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Intimal Problems: A Pictorial Review of Nontraumatic Aortic Disease at Multidetector Computed Tomography
Current Problems in Diagnostic Radiology ] (2015) ]]]–]]]
Current Problems in Diagnostic Radiology journal homepage: www.cpdrjournal.com
Intimal Problems: A Pictorial Review of Nontraumatic Aortic Disease at Multidetector Computed Tomography Abigail V. Berniker, MDn, Justin E. Mackey, MD, Oleg M. Teytelboym, MD Department of Radiology, Mercy Catholic Medical Center, Darby, PA
Nontraumatic aortic diseases include a spectrum of disorders, many of which result in high morbidity and mortality. This article highlights the multidetector computed tomography appearance of common and uncommon nontraumatic aortic entities: dissection, intramural hematoma, penetrating atherosclerotic ulcer, pseudoaneurysm, aneurysm, acute thrombus, chronic occlusion, and vasculitis. Additionally, classical imaging mimics and pitfalls are addressed. Radiologists should feel confident identifying these conditions and providing accurate diagnoses to expedite patient care and prevent devastating, even fatal outcomes. & 2015 Mosby, Inc. All rights reserved.
Introduction The normal aortic wall comprises 3 layers: intima (innermost), media (middle), and adventitia (outermost).1 The media contains vasa vasorum, small vessels that supply oxygen and nutrients to the aortic wall (Fig 1). Disruption of this fundamental wall integrity results in a range of aortic pathology. For instance, an intimal defect results in penetrating atherosclerotic ulcer or dissection; irregularity of intact vessel layers is seen in aneurysm and vasculitis; and rupture of the vasa vasorum leads to intramural hematoma. Multidetector computed tomography (MDCT) is the mainstay of aortic imaging owing to its fast acquisition time, high temporal and spatial resolution, widespread availability, and low cost. Typical computed tomography (CT) protocols include noncontrast and arterial, bolus-triggered phases. The noncontrast phase elucidates aortic calcification, hyperdense surgical material, and intramural hematoma, whereas the angiographic phase better evaluates the lumen of the aorta and branch vessels. Some institutions employ a venous phase routinely, whereas others include it selectively, for example, to assess for endoleaks in patients with prior endovascular repair. Electrocardiographic gating helps improve image quality of the aortic root and coronary arteries, as cardiac motion artifacts can mimic dissection flaps and degrade evaluation of these structures. Dual-source dual-energy CT can generate virtual noncontrast images from a single acquisition and may one day replace multiphase imaging with MDCT; however, this technology is not yet widely available and validated. Magnetic resonance (MR) angiography is an alternative to CT for aortic evaluation and is well suited for routine surveillance of known aortic pathology in hemodynamically stable patients. MR offers several potential benefits over CT, including lack of ionizing
n Reprint requests: Abigail V. Berniker, MD, Department of Radiology, Mercy Catholic Medical Center, 1500 Lansdowne Ave, Darby, PA 19023. E-mail address: [email protected] (A.V. Berniker).
http://dx.doi.org/10.1067/j.cpradiol.2015.08.007 0363-0188/& 2015 Mosby, Inc. All rights reserved.
radiation and ability to provide accurate functional and blood flow information. Additionally, unenhanced MR can be performed effectively in patients with contraindications to iodinated contrast due to allergy or renal failure. This article reviews the spectrum of nontraumatic aortic diseases and underscores areas of both overlap and difference to help radiologists better recognize these entities.
Aortic Dissection Dissection occurs when an intimal tear enables blood to communicate abnormally with the media and propagate longitudinally along an artery.2,3 An intimal flap separates true and false lumens; blood within the intima is termed the true lumen, whereas the abnormally blood-filled media is the false lumen (Fig 2).2 Various imaging clues can help distinguish the true and false lumens. Typically, the true lumen is smaller, enhances faster, and is demarcated by intimal calcifications, if present. The false lumen, on the contrary, tends to be larger, enhances slower, contains thrombus, and comprises the outer curve of the aortic arch in a Type A dissection.2 Additionally, the false lumen is associated with 2 classical signs: the beak sign, whereby the false lumen insinuates around the true lumen forming an acute angle resembling a bird's beak, and the cobweb sign, which refers to linear or serpiginous, hypodense media remnants that mimic a spider's web (Table 1).4,5 The Stanford classification subdivides aortic dissections into two major types based on the most proximal extent and management implications.6,2 Type A dissections account for most of the cases (60%-70%) and involve the ascending aorta with possible extension into the descending aorta.2 Type B dissections (30%-40%) are limited to the descending or abdominal aorta.2 Technically, Type B dissections begin at or distal to the ligamentum arteriosum; however, the left subclavian artery is used as a conventional landmark at cross-sectional imaging owing to conspicuity (Fig 3).7
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Classification becomes controversial when the aortic arch is the most proximal site of involvement: in such cases, it may be most helpful to describe a Type B dissection with arch extension as many of these cases can be managed conservatively. Dissection classification dictates treatment: Type A dissections are treated surgically or with endovascular repair to prevent extension to the aortic root, pericardium, or coronary arteries.2 If untreated, Type A dissections have more than 50% mortality at 48 hours.2 Type B dissections, on the contrary, are treated medically with hypertension control unless end-organ ischemia or persistent pain requires surgical intervention.2,8 Additionally, Type B dissections undergo surgical or endovascular treatment in certain emergent, high-risk situations such as ruptured aorta or uncontrollable hemodynamic instability.2 Although originally described for aortic dissection, the Stanford classification is now extrapolated to penetrating atherosclerotic ulcer and intramural hematoma.8,9 Aortic dissection has been linked with multiple underlying risk factors. Processes that alter flow dynamics can predispose to dissection, with the intimal tear typically occurring at the site of maximum shear stress. The most common risk factor is hypertension, accounting for 60%-90% of cases.2 Other entities include underlying aortic root anomalies such as bicuspid aortic valve, arteritis, and connective tissue diseases like Loeys-Dietz or Marfan syndrome (Fig 4).2 Of note, aortic dissection is not typically associated with atherosclerotic disease. Along with identifying the presence of dissection, it is critical to define the full extent of the intimal flap and assess for complications. In Type A dissections, it is important to evaluate for involvement of the aortic root or coronary arteries, which puts the patient at risk for imminent rupture, aortic valve insufficiency, or myocardial ischemia. Additionally, it is essential to assess for signs of rupture, namely hemomediastinum, hemopericardium, hemothorax, and hemoperitoneum. Dissection flaps may also obstruct or extend into branch vessels, resulting in end-organ damage, and any such findings should be reported. Finally, the aorta should be interrogated carefully on follow-up studies as patients with aortic dissection are at increased risk of developing subsequent aortic pathology, including dissection or aneurysm (Fig 5).
Intramural Hematoma Intramural hematoma results from rupture of vasa vasorum and hemorrhage within the media.9 The hemorrhage weakens the
Fig. 2. True and false lumens of aortic dissection in a 69-year-old woman. Axial contrast-enhanced computed tomography (CT) image shows an intimal flap separating true (T) and false (F) lumens. The true lumen is smaller and enhances quicker, whereas the false lumen is larger and enhances slower. The false lumen also demonstrates the beak sign: it insinuates around the true lumen, forming acute angles (arrow). The dissection flap involves the ascending and descending thoracic aorta (Type A). Note the hyperdense material reflecting hemomediastinum (asterisk).
media and the integrity of the aortic wall, with possible progression to aortic dissection, ulcer-like projections, or aneurysm.1,10 As with dissection, hypertension is the most common predisposing factor.9 In contradistinction to dissection (as well as penetrating atherosclerotic ulcer), however, the intima remains intact in intramural hematoma.9 On unenhanced CT, the classical appearance of acute intramural hematoma is crescentic hyperattenuating material in the media.9 If present, intimal calcifications appear displaced centrally from the outermost margin of the vessel because hemorrhage widens the media. Contrast-enhanced CT images reveal corresponding nonenhancing, crescentic wall thickening (Fig 6).9,10 Additionally, contrast-enhanced CT may reveal intramural blood pools or ulcer-like projections. An intramural blood pool is a contrast-filled area within the intramural hematoma that may have a tiny intimal orifice or connection with an intercostal or lumbar artery.11 Generally, intramural blood pools follow a relatively benign course, demonstrating either spontaneous resorption or stability in most patients.11 Ulcer-like projections, on the contrary, have a clearly visible, wider intimal orifice (4 3 mm) and have been associated with a poorer prognosis.11 The natural history of intramural hematoma varies and is unpredictable: it may stabilize, regress, or progress.9 In a small case series, 12 out of 36 patients with Type A intramural hematoma progressed to aortic dissection with the greatest risk occurring approximately 8-14 days after symptom onset.12 Generally, intramural hematoma is classified and managed in the same fashion as aortic dissection, whereby Type A intramural
Table 1 Clues for distinguishing true and false lumens in aortic dissection
Fig. 1. Normal aortic wall anatomy. The normal aortic wall comprises 3 layers: intima (innermost), media (middle), and adventitia (outermost). The media contains vasa vasorum, small vessels that supply oxygen and nutrients to the aortic wall. (Color version of figure is available online.)
True lumen
False lumen
Smaller
Larger Enhances slower
Enhances quicker
Usually contains thrombus Outer curve of aortic arch in Type A dissection
Surrounded by intimal calcifications, if present
Beak sign: false lumen insinuates around true lumen, forming acute margins Cobweb sign: linear or serpiginous, hypodense remnants in media
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Fig. 3. Type B aortic dissection arising just distal to the left subclavian artery in a 64-year-old woman. (A) Axial contrast-enhanced CT image shows a dissection flap arising in the aortic arch (arrow). Coronal (B) and 3D (C) reformatted CT images delineate the location of the dissection flap, arising just distal to the origin of the left subclavian artery (asterisk). The same patient developed a new Type A dissection 5 years later. (D) Axial contrast-enhanced CT image shows the new Type A dissection flap (arrowhead) alongside the old Type B flap (arrow). Hemopericardium was also present on the follow-up study (not shown). T ¼ true lumen; F ¼ false lumen. (Color version of figure is available online.)
Fig. 4. Type A aortic dissection in an adolescent male with Marfan syndrome. (A) Axial contrast-enhanced CT image shows a dissection flap in the ascending thoracic aorta (arrow). Hemopericardium is also partially visualized (asterisk). (B) Coronal contrast-enhanced CT image demonstrates annuloaortic ectasia, a finding present in 60%-80% of adults with Marfan syndrome.24 The dilated aortic root predisposes these patients to aortic valve insufficiency and subsequent dissection or rupture. The patient underwent emergent surgical repair. (C) Following surgical repair, including aortic valve replacement, the same patient later returned with a new Type B dissection (arrow) as shown on sagittal contrast-enhanced CT.
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Fig. 5. Type A aortic dissection with subsequent aneurysm and renal ischemia in a 72-year-old woman. (A) Initial axial contrast-enhanced CT image reveals a dissection flap involving the ascending and descending thoracic aorta consistent with Type A (arrows). The ascending thoracic aorta was surgically repaired. (B) Axial contrast-enhanced CT image performed 7 months later shows a new 5.3-cm descending thoracic aortic aneurysm (circle) superimposed on the old dissection flap. Aneurysms are a known complication of aortic dissection. End-organ damage is another important complication of dissection. (C) Axial contrast-enhanced CT image from the same patient shows a delayed right nephrogram (asterisk); the right renal artery is supplied by the false lumen, resulting in renal ischemia.
hematomas undergo surgery or endovascular repair and Type B cases are treated medically. Although classical intramural hematoma and aortic dissection can be distinguished, in actuality the imaging appearance of these entities may overlap (Fig 7). In particular, intramural hematoma and thrombosed aortic dissection may look similar; however, a few imaging clues can help. For instance, acute intramural hematoma tends to have higher density on unenhanced CT than that of thrombosed dissection. Additionally, intramural hematoma usually maintains a constant relationship with the longitudinal axis of the aortic wall, whereas dissection has a spiral configuration. Finally, the presence of intimal disruption favors dissection.
Penetrating Atherosclerotic Ulcer Penetrating atherosclerotic ulcer refers to a focal defect of the intima extending into the media (Fig 8). The intimal disruption results from atherosclerosis and enables pulsatile blood to enter the media, causing wall instability and possible hemorrhage. These ulcers occur in the descending thoracic aorta most frequently.13 At imaging, penetrating atherosclerotic ulcer may be difficult to distinguish from other entities, namely dissection, ulcerated plaque, and pseudoaneurysm. Classically, penetrating atherosclerotic ulcer is a focal, short-segment defect, whereas a longer segment
abnormality with 2 discrete lumens is termed dissection. In addition, penetrating atherosclerotic ulcers typically occur in elderly patients with severe atherosclerotic plaque, and aortic dissection is not commonly associated with atherosclerosis. Ulcerated plaque, on the contrary, is a defect of the arterial wall that is confined to the intima. As such, ulcerated plaque does not have the same risk of hemorrhage as penetrating atherosclerotic ulcer does, which extends through the intima into the media. Finally, pseudoaneurysm refers to disruption of arterial wall layers with frank bulging of the external vessel contour, whereas the external vessel contour is largely preserved in penetrating atherosclerotic ulcer. If untreated, penetrating atherosclerotic ulcers may progress to typical dissection, intramural hematoma, pseudoaneurysm, or rupture. Treatment remains controversial; however, symptomatic or complicated ulcers generally undergo percutaneous or surgical management, whereas asymptomatic, uncomplicated ulcers may be treated more conservatively.
Aortic Pseudoaneurysm In pseudoaneurysm, blood dissects through one or more layers of a damaged arterial wall, resulting in a perfused sac that communicates with the vessel lumen and results in bulging or enlargement of the external vessel contour. By definition, pseudoaneurysms represent a contained perforation held by an incomplete arterial wall; in some cases, the sac is encompassed only by adventitia or perivascular soft tissues.3,10 As such, pseudoaneurysms are at high risk for free rupture regardless of size. Pseudoaneuryms can result from any insult to the vessel wall. Common predisposing causes include atherosclerosis, penetrating atherosclerotic ulcer, trauma, iatrogenic injury, infection, and inflammation. At imaging, pseudoaneurysms typically have a narrow neck and arise eccentrically from the vessel lumen as a saccular protrusion with varying degrees of peripheral thrombus.3,10 Color Doppler ultrasound shows turbulent to-and-fro flow within the sac, the so-called ying-yang sign (Fig 9).
Aortic Aneurysm
Fig. 6. Subtle Type B intramural hematoma in a 71-year-old man. Axial unenhanced CT image demonstrates classical, albeit subtle, imaging findings of intramural hematoma in the descending thoracic aorta: crescentic intramural hyperdensity and displaced intimal calcifications (arrow).
Aneurysm is defined as permanent dilation of an artery, conventionally with at least 50% increase in diameter compared with normal. Unlike pseudoaneurysm, the dilated portion of the vessel has intact intima, media, and adventitial layers.3 Aortic dimensions demonstrate normal trends in the healthy, asymptomatic population. For example, the thoracic aorta is slightly
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Fig. 7. Large Type B intramural hematoma in an 81-year-old man. Axial (A) and sagittal (B) unenhanced CT images reveal extensive crescentic intramural hyperdensity in the descending thoracic aorta (arrows). Axial (C) and sagittal (D) contrast-enhanced CT images show corresponding nonenhancing, crescentic mural thickening (arrows). Intramural blood pools represent contrast-filled areas within the intramural hematoma (circle). As in this case, intramural hematoma, penetrating atherosclerotic ulcer, and dissection can have overlapping features and constitute a spectrum of pathology.
larger in males compared with females of the same age, and generally increases in size with age and body surface area for both genders.8,14 That said, simplified measurements are commonly used in daily practice for defining aortic enlargement: 4 cm or more for the ascending thoracic aorta, and 3 cm or more for the descending thoracic and abdominal aorta.3 The measurements should include the maximum diameter from external wall to external wall, perpendicular to the axis of luminal flow.8
Fig. 8. Penetrating atherosclerotic ulcer in an 84-year-old man. Axial contrastenhanced CT image shows a focal defect of the intima (arrow). Penetrating atherosclerotic ulcers most often arise in the descending thoracic aorta at the site of atherosclerotic plaque. As in this case, the appearances of penetrating atherosclerotic ulcer, ulcerated plaque, and pseudoaneurysm can be very similar.
Atherosclerosis causes most aortic aneurysms, typically in the descending thoracic and infrarenal abdominal aorta.3,13 Other common causes of aortic aneurysms include dissection, aortitis, and connective tissue disorders.3 Bicuspid aortic valve is also an independent risk factor for thoracic aortic aneurysms.8 Approximately 28% of patients with thoracic aortic aneurysms have a coexisting abdominal aortic aneurysm, so the entire course of the aorta should be interrogated when one is identified.3 Aortic aneurysm can lead to life-threatening conditions, namely dissection and rupture (Fig 10). Several signs of impending aortic rupture have been described, including the high-attenuation crescent sign, which represents fresh blood insinuating into mural thrombus before penetrating through the aortic wall; focal discontinuity of intimal calcification signifying wall weakening; and the so-called draped aorta sign, in which the posterior aortic wall follows the contour of the spine due to wall weakening or focal leak (Fig 11).3,9 Once ruptured, thoracic aortic aneurysms can cause hemomediastinum, hemopericardium, or hemothorax, and abdominal aortic aneurysms appear contiguous with hyperdense hemorrhage (Fig 12). To prevent rupture, patients with aortic aneurysms are managed with surveillance imaging and treated electively with endovascular or surgical repair according to general size thresholds: ascending thoracic aortic aneurysm 5.5 cm or greater; descending thoracic aortic aneurysm 5.5-6.5 cm or greater; any thoracic aortic aneurysm 4.0-5.0 cm or greater in high-risk patients, such as those with Marfan or Loeys-Dietz syndrome; abdominal aortic aneurysm 5.5 cm or greater; annual growth 0.5-1.0 cm or greater or
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Fig. 9. Abdominal aortic pseudoaneurysm in a 71-year-old man. (A) Axial unenhanced CT image shows dense atherosclerotic calcification in the abdominal aorta with a focal break anteriorly indicating intimal disruption. The intimal defect is contiguous with the pseudoaneurysm, delineated by a bulge of the external vessel contour (arrows). (B) Axial contrast-enhanced CT image again shows disruption of the aortic wall with contrast leaking from the vessel lumen into the pseudoaneurysm sac (arrow) through a narrow neck. Peripheral hypodense thrombus is seen. (C) Color Doppler ultrasound shows the narrow neck of the pseudoaneurysm (arrow) and the classical ying-yang appearance reflecting turbulent to-and-fro flow within the sac. Peripheral, hypoechoic thrombus is again noted. (Color version of figure is available online.)
Fig. 10. Ascending thoracic aortic aneurysm with subsequent dissection in a 68-year-old woman. (A) Axial unenhanced CT image shows a 6.3-cm ascending thoracic aortic aneurysm (arrow). The patient did not undergo repair and was lost to follow-up. Axial unenhanced (B) and contrast-enhanced (C) CT images performed 6 years later show development of a Type A dissection (arrows). Hemothorax represents rupture with active extravasation (asterisks).
Fig. 11. The high-attenuation crescent sign in a 58-year-old man. Axial unenhanced CT image shows an abdominal aortic aneurysm containing a hyperdense crescent (arrow). The hyperdense material represents acute hematoma and suggests impending rupture.
symptomatic aneurysms.3,8 Elective aneurysm repair before rupture has lower morbidity and mortality than emergent procedures do, but it is not without risk. As such, each patient much weigh the risk of the procedure against that of spontaneous rupture. There are a few subtypes of aortic aneurysm that are important to recognize to expedite appropriate management: inflammatory
and infected aneurysms. Inflammatory aneurysms most often arise in the infrarenal abdominal aorta and are characterized by a thickened aortic wall with dense fibrosis, typically with relative sparing of the posterior wall.15 The fibrosis often involves adjacent structures like bowel and ureter, resulting in secondary findings such as aortoenteric fistula or hydronephrosis (Fig 13).15,16 Inflammatory aneurysms may respond to corticosteroids or immunosuppressive medications, but surgery and endovascular treatment are required in some cases to prevent rupture and treat complications of periaortic fibrosis.16 Infected, or mycotic, aneurysms are an infectious break in an arterial wall with an irregular, saccular outpouching, overlapping the appearance of pseudoaneurysms (Fig 14).3 The presence of periaortic soft tissue mass, fluid, or fat stranding is an important clue to the diagnosis, and may be present before the vessel actually becomes enlarged.17,18 Periaortic gas and adjacent vertebral body changes are also helpful features, but occur infrequently.18 Infected aneurysms result from hematogenous spread via septic microemboli, contiguous involvement from an adjacent source, or by direct inoculation.3,15 The most common organisms include Streptococcus, Staphylococcus, Gonococcus, and Salmonella.3 Tuberculosis may also develop by direct spread from lymph nodes or the spine. Without early surgical intervention and antibiotics, infected aneurysms cause fulminant sepsis, aortic rupture, and death.3,17
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Fig. 12. Ruptured abdominal aortic aneurysms. (A and B) Axial unenhanced CT images from 2 patients depict infrarenal abdominal aortic aneurysms continuous with hyperdense retroperitoneal material (asterisks) reflecting aneurysm rupture. The first patient survived aortobiiliac stent graft repair, the second patient died shortly after the CT scan.
Fig. 13. Inflammatory abdominal aortic aneurysm in a 64-year-old man. (A) Axial contrast-enhanced CT image shows a mantle-like rind of soft tissue that encases the anterior aspect of an infrarenal abdominal aortic aneurysm (arrow). There is relative sparing of the posterior aortic wall, a typical finding of inflammatory aneurysms. (B) The fibrotic tissue compresses the proximal left ureter, resulting in left-sided hydronephrosis (asterisk) and a delayed left nephrogram (arrow).
Acute Aortic Thrombus
Chronic Aortic Occlusion
Acute aortic thrombus is uncommon, but may occur in patients with indwelling catheters, iatrogenic injury, slow-flow and hypercoagulable states, and in the setting of thromboemboli such as from a cardiac source or right-to-left shunt. Acute thrombus can cause peripheral emboli, in which patients might present with signs of extremity ischemia (Fig 15). Perhaps more concerning, however, are systemic and intracranial emboli that can result in devastating outcomes like bowel ischemia or stroke. Acute aortic thrombus requires immediate treatment with anticoagulation and thrombectomy to prevent such serious complications.19,20
Chronic aortic occlusion typically occurs in the setting of aortoiliac occlusive disease, also known as Leriche syndrome. Aortioiliac occlusive disease refers to complete aortic occlusion centered at the aortic bifurcation, most often from severe, longstanding atherosclerosis. Less common causes of aortic occlusion include emboli, surgical complication, or vasculitis.21 The classical Leriche syndrome has the triad of impotence, buttock claudication, and diminished femoral pulses.22 In chronic occlusion, collateral networks develop to supply the external iliac arteries and maintain perfusion to the lower extremities
Fig. 14. Infected abdominal aortic aneurysm. (A) Axial contrast-enhanced CT image demonstrates an irregular, saccular outpouching containing heterogeneous material arising from the abdominal aorta (arrow). (B) The aneurysm occluded the celiac artery, resulting in a splenic infarct (asterisk).
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Fig. 15. Acute aortic thrombus in a 53-year-old man presenting with right hand numbness. Axial (A) and sagittal oblique (B) contrast-enhanced CT images depict mural thrombus just proximal to the origin of the brachiocephalic artery (arrows). The thrombus sent emboli to the right arm, causing occlusion of the distal radial and ulnar arteries (not shown), accounting for the patient's numbness at presentation. (C) Emergent open thrombectomy confirms thrombus within the aortic lumen (arrow). (Color version of figure is available online.)
(Fig 16).21 These collateral pathways are important to describe to facilitate treatment planning and prevent clinicians from inadvertently interrupting them during a procedure.21 One of these collateral networks is termed the Winslow pathway, in which arterial flow reaches the external iliac arteries via the subclavian arteries: blood flows from the subclavian to internal mammary to superior epigastric to inferior epigastric to external iliac arteries.21,23 In a potential clinical situation, lower extremity perfusion would be compromised if a patient relies on the Winslow pathway and the internal mammary arteries are used for coronary artery bypass grafting.21,23
Aortic Vasculitis Large-vessel vasculitides include giant cell arteritis and Takayasu arteritis. Giant cell arteritis is the most common vasculitis affecting medium- and large-sized vessels. The external carotid artery branches are involved most frequently, but aortic involvement occurs in approximately 15% of cases.15 It usually afflicts white patients older than 50 years.15 Initially, the condition manifests with an inflammatory response including
multinucleated giant cells and lymphocytes that infiltrate the arterial wall. In later stages, progressive fibrosis is typical.15 Aortic involvement may present with annuloaortic ectasia, aneurysm, dissection, or aortic valve insufficiency.15 Takayasu arteritis, also known as pulseless disease, is an idiopathic inflammatory disorder that predominantly involves the aorta and its branches as well as the pulmonary arteries, typically arising in young adult women of Asian descent.3 The abdominal aorta is involved most commonly, followed by the descending aorta and arch.15 Aortic wall thickening is the main feature, reflecting media and adventitia inflammation with progressive intimal involvement.10 Fibrosis and calcification develop in later stages.3,15 Enhancement of the inflammatory material suggests active disease in patients with clinical and laboratory signs of active inflammation.10 Aortic and branch vessel disease can manifest with stenosis, occlusion, aneurysm, ulceration, or rupture, depending on the degree of inflammatory response and associated wall destruction (Fig 17).15 High-dose glucocorticoids are the mainstay for treatment of Takayasu arteritis.15 Other conditions can cause noninfectious aortitis, including rheumatoid arthritis, ankylosing spondylitis, relapsing polychondritis,
Fig. 16. Aortoiliac occlusive disease in an 84-year-old man. (A) Axial contrast-enhanced CT image shows a heavily calcified aorta with occlusion of the distal abdominal aorta at the bifurcation (arrow). (B) Coronal maximum-intensity projection illustrates an extensive collateral network that developed in this patient with chronic occlusion (arrows). The collaterals supply arterial flow to the external iliac arteries and maintain perfusion to the lower extremities.
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Fig. 17. Takayasu arteritis in a 52-year-old man. Axial unenhanced (A) and contrast-enhanced (B) CT images show circumferential soft tissue thickening and fat stranding around the aortic arch (arrows). The patient was not treated and returned 1 week later with worsening symptoms. (C) Repeat axial contrast-enhanced CT image shows increased soft tissue thickening and interval development of a pseudoaneurysm arising from the aortic arch between the brachiocephalic and left common carotid arteries (circle). Takayasu arteritis classically arises in young adult Asian women, making this case unusual. The differential diagnosis includes giant cell arteritis.
systemic lupus erythematosus, retroperitoneal fibrosis, and radiation.15 Each of these conditions can lead to an array of complications, including vessel stenosis, occlusion, aneurysm, or pseudoaneurysm. The imaging findings may be indistinguishable from the large-vessel vasculitides, but patient history can help narrow the differential diagnosis.
Mimics and Pitfalls A few anatomical variants occur in the thoracic aorta and are well-described mimics of aortic pathology. One common entity is a ductus diverticulum, or ductus bump, a neonatal remnant that appears as a bulge arising from the anterior undersurface of the aortic arch at the isthmus.3 A ductus diverticulum may mimic a posttraumatic aortic pseudoaneurysm, which most often occurs at the same location; however, a ductus diverticulum can be distinguished by its smooth margins, symmetrical shoulders, and obtuse angles.3 Pseudoaneurysms have irregular margins and form more acute angles. Another variant is an aortic spindle, which is a smooth, circumferential bulge below the isthmus in the proximal
descending aorta. A spindle may mimic an aneurysm, but again has smooth margins and always occurs in this portion of the aorta.3 Technical factors can also result in pitfalls, especially at CT. For instance, intravenous contrast can cause beam hardening artifacts that obscure or mimic a dissection flap. A way to decrease such artifacts in the aortic arch is to inject into the right upper extremity. A second pitfall is pulsation artifact from cardiac motion. This artifact can also mimic dissection flaps, especially around the aortic root, but can be mitigated with cardiac gating techniques.2,3
Conclusion Nontraumatic aortic diseases include a spectrum of conditions (Table 2). Many of these abnormalities are asymptomatic until an acute, often catastrophic, complication occurs. It is important for radiologists to recognize these entities and their complications to facilitate early intervention and reduce morbidity and mortality associated with aortic disease.
Table 2 Spectrum of nontraumatic aortic disease Entity
Key features
Entity
Key features
Aortic dissection Intimal tear that penetrates media, creating true (T) and false (F) lumens
Aneurysm
Dilation involving all 3 wall layers; 4 50% bigger diameter than normal
Intramural hematoma
Acute aortic Can cause emboli with devastating thrombus consequences, including to brain, bowel, kidneys, or extremities
Ruptured vasa vasorum in media or hemorrhage within atherosclerotic plaque
Schematic
Penetrating Focal defect at site of intimal atherosclerotic plaque atherosclerotic ulcer
Chronic aortic occlusion
Complete occlusion of aorta, typically centered at aortic bifurcation from atherosclerosis
Pseudoaneurysm Disrupted wall with extravasated blood contained by o 3 wall layers and periarterial connective tissue
Largevessel vasculitis
Idiopathic inflammatory disorders of large vessels
Schematic
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References 1. Macura KJ, Corl FM, Fishman EK, et al. Pathogenesis in acute aortic syndromes: Aortic dissection, intramural hematoma, and penetrating atherosclerotic aortic ulcer. Am J Roentgenol 2003;181:309–16. 2. McMahon MA, Squirrell CA. Multidetector CT of aortic dissection: A pictorial review. RadioGraphics 2010;30:445–60. 3. Agarwal PP, Chughtai A, Matzinger FRK, et al. Multidetector CT of thoracic aortic aneurysms. RadioGraphics 2009;29:537–52. 4. Williams DM, Joshi A, Dake MD, et al. Aortic cobwebs: An anatomic marker identifying the false lumen in aortic dissection—imaging and pathologic correlation. Radiology 1994;190(1):167–74. 5. LePage MA, Quint LE, Sonnad SS, et al. Aortic dissection: CT features that distinguish true lumen from false lumen. Am J Roentgenol 2001;177(1):207–11. 6. Daily PO, Trueblood HW, Stinson EB, et al. Management of acute aortic dissections. Ann Thorac Surg 1970;10(3):237–47. 7. Castaner E, Andreu M, Gallardo X, et al. CT in nontraumatic acute thoracic aortic disease: Typical and atypical features and complications. RadioGraphics 2003;23:S93–110. 8. Hiratzka LF, Bakris GL, Beckman JA, et al. ACCF/AHA/AATS/ACR/ASA/SCA/SCAI/SIR/ STS/SVM guidelines for the diagnosis and management of patients with thoracic aortic disease: Executive summary. J Am Coll Cardiol 2010;55:1509–44. 9. Chao CP, Walker TG, Kalva SP. Natural history and CT appearances of aortic intramural hematoma. RadioGraphics 2009;29:791–804. 10. Johnson PT, Horton KM, Fishman EK. Aortic valve and ascending thoracic aorta: evaluation with isotropic MDCT. Am J Roentgenol 2010;195:1072–81. 11. Wu MT, Wang YC, Huang YL, et al. Intramural blood pools accompanying aortic intramural hematoma: CT appearance and natural course. Radiology 2011;258 (3):705–13. 12. Estrera A, Miller C 3rd, Lee TY, et al. Acute Type A intramural hematoma: Analysis of current management strategy. Circulation 2009;120(suppl 11):S287–91.
13. Wadgaonkar AD, Black JH III, Weihe EK, et al. Abdominal aortic aneursyms revisited: MDCT with multiplanar reconstructions for identifying indicators of instability in the pre- and postoperative patient. RadioGraphics 2015;35 (1):254–68. 14. Wolak A, Gransar H, Thomson LEJ, et al. Aortic size assessment by noncontrast cardiac computed tomography: Normal limits by age, gender, and body surface area. J Am Coll Cardiol 2008;1:200–9. 15. Restrepo CS, Ocazionez D, Suri R, et al. Aortitis: Imaging spectrum of the infectious and inflammatory conditions of the aorta. RadioGraphics 2011;31: 435–51. 16. Ishizaka N, Sohmiya K, Miyamura M, et al. Infected aortic aneurysm and inflammatory aortic aneurysm—In search of an optimal differential diagnosis. J Cardiol 2012;59:123–31. 17. Lee WK, Mossop PJ, Little AF. Infected (mycotic) aneurysms: spectrum of imaging appearances and management. RadioGraphics 2008;28:1853–68. 18. Macedo TA, Stanson AW, Oderich GS, et al. Infected aortic aneurysms: imaging findings. Radiology 2004;231:250–7. 19. Kalangos A, Baldovinos A, Vuille C. Floating thrombus in the ascending aorta: A rare cause of peripheral emboli. J Vasc Surg 1997;26(1):150–4. 20. Sabetai MM, Conway AM, Hallward G, et al. Ascending aorta thrombus adjacent to a cholesterol-rich plaque as the source of multiple emboli. Interact Cardiovasc Thorac Surg 2013;16(3):389–90. 21. Hardman RL, Lopera JE, Cardan RA, et al. Common and rare collateral pathways in aortoilliac occlusive disease: A pictorial essay. Am J Roentgenol 2011;197: W519–W524. 22. Leriche R, Morel A. The syndrome of thrombotic obliteration of the aortic bifurcation. Ann Surg 1948;127:193–206. 23. Prager RJ, Akin JR, Akin GC, et al. Winslow's pathway: A rare collateral channel in infrarenal aortic occlusion. Am J Roentgenol 1977;128:485–7. 24. Ha HI, Seo JB, Lee SH, et al. Imaging of Marfan syndrome: Multisystemic manifestations. RadioGraphics 2007;27:989–1004.
Current Problems in Diagnostic Radiology journal homepage: www.cpdrjournal.com
Intimal Problems: A Pictorial Review of Nontraumatic Aortic Disease at Multidetector Computed Tomography Abigail V. Berniker, MDn, Justin E. Mackey, MD, Oleg M. Teytelboym, MD Department of Radiology, Mercy Catholic Medical Center, Darby, PA
Nontraumatic aortic diseases include a spectrum of disorders, many of which result in high morbidity and mortality. This article highlights the multidetector computed tomography appearance of common and uncommon nontraumatic aortic entities: dissection, intramural hematoma, penetrating atherosclerotic ulcer, pseudoaneurysm, aneurysm, acute thrombus, chronic occlusion, and vasculitis. Additionally, classical imaging mimics and pitfalls are addressed. Radiologists should feel confident identifying these conditions and providing accurate diagnoses to expedite patient care and prevent devastating, even fatal outcomes. & 2015 Mosby, Inc. All rights reserved.
Introduction The normal aortic wall comprises 3 layers: intima (innermost), media (middle), and adventitia (outermost).1 The media contains vasa vasorum, small vessels that supply oxygen and nutrients to the aortic wall (Fig 1). Disruption of this fundamental wall integrity results in a range of aortic pathology. For instance, an intimal defect results in penetrating atherosclerotic ulcer or dissection; irregularity of intact vessel layers is seen in aneurysm and vasculitis; and rupture of the vasa vasorum leads to intramural hematoma. Multidetector computed tomography (MDCT) is the mainstay of aortic imaging owing to its fast acquisition time, high temporal and spatial resolution, widespread availability, and low cost. Typical computed tomography (CT) protocols include noncontrast and arterial, bolus-triggered phases. The noncontrast phase elucidates aortic calcification, hyperdense surgical material, and intramural hematoma, whereas the angiographic phase better evaluates the lumen of the aorta and branch vessels. Some institutions employ a venous phase routinely, whereas others include it selectively, for example, to assess for endoleaks in patients with prior endovascular repair. Electrocardiographic gating helps improve image quality of the aortic root and coronary arteries, as cardiac motion artifacts can mimic dissection flaps and degrade evaluation of these structures. Dual-source dual-energy CT can generate virtual noncontrast images from a single acquisition and may one day replace multiphase imaging with MDCT; however, this technology is not yet widely available and validated. Magnetic resonance (MR) angiography is an alternative to CT for aortic evaluation and is well suited for routine surveillance of known aortic pathology in hemodynamically stable patients. MR offers several potential benefits over CT, including lack of ionizing
n Reprint requests: Abigail V. Berniker, MD, Department of Radiology, Mercy Catholic Medical Center, 1500 Lansdowne Ave, Darby, PA 19023. E-mail address: [email protected] (A.V. Berniker).
http://dx.doi.org/10.1067/j.cpradiol.2015.08.007 0363-0188/& 2015 Mosby, Inc. All rights reserved.
radiation and ability to provide accurate functional and blood flow information. Additionally, unenhanced MR can be performed effectively in patients with contraindications to iodinated contrast due to allergy or renal failure. This article reviews the spectrum of nontraumatic aortic diseases and underscores areas of both overlap and difference to help radiologists better recognize these entities.
Aortic Dissection Dissection occurs when an intimal tear enables blood to communicate abnormally with the media and propagate longitudinally along an artery.2,3 An intimal flap separates true and false lumens; blood within the intima is termed the true lumen, whereas the abnormally blood-filled media is the false lumen (Fig 2).2 Various imaging clues can help distinguish the true and false lumens. Typically, the true lumen is smaller, enhances faster, and is demarcated by intimal calcifications, if present. The false lumen, on the contrary, tends to be larger, enhances slower, contains thrombus, and comprises the outer curve of the aortic arch in a Type A dissection.2 Additionally, the false lumen is associated with 2 classical signs: the beak sign, whereby the false lumen insinuates around the true lumen forming an acute angle resembling a bird's beak, and the cobweb sign, which refers to linear or serpiginous, hypodense media remnants that mimic a spider's web (Table 1).4,5 The Stanford classification subdivides aortic dissections into two major types based on the most proximal extent and management implications.6,2 Type A dissections account for most of the cases (60%-70%) and involve the ascending aorta with possible extension into the descending aorta.2 Type B dissections (30%-40%) are limited to the descending or abdominal aorta.2 Technically, Type B dissections begin at or distal to the ligamentum arteriosum; however, the left subclavian artery is used as a conventional landmark at cross-sectional imaging owing to conspicuity (Fig 3).7
2
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Classification becomes controversial when the aortic arch is the most proximal site of involvement: in such cases, it may be most helpful to describe a Type B dissection with arch extension as many of these cases can be managed conservatively. Dissection classification dictates treatment: Type A dissections are treated surgically or with endovascular repair to prevent extension to the aortic root, pericardium, or coronary arteries.2 If untreated, Type A dissections have more than 50% mortality at 48 hours.2 Type B dissections, on the contrary, are treated medically with hypertension control unless end-organ ischemia or persistent pain requires surgical intervention.2,8 Additionally, Type B dissections undergo surgical or endovascular treatment in certain emergent, high-risk situations such as ruptured aorta or uncontrollable hemodynamic instability.2 Although originally described for aortic dissection, the Stanford classification is now extrapolated to penetrating atherosclerotic ulcer and intramural hematoma.8,9 Aortic dissection has been linked with multiple underlying risk factors. Processes that alter flow dynamics can predispose to dissection, with the intimal tear typically occurring at the site of maximum shear stress. The most common risk factor is hypertension, accounting for 60%-90% of cases.2 Other entities include underlying aortic root anomalies such as bicuspid aortic valve, arteritis, and connective tissue diseases like Loeys-Dietz or Marfan syndrome (Fig 4).2 Of note, aortic dissection is not typically associated with atherosclerotic disease. Along with identifying the presence of dissection, it is critical to define the full extent of the intimal flap and assess for complications. In Type A dissections, it is important to evaluate for involvement of the aortic root or coronary arteries, which puts the patient at risk for imminent rupture, aortic valve insufficiency, or myocardial ischemia. Additionally, it is essential to assess for signs of rupture, namely hemomediastinum, hemopericardium, hemothorax, and hemoperitoneum. Dissection flaps may also obstruct or extend into branch vessels, resulting in end-organ damage, and any such findings should be reported. Finally, the aorta should be interrogated carefully on follow-up studies as patients with aortic dissection are at increased risk of developing subsequent aortic pathology, including dissection or aneurysm (Fig 5).
Intramural Hematoma Intramural hematoma results from rupture of vasa vasorum and hemorrhage within the media.9 The hemorrhage weakens the
Fig. 2. True and false lumens of aortic dissection in a 69-year-old woman. Axial contrast-enhanced computed tomography (CT) image shows an intimal flap separating true (T) and false (F) lumens. The true lumen is smaller and enhances quicker, whereas the false lumen is larger and enhances slower. The false lumen also demonstrates the beak sign: it insinuates around the true lumen, forming acute angles (arrow). The dissection flap involves the ascending and descending thoracic aorta (Type A). Note the hyperdense material reflecting hemomediastinum (asterisk).
media and the integrity of the aortic wall, with possible progression to aortic dissection, ulcer-like projections, or aneurysm.1,10 As with dissection, hypertension is the most common predisposing factor.9 In contradistinction to dissection (as well as penetrating atherosclerotic ulcer), however, the intima remains intact in intramural hematoma.9 On unenhanced CT, the classical appearance of acute intramural hematoma is crescentic hyperattenuating material in the media.9 If present, intimal calcifications appear displaced centrally from the outermost margin of the vessel because hemorrhage widens the media. Contrast-enhanced CT images reveal corresponding nonenhancing, crescentic wall thickening (Fig 6).9,10 Additionally, contrast-enhanced CT may reveal intramural blood pools or ulcer-like projections. An intramural blood pool is a contrast-filled area within the intramural hematoma that may have a tiny intimal orifice or connection with an intercostal or lumbar artery.11 Generally, intramural blood pools follow a relatively benign course, demonstrating either spontaneous resorption or stability in most patients.11 Ulcer-like projections, on the contrary, have a clearly visible, wider intimal orifice (4 3 mm) and have been associated with a poorer prognosis.11 The natural history of intramural hematoma varies and is unpredictable: it may stabilize, regress, or progress.9 In a small case series, 12 out of 36 patients with Type A intramural hematoma progressed to aortic dissection with the greatest risk occurring approximately 8-14 days after symptom onset.12 Generally, intramural hematoma is classified and managed in the same fashion as aortic dissection, whereby Type A intramural
Table 1 Clues for distinguishing true and false lumens in aortic dissection
Fig. 1. Normal aortic wall anatomy. The normal aortic wall comprises 3 layers: intima (innermost), media (middle), and adventitia (outermost). The media contains vasa vasorum, small vessels that supply oxygen and nutrients to the aortic wall. (Color version of figure is available online.)
True lumen
False lumen
Smaller
Larger Enhances slower
Enhances quicker
Usually contains thrombus Outer curve of aortic arch in Type A dissection
Surrounded by intimal calcifications, if present
Beak sign: false lumen insinuates around true lumen, forming acute margins Cobweb sign: linear or serpiginous, hypodense remnants in media
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Fig. 3. Type B aortic dissection arising just distal to the left subclavian artery in a 64-year-old woman. (A) Axial contrast-enhanced CT image shows a dissection flap arising in the aortic arch (arrow). Coronal (B) and 3D (C) reformatted CT images delineate the location of the dissection flap, arising just distal to the origin of the left subclavian artery (asterisk). The same patient developed a new Type A dissection 5 years later. (D) Axial contrast-enhanced CT image shows the new Type A dissection flap (arrowhead) alongside the old Type B flap (arrow). Hemopericardium was also present on the follow-up study (not shown). T ¼ true lumen; F ¼ false lumen. (Color version of figure is available online.)
Fig. 4. Type A aortic dissection in an adolescent male with Marfan syndrome. (A) Axial contrast-enhanced CT image shows a dissection flap in the ascending thoracic aorta (arrow). Hemopericardium is also partially visualized (asterisk). (B) Coronal contrast-enhanced CT image demonstrates annuloaortic ectasia, a finding present in 60%-80% of adults with Marfan syndrome.24 The dilated aortic root predisposes these patients to aortic valve insufficiency and subsequent dissection or rupture. The patient underwent emergent surgical repair. (C) Following surgical repair, including aortic valve replacement, the same patient later returned with a new Type B dissection (arrow) as shown on sagittal contrast-enhanced CT.
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Fig. 5. Type A aortic dissection with subsequent aneurysm and renal ischemia in a 72-year-old woman. (A) Initial axial contrast-enhanced CT image reveals a dissection flap involving the ascending and descending thoracic aorta consistent with Type A (arrows). The ascending thoracic aorta was surgically repaired. (B) Axial contrast-enhanced CT image performed 7 months later shows a new 5.3-cm descending thoracic aortic aneurysm (circle) superimposed on the old dissection flap. Aneurysms are a known complication of aortic dissection. End-organ damage is another important complication of dissection. (C) Axial contrast-enhanced CT image from the same patient shows a delayed right nephrogram (asterisk); the right renal artery is supplied by the false lumen, resulting in renal ischemia.
hematomas undergo surgery or endovascular repair and Type B cases are treated medically. Although classical intramural hematoma and aortic dissection can be distinguished, in actuality the imaging appearance of these entities may overlap (Fig 7). In particular, intramural hematoma and thrombosed aortic dissection may look similar; however, a few imaging clues can help. For instance, acute intramural hematoma tends to have higher density on unenhanced CT than that of thrombosed dissection. Additionally, intramural hematoma usually maintains a constant relationship with the longitudinal axis of the aortic wall, whereas dissection has a spiral configuration. Finally, the presence of intimal disruption favors dissection.
Penetrating Atherosclerotic Ulcer Penetrating atherosclerotic ulcer refers to a focal defect of the intima extending into the media (Fig 8). The intimal disruption results from atherosclerosis and enables pulsatile blood to enter the media, causing wall instability and possible hemorrhage. These ulcers occur in the descending thoracic aorta most frequently.13 At imaging, penetrating atherosclerotic ulcer may be difficult to distinguish from other entities, namely dissection, ulcerated plaque, and pseudoaneurysm. Classically, penetrating atherosclerotic ulcer is a focal, short-segment defect, whereas a longer segment
abnormality with 2 discrete lumens is termed dissection. In addition, penetrating atherosclerotic ulcers typically occur in elderly patients with severe atherosclerotic plaque, and aortic dissection is not commonly associated with atherosclerosis. Ulcerated plaque, on the contrary, is a defect of the arterial wall that is confined to the intima. As such, ulcerated plaque does not have the same risk of hemorrhage as penetrating atherosclerotic ulcer does, which extends through the intima into the media. Finally, pseudoaneurysm refers to disruption of arterial wall layers with frank bulging of the external vessel contour, whereas the external vessel contour is largely preserved in penetrating atherosclerotic ulcer. If untreated, penetrating atherosclerotic ulcers may progress to typical dissection, intramural hematoma, pseudoaneurysm, or rupture. Treatment remains controversial; however, symptomatic or complicated ulcers generally undergo percutaneous or surgical management, whereas asymptomatic, uncomplicated ulcers may be treated more conservatively.
Aortic Pseudoaneurysm In pseudoaneurysm, blood dissects through one or more layers of a damaged arterial wall, resulting in a perfused sac that communicates with the vessel lumen and results in bulging or enlargement of the external vessel contour. By definition, pseudoaneurysms represent a contained perforation held by an incomplete arterial wall; in some cases, the sac is encompassed only by adventitia or perivascular soft tissues.3,10 As such, pseudoaneurysms are at high risk for free rupture regardless of size. Pseudoaneuryms can result from any insult to the vessel wall. Common predisposing causes include atherosclerosis, penetrating atherosclerotic ulcer, trauma, iatrogenic injury, infection, and inflammation. At imaging, pseudoaneurysms typically have a narrow neck and arise eccentrically from the vessel lumen as a saccular protrusion with varying degrees of peripheral thrombus.3,10 Color Doppler ultrasound shows turbulent to-and-fro flow within the sac, the so-called ying-yang sign (Fig 9).
Aortic Aneurysm
Fig. 6. Subtle Type B intramural hematoma in a 71-year-old man. Axial unenhanced CT image demonstrates classical, albeit subtle, imaging findings of intramural hematoma in the descending thoracic aorta: crescentic intramural hyperdensity and displaced intimal calcifications (arrow).
Aneurysm is defined as permanent dilation of an artery, conventionally with at least 50% increase in diameter compared with normal. Unlike pseudoaneurysm, the dilated portion of the vessel has intact intima, media, and adventitial layers.3 Aortic dimensions demonstrate normal trends in the healthy, asymptomatic population. For example, the thoracic aorta is slightly
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Fig. 7. Large Type B intramural hematoma in an 81-year-old man. Axial (A) and sagittal (B) unenhanced CT images reveal extensive crescentic intramural hyperdensity in the descending thoracic aorta (arrows). Axial (C) and sagittal (D) contrast-enhanced CT images show corresponding nonenhancing, crescentic mural thickening (arrows). Intramural blood pools represent contrast-filled areas within the intramural hematoma (circle). As in this case, intramural hematoma, penetrating atherosclerotic ulcer, and dissection can have overlapping features and constitute a spectrum of pathology.
larger in males compared with females of the same age, and generally increases in size with age and body surface area for both genders.8,14 That said, simplified measurements are commonly used in daily practice for defining aortic enlargement: 4 cm or more for the ascending thoracic aorta, and 3 cm or more for the descending thoracic and abdominal aorta.3 The measurements should include the maximum diameter from external wall to external wall, perpendicular to the axis of luminal flow.8
Fig. 8. Penetrating atherosclerotic ulcer in an 84-year-old man. Axial contrastenhanced CT image shows a focal defect of the intima (arrow). Penetrating atherosclerotic ulcers most often arise in the descending thoracic aorta at the site of atherosclerotic plaque. As in this case, the appearances of penetrating atherosclerotic ulcer, ulcerated plaque, and pseudoaneurysm can be very similar.
Atherosclerosis causes most aortic aneurysms, typically in the descending thoracic and infrarenal abdominal aorta.3,13 Other common causes of aortic aneurysms include dissection, aortitis, and connective tissue disorders.3 Bicuspid aortic valve is also an independent risk factor for thoracic aortic aneurysms.8 Approximately 28% of patients with thoracic aortic aneurysms have a coexisting abdominal aortic aneurysm, so the entire course of the aorta should be interrogated when one is identified.3 Aortic aneurysm can lead to life-threatening conditions, namely dissection and rupture (Fig 10). Several signs of impending aortic rupture have been described, including the high-attenuation crescent sign, which represents fresh blood insinuating into mural thrombus before penetrating through the aortic wall; focal discontinuity of intimal calcification signifying wall weakening; and the so-called draped aorta sign, in which the posterior aortic wall follows the contour of the spine due to wall weakening or focal leak (Fig 11).3,9 Once ruptured, thoracic aortic aneurysms can cause hemomediastinum, hemopericardium, or hemothorax, and abdominal aortic aneurysms appear contiguous with hyperdense hemorrhage (Fig 12). To prevent rupture, patients with aortic aneurysms are managed with surveillance imaging and treated electively with endovascular or surgical repair according to general size thresholds: ascending thoracic aortic aneurysm 5.5 cm or greater; descending thoracic aortic aneurysm 5.5-6.5 cm or greater; any thoracic aortic aneurysm 4.0-5.0 cm or greater in high-risk patients, such as those with Marfan or Loeys-Dietz syndrome; abdominal aortic aneurysm 5.5 cm or greater; annual growth 0.5-1.0 cm or greater or
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Fig. 9. Abdominal aortic pseudoaneurysm in a 71-year-old man. (A) Axial unenhanced CT image shows dense atherosclerotic calcification in the abdominal aorta with a focal break anteriorly indicating intimal disruption. The intimal defect is contiguous with the pseudoaneurysm, delineated by a bulge of the external vessel contour (arrows). (B) Axial contrast-enhanced CT image again shows disruption of the aortic wall with contrast leaking from the vessel lumen into the pseudoaneurysm sac (arrow) through a narrow neck. Peripheral hypodense thrombus is seen. (C) Color Doppler ultrasound shows the narrow neck of the pseudoaneurysm (arrow) and the classical ying-yang appearance reflecting turbulent to-and-fro flow within the sac. Peripheral, hypoechoic thrombus is again noted. (Color version of figure is available online.)
Fig. 10. Ascending thoracic aortic aneurysm with subsequent dissection in a 68-year-old woman. (A) Axial unenhanced CT image shows a 6.3-cm ascending thoracic aortic aneurysm (arrow). The patient did not undergo repair and was lost to follow-up. Axial unenhanced (B) and contrast-enhanced (C) CT images performed 6 years later show development of a Type A dissection (arrows). Hemothorax represents rupture with active extravasation (asterisks).
Fig. 11. The high-attenuation crescent sign in a 58-year-old man. Axial unenhanced CT image shows an abdominal aortic aneurysm containing a hyperdense crescent (arrow). The hyperdense material represents acute hematoma and suggests impending rupture.
symptomatic aneurysms.3,8 Elective aneurysm repair before rupture has lower morbidity and mortality than emergent procedures do, but it is not without risk. As such, each patient much weigh the risk of the procedure against that of spontaneous rupture. There are a few subtypes of aortic aneurysm that are important to recognize to expedite appropriate management: inflammatory
and infected aneurysms. Inflammatory aneurysms most often arise in the infrarenal abdominal aorta and are characterized by a thickened aortic wall with dense fibrosis, typically with relative sparing of the posterior wall.15 The fibrosis often involves adjacent structures like bowel and ureter, resulting in secondary findings such as aortoenteric fistula or hydronephrosis (Fig 13).15,16 Inflammatory aneurysms may respond to corticosteroids or immunosuppressive medications, but surgery and endovascular treatment are required in some cases to prevent rupture and treat complications of periaortic fibrosis.16 Infected, or mycotic, aneurysms are an infectious break in an arterial wall with an irregular, saccular outpouching, overlapping the appearance of pseudoaneurysms (Fig 14).3 The presence of periaortic soft tissue mass, fluid, or fat stranding is an important clue to the diagnosis, and may be present before the vessel actually becomes enlarged.17,18 Periaortic gas and adjacent vertebral body changes are also helpful features, but occur infrequently.18 Infected aneurysms result from hematogenous spread via septic microemboli, contiguous involvement from an adjacent source, or by direct inoculation.3,15 The most common organisms include Streptococcus, Staphylococcus, Gonococcus, and Salmonella.3 Tuberculosis may also develop by direct spread from lymph nodes or the spine. Without early surgical intervention and antibiotics, infected aneurysms cause fulminant sepsis, aortic rupture, and death.3,17
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Fig. 12. Ruptured abdominal aortic aneurysms. (A and B) Axial unenhanced CT images from 2 patients depict infrarenal abdominal aortic aneurysms continuous with hyperdense retroperitoneal material (asterisks) reflecting aneurysm rupture. The first patient survived aortobiiliac stent graft repair, the second patient died shortly after the CT scan.
Fig. 13. Inflammatory abdominal aortic aneurysm in a 64-year-old man. (A) Axial contrast-enhanced CT image shows a mantle-like rind of soft tissue that encases the anterior aspect of an infrarenal abdominal aortic aneurysm (arrow). There is relative sparing of the posterior aortic wall, a typical finding of inflammatory aneurysms. (B) The fibrotic tissue compresses the proximal left ureter, resulting in left-sided hydronephrosis (asterisk) and a delayed left nephrogram (arrow).
Acute Aortic Thrombus
Chronic Aortic Occlusion
Acute aortic thrombus is uncommon, but may occur in patients with indwelling catheters, iatrogenic injury, slow-flow and hypercoagulable states, and in the setting of thromboemboli such as from a cardiac source or right-to-left shunt. Acute thrombus can cause peripheral emboli, in which patients might present with signs of extremity ischemia (Fig 15). Perhaps more concerning, however, are systemic and intracranial emboli that can result in devastating outcomes like bowel ischemia or stroke. Acute aortic thrombus requires immediate treatment with anticoagulation and thrombectomy to prevent such serious complications.19,20
Chronic aortic occlusion typically occurs in the setting of aortoiliac occlusive disease, also known as Leriche syndrome. Aortioiliac occlusive disease refers to complete aortic occlusion centered at the aortic bifurcation, most often from severe, longstanding atherosclerosis. Less common causes of aortic occlusion include emboli, surgical complication, or vasculitis.21 The classical Leriche syndrome has the triad of impotence, buttock claudication, and diminished femoral pulses.22 In chronic occlusion, collateral networks develop to supply the external iliac arteries and maintain perfusion to the lower extremities
Fig. 14. Infected abdominal aortic aneurysm. (A) Axial contrast-enhanced CT image demonstrates an irregular, saccular outpouching containing heterogeneous material arising from the abdominal aorta (arrow). (B) The aneurysm occluded the celiac artery, resulting in a splenic infarct (asterisk).
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Fig. 15. Acute aortic thrombus in a 53-year-old man presenting with right hand numbness. Axial (A) and sagittal oblique (B) contrast-enhanced CT images depict mural thrombus just proximal to the origin of the brachiocephalic artery (arrows). The thrombus sent emboli to the right arm, causing occlusion of the distal radial and ulnar arteries (not shown), accounting for the patient's numbness at presentation. (C) Emergent open thrombectomy confirms thrombus within the aortic lumen (arrow). (Color version of figure is available online.)
(Fig 16).21 These collateral pathways are important to describe to facilitate treatment planning and prevent clinicians from inadvertently interrupting them during a procedure.21 One of these collateral networks is termed the Winslow pathway, in which arterial flow reaches the external iliac arteries via the subclavian arteries: blood flows from the subclavian to internal mammary to superior epigastric to inferior epigastric to external iliac arteries.21,23 In a potential clinical situation, lower extremity perfusion would be compromised if a patient relies on the Winslow pathway and the internal mammary arteries are used for coronary artery bypass grafting.21,23
Aortic Vasculitis Large-vessel vasculitides include giant cell arteritis and Takayasu arteritis. Giant cell arteritis is the most common vasculitis affecting medium- and large-sized vessels. The external carotid artery branches are involved most frequently, but aortic involvement occurs in approximately 15% of cases.15 It usually afflicts white patients older than 50 years.15 Initially, the condition manifests with an inflammatory response including
multinucleated giant cells and lymphocytes that infiltrate the arterial wall. In later stages, progressive fibrosis is typical.15 Aortic involvement may present with annuloaortic ectasia, aneurysm, dissection, or aortic valve insufficiency.15 Takayasu arteritis, also known as pulseless disease, is an idiopathic inflammatory disorder that predominantly involves the aorta and its branches as well as the pulmonary arteries, typically arising in young adult women of Asian descent.3 The abdominal aorta is involved most commonly, followed by the descending aorta and arch.15 Aortic wall thickening is the main feature, reflecting media and adventitia inflammation with progressive intimal involvement.10 Fibrosis and calcification develop in later stages.3,15 Enhancement of the inflammatory material suggests active disease in patients with clinical and laboratory signs of active inflammation.10 Aortic and branch vessel disease can manifest with stenosis, occlusion, aneurysm, ulceration, or rupture, depending on the degree of inflammatory response and associated wall destruction (Fig 17).15 High-dose glucocorticoids are the mainstay for treatment of Takayasu arteritis.15 Other conditions can cause noninfectious aortitis, including rheumatoid arthritis, ankylosing spondylitis, relapsing polychondritis,
Fig. 16. Aortoiliac occlusive disease in an 84-year-old man. (A) Axial contrast-enhanced CT image shows a heavily calcified aorta with occlusion of the distal abdominal aorta at the bifurcation (arrow). (B) Coronal maximum-intensity projection illustrates an extensive collateral network that developed in this patient with chronic occlusion (arrows). The collaterals supply arterial flow to the external iliac arteries and maintain perfusion to the lower extremities.
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Fig. 17. Takayasu arteritis in a 52-year-old man. Axial unenhanced (A) and contrast-enhanced (B) CT images show circumferential soft tissue thickening and fat stranding around the aortic arch (arrows). The patient was not treated and returned 1 week later with worsening symptoms. (C) Repeat axial contrast-enhanced CT image shows increased soft tissue thickening and interval development of a pseudoaneurysm arising from the aortic arch between the brachiocephalic and left common carotid arteries (circle). Takayasu arteritis classically arises in young adult Asian women, making this case unusual. The differential diagnosis includes giant cell arteritis.
systemic lupus erythematosus, retroperitoneal fibrosis, and radiation.15 Each of these conditions can lead to an array of complications, including vessel stenosis, occlusion, aneurysm, or pseudoaneurysm. The imaging findings may be indistinguishable from the large-vessel vasculitides, but patient history can help narrow the differential diagnosis.
Mimics and Pitfalls A few anatomical variants occur in the thoracic aorta and are well-described mimics of aortic pathology. One common entity is a ductus diverticulum, or ductus bump, a neonatal remnant that appears as a bulge arising from the anterior undersurface of the aortic arch at the isthmus.3 A ductus diverticulum may mimic a posttraumatic aortic pseudoaneurysm, which most often occurs at the same location; however, a ductus diverticulum can be distinguished by its smooth margins, symmetrical shoulders, and obtuse angles.3 Pseudoaneurysms have irregular margins and form more acute angles. Another variant is an aortic spindle, which is a smooth, circumferential bulge below the isthmus in the proximal
descending aorta. A spindle may mimic an aneurysm, but again has smooth margins and always occurs in this portion of the aorta.3 Technical factors can also result in pitfalls, especially at CT. For instance, intravenous contrast can cause beam hardening artifacts that obscure or mimic a dissection flap. A way to decrease such artifacts in the aortic arch is to inject into the right upper extremity. A second pitfall is pulsation artifact from cardiac motion. This artifact can also mimic dissection flaps, especially around the aortic root, but can be mitigated with cardiac gating techniques.2,3
Conclusion Nontraumatic aortic diseases include a spectrum of conditions (Table 2). Many of these abnormalities are asymptomatic until an acute, often catastrophic, complication occurs. It is important for radiologists to recognize these entities and their complications to facilitate early intervention and reduce morbidity and mortality associated with aortic disease.
Table 2 Spectrum of nontraumatic aortic disease Entity
Key features
Entity
Key features
Aortic dissection Intimal tear that penetrates media, creating true (T) and false (F) lumens
Aneurysm
Dilation involving all 3 wall layers; 4 50% bigger diameter than normal
Intramural hematoma
Acute aortic Can cause emboli with devastating thrombus consequences, including to brain, bowel, kidneys, or extremities
Ruptured vasa vasorum in media or hemorrhage within atherosclerotic plaque
Schematic
Penetrating Focal defect at site of intimal atherosclerotic plaque atherosclerotic ulcer
Chronic aortic occlusion
Complete occlusion of aorta, typically centered at aortic bifurcation from atherosclerosis
Pseudoaneurysm Disrupted wall with extravasated blood contained by o 3 wall layers and periarterial connective tissue
Largevessel vasculitis
Idiopathic inflammatory disorders of large vessels
Schematic
10
A.V. Berniker et al. / Current Problems in Diagnostic Radiology ] (2015) ]]]–]]]
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