intramural hematoma, penetrating atherosclerotic ulcer, and aortic aneurysm rupture. Additionally, aortic wall can be disrupted as a result of acute trauma, commonly manifesting as acute intimal injury, traumatic dissection or contained aortic rupture/pseudo aneurysm. Some of the conditions above may co-exist or arise from each other. For example, penetrating atherosclerotic ulcer can result in dissection or pseudo aneurysm formation, blunt trauma to the chest can lead to an intramural hematoma, and an intramural hematoma may progress to dissection. Finally, acute aortitis of inflammatory or infectious etiology may present with symptoms and clinical signs indistinguishable from AAS. Radiological imaging plays a key role in diagnosis of AAS and related conditions.
Aortic dissection
Dissection results from the flowing blood within the lumen of the vessel entering the substance of the arterial wall (media) through an intimal defect, causing separation of a layer of the vessel wall (the intima and portion of the media) from the rest of it. This in turn leads to creation of two channels of flowing blood - true and false lumens. Arterial hypertension, connective tissue disorders, bicuspid aortic valve, large vessel vasculitides and atherosclerosis are considered predisposing factors to spontaneous aortic dissection [1]. Depending on its location, aortic dissection is classified as type A - if either the ascending aorta or the aortic arch are involved, or as type B - if only the descending aorta is involved (Stanford classification) [2]. Whereas type B aortic dissection is usually treated conservatively with aggressive blood pressure control, type Aaortic dissection is considered a surgical emergency, as if left untreated; it can lead to occlusion of the coronary or carotid arteries, or cardiac tamponade secondary to hemopericardium.
On contrast-enhanced computed tomography (CT), the aortic dissection appears as a linear filling defect in the lumen of the vessel, the so-called “intimal flap”, which can extend over variable length along the course of the aorta, dividing the aortic lumen into two, usually unequal, parts (Figure 1). The true lumen, the one surrounded by the intima, is usually smaller. The degree of perfusion of the true and false lumens can be different (Figure 2), and occasionally one of the lumens may thrombose. Aortic dissections commonly extend into the branches of the aorta - coronary arteries, branches of the arch, visceral branches of the abdominal aorta, iliac and femoral arteries (Figure 1,2).
Long term complications of untreated dissection include aneurysmal dilatation of the affected portion of the vessel and occlusion of the branch vessels.
Aortic intramural hematoma
Intramural hematoma is a condition similar to dissection. It results from acute bleed into the media from disrupted vasa vasorum - small arteries which feed the aortic wall, in the absence of intimal defect [1]. Similar to dissection, intramural hematomas are classified as type A or type B, depending on their location - proximal or distal to the take-off of the left subclavian artery. The former is considered a surgical emergency, whereas the latter is usually treated conservatively.
On non-contrast enhanced CT, the intramural hematoma appears as a crescent of hyper-attenuation along the aortic wall (Figure 3,4). On contrast-enhanced CT, the intramural hematoma appears as aortic wall thickening which is hypo-dense compared to the intraluminal contrast material (Figure 4). In type A aortic intramural hematoma, the blood can extend along the wall of the central pulmonary arteries and result in their luminal narrowing [3] (Figure 4).
Penetrating atherosclerotic ulcer (PAU)
Atherosclerotic disease is a common condition affecting the aorta of older individuals. Atherosclerotic plaques may occasionally ulcerate. When the ulcer extends through significant thickness of the aortic wall media, symptoms related to pain and bleeding may arise. On cross-sectional imaging, PAU appears as a focal excavating defect in the wall of the atheromatous vessel (Figure 5). Most of the PAUs arise in the descending thoracic aorta [4].
PAUs may regress spontaneously, remain stable or progress to dissection or pseudoaneurysm.
Aneurysmal rupture
Aortic aneurysms, and consequently their rupture, occur more commonly in the abdominal aorta. The aneurysmal rupture happens when the intra-arterial pressure exceeds the tensile strength of the aneurysmal wall. Thus the risk of rupture is proportional to the size of the aneurysm [5].
On imaging of aneurysmal rupture, one usually finds irregular contour of the aneurysm, peri-aneurysmal hematoma plus/ minus areas of active contrast extravasation (Figure 6).
Aneurysmal rupture is a surgical emergency and is associated with high mortality rate.
Acute traumatic aortic injury (ATAI)
Of all cases of ATAI, motor vehicle collisions account for majority, followed by falls from height, and crush injuries [6]. The aortic isthmus is the most common location of ATAI. This is thought to be due to relatively immobile position of the isthmus, being tethered by the ligamentum arteriosum. The severity of ATAI may range from minimal intimal injury to frank aortic rupture.
In the context of blunt or acute deceleration trauma to the chest, minute tears of the aortic intima may occur. The intimal defects themselves are usually occult on CT imaging of the chest. Acute intimal injury is usually inferred from the presence of an endoluminal filling defect, thought to represent a mural thrombus forming at the site of injury (Figure 7,8). If aortic injury is more extensive, traumatic dissection with an intimal flap may be seen.
In cases of even more severe injury to the aorta, abnormal aortic contour and traumatic pseudo aneurysm/contained rupture can occur (Figure 9,10). Active extravasation of intravenous contrast material is sometimes observed, indicative of impending exsanguination.
Indirect findings of ATAI generally consist of mediastinal hematoma and hemothorax (Figure 9,10).
Aortic pseudoaneurysm/contained rupture can also be seen secondary to surgical procedures, such as aortic valve replacement or aorto-coronary bypass grafts (Figure 11,12).
Mycotic aneurysm
The term “mycotic aneurysm” refers to an aneurysm which results from an infection, most commonly bacterial. Mycotic aneurysms are estimated to account for roughly 1% of all aortic aneurysms [7]. Approximately a quarter of aortic mycotic aneurysms result in rupture. The infra-renal abdominal aorta is the most frequently involved.
A mycotic aortic aneurysm appears as a focal, contrastenhancing dilatation that is usually saccular (Figure 13). Ruptured infected aortic aneurysms may show active extravasation of intravascular contrast, and hematoma formation adjacent to the aneurysm can occur.
Noninfectious aortitis
Noninfectious aortitis can be seen in patients with largevessel vasculitides, of which Takayasu’s arteritis (TA) and giant cell arteritis are the most common [8].
TA is an idiopathic large-vessel granulomatous inflammation. It is more commonly encountered in younger Asians females [9]. The aortic arch and its major vessels are most commonly affected. TA has three recognized clinical phases: a pre-pulse less phase characterized by constitutional symptoms, an acute inflammatory phase where arterial wall thickening develops, and a chronic occlusive phase. Since the disease is often recurrent, different phases may coexist.
CT findings in the acute phase of TA include arterial wall thickening, which is hyper-attenuating on non-infused studies, and features delayed post intravenous contrast enhancement, and surrounding fat stranding (Figure 14). In the chronic phase one may find arterial stenosis, occlusion, post-stenotic dilatation, pseudo aneurysms, mural ulcerations, dissection, thrombosis, and mural calcifications.
Pitfalls
Respiratory and cardiac motion during chest imaging can result in artifactual images of aortic wall irregularities and intraluminal filling defects simulating dissection, intimal injury or pseudo aneurysm (Figure 15). Usually motion artifacts can be identified as such by carefully assessing adjacent anatomical structures, such as pulmonary airways and vessels, which will appear ill-defined and blurred when motion is present. When doubt remains whether an abnormality represents a motion artifact, the study can be repeated and ECG gating can be performed (Figure 15).
Small out pouching of the aortic wall at the origin of small vessels, such as bronchial and intercostal arteries, can simulate (pseudo)aneurysms (Figure 16). These can be recognized as such by tracing the corresponding vessels to the suspected abnormality (Figure 16).
Another common pitfall is the ductus diverticulum - a developmental out pouching of the undersurface of the distal arch at the site of ligamentum arteriosum (Figure 17). This is also the site of most of traumatic aortic injuries; therefore, differentiation of ductus diverticulum from a pseudoaneurysm is crucial. In contrast to pseudoaneurysm, which usually forms sharp margins with the aorta, ductus diverticulum usually appears as a smooth focal bulge forming gentle obtuse angles with the aortic wall [10] (Figure17).
CONCLUSION
CT plays critical role in assessment of patients with suspected acute aortic abnormalities. Knowledge of typical radiological appearances of these conditions is important as many of them are potentially life threatening and require prompt diagnosis and management.
