Aortic Aneurysm

An aortic aneurysm is defined as an abnormal permanent dilatation of the aorta. Since the aorta normally dilates with advancing age, the ''normal'' diameter is age dependent, but always less than 4 cm in diameter in the ascending aorta and less than 3 cm in the descending aorta [34]. Approximately 75% of thoracic aortic aneurysms (TAAs) affect the descending aorta, where atherosclerotic disease is most common. In addition, the aortic diameter tapers as it extends distally, with the descending aorta never larger than the ascending aorta. The prevalence of aneurysm increases with age, with an overall incidence of approximately 450 per 100,000 people and a male:female ratio of approximately 3:1 [35]. Concommitant abdominal aortic aneurysms is seen in up to 28% of patients, so the entire thoracoabdominal aorta should be imaged when evaluating TAAs [35].

The etiologies of thoracic aortic aneurysm (TAA) include atherosclerosis, connective tissue diseases such as Marfan's and Ehlers-Danlos syndromes, trauma, aortitis (including syphilis, mycotic aneurysms, and noninfectious inflammatory diseases) (Fig. 13), and aortic dissection. A genetic predisposition to TAA is supported by studies that show a familial aggregation of cases [36].

Thoracic aortic aneurysms are generally divided into true aneurysms and pseudoaneurysms (i.e., false aneurysms) based on their gross and pathological appearance. True aortic aneurysms are usually fusiform in shape and are composed of all three anatomic layers (intima, media, and adventitia). These most commonly arise in the descending thoracic aorta as the result of atherosclerotic disease. Pseudoaneurysms have an absent intimal layer and are contained by the adventitia or periadventitial tissues. They are usually saccular in shape; have a narrow neck where they arise from the aorta; and develop as the result of blunt or penetrating trauma, a penetrating atherosclerotic ulcer, or infection. Pseudoaneurysms may affect any portion of the thoracic aorta (Fig. 14), although traumatic pseudoaneurysms are most often found at the aortic isthmus and those resulting as a complication of PAU are usually seen in the descending aorta.

Plain film findings of aortic aneurysm include a mediastinal mass with peripheral calcification that is contiguous with the aorta, widening of the aortic knob or prominence of the ascending or descending aorta, and mediastinal widening. Aortography can considerably underestimate the size of the aneu-rysm due to mural thrombus and is rarely obtained in the evaluation of TAA.

Spiral CT aortography and MR have virtually replaced conventional aor-tography for the detection and characterization of thoracoabdominal aortic aneurysms. Spiral CT allows accurate diagnosis of aortic aneurysm and readily distinguishes this from other mediastinal masses. It demonstrates all features of TAAs, including an accurate assessment of the shape, length, and diameter of the aneurysm; the presence of mural thrombus and calcification; and the relationship of the aneurysm to adjacent intrathoracic structures [37]. Spiral CT is limited by an inability to evaluate the coronary arteries in patients with ascending aortic aneurysms or delineate the intercostal supply to the spinal cord at T8-L1 in those with descending TAAs, information more readily provided by angiography.

While it may be difficult to distinguish mural thrombus in an aortic aneurysm from an intramural hematoma (IMH) or thrombosed aortic dissection, there are several CT findings that aid in this distinction. In a thrombosed

Aortic Aneurysm Imh

Figure 13 Ascending aortic and arch aneurysm. (A) Contrast-enhanced CT at the level of the top of the left atrium in a patient with Reiter's syndrome shows an ascending aortic aneurysm. (B) Scan at the level of the aortic arch shows mild aneurysmal dilatation of the arch that tapers distally.

Figure 13 Ascending aortic and arch aneurysm. (A) Contrast-enhanced CT at the level of the top of the left atrium in a patient with Reiter's syndrome shows an ascending aortic aneurysm. (B) Scan at the level of the aortic arch shows mild aneurysmal dilatation of the arch that tapers distally.

Mural Thrombus Aorta

Figure 14 Pseudoaneurysm of the descending aorta. (A) Contrast-enhanced CT through the lower chest shows a large aneurysm of the descending aorta with a large amount of mural thrombus. Note the relatively narrow aneurysm neck (arrows). (B) Coronal reconstruction shows the aneurysm projecting into the left lung and a surgically confirmed pseudoaneurysm complicating a penetrating atherosclerotic ulcer.

Figure 14 Pseudoaneurysm of the descending aorta. (A) Contrast-enhanced CT through the lower chest shows a large aneurysm of the descending aorta with a large amount of mural thrombus. Note the relatively narrow aneurysm neck (arrows). (B) Coronal reconstruction shows the aneurysm projecting into the left lung and a surgically confirmed pseudoaneurysm complicating a penetrating atherosclerotic ulcer.

aneurysm, the residual aortic lumen is generally smooth and the thrombus circumferential (Fig. 14), while IMH and thrombosed dissection produces a more irregular interface with the aortic lumen. When present, atherosclerotic intimal calcifications are seen at the peripheral edge of thrombus, whereas they are displaced medially by IMH and thrombosed dissection. Despite these characteristic features, the distinction between a thrombosed aneurysm and IMH or thrombosed dissection may be difficult in selected cases, particularly when the thrombus within an aneurysm is calcified [38,39].

While there is no direct relationship between the etiology of a TAA and its radiologic appearance, the appearance and location of a TAA as depicted on spiral CT should suggest specific etiologies. Patients with cystic medial necrosis typically have annuloaortic ectasia with dilated sinuses of Valsalva and the classic Marfanoid pear-shaped aneurysmal aorta with a smooth taper to a normal aortic arch. This appearance is best appreciated on coronal reformations through the ascending aorta. Atherosclerotic aneurysms appear as continuous fusiform dilatations of the descending aorta with smooth mural thrombus that can be either crescentic or concentric. Mycotic aneurysms arising from bacterial infection of a diseased aortic wall are most often saccular with focal dilatation and eccentric thrombus and mural calcification (Fig. 15) [40]. They have a propensity to affect the ascending aorta likely due to its proximity to the regions affected by endocarditis, which is often an associated condition [41]. Traumatic pseudoaneurysms following blunt trauma most often develop near the aortic isthmus and are usually saccular with mural calcification common. Aneurysms arising as a complication of penetrating atherosclerotic ulcers are most often seen as saccular aneurysms of the descending aorta. Aortitis due to noninfectious diseases, particularly connective tissue diseases such as rheumatoid arthritis; Reiter's syndrome; and ankylosing spondylitis usually produces fusiform aneurysms of the ascending aorta (Fig. 13).

As in other aortic diseases, the axial reconstructions provide the primary means of helical CT interpretation. However, there are advantages of specific types of reformations unique to the evaluation of aortic aneurysms that differ from those performed for aortic dissection and its variants. Multiplanar reformations (MPRs) of volumetric data provide a more accurate measurement of the diameter of a TAA than axial scans, particularly in the descending aorta where the dilated lumen may course oblique to the scan plane. MPRs also allow display of intraluminal contents, particularly mural thrombus and atherosclerotic changes [10]. In patients with aneurysms complicated by dissection, curved planar reformats (CPRs) best depict the relationship of the intimal flap to the great vessel ostia. Shaded surface displays, a three-dimensional volume-rendering technique, is useful in displaying complex relationships of aneu-

Figure 15 Mycotic aneurysm of the aorta. (A) Contrast-enhanced CT at the level of the aortic arch shows an aneurysm projecting anteriorly from the proximal arch. Note the narrow aneurysm neck (arrows) suggesting a pseudoaneurysm. (B) Sagittal reconstruction shows the aneurysm (a) with narrow neck projecting anteriorly from the arch (A) arising just proximal to the brachiocephalic artery (curved arrow). (C) Corresponding lateral digital subtraction confirms the findings in (B). Surgically confirmed mycotic aneurysm.

Figure 15 Mycotic aneurysm of the aorta. (A) Contrast-enhanced CT at the level of the aortic arch shows an aneurysm projecting anteriorly from the proximal arch. Note the narrow aneurysm neck (arrows) suggesting a pseudoaneurysm. (B) Sagittal reconstruction shows the aneurysm (a) with narrow neck projecting anteriorly from the arch (A) arising just proximal to the brachiocephalic artery (curved arrow). (C) Corresponding lateral digital subtraction confirms the findings in (B). Surgically confirmed mycotic aneurysm.

Aortic Spin Technique

Figure 15 Continued

Figure 15 Continued rysms to adjacent mediastinal vessels but does not provide visualization of aneurysmal contents.

Magnetic resonance using standard spin-echo techniques can provide information regarding TAAs analogous to spiral CT, though without the use of ionizing radiation or intravenous contrast. Magnetic resonance depicts mural thrombus as intermediate signal material on T1-weighted images but cannot detect calcification within the wall of an aneurysm. Oblique sagittal MR scans of the aorta allow simultaneous display of the entire length of the thoracic and upper abdominal aorta, which is beneficial in patients with thoracoabdominal aneurysms [42]. Magnetic resonance is particularly useful in the follow-up of patients with known TAAs and in those with contraindications to intravenous contrast administration.

Complications of TAA, including progressive dilatation, dissection, and hemorrhage, are easily assessed by CT. In particular, spiral CT is the modality of choice in the setting of suspected acute aortic rupture, as it can detect active contrast extravasation (Fig. 16) or high-attenuation hematoma within the pleural or pericardial space or mediastinum [43]. A crescent of high-attenuation material within the mural thrombus of a TAA represents acute or impending contained aneurysm rupture [44]. A contained rupture of the posterior aortic wall in close apposition to the spine may show a draped aorta sign, thought

Aortobronchial Fistula
Figure 16 Ruptured aortic aneurysm. Contrast-enhanced scan through the lower chest in a patient with sudden onset of chest pain shows active extravasation of contrast (arrow) from a descending aortic aneurysm into a retrocardiac hematoma.

to be indicative of a deficient aortic wall [45]. Aortobronchial fistula results from communication between the aorta and bronchial tree and most often arises as a complication of an atherosclerotic TAA or as a postsurgical complication of aneurysm repair. The fistulous communication in an aortobronchial fistula is between the descending aorta and left bronchopulmonary tree in nearly 90% of cases [46]. Patients usually present with massive hemoptysis. Spiral CT does not often demonstrate the fistula itself but shows an aortic aneurysm adjacent to consolidated lung [47]. Management consists of surgical repair of the aortic and bronchial defects, with approximately three-fourths of patients successfully repaired. Similarly, aortoesophageal fistula is a catastrophic and often fatal complication of TAA, resulting from communication between the descending aorta and esophagus. Patients present with massive upper gastrointestinal hemorrhage with endoscopy, often failing to delineate the source of bleeding. Computed tomography may demonstrate the aneurysm and its intimate relationship to the esophagus; mediastinal hematoma or rarely contrast extravasation into the esophagus may be seen. Immediate surgical repair is mandatory [48].

The natural history of TAA is directly related to its size. While TAAs can present due to mass effect on adjacent mediastinal structures, the most serious complication is aortic rupture, which occurs in up to 70% of affected patients [49]. While aortic rupture usually results in exsanguinating hemorrhage into the mediastinum, lung, or pleural space, sometimes a communication will develop with the tracheobronchial tree or esophagus producing hemoptysis or hematemesis respectively. The risk of rupture increases with increasing size of the aneurysm, and TAAs have been observed to dilate at a mean rate of 0.12 cm/year [50]. Based on these data, elective surgical repair has been recommended for ascending aortic aneurysm diameters of 5-5.5 cm and descending aortic aneurysms of 5.5-6.5 cm [50]. Surgical treatment generally consists of the placement of a Dacron graft within the diseased aortic segment: Ascending aortic aneurysms that involve the aortic annulus and valve, most often seen in patients with annuloaortic ectasia due to Marfan's syndrome, usually require a composite graft (i.e., combined aortic valve with prosthetic ascending aortic graft) with reimplantation of the coronary arteries.

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