EDUCATION EXHIBIT 537 Multidetector CT of Thoracic Aortic ......EDUCATION EXHIBIT 537 Prachi P....

537EDUCATION EXHIBIT

Prachi P. Agarwal, MD • Aamer Chughtai, MD • Frederick R. K. Matzinger, MD • Ella A. Kazerooni, MD, MS

Thoracic aortic aneurysms (TAAs) can be broadly divided into true aneurysms and false aneurysms (pseudoaneurysms). True aneurysms contain all three layers of the aortic wall (intima, media, and adven-titia), whereas false aneurysms have fewer than three layers and are contained by the adventitia or periadventitial tissues. Multidetector computed tomographic (CT) angiography allows the comprehensive evaluation of TAAs in terms of morphologic features and extent, pres-ence of thrombus, relationship to adjacent structures and branches, and signs of impending or acute rupture, and is routinely used in this setting. Knowledge of the causes, significance, imaging appear-ances, and potential complications of both common and uncommon aortic aneurysms, as well as of the normal postoperative appearance of the thoracic aorta, is essential for prompt and accurate diagnosis. Supplemental material available at http://radiographics.rsnajnls.org/cgi /content/full/29/2/537/DC1.©RSNA, 2009 • radiographics.rsnajnls.org

Multidetector CT of Thoracic Aortic Aneurysms1

LEARNING OBJECTIVES FOR TEST 5After reading this article and taking the test, the reader

will be able to:

Describe the CT ■

features that help differentiate be-tween true and false aneurysms.

List the causes of ■

thoracic aortic aneu-rysms and identify potential complica-tions.

Discuss the role of ■

CT in the compre-hensive evaluation of thoracic aortic aneurysms.

Abbreviations: ECG = electrocardiographic, TAA = thoracic aortic aneurysm, VR = volume-rendered

RadioGraphics 2009; 29:537–552 • Published online 10.1148/rg.292075080 • Content Codes: 1From the Department of Radiology, Division of Cardiothoracic Radiology, University of Michigan Health System, 1500 E Medical Center Dr, Ann Arbor, MI 48109 (P.P.A., A.C., E.A.K.); and Department of Radiology, University of Ottawa, Ottawa, Ontario, Canada (F.R.K.M.). Recipient of a Cum Laude award for an education exhibit at the 2006 RSNA Annual Meeting. Received April 23, 2007; revision requested July 26; final revision received July 21, 2008; accepted August 22. E.A.K. is a consultant with GE and Vital Images; all other authors have no financial relationships to dis-close. Address correspondence to P.P.A. (e-mail: [email protected]).

©RSNA, 2009

CME FEATURESee accompanying

test at http://www.rsna.org

/education/rg_cme.html

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538 March-April 2009 radiographics.rsnajnls.org

IntroductionAn aneurysm is defined as an abnormal focal dilatation of a blood vessel. Multidetector com-puted tomographic (CT) angiography is rou-tinely performed for the diagnosis and evaluation of thoracic aortic aneurysms (TAAs), having es-sentially replaced diagnostic angiography. Unlike conventional angiography, which demonstrates only the lumen of an aneurysm, CT angiography also demonstrates the wall and contents of an aneurysm, including thrombus, thereby allowing a more accurate measurement of aneurysm size and the evaluation of morphologic features and surrounding structures. In this article, we discuss and illustrate common and uncommon TAAs with an emphasis on their causes, significance, CT features, and potential complications.

DefinitionsThe thoracic aorta consists of the aortic root, ascending aorta, aortic arch, and descending tho-racic aorta (Fig 1). The ascending aorta extends from the root to the origin of the right brachio-cephalic artery; the arch, from the right brachio-cephalic artery to the attachment of the ligamen-tum arteriosum; and the descending aorta, from the ligamentum arteriosum to the aortic hiatus in the diaphragm (1). The aortic root is defined as that part of the ascending aorta that contains the valve, annulus, and sinuses (1). The arch may be subdivided into proximal (right brachiocephalic artery to left subclavian artery) and distal (left subclavian artery to attachment of the ligamen-tum arteriosum) segments (1). The distal arch, also referred to as the isthmus, may be narrower than the proximal descending aorta (1).

A TAA is defined as a permanent abnormal dilatation of the thoracic aorta (2). Although the aortic diameter increases slightly with age, the normal diameter of the midascending aorta should always be less than 4 cm, and that of the descending aorta no more than 3 cm (3).

CausesAtherosclerosis is the cause of approximately 70% of all TAAs (Fig 2) (4); most of these ath-erosclerotic TAAs occur in the descending tho-racic aorta. Because an abdominal aortic aneu-rysm occurs in 28% of patients with a TAA, it is important that the initial evaluation include the entire thoracoabdominal aorta (5). The causes

Figure 1. Three-dimensional volume-rendered (VR) image shows the anatomic segments of the thoracic aorta. A = arch, AA = ascending aorta, DA = de-scending aorta, I = isthmus, IA = innominate artery (brachiocephalic trunk), LCA = left common carotid artery, LSA = left subclavian artery.

Table 1 Causes of TAAs

AtherosclerosisAortic dissectionMedial degeneration (genetic)Marfan syndromeEhlers-Danlos syndromeOutside influences (acquired)TraumaSyphilisMycosis (infection)Noninfective aortitisRheumatic feverRheumatoid arthritisAnkylosing spondylitisGiant cell arteritisRelapsing polychondritisTakayasu arteritisReiter syndromeSystemic lupus erythematosusSclerodermaPsoriasisUlcerative colitisRadiationBehçet diseaseCongenital aneurysm (rare)

Source.—Reference 6.

RG ■ Volume 29 • Number 2 Agarwal et al 539

of TAAs are listed in Table 1 (6). The reported prevalence of TAAs varies depending on the cause. Also, accurate subclassification of aneu-rysms according to cause can be difficult, since it may not be possible to determine the exact cause with certainty in all cases (7). In one study of 51 TAAs with pathologic correlation, the cause was ascribed to aortic dissection in 53% of cases, ath-erosclerosis in 29%, aortitis in 8%, cystic medial necrosis in 6%, and syphilis in 4% (5).

Annuloaortic ectasia, a condition characterized by dilated sinuses of Valsalva with effacement of the sinotubular junction producing a pear-shaped aorta that tapers to a normal aortic arch, is most commonly associated with Marfan syndrome (Fig 3) (2,8). Other causes include homocystinuria, Ehlers-Danlos syndrome, and osteogenesis im-perfecta; however, annuloaortic ectasia can be idiopathic in approximately one-third of cases.

Figure 2. Fusiform descending TAA in an 80-year-old man. (a) Contrast mate-rial–enhanced CT scan shows an an-eurysm containing thrombus (arrow). (b) Three-dimen-sional VR image shows the overall ex-tent of the atheroscle-rotic changes, which are predominantly in the descending tho-racic aorta and aortic arch branches.

Figure 3. Marfan syndrome and annu-loaortic ectasia in a 40-year-old man. Contrast-enhanced CT scan (a) and three-dimensional VR image (b) show a pear-shaped aorta that tapers to a nor-mal aortic arch, a finding characteristic of Marfan syndrome and annuloaortic ectasia.


The presence of bicuspid aortic valve is an in-dependent risk factor for TAA formation (Fig 4; see also Movie at http://radiographics.rsnajnls.org /cgi/content/full/29/2/537/DC1), and not merely a consequence of poststenotic dilatation secondary to aortic stenosis (12). Although aortic stenosis is a frequent complication of bicuspid aortic valve because the dysfunctional valves are prone to pre-mature fibrosis and calcium deposition (12), aortic aneurysms associated with bicuspid aortic valve

Although the appearance of the aorta in patients with Marfan syndrome is identical to that in pa-tients with idiopathic medial degeneration, there is a striking difference in the natural history of these two conditions, with both onset and progression being more rapid in Marfan syndrome (9).

Aneurysms due to syphilis are now rare, with effective treatment available for this infectious disease. Cardiovascular disease has been reported in up to 12% of patients with untreated syphilis, usually with a latency period of 10–30 years af-ter the primary infection (10). Syphilitic aortitis causes focal destruction of the media with loss of elastic and smooth muscle fibers and scarring, leading to aortic dilatation and aneurysms. The most common site of these TAAs is the ascend-ing thoracic aorta (36% of cases), followed by the aortic arch (34%), proximal descending thoracic aorta (25%), and distal descending thoracic aorta (5%). Aortic sinus involvement occurs in less than 1% of cases and is most often asymmetric, as opposed to the symmetric enlargement seen in annuloaortic ectasia (6,11). A less common man-ifestation of syphilitic aortitis is narrowing of the coronary ostia due to subintimal scarring, result-ing in myocardial ischemia; this condition carries a poor prognosis, with an average survival time of only 6–8 months from the onset of cardiac symptoms (11). Syphilitic aneurysms are at high risk for rupture, with death due to aortic rupture reported in 40% of cases (11). Dissection is less common due to the presence of medial scar.

Figure 4. Ascending aortic aneurysm and bicuspid aortic valve in a 40-year-old woman. (a, b) Contrast-enhanced CT scan (a) and VR image (b) show an ascending aortic aneurysm. (c) Oblique axial image through the plane of the aortic valve shows the bicuspid nature of the valve.

Figure 5. Contrast-enhanced CT scan obtained in a 50-year-old man shows a retroesophageal mediastinal abscess and a mycotic pseudoaneurysm of the de-scending thoracic aorta (arrow).

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culosis can involve the aorta by contiguous spread from lymph nodes and spine (15).

Several causes of noninfective aortitis can lead to an aneurysm. Aortitis most commonly af-fects the ascending aorta in rheumatoid arthritis, ankylosing spondylitis, giant cell arteritis, and relapsing polychondritis (6). These conditions may also be associated with aortic valve insuf-ficiency. Aortitis is a known but rare feature of rheumatic fever and can be segmental, limited to the ascending aorta, involve the abdominal aorta, or involve the entire aorta (6,10). Takayasu arteritis, a vasculitis usually encountered in Asian women, commonly affects the aortic arch and its major branches, with variable involvement of the abdominal aorta and pulmonary arteries. Al-though Takayasu arteritis typically causes arterial stenosis and occlusion, aneurysms may also occur (Fig 6). CT features include high attenuation of the thickened aortic wall with calcifications on

are not secondary to valve dysfunction and have been described in the absence of significant aortic stenosis and aortic insufficiency, as well as in pa-tients who have undergone successful prosthetic valve replacement for bicuspid aortic valve (12).

Aortitis may be infective or noninfective. Non-syphilitic infection of the arterial wall with aneu-rysmal dilatation is referred to as mycotic aneu-rysm. Although the intima is extremely resistant to infection, any condition that causes damage to the aortic wall predisposes the patient to mycotic an-eurysm, including contiguous bacterial endocardi-tis, atherosclerosis, drug abuse, and aortic trauma (6). Immunocompromised patients also have an increased prevalence of mycotic aneurysms (13). Mycotic aneurysms are usually saccular and con-tain eccentric thrombus (Fig 5) (14). They have a propensity to involve the ascending aorta, which is in proximity to regions affected by endocarditis (6). The most common infectious agents are non-hemolytic Streptococcus, Pneumococcus, Staphylococ-cus, Gonococcus, and Salmonella species (6). Tuber-

Figure 6. Taka-yasu arteritis in a 35-year-old woman. (a, b) Contrast-enhanced CT scans obtained at the level of the ascending (a) and distal descend-ing (b) aorta show diffuse aortic wall thickening and an ascending aortic an-eurysm. (c) VR im-age shows multiple areas of stenosis and aneurysm formation involving the aorta and its branches.


ties; this is particularly important with higher heart rates and in areas that move the most with cardiac motion, such as the ascending aorta. In addition, ECG gating can facilitate evaluation of at least the proximal coronary arteries (if not the entire coronary artery system) if the specified acquisition parameters provide the appropri-ate spatial and temporal resolution. In cases of suspected aortic dissection, it may be useful for determining coronary artery involvement. We routinely use ECG gating for the thoracic por-tion of our CT examinations of the aorta, which are performed on either a 16- or 64-detector CT scanner. Roos et al (19) compared ECG-gated with nongated scans of the thoracic aorta and found significant reduction in motion artifacts with the use of gating. Although motion artifacts decrease with increasing distance from the heart, the authors found significant reduction in motion artifacts for the entire thoracic aorta. However, the maximum benefit was seen at the level of the aortic valve and ascending aorta (19). We per-form scanning in the craniocaudal direction, and gating is turned off at the diaphragm, which re-duces the breath-hold time and radiation dose.

In the past, ECG gating has primarily been retrospective gating, with which data are collected over the entire cardiac cycle. This permits review of aortic valve morphologic features on static im-

nonenhanced scans (2). Arterial enhancement is considered a sign of active disease (2).

Posttraumatic aneurysms following blunt trauma may result from rapid deceleration, a generally accepted mechanism of injury. Ac-cording to this theory, the distal transverse arch moves forward while the proximal descending thoracic aorta remains stationary, held back by the ligamentum arteriosum and the intercostal vessels (16). Another proposed mechanism is the “osseous pinch,” wherein an anteroposte-rior compression force results in posteroinferior displacement of the manubrium, first rib, and medial clavicle, which impinge on the aorta and compress it against the thoracic spine posteriorly (17). The site of injury most commonly seen in trauma victims who survive to reach the hospital is the aortic isthmus (90% of cases), followed by the ascending aorta and the descending aorta near the diaphragmatic hiatus (1). Chronic pseudoaneurysms develop in 2.5% of patients who survive the initial trauma. These often cal-cify, may contain thrombus (Fig 7) (18), and have the potential to enlarge progressively, rup-turing even years after the initial trauma (6).

Aortic dissection is an abnormal passage of blood into the media through an intimal tear. This produces a false lumen that is separated from the true lumen by an intimal flap. A previ-ous aortic dissection with a persistent false chan-nel may produce aneurysmal dilatation of the false lumen. These false aneurysms are contained only by the outer media and adventitia and tend to enlarge over time (Fig 8).

CT TechniqueWhen an acute aortic syndrome is suspected (owing either to clinical signs and symptoms or to chest radiographic findings), nonenhanced CT is usually performed first to look for a high-atten-uation acute intramural hematoma. The contrast-enhanced scanning that follows is the key part of the CT examination. The correct timing of the arrival of contrast material in the aorta is critical to image quality. Approaches to accomplishing this include a timing bolus or a bolus tracking technique. Electrocardiographic (ECG) gating is commonly used to reduce motion artifacts, which can mimic dissection or luminal irregulari-

Figure 7. Contrast-enhanced CT scan obtained in a 28-year-old man shows a posttraumatic saccular pseu-doaneurysm at the aortic isthmus (arrow).


ages at end systole and end diastole, measurement of aortic valve surface area (Fig 9), and the viewing of valve leaflet motion in cine mode. Incomplete coaptation of the valve leaflets corresponds to re-gurgitation, and a restricted opening corresponds to stenosis (20). For example, an ascending aortic aneurysm can be associated with an unsuspected bicuspid aortic valve or calcific aortic stenosis.

However, retrospectively gated scanning is associ-ated with a high radiation dose compared with nongated scanning. In the study by Roos et al (19), the radiation doses with retrospectively gated and nongated scanning of the thoracic aorta were 8.85 and 4.5 mSv, respectively. Scanning covered a craniocaudal range of 15 cm, with a tube po-tential of 120 kVp, a collimation of 1 mm, and a section width of 1.25 mm. The tube current and pitch used for gated and nongated scans were 140 mAs/1.5 and 250–400 mAs/0.38–0.75, respectively (19). Tube current modulation, with which the tube output is reduced during systole, can reduce the radiation dose associated with a retrospec-tively gated CT acquisition and is routinely used at our institution. A mean dose reduction of 48% for males and 45% for females has been reported with this technique (21). Also, the newer prospec-tive triggering technique collects CT data only at a specified point or cluster of points in the cardiac cycle, reducing the time the CT beam is on to a fraction of what it was with retrospective gating, thus substantially reducing the radiation dose. The mean patient radiation dose reduction has been shown to be 77%–83% (22,23) for prospectively gated versus retrospectively gated CT angiography (with tube current modulation) of the coronary ar-teries performed on a 64-detector scanner.

Figure 8. Contrast-enhanced axial (a) and oblique sagittal reformatted (b) CT images obtained in a 65-year-old man show a type B aortic dissection with a partially thrombosed false lumen and a descending TAA.

Figure 9. Aortic stenosis in a 74-year-old woman. On an oblique axial CT scan through the aortic valve leaflets obtained at end systole, the aortic valve surface area measures 0.9 cm2.


In the evaluation of the thoracic aorta for en-dovascular repair, craniocaudal coverage should extend from the neck to the femoral heads. Assess-ment of access to the common femoral artery is essential to determine the feasibility of large-bore sheath access. Knowledge of the relationship be-tween the aortic aneurysm and the aortic branches is necessary to assess the adequacy of the “landing zone” (the aortic segments proximal and distal to the lesion where the stent extremities will be posi-tioned) (24). To ensure an adequate neck for graft attachment, the following conditions are desirable (25): (a) a minimum distance of 15 mm from the aneurysm to the left subclavian artery and to the celiac trunk, (b) a maximum aortic landing zone diameter of 40 mm, and (c) the absence of circumferential thrombus or atheroma within the landing zone. If the lesion is very close to the left subclavian artery, it may be necessary to cover the origin of the subclavian artery to ensure an ad-equate landing zone; however, patency of both ver-tebral arteries must be demonstrated prior to the procedure (25). For the assessment of stent-graft repair of aortic aneurysms, it is important that de-layed views be evaluated for endoleak. We typically acquire these views 60 seconds after the arterial phase acquisition.

CT Data ManipulationCT is the primary modality for evaluating abnor-malities of the thoracic aorta. Multidetector CT,

with its multiplanar capability, can be used to eval-uate an aneurysm in any plane, determine its size and morphologic features, clarify its relationship to branch vessels, evaluate its effect on adjacent structures, and identify complications such as dis-section and rupture. These options give multide-tector CT a decided advantage over conventional angiography, which provides information primarily about the aortic lumen. In a series of examinations

Figure 10. Measurement of aortic diameter. (a) Axial contrast-enhanced CT scan of the descending thoracic aorta indicates an aortic diameter of 23.4 mm (3D = three-dimensional). (b) Double oblique reformatted CT image of the descending thoracic aorta obtained at the same level shows the aorta in true cross-section, with a diameter of 18.5 mm (3D = three-dimensional). The aortic diameter was over-estimated on the axial scan, which showed the aorta coursing obliquely.

Table 2 Anatomic Locations of Measurements in a Standard Report Describing the Thoracic Aorta

SinusSinoaortic junctionMidascending aorta (midpoint between sinoaortic

junction and proximal aortic arch)Proximal aortic arch (aorta at origin of brachio-

cephalic trunk)Midaortic arch (just distal to left common carotid

artery)Proximal descending aorta (2 cm distal to left

subclavian artery)Middescending aortaAorta at diaphragm (2 cm above celiac axis origin)Abdominal aorta at celiac axis originAbdominal aorta at most cephalic renal arteryAbdominal aorta at most caudal renal arteryInfrarenal abdominal aorta (15 mm below most

caudal renal artery)Aorta just above bifurcationAneurysm (maximum diameter [location specified])

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port describing the thoracic aorta includes mea-surements of aortic diameter (mean, minimum, and maximum) at specific locations (Table 2), allowing documentation of size at these locations and change over time. A potential drawback to using the measurements obtained from the more recently available double oblique reformatted im-ages is the fact that data regarding aortic size and risk of rupture are based on measurements taken from axial sections (28). Curved planar refor-matted images may be useful in aneurysms with dissection, depicting the ostia of aortic branches with respect to the intimal flap.

Aneurysm Morphologic FeaturesTAAs can be classified as either true aneurysms or false aneurysms (pseudoaneurysms). True aneurysms contain all three anatomic layers—the intima, media, and adventitia—are usually associated with fusiform dilatation of the aorta, and are most commonly due to atherosclerosis. Although the majority of atherosclerotic aneu-rysms are fusiform, up to 20% may be saccular (6). Pseudoaneurysms have fewer than three lay-ers and are contained by the adventitia or periad-ventitial tissues. They are typically saccular with a narrow neck, and are most commonly due to trauma (Fig 7), penetrating atherosclerotic ulcers, or infection (mycotic aneurysms) (4).

The location of an aneurysm can provide a clue to its cause. In a study of 249 aneurysms of the aorta and its branches by Fomon et al (7), most of the aneurysms were found in the abdominal aorta (30.9% of cases), whereas the TAAs were most fre-quently seen in the ascending aorta (22.1%). Arch aneurysms, descending aortic aneurysms, and thoracoabdominal aneurysms were seen in 11.6%, 7.6%, and 2.8% of cases, respectively (7).

Involvement of the ascending aorta alone is usually associated with annuloaortic ectasia, syphilis, postoperative aneurysms (at the aortic suture line or at the site of aortic cannulation), aortic valve disease, or infectious or noninfectious aortitis. In contrast, atherosclerosis is a more dif-fuse process and rarely involves only the ascend-ing aorta (4). Postoperative ascending aortic pseudoaneurysms can occur at an aortotomy site, cannulation site for cardiopulmonary bypass, or needle puncture site (needle inserted for pressure measurement, to purge the aorta of air, or to inject cardioplegic solution), or at incompetent suture lines (29,30). Cross-clamping an atherosclerotic ascending aorta may also cause an iatrogenic aor-tic dissection or pseudoaneurysm (30). Figure 11 shows the potential sites of these postoperative

that included 33 thoracic aneurysms, three rup-tured TAAs, six penetrating atherosclerotic ulcers, five aortic dissections, and two pseudoaneurysms, Quint et al (26) demonstrated that CT has a 92% accuracy for diagnosing abnormalities of the thoracic aorta. In addition, CT correctly helped predict the need for hypothermic circulatory arrest during surgical repair in 94% of patients (26).

Although axial sections are still the mainstay of interpretation, two-dimensional and three-dimen-sional reformatting techniques such as maximum intensity projection, curved planar reformation, multiplanar reformation, and VR may facilitate interpretation and improve communication with referring physicians (27). To our knowledge, it has not been scientifically shown that the use of these tools increases diagnostic accuracy or di-agnostic confidence. For example, in the study by Quint et al (26), the use of multiplanar refor-matted images along with axial images changed the interpretation in only one case. It should be noted that their study involved CT examinations performed on single-section helical scanners and interpreted by thoracic radiology specialists, who may be more experienced in the evaluation of thoracic CT examinations.

Double oblique reformatted images obtained perpendicular to the aortic lumen (ie, true short-axis images of the aorta) allow more accurate measurement of aortic diameter than does rely-ing on axial CT scans, on which the aorta has an oblique course (Fig 10) (28). Our standard re-

Figure 11. Three-dimensional VR image shows the potential sites of postoperative ascending aortic pseudoaneurysms. C = clamping site, Cn = cannula-tion site, G = graft, GA = graft anastomosis, N = needle puncture site, v = valvulotomy site.


ascending aortic pseudoaneurysms. Saccular trau-matic aneurysms are most common at the aortic isthmus, whereas those secondary to penetrating ulcers can occur anywhere in the descending aorta.

TAA MimicsIt is important to be aware of normal variants that can mimic an aortic aneurysm, two of which are ductus diverticulum and aortic spindle.

Ductus DiverticulumDuctus diverticulum consists of a convex fo-cal bulge along the anterior undersurface of the isthmic region of the aortic arch (31). Although ductus diverticulum is commonly believed to be a remnant of the closed ductus arteriosus, it has been suggested that this entity may actually repre-sent a remnant of the right dorsal aortic root (32). It is particularly important to differentiate ductus diverticulum from a posttraumatic aortic pseu-doaneurysm, which most commonly occurs at the aortic isthmus. In contrast to a pseudoaneurysm, ductus diverticulum has smooth margins with gently sloping symmetric shoulders and forms obtuse angles with the aortic wall (Fig 12) (31).

Aortic SpindleAortic spindle is a smooth, circumferential bulge below the isthmus in the first portion of the de-scending aorta (Fig 13) and should not be con-fused with an aneurysm.

Complications

RuptureThe risk of rupture of TAAs increases with the size of the aneurysm (31). This is in accordance with the law of Laplace, which states that wall tension increases with the diameter of the aorta. Elective aneurysm repair has a lower mortal-ity rate (9%) than does emergent repair (22%); therefore, aneurysms are considered for repair when they are either symptomatic or exceed 5–6 cm in diameter (33–35). Coady et al (36,37) described the median size of rupture-dissection of ascending and descending aortic aneurysms as 5.9 and 7.2 cm, respectively, and advocated surgical intervention for ascending TAAs exceed-ing 5.5 cm and for descending TAAs exceeding 6.5 cm. Earlier intervention is recommended in patients with Marfan syndrome and is advocated at an aortic diameter of 5 cm (36). It is important to monitor the size of aneurysms with CT annu-ally, since there is variability in the annual growth

Figure 12. Duc-tus diverticulum in a 35-year-old man. The entity was seen at CT angiography of the thoracic aorta. Axial (a) and sagittal reformatted (b) CT images show a focal convex bulge (arrow) along the anterior aspect of the isthmus. Note the obtuse angles with the aortic wall, unlike with a pseudoaneurysm.

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on nonenhanced scans and even as contrast material extravasation from the aortic lumen on contrast-enhanced scans. A high-attenuation “crescent” in the mural thrombus of a TAA may represent an acute contained or impending rupture, analogous to that described in abdominal aortic aneurysms (Fig 15) (2,39). Another sign

rate of aneurysms (0.07–0.42 cm) (31,33). An annual growth rate greater than 1 cm is an ac-cepted indication for surgical repair (38).

CT is the modality of choice for identify- ing aneurysm rupture. Aortic aneurysms can rupture into the mediastinum, pleural cavity (Fig 14), pericardium, or adjacent luminal structures such as the airway or esophagus, manifesting as a high-attenuation hematoma

Figure 13. Aortic spindle. Three-dimen-sional VR image shows an aortic spindle (ar-row) as a circumferential bulge in the proxi-mal descending thoracic aorta.

Figure 14. Aneurysm rupture in a 65-year-old man. Nonenhanced CT scan shows a ruptured atheroscle-rotic aneurysm of the descending thoracic aorta. Note the high-attenuation fluid in the left pleural space, a finding that represents acute hemothorax.

Figure 15. Abdominal aortic aneurysm in a 75-year-old man. Nonenhanced (a) and contrast-enhanced (b) CT scans show a high-attenuation crescent in the mural thrombus of an aortic aneurysm, a sign of impending rupture or contained leak.


aortobronchial fistula, which manifests clinically as hemoptysis (4) and at CT as consolidation in the adjacent lung due to hemorrhage (Fig 16); the fistulous communication itself is not com-monly seen at CT (41). Most aortobronchial fis-tulas (90%) occur between the descending aorta and the left lung (42). Communication with the esophagus (aortoesophageal fistula) is less com-

of contained rupture is the “draped aorta sign,” wherein the posterior aortic wall is closely ap-posed to the spine; this condition is thought to be a consequence of a deficient aortic wall (40). A TAA can develop fistulous communication with the tracheobronchial tree, known as an

Figures 16, 17. (16) Aortobronchial fistula in a 50-year-old man with hemoptysis. Contrast-enhanced CT scan shows a focal rupture of the descending TAA, consolidation in the adjacent left lower lobe of the lung, and endobronchial blood in the left lower lobe segmental bronchus (arrow), findings that are compatible with an aortobronchial fistula. (17) Aortoesophageal fistula in a 73-year-old man. Nonenhanced (a, b) and contrast-enhanced (c) CT scans show an aortoesophageal fistula and intraesophageal rupture of a saccular de-scending TAA. High-attenuation blood is seen within the mediastinum in a and within the esophagus in b.


mon and is usually associated with hematemesis and dysphagia (43). An aortoesophageal fistula is a catastrophic complication whose CT find-ings include mediastinal hematoma, an intimate relationship of the aneurysm to the esophagus, and, rarely, contrast material extravasation into the esophagus (Fig 17) (2).

Compression of Adjacent StructuresTAAs can be asymptomatic, but when large enough, they can produce symptoms by com-pressing adjacent structures—for example, supe-rior vena cava syndrome due to compression of the superior vena cava, stridor or dyspnea due to airway compression, hoarseness due to compres-sion of the recurrent laryngeal nerve, and dys-phagia due to esophageal compression (6).

Postoperative ImagingThe normal postoperative appearance of the thoracic aorta can be confusing and may mimic disease; hence, knowledge of surgical details is

of paramount importance prior to interpreta-tion. The type of surgical repair used is based on a variety of factors, including disease extent, status of the aortic tissue and valve, preference of the patient and surgeon, need for long-term anticoagulation therapy, and type of prior sur-gery (if applicable) (44). Aortic grafts may be tissue (porcine) grafts or synthetic in nature. Tissue grafts are indistinguishable from native aortic tissue at CT, whereas synthetic grafts have a higher attenuation that is readily seen at non-enhanced CT (44). Two common techniques of aortic root graft repair are interposition graft and inclusion graft (1).

After the diseased segment has been excised, an interposition graft is sewn end to end and vas-cular branches (such as coronary arteries) are re-implanted. Felt rings and pledgets are often used to reinforce the site of anastomosis and the site of cannula placement. These objects can mimic pseudoaneurysms on contrast-enhanced scans but can easily be identified because of their high attenuation on nonenhanced scans.

An inclusion graft is inserted into the aortic lumen, leaving a potential space between the native aorta and the graft that may thrombose or even show persistent blood flow (Fig 18). In the absence of hemodynamic instability, blood flow in the perigraft space does not require in-tervention (1).

When the descending aorta is repaired with a graft, the native aorta may be left in situ and ap-pears as an irregular curvilinear area of dense cal-cification or a rind of soft tissue, often with fluid between it and the graft (44).

Complications that should be monitored in the postoperative period include graft dehis-cence and infection. Dehiscence of the surgical suture line may lead to pseudoaneurysm forma-tion, which can also involve the reimplanted coronary arteries (1).

The “elephant trunk” technique is used in patients with diffuse aneurysmal disease and involves graft replacement of the ascending aorta and aortic arch with or without valve re-placement. The free segment of the graft is left projecting into the proximal descending aorta,

Figure 18. Contrast material in the perigraft space in a 75-year-old man with an inclusion root graft. Routine follow-up CT scan shows contrast material (curved arrow) in the space between the inclusion root graft and the surrounding native aortic wrap, arising from a right coronary artery “button” anastomosis (straight arrow).


Figure 21. Endoleak in a 69-year-old man who had undergone endovascular repair of the thoracic aorta for an aneurysm. Coronal oblique reformatted CT image shows a type 2 endoleak (arrow) in continuity with an inter-costal artery (arrowhead).

Figure 20. Drawings illustrate the various types of endoleak: type I, leak at the attachment site; type II, leak from a branch artery; type III, graft defect; and type IV, graft porosity.

Endovascular repair of the thoracic aorta is an alternative surgical procedure in poor surgi-cal candidates or in emergency situations (1). A combined endovascular-surgical procedure can be performed in patients with aortic arch in-volvement to allow treatment of a wider range of

which can then be repaired at a later date (1). Knowledge of the surgical procedure can pre-vent mistaking the free segment of the graft for a dissection flap (Fig 19).

Figure 19. Aortic repair with the elephant trunk technique in a 53-year-old woman. (a) Contrast-enhanced CT scan shows the “elephant trunk” simulating a dissection flap. Note the felt strip at the distal aortic anastomosis (ar-row). (b) Sagittal reformatted CT image clearly depicts the distal end of the aortic arch graft dangling into the de-scending thoracic aorta.

TeachingPoint


graft, and thrombus providing an ineffective bar-rier to pressure transmission (45).

ConclusionsMultidetector CT angiography is routinely used to evaluate the spectrum of TAAs. Knowledge of the causes, significance, imaging appearances, and potential complications of both common and uncommon aortic aneurysms is essential for prompt and accurate diagnosis.

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A unique complication of endovascular repair is an endoleak, defined as contrast enhancement outside the stent-graft. Endoleaks have been di-vided into four types on the basis of the source of blood flow: type I, leak at the attachment site; type II, leak from a branch artery; type III, graft defect; and type IV, graft porosity (Figs 20, 21) (1). Unlike in the infrarenal aorta, type 2 endoleak is uncommon in the thoracic aorta and type 1 is more prevalent (1,45). There are several CT find-ings that may help distinguish between different types of endoleaks. Type 1 endoleak is seen com-municating with the proximal or distal attachment site of the stent-graft, whereas type 2 endoleak is located in the periphery of the aneurysm sac with-out contact with the stent (45). CT can also help visualize vessels in communication with the en-doleak cavity (Fig 21); however, contrast enhance-ment in these vessels may represent inflow (as in type 2 endoleak) or outflow (from endoleaks other than type 2). Type 3 endoleaks usually manifest around the graft while sparing the sac periphery (46). When type 3 endoleaks are suspected, CT can be used to evaluate for stent-graft integrity as well. Type 4 endoleaks secondary to graft po-rosity are uncommon with today’s stent-grafts and are identified as a “blush” on the immediate postdeployment angiogram when the patient is fully anticoagulated (45). The diagnosis of type 4 endoleak is one of exclusion (45), since other types of endoleak can be present on the postimplanta-tion angiogram and should be excluded.

Identification of the correct type of endoleak has important treatment implications. Type 1 and type 3 endoleaks are repaired immediately, the former by securing the attachment sites with angioplasty balloons, stents, or stent-graft exten-sions and the latter by covering the defect with a stent-graft extension (45). The management of type 2 endoleak is controversial, and, although some authors follow up this type of endoleak as long as the size of the aneurysm does not increase, others prefer to repair it (45). Type 4 endoleaks are self-limited, require no treatment, and resolve with normalization of the patient’s coagulation status (45).

Aneurysm expansion without endoleak is known as endotension or type 5 endoleak (45). Although the exact cause of endotension is un-known, possible causes include an endoleak that cannot be visualized with traditional imaging techniques, ultrafiltration of blood across the


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This article meets the criteria for 1.0 credit hour in category 1 of the AMA Physician’s Recognition Award. To obtaincredit, see accompanying test at http://www.rsna.org/education/rg_cme.html.

RG Volume 29 • Number 2 • March-April 2009 Agarwal et al

Multidetector CT of Thoracic Aortic Aneurysms Prachi P. Agarwal, MD, et al

Page 540 The presence of bicuspid aortic valve is an independent risk factor for TAA formation...and not merely a consequence of poststenotic dilatation secondary to aortic stenosis. Page 544 Knowledge of the relationship between the aortic aneurysm and the aortic branches is necessary to assess the adequacy of the “landing zone” (the aortic segments proximal and distal to the lesion where the stent extremities will be positioned). Page 546 In contrast to a pseudoaneurysm, ductus diverticulum has smooth margins with gently sloping symmetric shoulders and forms obtuse angles with the aortic wall. Page 546 Elective aneurysm repair has a lower mortality rate (9%) than does emergent repair (22%); therefore, aneurysms are considered for repair when they are either symptomatic or exceed 5-6 cm in diameter. Page 550 Knowledge of the surgical procedure can prevent mistaking the free segment of the graft for a dissection flap.

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