October 2009, VOLUME 193
NUMBER 4

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October 2009, Volume 193, Number 4

Vascular and Interventional Radiology

Review

CT and MRI in Diseases of the Aorta

+ Affiliation:
1All authors: Department of Diagnostic Radiology, Beth Israel Deaconess Medical Center and Harvard Medical School, 330 Brookline Ave., TCC-4, Boston, MA 02215.

Citation: American Journal of Roentgenology. 2009;193: 928-940. 10.2214/AJR.08.2166

ABSTRACT
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OBJECTIVE. This review focuses on the role of CT and MRI in the diagnosis, follow-up, and surgical planning of aortic aneurysms and acute aortic syndromes, including aortic dissection, intramural hematoma, and penetrating aortic ulcer. It also provides a systematic approach to the definition, causes, natural history, and imaging principles of these diseases.

CONCLUSION. An understanding of the pathophysiology, natural history, and imaging features is the key to successful diagnosis and appropriate management of patients with these aortic diseases.

Keywords: aortic aneurysm, aortic dissection, intramural hematoma, MDCT, MRI, penetrating aortic ulcer

Introduction
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During the past decade, advances in MDCT and MRI have led to their current role as the techniques of choice for the evaluation of the entire spectrum of aortic diseases [1]. Aortic aneurysm and acute aortic syndrome are the most common aortic diseases encountered in daily practice. This review focuses on the role of MDCT and MRI in the diagnosis, follow-up, and surgical planning of these entities. It also provides a systematic approach to the definition, causes, natural history, and imaging principles of aortic aneurysm and acute aortic syndrome.

Normal Anatomy of the Thoracic and Abdominal Aorta
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The ascending aorta extends from the aortic valve to the origin of the innominate artery, with its proximal portion referred to as the “aortic root.” The aortic arch begins at the innominate artery and ends at the ligamentum arteriosum. Its most distal aspect, which is often slightly narrowed, is termed the “aortic isthmus.” The descending aorta begins at the ligamentum. Its proximal portion may appear slightly dilated and has been termed the “aortic spindle.”

Three great arterial branches arise sequentially from the aortic arch. The innominate artery is the first and typically the largest branch and is usually seen more caudally than the other branches on transverse CT images. It gives rise to the right subclavian and right common carotid arteries; the right vertebral artery subsequently originates from the right subclavian artery. The left common carotid artery arises next at a more cephalad level and has the smallest diameter of the three major arterial branches. The left subclavian artery is the third branch and arises from the posterior superior portion of the aortic arch.

This normal branching pattern is seen in about 70% of individuals [2, 3]. The most common variation is a combined origin of the innominate and left common carotid arteries, which is seen in about 20–30% of individuals (Fig. 1). In about 5% of cases, the left vertebral artery arises as a separate branch directly from the aorta, between the left common carotid artery and the left subclavian artery.

The abdominal portion of the aorta extends from the diaphragm to the level of the fourth lumbar vertebra, where it bifurcates into the right and left common iliac arteries. The abdominal aorta gives rise to important single and paired branches. The single branches arise anteriorly and supply the anterior abdomen, whereas the paired branches arise laterally and supply the posterior abdomen. The single branches include the celiac trunk, which arises close to the diaphragmatic crus; the superior mesenteric artery, which arises immediately caudally; and the inferior mesenteric artery, which arises just above the aortic bifurcation [4]. The unpaired middle sacral artery arises from the posterior terminal portion of the abdominal aorta [4]. The paired abdominal aorta branches are, from cephalad to caudad, the inferior phrenic arteries, suprarenal and renal arteries, gonadal arteries, and several paired lumbar arteries. Variations of the abdominal aorta branches are common and knowledge of these variations facilitates their accurate identification and protection during surgery [5].

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Fig. 1 Three-dimensional volume rendering of aortic arch in 52-year-old man shows most common anatomic variant: common origin of innominate artery and left common carotid artery. AA = ascending aorta, DA = descending aorta, asterisk = common origin of innominate artery and left common artery, arrow = left subclavian artery.

Based on normative data collected from a large sample population, the intraluminal diameter of the ascending aorta (mean ± 2 SDs) in young adults (20–40 years) has been shown to vary between 35.6 and 37.8 mm for women and men, respectively, with a statistically significant linear association with age, sex, descending aortic diameter, and pulmonary artery diameter [6]. The aorta tapers distally, with the normal descending aorta always smaller in caliber than the ascending aorta in healthy individuals. The proximal descending aorta is considered abnormal when it exceeds 2.63 cm, and the distal descending aorta is considered abnormal when it exceeds 2.43 cm in diameter [7, 8]. The normal abdominal aorta usually does not exceed 2 cm in diameter in healthy individuals [9]. Table 1 provides practical values of the generally accepted maximal normal diameters of the ascending, descending, and abdominal aorta.

TABLE 1: Maximal Normal Aortic Diameter

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Fig. 2A 55-year-old asymptomatic woman with incidentally detected ascending aortic aneurysm. Contrast-enhanced CT image shows incidental ascending aorta aneurysm with widening of aorta to 5.5 cm (arrow).

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Fig. 2B 55-year-old asymptomatic woman with incidentally detected ascending aortic aneurysm. Axial T1-weighted MR image obtained 6 months after A confirms stability of luminal diameter (arrow).

Aortic Aneurysm
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Thoracic Aortic Aneurysm

The normal CT dimensions of the aorta have been defined based on normative measurements performed in large patient populations [7, 10]. An aneurysm is defined as a permanent dilatation of the aorta exceeding the normal measurements by more than 2 SDs at a given anatomic level. For example, an ascending aortic diameter greater than 3.91 cm (mean + 2 SDs) and a descending aorta diameter greater than 3.13 (mean + 2 SDs) have been determined as an upper threshold for normality by two independent studies [7, 10] (Fig. 2A, 2B). As a rule, an ascending aortic diameter equal to or greater than 4 cm (in individuals younger than 60 years old) and a descending aortic diameter larger than 3 cm is considered to indicate dilatation and a diameter equaling or exceeding 1.5 times the expected normal diameter is considered an aneurysm (Table 2).

TABLE 2: Aortic Aneurysm Size Criteria a

The prevalence of thoracic aortic aneurysms increases with age, with an overall incidence approximating 450 per 100,000 and a 3:1 male predominance [11]. In up to one third of cases, the abdominal aorta is also involved [11].

Although atherosclerosis is, overall, the most frequent cause of thoracic aneurysms [12], cystic median necrosis is the most common cause of an aneurysm isolated to the ascending aorta, especially when annuloaortic ectasia is present. It is most frequently caused by Marfan syndrome, but in one third of the cases, it is idiopathic [13] (Appendix 1). In Marfan syndrome, the classic imaging features include a pear-shaped aneurysmal ascending aorta with smooth tapering to a normal aortic arch. In addition to these risk factors, a genetic predisposition to develop thoracic aortic aneurysm has been shown by Coady et al. [14].

Thoracic aortic aneurysms are divided into two main categories: true and false, according to their pathologic features. In true aneurysms, all three layers of the aortic wall (intima, media, and adventitia) are involved in aneurysm formation without disruption of any layers [15]. In false aneurysms (also referred to as pseudoaneurysms), the intima is disrupted (and often, the media as well), and blood is contained by the adventitia and periadventitial tissues. When resulting from trauma, pseudoaneurysms are usually seen in the aortic isthmus, whereas penetrating aortic ulcer occurs in the descending aorta in most cases [15].

True aortic aneurysms are usually fusiform in shape, involving the entire circumference of the aorta, and often extend over a significant length of the vessel. Pseudoaneurysms are usually saccular, with a narrow neck at the origin in the aorta. The presence of a wide neck in a saccular aneurysm suggests a mycotic origin. These aneurysms have a tendency to involve the ascending aorta, likely because of its proximity to regions affected by endocarditis [15].

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Fig. 3 70-year-old man with diffuse abdominal pain. Three-dimensional volume rendering of contrast-enhanced CT scan shows fusiform aneurysm of distal abdominal aorta without extension into iliac arteries. Note narrowing of right common iliac artery (arrow). Atherosclerotic plaque is highlighted in blue and contrast-opacified lumen in pink.

Abdominal Aortic Aneurysm

The vast majority of abdominal aneurysms are caused by atherosclerosis. Dilatation of the abdominal aorta greater than 2 cm is considered abnormal, but a diameter of more than 3 cm is considered diagnostic of an abdominal aortic aneurysm [9]. Abdominal aortic aneurysm is rare before the age of 50 years, but it is found in up to 4% of the population above that age [16]. Abdominal aortic aneurysm most often involves the infrarenal segment of the aorta (Fig. 3).

Thoracoabdominal Aneurysms

Thoracoabdominal aneurysms have been classified into four types by Crawford and DeNatale [17] on the basis of their anatomic location. Type I involves the descending thoracic aorta below the origin of the left subclavian artery and the upper abdominal aorta. Type II involves both the thoracic descending aorta and most of the abdominal aorta. Type III is restricted to the lower portion of the thoracic aorta. Type IV begins at the diaphragm and extends caudally. Of these four types, types II and III represent the greatest therapeutic challenge [18].

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Fig. 4 70-year-old woman with shortness of breath. Contrast-enhanced CT image shows large fusiform descending aortic aneurysm (AA) causing extrinsic compression of adjacent bronchi with luminal narrowing (arrows).

Imaging

On both MDCT and MRI, the key features for imaging assessment of aneurysms are maximal aortic diameter; shape and extent; involvement of aortic branches; relationship to adjacent structures, such as the bronchi and esophagus (Fig. 4); and presence of mural thrombus (especially if the patient presents with peripheral embolization symptoms) [19]. MDCT can also be useful in detection of aortic calcifications. In addition, other relevant structures should be carefully evaluated, such as the coronary arteries and the intercostal arterial supply to the spinal cord at the lower thoracic–upper lumbar level [20, 21]. Both CT and MRI show excellent accuracy for characterizing these important features of aneurysms [22].

MDCT—Although the specific acquisition parameters will vary slightly on MDCT scanners with varying detector arrays, several common principles should be applied to all imaging protocols to provide optimal spatial and temporal resolution: utilization of the maximum detector array, bolus-timing software for optimization of contrast delivery timing, thin-section acquisition, and administration of at least 100 mL of nonionic contrast material (preferably 350 mg I/mL) injected at rapid infusion rates (4–5 mL/s) with a target opacification of the aorta greater than 250 HU. More specific details regarding various protocols from MDCT scanners of various vendors have been widely described in the literature and are beyond the scope of this review [12, 18, 23].

On MDCT images, a thrombus is usually smoothly marginated and either crescenteric or concentric with atherosclerotic calcifications along its periphery. Both thrombus and atherosclerotic plaque are usually peripheral to the patent lumen, low in density, and located internal to mural calcifications from the luminal side. An increase in plaque thickness to greater than 4 mm and in plaque ulcerations to greater than 2 mm, and the presence of mobile thrombi, have a well-known association with a higher risk for stroke [24, 25] and are also markers for coronary artery disease [26, 27].

Although the initial detection and evaluation of an aneurysm is usually based on axial images, postprocessing of MDCT data can be helpful for optimal characterization. Widely available postprocessing techniques include multiplanar reformation (MPR), maximum intensity projection (MIP), curved planar reformation, and 3D volume rendering of the volumetric data. MPR images, which include curved planar reformation and sagittal, coronal, and oblique reformations, are typically isotropic tomographic images [23]. Single-voxel-thick MPRs, notably curved planar reformations, are the most useful MPR technique [28]. For example, MPRs (curved planar reformations, in particular) are more accurate in estimating the aneurysmal diameter compared with axial images, especially in the descending aorta where the dilated lumen often courses obliquely to the scanning plane. Mural thrombi and atherosclerotic plaques are also better appreciated on coronal, sagittal, and oblique MPRs than on standard axial images [29]. Curved planar reformations are particularly useful for evaluating known or suspected aortic dissections.

MIP, a technique that highlights the highest-attenuation voxels, has relatively limited applications in evaluation of the thoracic aorta because simultaneous display and distinction of the contrast-enhanced flow lumen and mural calcifications with MIP images are usually not possible with this method. Another disadvantage of the MIP technique is its inability to discern overlapping vascular anatomy because of its lack of depth perception. Additionally, advanced postprocessing techniques, such as shaded-surface display 3D volume rendering, are useful in depicting the spatial relationship of true or false aneurysms to adjacent structures, helping to display critical anatomic relationships to surgeons (Fig. 5). The major drawback of 3D volume rendering is an underestimation of aneurysm size because of only partial inclusion of the mural thrombus into reconstructions that are based on a particular Hounsfield unit threshold.

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Fig. 5 80-year-old man with incidental finding on chest radiograph that prompted evaluation with CT. Sagittal oblique 3D volume rendering of contrast-enhanced CT scan shows saccular false aneurysm of proximal descending aorta (asterisk).

Calculation of aneurysm volume provides the most accurate measure of aneurysm size, but this method is time-consuming and, to the best of our knowledge, there are no current data regarding the risk of aneurysm rupture and guidelines for intervention based on aneurysm volume expansion. Rather, available guidelines are based on traditional 2D measurements.

MRI—MRI is a robust tool for evaluation of aortic aneurysms. Three-dimensional contrast-enhanced MR angiography (MRA) is highly accurate at depicting the location, extent, and precise diameter of an aneurysm and its relationship to the aortic branch vessels [3033]. When interpreting MR images, it is important to recognize that measurements should be obtained from source images on which the vessel wall is visible, because MIP images represent a cast of the lumen alone and will lead to underestimation of aneurysm size [34]. MRI displays mural thrombus as intermediate signal material on standard spin-echo T1-weighted images, but it does not reliably detect calcifications in the aortic wall. With this technique, the patent lumen can be seen as a flow void. However, a known pitfall of this technique is an intraluminal signal produced by slow flow that may mimic intraluminal thrombus. This problem may be avoided by combining spin-echo with gradient-recalled echo (GRE) cine MRI or contrast-enhanced MRA [35]. Recently, real-time, bright blood, cine imaging has been introduced and is referred to as balanced steady-state free precession (SSFP) imaging, true FISP (fast imaging with steady-state precession), and FIESTA (fast imaging employing steady-state acquisition) by various vendors [36]. FISP, for example, is a coherent steady-state technique that uses a fully balanced gradient waveform to recycle transverse magnetization. Contrast enhancement is determined on the basis of the ratio of T2 to T1 rather than on the basis of inflow effects, as in spoiled gradient echo methods. This difference eliminates sensitivity to saturation effects from absent or slow flow [37, 38]. Oblique sagittal MRI of the aorta allows almost the entire length of the aorta to be displayed, an advantage for imaging patients with thoracoabdominal aneurysms [39]. Concomitant evaluation of the aortic valve in patients with ascending aortic aneurysms with the cine MRI technique may provide crucial information for treatment planning [34]. Because of its lack of ionizing radiation, MRI is especially useful in patients with either contraindications to iodinated contrast material or in those with known aneurysms who require sequential follow-up with multiple examinations. The high level of reproducibility of MRI measurements enhances its role in monitoring the aneurysm expansion rate over time [40].

Treatment and Outcome

The most frequent complications of aortic aneurysm—mass effect, dissection, and rupture—are directly related to size. The mean rate of dilatation for thoracic aortic aneurysm is 0.12 cm per year [41]. The risk of rupture increases with increasing aortic diameter, with a high risk of complications (rupture and dissection) at 6 cm for the ascending and 7 cm for the descending aorta [42, 43]. For example, the yearly risk of rupture, dissection, or death for a thoracic aneurysm larger than 6 cm is as high as 14% [42]. As for abdominal aortic aneurysm, a cross-sectional diameter of greater than 7 cm is associated with a risk of rupture of up to 20% per year [44] (Table 3).

TABLE 3: Yearly Risk of Abdominal Aneurysm Rupture

CT angiography (CTA) is the technique of choice for evaluation of suspected aortic rupture because of its fast speed, widespread availability, and excellent sensitivity and specificity for this complication [45]. Diagnostic findings include active contrast extravasation or high-attenuation hemorrhagic collections in the pleura, pericardium, or mediastinum [46] (Fig. 6). A draped aorta sign, defined as the posterior wall of the aorta being indistinct from adjacent structures or closely following the contour of adjacent vertebral bodies, may be seen in a contained rupture of the posterior aortic wall [47]. In the abdomen, rupture usually involves the posterolateral aorta, with hemorrhage into the retroperitoneum [48]. Impending rupture in both thoracic and abdominal aneurysms may be seen as a crescent of high-attenuation contrast material within the mural thrombus [49, 50]. Another complication of abdominal aortic aneurysm is aortoduodenal fistula, which may be associated with recurrent and potentially catastrophic bleeding. CT can detect this condition by showing loss of the normal fat plane between the aorta and duodenum and the presence of air in the aorta [51].

Elective surgical repair has been recommended for ascending aortic aneurysms of 5.0–5.5 cm in diameter and descending aortic aneurysms of 5.5–6.5 cm [41]. For abdominal aortic aneurysms, surgical intervention is generally considered when the diameter of the aneurysm exceeds 5.5 cm or when the diameter is greater than 4.5 cm and has increased at least 0.5 cm in the preceding 6 months. Although the specific techniques of aneurysm treatment are beyond the scope of this review, it should be noted that a paradigm shift is occurring in treatment options, with endovascular aortic repair (Fig. 7) increasingly used as an alternative to open surgical repair, especially among patients who are poor surgical candidates [18].

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Fig. 6 69-year-old man with worsening abdominal pain. Contrast-enhanced CT image shows aortic aneurysm with active extravasation of contrast material within aneurysm (arrow). Left-sided hemothorax (asterisk) is secondary to rupture.

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Fig. 7 83-year-old man with infrarenal abdominal aortic aneurysm. Color-coded 3D volume rendering of contrast-enhanced CT scan shows successful treatment of abdominal aortic aneurysm with endovascular stent, which extends into common iliac arteries.

Acute Aortic Diseases
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In contrast to uncomplicated aneurysms, which are often detected in asymptomatic patients, acute aortic diseases, including aortic dissection, intramural hematoma, and penetrating aortic ulcer, are usually diagnosed in patients with acute chest pain. MDCT and MRI play a major role in detecting these complications, followed by conventional angiography and transesophageal echocardiography (TEE) [52].

Aortic Dissection

Definition, clinical symptoms, and risk factors—Aortic dissection is the most common acute aortic disorder, with an incidence up to 0.2–0.8%, and also carries the highest mortality rate [53]. Thus, prompt diagnosis is critical. Aortic dissection is caused by an intimal tear within an abnormal, weakened vessel wall. This leads to blood entering the wall, with subsequent propagation in the media both proximally and distally, displacing the intima inward. The two well-known classification systems, De Bakey and Stanford, are based on two parameters: the origin of the intimal tear and the extent of involvement of the aorta. The Stanford system classifies aortic dissection into two types, A and B. Type A involves the ascending aorta (regardless of the origin of the intimal tear), with or without involvement of descending aorta. Surgical intervention is required for type A dissection. In contrast, type B affects only the descending aorta and generally requires only conservative medical treatment [54]. Because the Stanford system reflects the treatment approach, it has superseded the original DeBakey system, in which type 1 (origin at ascending) and type 2 (origin at arch) were equivalent to type A in the Stanford classification, and type 3 was equivalent to type B (Fig. 8A, 8B, 8C, 8D).

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Fig. 8A Examples of ascending aortic dissection on CT angiography, gadolinium-enhanced MRI, and steady-state acquisition. (Courtesy of Ivan Pedrosa, Boston, MA) 80-year-old man with chest pain. Sagittal image of gadolinium-enhanced MR angiography of aorta (A) shows Stanford type B dissection with intimal flap (arrow). Sagittal reformation of contrast-enhanced CT (B) better delineates intimal flap (solid arrow) and anteriorly located true lumen (open arrow).

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Fig. 8B Examples of ascending aortic dissection on CT angiography, gadolinium-enhanced MRI, and steady-state acquisition. (Courtesy of Ivan Pedrosa, Boston, MA) 80-year-old man with chest pain. Sagittal image of gadolinium-enhanced MR angiography of aorta (A) shows Stanford type B dissection with intimal flap (arrow). Sagittal reformation of contrast-enhanced CT (B) better delineates intimal flap (solid arrow) and anteriorly located true lumen (open arrow).

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Fig. 8C Examples of ascending aortic dissection on CT angiography, gadolinium-enhanced MRI, and steady-state acquisition. (Courtesy of Ivan Pedrosa, Boston, MA) 68-year-old woman with shortness of breath. Axial images obtained using fast imaging employing steady-state acquisition at level of aortic arch (C) and ascending aorta (D) show intimal flap separating true and false lumen (black arrows). White arrow in D indicates site of fenestration.

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Fig. 8D Examples of ascending aortic dissection on CT angiography, gadolinium-enhanced MRI, and steady-state acquisition. (Courtesy of Ivan Pedrosa, Boston, MA) 68-year-old woman with shortness of breath. Axial images obtained using fast imaging employing steady-state acquisition at level of aortic arch (C) and ascending aorta (D) show intimal flap separating true and false lumen (black arrows). White arrow in D indicates site of fenestration.

Patients with acute aortic dissection usually present with one or a combination of the following symptoms: chest pain radiating to the back, syncope, and shortness of breath. Acute aortic dissection is defined as dissection detected within 2 weeks of the onset of symptoms, whereas chronic dissection is defined as older than 2 weeks [55].

The major risk factor for aortic dissection is systemic hypertension, followed by cystic medial necrosis, congenital lesions (such as bicuspid aortic valve and aortic coarctation), pregnancy, trauma, and arteritis. Thoracic aortic aneurysm is associated with a higher risk of developing dissection than abdominal aneurysm [55].

Imaging—When aortic dissection is suspected clinically, urgent cross-sectional imaging is required. CT, MRI, and TEE have been shown to be equally reliable for confirming or excluding the diagnosis of dissection [56].

Historically, in the early 1990s, MRI was considered the leading technique for diagnosis of aortic dissection, with a reported sensitivity and specificity of 98% [52]. Major MRI advantages include lack of ionizing radiation or iodinated contrast material and ability to concurrently evaluate the entire aorta, aortic valve, great vessels, heart, and pericardial space. When aortic dissection is suspected, the MRI examination usually begins with spin-echo black blood sequences that depict the intimal flap as a linear structure. The true lumen shows signal void, whereas the false lumen shows higher signal intensity indicative of turbulent flow [34]. In stable patients, adjunctive GRE sequences can be performed to differentiate slow flow from thrombus in the false lumen. The relatively new steady-state free precession technique provides diagnostic image quality even when the breath-hold is suboptimal [38]. The recent development of contrast-enhanced MRA has provided greater imaging details of the aorta and arch vessels while also requiring a significantly decreased time of acquisition [33, 37, 57, 58]. With current technology, gadolinium-enhanced 3D MRA can be performed rapidly, without the need for ECG-gating, and it can be performed even in severely ill patients. It is important to remember that evaluation of both source and MIP images is crucial for diagnosis because MIP images can occasionally fail to show the intimal flap [59].

As discussed later, advances in MDCT coupled with its greater accessibility and ease of monitoring unstable patients, have resulted in a less prominent role for MRI in the acute setting at most centers. Currently, the major role of MRI is in monitoring patients treated conservatively and in postoperative follow-up [22].

Recent advances in MDCT resulting in shorter scanning time, in addition to the easier ability to monitor unstable patients in CT compared with MRI, have led to its rise to the most commonly used imaging technique for evaluation of suspected acute aortic syndrome and aortic dissection in particular [6062]. As shown by Sommer and colleagues [60], single-detector CT has 100% sensitivity and specificity for the detection of aortic dissection. MDCT can also reliably predict the progression of dissection [61, 62]. Although contrast material injection rates and technical parameters may vary based on institutional preferences and different MDCT vendors and configuration of scanners, features that should be part of all protocols for aortic dissection include both unenhanced and contrast-enhanced image acquisition sequences, imaging of the entire aorta from the proximal arch vessels to the common iliac arteries distally, and appropriate bolus-tracking technique to optimize aortic true lumen enhancement.

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Fig. 9A 65-year-old man with abdominal pain. Unenhanced, low-dose CT image shows displaced intimal calcifications (arrow) in Stanford type B dissection.

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Fig. 9B 65-year-old man with abdominal pain. After contrast administration, intimal flap is seen separating true and false lumen (arrow).

Imaging findings—The primary role of unenhanced acquisition is to detect medially displaced aortic calcifications or the intimal flap itself, especially in patients with severe anemia [63] (Fig. 9A, 9B). Unenhanced images are also important for detecting intramural hematoma, an entity that may mimic aortic dissection clinically and may sometimes coexist with dissection [61, 63] (Table 4). In patients with impaired renal function, such findings may be sufficient for diagnosis or for serial monitoring without administration of potentially nephrotoxic iodinated IV contrast material.

TABLE 4: CT Appearance of Acute Aortic Diseases

The key finding on contrast-enhanced images is an intimal flap separating two lumens (Fig. 9A, 9B). The convexity of the intimal flap is usually toward the false lumen that surrounds the true lumen. The false lumen usually has slower flow and a larger diameter and may contain thrombi [64]. The extent of the thrombus in the false lumen is an indirect indicator of the degree of communication between the true and false lumen, with the false lumen enlarging over time because of intraluminal pressure and wall stress, with risk of aneurysm formation and rupture. The differentiation between false and true lumen can sometimes be challenging, especially when both lumens appear similar in degree of opacification and size.

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Fig. 10A 66-year-old woman with chest pain. Contrast-enhanced CT shows descending aortic dissection with small anterior true lumen and posterior false lumen. Arrow points to cobwebs in false lumen. Note is made of incidental spinal bone island.

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Fig. 10B 66-year-old woman with chest pain. Sagittal reformation image confirms posteriorly located false lumen with cobwebs (arrow) and well-opacified anterior true lumen.

Usually, in type A aortic dissection, the false lumen is located along the right anterolateral wall of the ascending aorta and extends distally in a spiral fashion along the left posterolateral wall of the descending aorta. Slender linear areas of low attenuation may be observed in the false lumen and represent the residua of incompletely dissected media and are known as the cobweb sign, a specific finding for identifying the false lumen (Fig. 10A, 10B). In most cases, the lumen that extends more caudally is the true lumen. Accurate distinction between the false and true lumens is of importance when endovascular therapy is considered because the endograft must be positioned in the true lumen [65]. However, this distinction is less significant when an open surgical approach is planned. Both approaches require accurate localization of the intimal tear site (the most proximal split in the intimal flap or an ulcerlike projection within an intramural hematoma on contrast-enhanced CT) because both open surgical repair and stent-graft implantation usually attempt to occlude the tear and to induce thrombosis of the false lumen [66, 67]. The major role of MDCT is in providing specific, precise measurements of the extent of dissection, including length, diameter of the aorta and the true and false lumens, involvement of vital vasculature, and distance from the intimal tear to the vital vascular branches [65] (Fig. 11A, 11B, 11C, 11D, 11E).

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Fig. 11A Examples of involvement of aortic branches by dissection in five different patients. 72-year-old woman with abdominal pain. Contrast-enhanced CT shows celiac trunk originating from false lumen (arrow).

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Fig. 11B Examples of involvement of aortic branches by dissection in five different patients. 81-year-old man. Contrast-enhanced CT shows Stanford type B dissection extending into celiac trunk (arrow).

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Fig. 11C Examples of involvement of aortic branches by dissection in five different patients. 65-year-old woman with high serum creatinine levels. Contrast-enhanced CT shows extension of Stanford type B dissection in left renal artery (arrow).

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Fig. 11D Examples of involvement of aortic branches by dissection in five different patients. 77-year-old man. Contrast-enhanced CT shows known Stanford type B dissection that extends into both iliac arteries (arrows).

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Fig. 11E Examples of involvement of aortic branches by dissection in five different patients. 68-year-old man with vision disturbances. Contrast-enhanced CT shows direct continuation of Stanford type A dissection into aortic arch vessels (arrows).

On MRI, an intimal flap remains the key finding, usually seen first on spin-echo black blood sequences. Additional findings include signal void in the true lumen and higher signal intensity indicative of turbulent flow in the false lumen [34]. Slow flow can be differentiated from thrombus in the false lumen using steady-state cine MRI [38]. Combining the spin-echo images with gadolinium-enhanced 3D contrast-enhanced MRA completes the diagnosis and provides detailed imaging of the aorta and arch vessels [59]. As previously mentioned, evaluation of both source and MIP images is crucial for diagnosis because MIP images can occasionally fail to show the intimal flap [59]. High signal intensity of pericardial effusion is considered a sign of impending rupture of the ascending aorta into the pericardial space [34].

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Fig. 12 75-year-old man with chest pain and hypotension. Intimal flap (black arrow) in Stanford type B dissection separates smaller anterior true lumen from posterior false lumen of descending aortic aneurysm complicated by rupture, seen as active extravasation of contrast material (white arrow).

Once aortic dissection is diagnosed, the radiologist should carefully assess for complications. The four life-threatening complications of type A aortic dissection are pericardial hemorrhage and tamponade, aortic valve rupture and acute aortic insufficiency, coronary artery dissection or origin from the false lumen with subsequent myocardial perfusion compromise and infarction, and carotid artery dissection and stroke. Other potentially fatal complications of dissection include continuation of the dissection into the aortic branches; aortic branch occlusion causing renal, bowel, and splenic infarction; aneurysm formation; and hemothorax [55] (Fig. 12).

As in aortic aneurysm, the diagnosis of dissection can be made on transverse CT images, but MPR images play an important complementary role in confirming the diagnosis and determining the extent of involvement, especially with regard to involvement of aortic branch vessels [29, 68] (Fig. 13A, 13B).

Common pitfalls in image acquisition and interpretation—When interpreting CTA studies, it is important to be aware that insufficient vascular enhancement may preclude detection of the intimal flap. Additionally, it is important to recognize several artifacts that may mimic an intimal flap [69]. The so-called pulsation artifact is the most common cause of a pseudodissection [70]. It is caused by pulsatile movement of the ascending aorta during the cardiac cycle between end-diastole and end-systole. The right posterior and left anterior aspects of the ascending aorta are most commonly affected. Potential solutions for avoiding this artifact include retrospective ECG-modulated cardiac gating [71] or a 180° linear interpolation reconstruction algorithm [72]. Dense contrast enhancement in the left brachiocephalic vein or superior vena cava, mediastinal clips, and indwelling catheters can all produce streak artifacts in the aorta that may potentially simulate dissection. Enhanced left brachiocephalic, superior intercostal, or left inferior pulmonary veins can also simulate an enhancing false lumen. In addition to these artifacts, insufficient vascular enhancement may yield a false-negative diagnosis of dissection. This difficulty can be avoided by careful attention to injection rate and volume of IV contrast material administered. In nondiagnostic cases, the contrast-enhanced study will need to be repeated, or an alternative test, such as TEE or MRA, may need to be performed [73].

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Fig. 13A 58-year-old man with chest pain. Curved multiplanar reconstruction (A) and 3D volume rendering (B) of contrast-enhanced CT show type B dissection originating just distal to left subclavian artery, continuing into left common iliac artery.

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Fig. 13B 58-year-old man with chest pain. Curved multiplanar reconstruction (A) and 3D volume rendering (B) of contrast-enhanced CT show type B dissection originating just distal to left subclavian artery, continuing into left common iliac artery.

Triple-rule out—“Triple-rule out” is a relatively new term that describes an ECG-gated 64-MDCT angiography study to evaluate patients with acute chest pain for three potential causes: aortic dissection, pulmonary embolism, and coronary artery dissection. MDCT is a feasible approach to provide a comprehensive chest pain evaluation in the emergency department, providing reliable information regarding the triage of patients with acute coronary, aortic, and pulmonary syndromes [74]. The ability to scan the entire thorax and to simultaneously image the thoracic aorta, pulmonary arteries, and coronary arteries provides an exciting new approach to the assessment of patients with acute chest pain (Fig. 14A, 14B). The inherent advantage of MDCT with state-of-the-art cardiac capabilities is the rapid investigation of the main sources of acute chest pain with a high negative predictive value [75].

The potential problem of pulsation artifacts that may lead to a false-positive diagnosis of type A aortic dissection can be eliminated with ECG gating [71, 75, 76]. With dedicated multiphase reconstruction, the aortic and mitral valves can be evaluated [71, 77].

The major drawback of this protocol is its relatively high radiation dose [75]. Therefore, this protocol should be used only when there is a clinical indication for imaging of the pulmonary vessels and the coronary arteries, aorta, or both. On the other hand, this protocol allows evaluation of the entire chest in one scan and may therefore help to avoid additional diagnostic procedures among patients presenting with nonspecific chest pain.

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Fig. 14A ECG-gated CT angiography of aorta in 68-year-old man after aortic valve replacement. Sagittal (A) and coronal (B) reformation images show extent of dissection as well as relationship to replaced aortic valve. Absence of pulsation artifacts is due to ECG-gating technique. Narrow false lumen (black arrow, A) and intimal flap (white arrow, A) and replaced aortic valve (arrow, B) are shown.

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Fig. 14B ECG-gated CT angiography of aorta in 68-year-old man after aortic valve replacement. Sagittal (A) and coronal (B) reformation images show extent of dissection as well as relationship to replaced aortic valve. Absence of pulsation artifacts is due to ECG-gating technique. Narrow false lumen (black arrow, A) and intimal flap (white arrow, A) and replaced aortic valve (arrow, B) are shown.

Treatment options and outcome—A detailed description of the current treatment options is beyond the scope of this review. In summary, surgical therapy is the treatment of choice for type A aortic dissection [78]. Currently, the alternative therapeutic approach of thoracic aortic stent placement for type A aortic dissection is recommended by Svensson and colleagues [79] only when the risk of stent placement is lower than that of open surgical repair or medical management. For type B aortic dissection, patients with no complications are at present treated conservatively, with the main objective to control blood pressure [78]. Endoluminal stent-graft placement has recently emerged as a viable alternative to surgery, but it is generally limited to relief of life-threatening complications such as impending aortic rupture, ischemia of limbs and organ systems, persistent or recurrent intractable pain, progression of dissection, aneurysm expansion, and uncontrolled hypertension. Studies have shown that endovascular stent-graft treatment is better than open aortic surgery for patients with complicated type B dissection [65, 67, 80]. A novel fenestration technique appears to be a safe and effective alternative to medical management and aortic replacement surgery in selected patients [81].

Intramural Hematoma

Definition, risk factors, and clinical symptoms—Intramural hematoma is defined as a bleeding of the vasa vasorum in the medial layer of the aorta, with no blood flow within the media [82]. The most frequent source of intramural hematoma is in the media itself, representing spontaneous hemorrhage from the vasa vasorum of the medial layer with subsequent formation of hematoma within the media causing focal, most often circumferential, wall thickening. Other potential causes include aortic trauma or a penetrating aortic ulcer that has bled into the aortic wall [83]. The importance of intramural hematoma is that it can be a precursor of aortic dissection, representing either an early stage or a variant of dissection [84]. For example, up to 8% of patients originally classified as having dissection were eventually shown to have an intramural hematoma [85], and this entity constitutes 10–20% of acute aortic syndromes [52, 8688]. Similar to aortic dissection, systemic hypertension is the leading risk factor for intramural hematoma [88].

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Fig. 15A 50-year-old woman with acute chest pain. Unenhanced low-dose CT image shows relatively high-density intramural hematoma surrounding descending aorta (arrow).

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Fig. 15B 50-year-old woman with acute chest pain. On contrast-enhanced CT image, intramural hematoma is visually less conspicuous (arrow).

Intramural hematoma most frequently involves the ascending or proximal descending aorta—up to 70% of cases. The clinical presentation is similar to dissection: severe chest pain radiating to the back. Because of the similarity between the two entities, intramural hematoma is classified in the same way as aortic dissection: type A when the ascending aorta is involved and type B when involvement is limited to the descending aorta.

Imaging techniques and features—CT and MRI are the leading techniques for diagnosis and classification of intramural hematoma. When evaluating the aorta using MDCT, an unenhanced acquisition is crucial for diagnosis. A high-attenuation crescenteric thickening of the aortic wall that extends in a longitudinal, nonspiral fashion is the hallmark of this entity (Fig. 15A, 15B). In contrast to dissection, the aortic lumen is rarely compromised, and no intimal flap or enhancement of the aortic wall is seen after contrast administration. The combination of an unenhanced acquisition followed by a contrast-enhanced acquisition is associated with a sensitivity as high as 96% for detection of intramural hematoma using MDCT [89]. Infrequently, however, the differentiation of intramural hematoma from either atherosclerotic thickening of the aorta, thrombus, or thrombosed dissection may be difficult at CT. In such settings, MRI can be a valuable problem-solving tool, especially when dynamic cine gradient-echo sequences are applied [90, 91] (Fig. 16A, 16B). MRI may also provide a determination of the age of a hematoma based on the signal characteristics of different degradation products of hemoglobin. For example, T1-weighted spin-echo images show intermediate signal intensity caused by the presence of oxyhemoglobin in the acute stage and high signal intensity caused by the presence of methemoglobin in the subacute stage [34].

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Fig. 16A 75-year-old man with atypical chest pain. Axial MR image with fast imaging employing steady-state acquisition shows low-signal intramural hematoma surrounding ascending aorta (arrow) and bilateral pleural effusions (asterisks).

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Fig. 16B 75-year-old man with atypical chest pain. Contrast-enhanced CT image shows low-density intramural hematoma of ascending aorta (arrow).

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Fig. 17A Penetrating aortic ulcer in two different patients. 78-year-old man with known aortic atherosclerotic disease. Coronal reformation of contrast-enhanced CT scan shows penetrating atherosclerotic ulcer in tortuous descending aorta (arrow).

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Fig. 17B Penetrating aortic ulcer in two different patients. 72-year-old man who presented for follow-up of known penetrating atherosclerotic ulcer. Sagittal oblique gadolinium-enhanced MR angiography of aorta 1 year later shows progression of penetrating atherosclerotic ulcer (arrow).

Treatment and clinical outcome—The clinical outcome of intramural hematoma may be favorable, with spontaneous regression over time, or it may be complicated by progression to overt aortic dissection. Given the potentially fatal complications of ascending aortic dissection, there is general agreement that surgical management is the treatment of choice for type A intramural hematoma because complications are common when left untreated [85, 91, 92]. For example, in a multicenter study, Kodolitsch et al. [93] showed a 50% complication rate of intramural hematoma within 30 days of the initial presentation, including overt dissection, contained rupture or aneurysm. A maximal aortic diameter of greater than 5 cm has been suggested by Kaji et al. [62] as an independent predictor of progression, requiring frequent monitoring. As with aortic dissection, most patients with type B intramural hematoma have a good short-term outcome with aggressive control of hypertension. Several findings on initial CT examination have been shown to predict progression, including aneurysm formation, ulcer formation—in particular, penetrating aortic ulcer, frank dissection or aortic rupture, and wall thickness of 10 mm or greater [86, 94]. Close follow-up imaging during the first 30 days of presentation is recommended for all patients treated medically. If there is longitudinal progression of aortic involvement, progressive luminal dilatation, penetrating ulcer, enlarging intramural hematoma, or overt dissection, surgical or endovascular treatment should be strongly considered [95].

Penetrating Aortic Ulcer

Definition, clinical symptoms and risk factors—Penetrating aortic ulcer occurs when ulceration of atherosclerotic plaque disrupts the intima with subsequent extension of the blood into the media. Penetrating aortic ulcer can cause intramural hematoma, extend along the media, or rarely may progress into frank dissection or rupture through the adventitia. Embolization of material from the ulcer to the distal arterial circulation is an additional recognized complication. Elderly hypertensive patients are most frequently affected, and the mid descending thoracic aorta is most commonly (90%) involved [85]. The clinical presentation is very similar to aortic dissection and intramural hematoma: severe chest pain frequently radiating to the back.

Imaging findings—On unenhanced MDCT, penetrating aortic ulcer appears as an intramural hematoma. Contrast-enhanced MDCT, including axial and multiplanar reformations, is the technique of choice for diagnosis of penetrating aortic ulcer [96, 97]. Localized ulceration penetrating through the aortic intima into the aortic wall is the characteristic finding, usually in the mid to distal third of the descending aorta (Fig. 17A, 17B). Focal thickening or high attenuation of the adjacent aortic wall suggests associated intramural hematoma (Fig. 18A, 18B). A potential disadvantage of MRI, compared with CT, is its inability to reveal dislodgement of intimal calcifications, which frequently accompany penetrating aortic ulcer.

Treatment and outcome—Although most patients with penetrating aortic ulcer in the descending aorta can be managed conservatively (similar to patients with type B dissection) [84], progression to aneurysm formation, rupture, or dissection has been reported in up to 40% of patients [97, 98]. Thus, if a conservative approach (controlling hypertension) is chosen, close imaging follow-up within the first 30 first days of presentation is strongly advised.

Summary
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CT and MRI are robust tools for the evaluation of atherosclerotic aortic diseases, including aortic aneurysm and acute aortic syndromes. Because these noninvasive techniques provide crucial information about the vessel size, wall, and relationship to surrounding structures, they are the preferred primary diagnostic imaging methods for patients with suspected or known aortic diseases. An understanding of the pathophysiology and natural history of aortic diseases and their characteristic imaging findings, along with careful attention to optimal imaging technique, is the key to successful diagnosis and proper management of affected patients.

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Fig. 18A 73-year-old man with acute chest pain. Unenhanced CT image shows relatively high-density intramural hematoma of aortic arch (arrow) with associated hemothorax (asterisk), indicating rupture.

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Fig. 18B 73-year-old man with acute chest pain. Contrast-enhanced CT image confirms intramural hematoma (black arrow) and depicts causative penetrating atherosclerotic ulcer (white arrow). Asterisk shows adjacent hemothorax.

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APPENDIX 1: Thoracic Aortic Aneurysm Causes

Address correspondence to D. Litmanovich ().

The authors thank Maryellen O'Rourke for her substantial contribution to imaging postprocessing and Meredith Cunningham for her secretarial assistance.

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American Journal of Roentgenology. 2011;196:1206-1213. 10.2214/AJR.10.4877
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