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Plain-film evaluation of the thoracic aorta
Plain-Film Evaluation of the Thoracic Aorta Eric B. Friedberg and Lawrence M. Boxt
HE AORTA is the largest blood vessel in the body, serving as the origin of all systemic arteries. Like other organs in the body, it is subject to acquired aging changes as well as from congenital developmental malformation. The former diseases include atherosclerosis and aortic dissection. Atherosclerosis is the most common disease of the aorta. It is prevalent in all societies where the serum cholesterol level is greater than 200 mg/dL.1 Atherosclerosis and aortic dissection manifest themselves morphologically as dilatation of the aorta. The incidence of congenital aortic malformations in unknown because many patients may be asymptomatic, and therefore go undetected. However, many of these lesions may be lethal if prompt medical and surgical treatment is not instituted. Congenital malformations often manifest themselves radiographically in terms of decreased aortic caliber or as the consequences of the unusual course of the malformed aorta or its branches. In an era of computed tomography, magnetic resonance, and angiographic aortic imaging, the use of plain-film diagnosis has changed significantly, but nevertheless continues to play a valuable role in the detection of aortic disease. Radiographic changes in patients with aortic pathology are often readily apparent, but may also be quite subtle. This article provides an overview of acquired and congenital abnormalities that may involve the aorta and their various plain-film manifestations. Our emphasis will be the identification of general radiographic abnormalities, their differentiation from the normal, and the differential diagnosis of congenital or acquired diseases causing those findings.
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ANATOMY OF THE AORTA
The aorta is attached to the heart by the aortic annulus, which contains the aortic valve and occupies a position approximately in the center of the heart. It lies in an oblique plane, with blood ejected into the aorta directed superiorly, anteriorly, and to the patient's right. On posteroanterior (PA) examination, the annulus is just to the left of the midline, projected over the thoracic spine. In lateral examination, it resides nearly midway between the posterior border of the heart and the sternum, halfway up from the diaphragm to the beginning of the retroster-
nal clear space (Fig 1). Unless calcified, the annulus will not be radiographically apparent. The aortic valve contains three sinuses of Valsalva. The anterior sinus gives rise to the right coronary artery and is often referred to as the "right sinus of Valsalva." There are two posterior sinuses. The posterior left sinus contains the ostium of the left main coronary artery; it is referred to as the "left sinus of Valsalva." The posterior right sinus of Valsalva does not give rise to a coronary artery and is referred to as the "noncoronary sinus." There is a transition in caliber between the aortic sinuses and ascending aorta, the sinotubular junction. The origins of the coronary arteries are usually just below this junction. After leaving the heart, the aorta ascends behind the fight atrial appendage and then in front of the fight hilum, medial, and slightly anterior to the superior vena cava. The ascending aorta begins to course away from the anterior chest wall and back toward the left to form the aortic arch (Fig 2). The width of the ascending aorta is between 3 and 3.7 cm. It is surrounded by pericardium to approximately the level of the junction of the ascending aorta and arch. The aortic arch lies almost entirely behind the manubrium sternum. The innominate artery originates from the proximal arch and takes a slightly rightward course anterior to the trachea. The left common carotid artery originates to the left and slightly dorsal to the innominate, to the left of the trachea. The left subclavian artery originates from the dorsal aspect of the arch, posterior and to the left of the trachea. The normal left-sided aortic arch displaces the trachea to the right. This displacement is important to note when trying to distinguish between left-sided and right-sided aortic arches in patients with suspected congenital heart disease. Descending along the left lateral border of the spine, the aorta enters the abdomen at the left-sided
Seminars in Roentgenology, Vol XXXlV, No 3 (July), 1999: pp 181-194
From the Department of Radiology, Columbia University College of Physicians and Surgeons, New York; and the Division of Cardiovascular MRI, Department of Radiology, Beth Israel Medical Center, New York, NY Address reprint requests to Lawrence M. Boxt, MD, Division of Cardiovascular MRI, Department of Radiology, Beth Israel Medical Center, FirstAve and 16th St, New York, NY 10003. Copyright © 1999 by W.B. Saunders Company 0037-198X/99/3403-0005510.00/0 181
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Fig 1. Lateral chest film from a 48-year-old woman with valvular aortic stenosis is shown. The anterior aspect of the ascending aorta and superior aspect of the aortic arch (small arrows) are outlined by the air-filled lung. Note the calcified aortic valve (arrowheads). The left ventricle (large arrows) is not enlarged.
Fig 2. (A) Anteroposterior ICU chest film from a 6g-year-old man with congestive heart failure is shown. Extensive calcification of the ascending aorta (arrowheads), the aortic arch (Ao}, and descending aorta (short arrows) demonstrates the projected course of the thoracic aorta. There is mild tortuosity of the descending aorta. (Reprinted with permission. 4z) (B) Lateral chest film from a 58-year-old woman with emphysema. The hyperaerated lungs sharply outline the course of the aorta (small arrows) from its origin in the heart, around the arch to its descending portion.
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aortic hiatus, regardless of whether there is a fight-sided or left-sided aortic arch. The aorta only enters the abdomen through a fight-sided aortic hiatus in situs inversus. In general, the diameter of the aorta must be estimated on plain film because only the lateral border of the aorta may be visualized in the frontal projection. When the lateral border of the ascending aorta extends beyond the vascular pedicle of the right hilum or the shadow of the superior vena cava, abnormal enlargement of the ascending aorta should be considered. However, widening of the mediastinum in not uncommonly caused by mediasfinal masses. Careful examination of the film or awareness of a relevant medical history will direct the observer to consider alternative diagnoses (Fig 3). On the lateral film, observation of increased curvature in the middle third of the right heart border, or of a forward bulge in the anterior border of the aorta, would further confirm abnormal enlargement of the ascending aorta. The aortic arch diameter 2 is equal to the sum of the distances between the midline and right-most ascending aortic contour and left-most aortic shadow, measured from above the base of the heart.
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Fig 3. Mediastinal hematoma mimicking an aortic aneurysm is shown, (A) Anteroposterior chest film in a 50-year-old woman with a widened mediastinum. There is a sharply defined curvilinear density (small arrows) extending from the level of the right hilum to above the clavicle, apparently displacing the trachea toward the left. The left-sided aortic arch is obscurred. (B) Anteroposterior chest film obtained 3 days earlier shows a malplaced central venous catheter (small arrows). Multiple attempts at line placement resulted in vein perforation and mediastinal hemorrhage.
In normal individuals, the diameter of the aortic arch should not exceed 4 cm. However, the ascending aorta--for which there are no "normal" v a l u e s - i s most frequently enlarged in disease. The sinotubular junction (the junction between the aorta and the sinuses of valsalva) is a site of possible aneurysmal dilatation in individuals with Marfan's syndrome and other connective tissue diseases. Peripheral, discrete calcification of the aorta is usually secondary to degenerative intimal change, primarily related to atherosclerosis. Careful attention must be paid to the apparent distance between the outer wall of the aorta and intimal calcifications. When this distance is greater than 1 cm, a diagnosis of aortic dissection may be made. 3 However, it is important to consider the patient's position when evaluating the distance between the aortic intima and outer wall because slight differences in obliquity may have a significant effect on the appearance of the wall thickness between intimal calcifications and the outer aortic silhouette. CLASSIFICATION OF AORTIC DISEASES
Aortic disease may be diagnosed on plain-film examination by identification of changes in the aorta, which manifest themselves as changes in the heart borders formed by the aorta itself. Unless the abnormal portion of the aorta forms a portion of the
heart border, it will not be identified. Thus, other than direct demonstration of the presence of calcification, or presence of an abnormality great enough to significantly displace or alter the appearance of a heart border-forming structure, only changes in the aorta that are heart border-forming are identified on plain-film examination. A scheme for identification and classification of aortic disease on plain-film examination may be based on whether the contour of the ascending aorta, aortic arch, and proximal descending aorta (the heart border forming portions of the thoracic aorta) are altered, that is, increased or decreased in caliber. Using this general scheme, one may begin the process of differential diagnosis by correlation of the nature of the abnormality (increase or decrease in caliber) with the location of the abnormality (reflecting the portion of aorta that is diseased). CONDITIONS CAUSING INCREASED AORTIC ARCH CALIBER Aortic Dissection
Aortic dissection is most commonly caused by systemic hypertension. The likelihood of dissection is directly related to the severity of the systemic blood pressure. 4 Although only about 70% of patients with aortic dissection have elevated systemic blood pressure, aortic dissection is often their first serious illness. The dissecting channel, or false
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lumen, is nearly always in the outer half of the aortic wall. Thus, the outer aortic wall is very thin. The partition between true and false lumen is usually about three times as thick as the outer wall. Therefore, the outer wall frequently ruptures; extravasation of blood outside the aorta is more common than rerupture of the inner wall into the aortic true lumen. 5 Most aortic dissections originate in the descending aorta immediately distal to the origin of the left subclavian artery (aortic isthmus) (Fig 4) or approximately 1 cm superior to the sinotubular junction in the ascending aorta. When the dissection involves the aortic arch, approximately 80% to 90% of these patients will have an abnormal appearing aortic arch. 6 Abnormal chest radiograph findings suggestive of aortic dissection include an abnormally wide aortic arch just distal to the left subclavian artery, and intimal aortic calcifications separated by more than 6 mm 3,7 to 10 mm s4° from the outer contour of the arch shadow. It is important to recognize that calcifications within the ascending aorta may be projected medially to the outer contour of the distal arch, and that calcification of mural thrombus or inflammatory thickening of the aorta wall, may produce a falsepositive diagnosis. However, the most important factor in diagnosing aortic dissection remains a high index of suspicion, 3 especially in patients with
acute onset of severe chest pain associated with clinically significant asymmetric peripheral pulses or blood pressure measurements, because the plain film may appear entirely normal, u Any subtle or questionable finding becomes significant when a change in appearance from a previous examination is demonstrated.
Systemic Hypertension On the frontal chest film (Fig 5), an increase in the caliber of the ascending aorta may be secondary to aortic tortuosity or may represent a true increase in aortic caliber. Tortuosity of the aorta may be secondary to systemic hypertension or to longstanding atherosclerosis. On the lateral chest film, retrosternal filling may be the only clue to the presence of a dilated or tortuous aorta because the changes may not be readily apparent on the frontal examination if the predominant direction of the aortic curvature is anteriorly into the mediastinum.
Aortic Stenosis A true increase in aortic caliber (Fig 6) may be seen with valvar aortic stenosis, which can cause focal poststenotic dilation of the proximal ascending aorta. ~2 Aortic stenosis is most frequently secondary to degeneration of a congenital bicuspid valve (seen in 0.5 to 2% of the population). ~3
Ao
Fig 4. Acute aortic dissection in a 62-year-old hypertensive man complaining of acute onset of upper back pain is shown. (A) Posteroanterior chest film obtained immediately after surgery (note left upper rib fracture, small black arrow) for aortic dissection. The left ventricular portion of the left heart border (white arrows} is increased in curvature. The ascending aortic contour extends beyond the right hilum. The aortic arch {Ao) and proximal descending aorta (long arrows) are dilated, causing widening of the mediastinum. (B) Posteroanterior examination obtained 6 months earlier shows that the left ventricular portion of the left heart border as well as the ascending aorta have not changed in appearance since that time, However, the caliber of the aortic arch (Ao) and size of descending aorta (short arrows} were very much smaller at that time.
PLAIN-FILM EVALUATION OF THE THORACIC AORTA
Fig 5. A 62-year-old man with systemic hypertension is shown. Greater than half the perimeter of the aortic arch (Ao} is clearly identified. Although overall heart size is normal, the left ventricular portion of the left heart border is increased in caliber, resulting in a "left ventricular configuration." The contour of the ascending aorta (arrows) extends beyond the right hilum.
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individuals with rheumatic aortic valvular disease, the aortic valve cusps become infiltrated with fibrous tissue and retract, which prevents their closure and commonly leads to aortic regurgitation. 16 The same fibrotic process may also restrict opening of the aortic valve as a result of commissural fusion, thus causing mixed aortic stenosis and regurgitation. Infective endocarditis destroys the valve or leaves vegetations, which may interfere with proper coaptation of the cusps. In addition, trauma may cause the ascending aorta to tear. The aorta then may extend inferiorly to the valve plane, causing prolapse of the cusps from loss of commissural support. Aortic regurgitation also may be seen in approximately 7% to 20% of individuals with bicuspid aortic valves. 16 Due to the excessive length of the conjoined cusp, the free edge of this cusp lies lower than the smaller cusp. During diastole, the improper apposition of the cusps thus leads to aortic regurgitation. Aortoannular ectasia and Marfan's syndrome cause dilation of the aortic root (Fig 7). Marfan's syndrome is a generalized disorder of connective tissue involving skeletal, ocular, and cardiovascular systems. Cardiovascular manifestations include dilation, aneurysm, and dissection of the aorta from
Although less common, aortic stenosis may be secondary to rheumatic heart disease or to degeneration of a tricuspid aortic valve in patients over the age of 65 years. 14The abnormal valvar architecture induces turbulent flow, which ultimately damages the leaflets and causes fibrosis, calcification, and narrowing of the aortic orifice. Thus, calcification of the aortic valve in the presence of focal dilation of the ascending aorta is highly suggestive of aortic stenosis. If infective endocarditis develops on these valves, aortic regurgitation may occur. Combined aortic and mitral valvar calcifications are typically found in rheumatic heart disease.
Aortic Regurgitation Aortic regurgitation and aortic dissection may produce a more generalized dilation of the ascending aorta compared with aortic stenosis. Aortic regurgitation can be seen with primary disease of the aortic valve, with that of the aorta, or with both. Rheumatic heart disease had accounted for nearly 50% of cases of aortic regurgitation. 15 The disease is still common but is declining in frequency. In
Fig 6. Posteroanterior film of the woman with aortic stenosis in Figure 1 is shown. Although overall heart size is not increased, the contour of the left ventricular portion of the left heart border is increased in curvature. The ascending aorta (small arrows) extends over the right hilum. Notice that the portion of the perimeter of the aortic arch (Ao) visible in this figure is less than that visualized in Figure 5.
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Fig 7. A 4g-year-old man with exercise-induced dyspnea is shown. (A) Posteroanterior chest film shows enlargement of the left ventricular portion of the left heart border, but little evidence of increased curvature of the ascending aorta. (B) Anteroposterior aortogram shows dilatation of the ascending aorta (Ao). Opacification of the left ventricle (LV) indicates aortic regurgitation.
cystic medial necrosis (Fig 8). 17 Aortic regurgitation may result from aortic dilation or myxomatous degeneration of the aortic valve, is Cardiovascular disease is the most common cause of death in patients with the Marfan syndrome. Death is usually premature. 19,2°Roberts 19 personally examined
18 patients whom he divided into three groups: Group I included 12 of 18, with fusiform aneurysms of the ascending aorta; all 12 had aortic regurgitation, and 6 of 12 had associated mitral regurgitation. In each case, the ascending aortic aneurysm involved the sinus portion and proximal
Fig 8. A 32-year-old patient with Ntarfan's syndrome is shown. [A) Posteroanterior chest film obtained at the time of acute onset of tearing back pain. The aortic arch (Ao) and descending aorta (arrow) are dilated, causing widening of the superior mediastinum. (B) Previous posteroanterior chest film obtained 2 years earlier shows that the aortic arch and proximal descending aorta were abnormal, but the mediastinum was much narrower.
PLAIN-FILM EVALUATION OF THE THORACIC AORTA
tubular portion of the ascending aorta. Group 2 included 3 of 18 patients with dissection of the entire aorta. Before dissection, the ascending aorta was normal in caliber. Aortic regurgitation was seen in only one of these patients, after their dissection. Group 3 included 2 of 18 patients without apparent aortic disease and marked mitral regurgitation. Aortic regurgitation can initiate a vicious cycle. When the ascending aorta and aortic annulus become dilated, the aortic leaflets become stretched apart leading to aortic regurgitation. In turn, the aortic regurgitation causes the left ventricle to dilate secondary to an increased volume load. Left ventricular stroke volume is then increased as the left ventricle moves along the Frank-Starling curve. This increased stroke volume leads to further poststenotic dilatation of the aorta, and ultimately causes increased aortic regurgitation.
Atherosclerotic Aneurysm In general, a diffuse increase in the caliber of the aorta is commonly associated with atherosclerosis
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or systemic hypertension. These disease processes often cause the aorta to appear prominent but not necessarily dilated to pathological size. Focal dilation of the aorta may result from aortic dissection, post-traumatic pseudoaneurysm, 21 or focal atherosclerotic anenrysms. Historically, aneurysms of the thoracic aorta were mostly secondary to inflammatory disease, most notably syphilis (Fig 9)22,23 Today, atherosclerosis is the most common cause of thoracic aneurysms (Fig 10) 24,25 and tends to spare the ascending aorta. 26 Thus, most atherosclerotic thoracic aortic aneurysms involve the descending aorta (Fig 11). Approximately 50% of atherosclerotic aneurysms involve the aortic arch. 27 Generally, these aneurysms project into the left chest on frontal films and typically arise immediately distal to the origin of the left subclavian artery. They ordinarily appear as fusiform enlargement of the aortic contour. Usually focal, these aneurysms can elongate and extend into the abdominal aorta. The most common complication is acute rupture, which can be catastrophic. As a general rule, descending thoracic
Ao
Fig 9. A 54-year-old man with syphillis is shown. (A) Posteroanterior radiograph shows marked dilatation of the ascending aorta (arrows) and aortic arch (Ao). (B) Lateral view shows filling of the retrosternal clear space by the dilated ascending aorta. Typical intimal calcification (arrows) of inflammatory aortic disease outlines the course of the ascending aorta.
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Fig 10. Atherosclerotic thoracic aortic aneurysm is shown. (A) Posteroanterior film of a 74-year-old man without acute clinical complaints. The increased caliber of the aortic arch (Ao) and proximal descending aorta (arrows) cause widening of the mediastinum. (B) Same patient, lateral view. The normal course and curvature of the ascending aorta (short arrows) is in marked contrast to the profound widening and tortuous course of the descending aorta.
Fig 11, Atherosclerotic descending thoracic aortic aneurysm mimicking left atrial enlargement in a 54-year-old woman is shown. (A) Posteroanterior film shows a large double density superimposed on the middle of the heart, The aortic arch (Ao) and proximal descending aorta (small arrows) are dilated. (B) Lateral view shows the high, posterior location of the mass. The anterior border of the mass (arrows) is outlined by lung and displaces the carina (long arrow) anteriorly.
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aneurysms rupture into the adjacent left pleural space, whereas arch aneurysms tend to rupture into the contiguous mediastinum.
Pseudocoarctation of the Aorta Pseudocoarctation is the term used to describe a lesion of the aorta that may have the radiological appearance of a coarctation but does not possess a significant pressure gradient across the lesion. However, in coarctation of the aorta, the aortic arch is decreased in expected caliber (vida infra). Rather, in pseudocoarctation, the contour of the aortic arch is elongated (Fig 12) and appears to extend cephalad, above the clavicle into the pulmonary apex, and buckles anteriorly at the level of the aortic isthmus? 8 Because there is no pressure gradient across the aorta, collateral flow is not present and rib notching is not seen.
False Aneurysm of the Aorta A false aneurysm (pseudoaneurysm) is a focal aortic dilatation caused by perforation of the intima
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and media, creating an encapsulated periadventitial hematoma. False aneurysms are differentiated from true aneurysms by the absence of all layers of the aorta in the pseudoaneurysm wall. They are most commonly caused by trauma, but may be complications of surgery or catheter arteriography. Inflammatory insult to the aortic wall may result in a mycotic aneurysm. Such aneurysms are typically associated with a history of sepsis, septic embolism, or bacterial endocarditis. Traumatic aortic rupture is not uncommon. Aortic interruption in the supravalvular ascending aorta are almost immediately fatal. The most common location is in the aortic isthmus in the region of the ligamentum arteriosum (Fig 13). In a review of 125 cases of traumatic rupture of the aorta, 56 (45%) occurred in this area. Nearly half of these patients survived their initial insult. 29 Isthmus rupture is probably caused by horizontal decelleration, most commonly seen in automobile accidents. Among those cases that survive the initial traumatic episode, spontaneous healing may occur, resulting in
Fig 12. An asymptomatic 63-year-old man is shown. (A) Posteroanterior radiograph shows a sharply defined, peripherally calcified mass extending from the region of the aortic arch into the pulmonary apex (arrow). (B) Left anterior oblique view from an ascending aortogram. The catheter enters from the right subclavian artery and passes through a dilated innominate artery. The mid-aortic arch (Ao) is elongated. The distal arch (arrowheads) takes a hairpin turn before forming the descending aorta.
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Fig 13, This 26-year-old man was in a motor vehicle accident 2 years previously. (A) Posteroanterior radiograph shows a sharply defined soft-tissue density in the superior mediastinum. (B) Image from an anteroposterior ascending aortogram shows a Iobulated contrast containing pseudoaneurysm extrinsically compressing the proximal descending aorta. In this patient with a double aortic arch, the right subclavian (double small arrows) and right common carotid (double long arrows) arise from the superior right-sided arch (RAo) and the left common carotid (single long arrow) and left subclavian (single short arrow) arteries arise from the smaller, left-sided aortic arch (LAo).
late development of a saccular paeudoaneurysm. 3°,~ Such pseudoaneurysms may calcify or enlarge over time. CONDITIONS CAUSING DECREASED AORTIC ARCH CALIBER
Coarctation of the Aorta In patients with coarctation of the aorta, the aortic arch segment is focally decreased in caliber (Fig 14). The focal coarctation presents as a concave (to the right) notch in the proximal descending aorta, generally in the region of the aortic isthmus (the portion between the left subclavian artery and the ductus/ligamentum arteriosum). The descending aortic shadow may usually be followed from below, in a cephalad direction just to the left of the spine, continuous with the aortic arch shadow. In patients with coarctation, the shadow of
the descending aorta is interrupted caudal to its continuity with the aortic arch. In many individuals with coarctation, associated poststenotic dilatation of the aorta occurs just distal to this apparent interruption in the aortic shadow. It is this configuration that has been described as the "figure three sign." Rib notching is a reliable sign of retrograde collateral flow to the post-coarctation aorta by dilated intercostal arteries. Unilateral, left-sided rib notching suggests an anomalous right subclavian artery. Unilateral, right-sided rib notching is seen with coarctations that are proximal to the left subclavian artery. Coarctation of the aorta is frequently associated with bicuspid aortic valve, 32 predisposing them to aortic stenosis and bacterial endocarditis, mitral valve abnormalities, 33 and other aortic arch anomalies (A.L. Schaffer, K. Ellis, personal communication, 1993).
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Right Aortic Arch
Fig 14. A 25-year-old woman with differential pulses between upper and lower extremities. The left ventrieular portion of the lower left heart border is increased in curvature, The aortic arch (Ao) is inconspicuous. Left subclavian artery dilatation is seen as a sharply defined density ascending from the aortic arch (small arrows). Note the break in the shadow of the descending aorta (arrow) just inferior to the aortic arch at the site of the aortic narrowing. Irregular erosions (arrowheads) on the underside of the left and right fourth ribs (rib notching) are seen. (Reprinted with permission. 43)
Right-sided aortic arches may be classified 34-36 into anterior or posterior arches, depending on their relationship to the esophagus. The distinction between the two arches is important. Anterior arches are almost uniformly associated with congenital heart disease (Fig 15). Mirror-image branching is frequently seen in anterior right aortic arches and, in this instance, commonly associated with congenital heart disease. 37 The order of branching of the great vessels most commonly is as follows: left brachiocephalic artery, fight carotid artery, and right subclavian artery. On the lateral chest radiograph, there is no anterior bowing of the tracheal air column because the aorta is anterior to the trachea and esophagus. In anterior right aortic arches, the left subclavian artery is also anterior to the trachea and thus the ductus arteriosum, which connects the left subclavian artery and the left pulmonary artery; it does not form a vascular ring. The aortic arch is found to the right of the trachea, causing asymmetric widening of the superior mediastinum by rightward displacement of the superior vena cava. On lateral chest examination, the trachea is not displaced. Approximately 18% 38 to 34% 39 of anterior
Fig 15. A 32-year-old man with an isolated right aortic arch. (A) Posteroanterior examination shows the aortic arch (Ao) on the right displacing the tracheal air column toward the left, (B) Lateral examination shows no anterior displacement of the trachea (arrowheads). (Reprinted with permission. 43)
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right aortic arches are associated with tetralogy of Fallot, 33% 40 to 60% 37 are associated with truncus arteriosus and 4.9% 4t to 6.7% 37 are associated with D-transposition of the great arteries. Plain-film observations of the superior mediastinum, pulmonary artery segment, and pulmonary vasculature may be combined to differentiate these various lesions. For example, tetralogy of Fallot has a small pulmonary artery segment with decreased pulmonary vascularity. In D-transposition, the pulmonary artery segment is not a left heart border-forming structure but, instead, is posterior to the aorta causing a narrow mediastinal shadow on frontal radiographs and a wide mediastinal shadow on lateral chest radiograph. Furthermore, in D-transposition and persistent truncus arteriosus, the pulmonary artery segment is not found on the left heart border, and there is increased pulmonary vascularity. Posterior aortic arches are generally associated with a normal heart (Fig 16). Posterior arches are present in approximately 0.1% of the general population. 3%42 On the lateral chest radiograph,
Fig 16. A 78-year-old man without cardiovascular complaints is shown. (A) Posteroanterior chest radiograph shows the convexity of the right aortic arch (Ao) displacing the trachea to the left (arrows}. (B) In lateral view, the trachea is bowed anteriorly by the calcified (arrows) arch (Ao). (Reprinted with permission. 43}
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anterior bowing of the tracheal air column provides a useful clue that a posterior arch may be present. Patients with posterior, fight-sided aortic arches are commonly asymptomatic and have an aberrant left subclavian artery. If the ductus arteriosum connects this aberrant, retroesophageal left subclavian artery to the anterior left pulmonary artery, then a vascular ring is formed. These patients have a slight risk of developing symptomatic tracheal and esophageal compression if the ductus persists. Double Aortic Arch
Although the double aortic arches are rare (Fig 13), it is the most common form of symptomatic vascular ring. It results from persistence of the embryonic right and left aortic arches or their remnants. The ascending aorta generally divides into a larger and more superior fight arch, which circles to the right of the trachea and esophagus. This right arch then joins the smaller, or atretic, and more inferior left arch posterior to the trachea and esophagus. These arches then fuse to form a single descending aorta. The bilateral paratracheal masses
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s e e n o n t h e f r o n t a l c h e s t film m a y b e difficult to d i s t i n g u i s h f r o m a r i g h t - s i d e d aortic a r c h w i t h a n a b e r r a n t left s u b c l a v i a n artery a r i s i n g f r o m a large diverticulum. Bilateral and posterior compression o f t h e b a r i u m - f i l l e d e s o p h a g u s at t h e l e v e l o f t h e aortic a r c h e s is d i a g n o s t i c o f this c o n d i t i o n .
U n l i k e c o a r c t a t i o n o f the aorta, t h e r e is n o c o n n e c t i o n b e t w e e n t h e a s c e n d i n g a n d d e s c e n d i n g aorta. W h e n t h e c o n v e x s h a d o w o f the aortic arch lies to t h e r i g h t o f t h e t r a c h e a l air c o l u m n , d e v i a t i n g the t r a c h e a a n d e s o p h a g u s to t h e left at a p p r o x i m a t e l y the l e v e l o f t h e t h o r a c i c inlet, a r i g h t - s i d e d aortic arch is present. T h e d e s c e n d i n g t h o r a c i c aortic s i l h o u e t t e is p r e s e n t o n t h e r i g h t side o f t h e
Aortic Interruption A o r t i c a r c h i n t e r r u p t i o n is a rare c o n g e n i t a l a n o m a l y a n d p r i m a r i l y is d i a g n o s e d in n e o n a t e s .
m e d i a s t i n u m . I n f e r i o r l y in the thorax, a b o v e the d i a p h r a g m , t h e a o r t a c o u r s e s to t h e left to e n t e r t h e a b d o m e n v i a a n o r m a l l e f t - s i d e d aortic hiatus.
REFERENCES 1. Roberts WC: The aorta: Its acquired diseases and their consequences as viewed from a morphologic perspective, in Lindsay J Jr, Hurst JW (eds): The Aorta. New York, NY, Grune and Stratton, 1979, p 56 2. Baron MG: Radiologic notes in cardiology: Dissecting aneurysm of the aorta. Circulation 43:933-943, 1971 3. Crawford ES: The diagnosis and management of aortic dissection. JAMA 264:2537-2541, 1990 4. Roberts WC: The hypertensive diseases: Evidence that systemic hypertension is a greater risk factor to the development of cardiovascular diseases than previously suspected. Am J Med 59:523, 1975 5. Schlatmann TJM, Becker AE: Pathogenesis of dissecting aneuryms of the aorta. Am J Cardiol 39:21, 1977 6. Slater EE, DeSanctis RW: Clinical recognition of dissecting aortic aneurysm. Am J Med 60:625-633, 1976 7. Dow J, Roebuck EJ, Cole F: Dissecting aneurysms of the aorta. Br J Radiol 39:915-927, 1966 8. Earnest F IV, Muhm JR, Sheedy PF II: Roentgenographic findings in thoracic aortic dissection. Mayo Clinic Proc 54:4350, 1979 9. Eyler WR, Clark MD: Dissecting aneurysms of the aorta: Roentgen manifestations including a comparison with other types of aneurysms. Radiology 85:1047-1057, 1965 10. Beachley MC, Ranniger K, Roth FJ: Roentgenographic evaluation of dissecting aneurysms of the aorta. AJR Am J Roentgenol 121:617-625, 1974 11. Stein HL, Steinberg I: Selective aortography, the definitive technique for diagnosis of dissecting aneurysm of the aorta. AJR Am J Roentgenol 102:333-348, 1968 12. Jarchow BH, Kinkaid OW: Poststenotic dilation of the ascending aorta: Its occurrence and significance as a Roentgenologic sign of aortic stenosis. Mayo Clin Proc 36:23-33, 1961 13. Roberts WC: The congenitally bicuspid aortic valve: A study of 85 autopsied patients. Am J Cardio126:72-83, 1970 14. Morrow AG, Goldblatt A, Braunwald E: Congenital aortic stenosis II: Surgical treatment and results of operation. Circulation 27:426-432, 1963 15. Stapleton JF: Natural history of chronic valvular disease, in Frankl WS, Brest AN (eds): Cardiovascular Clinics, Valvular Heart Disease: Comprehensive Evaluation and Management. Philadelphia, FA Davis, 1986, pp 105-114 16. Waller BF: Rheumatic and nonrheumatic conditions producing valvular heart disease, in Frankl WS, Brest AN (eds):
Cardiovascular Clinics, Valvular Heart Disease: Comprehensive Evaluation and Management. Philadelphia, FA Davis, 1986, pp 30-31 17. Brown OR, DeMots H, Kloster FE, et al: Aortic root dilation and mitral valve prolapse in marfanfs Syndrome. Circulation 52:651-657, 1975 18. Read RC, Thai AE Wendt VE: Symptomatic valvular myxomatous transformation (the floppy valve syndrome). Circuiation 32:897-910, 1965 19. Roberts WC: Congenital cardiovascular abnormalities usually silent until adulthood: Morphological features of the floppy mitral valve, valvular aortic stenosis, discrete subvalvular aortic stenosis, hypertrophic cardiomyopathy, sinus of Valsalva aneurysm, and the Marfan syndrome, in Roberts WC (ed): Congenital Heart Disease in Adults. Philadelphia, PA, 1979, FA Davis, p 407 20. Murdoch JL, Walker BA, Halpern BL, et al: Life expectancy and causes of death in the Marfan syndrome. N Engl J Med 286:804, 1972 21. Wilkinson GA, Fraser RG: Roentgenography of the chest. Appl Radiol 4:41-49, 1975 22. Brindley P, Stembridge VA: Aneurysms of the aorta: A clinicopathotogic study of 369 necropsy cases. Am J Pathol 32:67-82, 1956 23. Fomon JJ, Kurtzweg FT, Broadway RK: Aneurysms of the aorta: A review. Ann Surg 165:557-563, 1967 24. DeBakey ME, Noon GP: Aneurysms of the thoracic aorta. Modern Concepts in Cardiovascular Disease 44:53-82, 1975 25. Dillon ML, Young WG, Sealy WC: Aneurysms of the descending thoracic aorta. Ann Thorac Surg 3:430-438, 1967 26. Joyce JW, Fairbairn JF II, Kinkald OW, et al: Aneurysms of the thoracic aorta. Circulation 29:176-181, 1964 27. Blakemore AH, Voorhees HB Jr: Aneurysms of the aorta. Angiology 5:209-224, 1954 28. Souders CR, Pearson CM, Adams HD: An aortic deformity simulating mediastinal tumors: A subclinical form of coarctation. Dis Chest 20:35-45, 1951 29. Parmley LF, Mattingly TW, Manion WC, et al: Nonpenetrating traumatic injury of the aorta. Circulation 17:1086-1101, 1958 30. Molnar W, Pace WG: Traumatic rupture of the thoracic aorta. Radiol Clin North Am 4:403-414, 1966
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31. MacImyre RS: Traumatic aneurysm of the thoracic aorta. AJRAm J Roentgenol 83:1011-1019, 1960 32. Tawes RL, Bert CL, Aberdern E: Congenital bicuspid aortic valves associated with coarctation of the aorta in children. Br Heart J 31:127-128, 1969 33. Rosenqnist GC: Congenital mitral valve disease associated with coarctation of the aorta. Circulation 49:985-993, 1974 34. Stewart JR, Kincaid OW, Tiros JL: Right aortic arch: Plain film diagnosis and significance. AJR Am J Roentgenol 97:377-389, 1966 35. Shuford WH, Sybors RG, Edwards FK: The three types of right aortic arch. AJR Am J Roentgenol 109:69-74, 1970 36. Felson B, Palayew MJ: Two types of right aortic arch. Radiology 81:745-759, 1963 37. Knight L, Edwards J: Right aortic arch: Types and associated cardiac anomalies. Circulation 50:1047-1051, 1974
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38. Keith JD, Rowe RD, Vlad P (eds): Heart Disease in Infancy and Childhood. New York, Macmillan, 1958 39. Hastreiter AR, D'Cruz IA, Cantez T: Right-sided aorta. Br Heart J 28:722-737, 1966 40. Tandon R, Hauck AJ, Nadas AS: Persistent trnncus arteriosus: A clinical, hemodynamic and autopsy study of nineteen cases. Circulation 28:1050-1060, 1963 41. Jue KL, Amplatz K, Adams P Jr, et al: Anomalies of great vessels associated with lung hypoplasia. Am J Dis Child 111:35-44, 1966 42. Liechty JD, Shields TW, Anson BJ: Variations pertaining to the aortic arches and their branches. Quarterly Bulletin of the Northwestern University Medical School 31:136-143, 1957 43. Boxt LM: Plain film examination of the chest, in Topol EJ (ed): Textbook of Cardiovascular Medicine. Philadelphia, Lippincott-Raven, 1998
HE AORTA is the largest blood vessel in the body, serving as the origin of all systemic arteries. Like other organs in the body, it is subject to acquired aging changes as well as from congenital developmental malformation. The former diseases include atherosclerosis and aortic dissection. Atherosclerosis is the most common disease of the aorta. It is prevalent in all societies where the serum cholesterol level is greater than 200 mg/dL.1 Atherosclerosis and aortic dissection manifest themselves morphologically as dilatation of the aorta. The incidence of congenital aortic malformations in unknown because many patients may be asymptomatic, and therefore go undetected. However, many of these lesions may be lethal if prompt medical and surgical treatment is not instituted. Congenital malformations often manifest themselves radiographically in terms of decreased aortic caliber or as the consequences of the unusual course of the malformed aorta or its branches. In an era of computed tomography, magnetic resonance, and angiographic aortic imaging, the use of plain-film diagnosis has changed significantly, but nevertheless continues to play a valuable role in the detection of aortic disease. Radiographic changes in patients with aortic pathology are often readily apparent, but may also be quite subtle. This article provides an overview of acquired and congenital abnormalities that may involve the aorta and their various plain-film manifestations. Our emphasis will be the identification of general radiographic abnormalities, their differentiation from the normal, and the differential diagnosis of congenital or acquired diseases causing those findings.
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The aorta is attached to the heart by the aortic annulus, which contains the aortic valve and occupies a position approximately in the center of the heart. It lies in an oblique plane, with blood ejected into the aorta directed superiorly, anteriorly, and to the patient's right. On posteroanterior (PA) examination, the annulus is just to the left of the midline, projected over the thoracic spine. In lateral examination, it resides nearly midway between the posterior border of the heart and the sternum, halfway up from the diaphragm to the beginning of the retroster-
nal clear space (Fig 1). Unless calcified, the annulus will not be radiographically apparent. The aortic valve contains three sinuses of Valsalva. The anterior sinus gives rise to the right coronary artery and is often referred to as the "right sinus of Valsalva." There are two posterior sinuses. The posterior left sinus contains the ostium of the left main coronary artery; it is referred to as the "left sinus of Valsalva." The posterior right sinus of Valsalva does not give rise to a coronary artery and is referred to as the "noncoronary sinus." There is a transition in caliber between the aortic sinuses and ascending aorta, the sinotubular junction. The origins of the coronary arteries are usually just below this junction. After leaving the heart, the aorta ascends behind the fight atrial appendage and then in front of the fight hilum, medial, and slightly anterior to the superior vena cava. The ascending aorta begins to course away from the anterior chest wall and back toward the left to form the aortic arch (Fig 2). The width of the ascending aorta is between 3 and 3.7 cm. It is surrounded by pericardium to approximately the level of the junction of the ascending aorta and arch. The aortic arch lies almost entirely behind the manubrium sternum. The innominate artery originates from the proximal arch and takes a slightly rightward course anterior to the trachea. The left common carotid artery originates to the left and slightly dorsal to the innominate, to the left of the trachea. The left subclavian artery originates from the dorsal aspect of the arch, posterior and to the left of the trachea. The normal left-sided aortic arch displaces the trachea to the right. This displacement is important to note when trying to distinguish between left-sided and right-sided aortic arches in patients with suspected congenital heart disease. Descending along the left lateral border of the spine, the aorta enters the abdomen at the left-sided
Seminars in Roentgenology, Vol XXXlV, No 3 (July), 1999: pp 181-194
From the Department of Radiology, Columbia University College of Physicians and Surgeons, New York; and the Division of Cardiovascular MRI, Department of Radiology, Beth Israel Medical Center, New York, NY Address reprint requests to Lawrence M. Boxt, MD, Division of Cardiovascular MRI, Department of Radiology, Beth Israel Medical Center, FirstAve and 16th St, New York, NY 10003. Copyright © 1999 by W.B. Saunders Company 0037-198X/99/3403-0005510.00/0 181
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Fig 1. Lateral chest film from a 48-year-old woman with valvular aortic stenosis is shown. The anterior aspect of the ascending aorta and superior aspect of the aortic arch (small arrows) are outlined by the air-filled lung. Note the calcified aortic valve (arrowheads). The left ventricle (large arrows) is not enlarged.
Fig 2. (A) Anteroposterior ICU chest film from a 6g-year-old man with congestive heart failure is shown. Extensive calcification of the ascending aorta (arrowheads), the aortic arch (Ao}, and descending aorta (short arrows) demonstrates the projected course of the thoracic aorta. There is mild tortuosity of the descending aorta. (Reprinted with permission. 4z) (B) Lateral chest film from a 58-year-old woman with emphysema. The hyperaerated lungs sharply outline the course of the aorta (small arrows) from its origin in the heart, around the arch to its descending portion.
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aortic hiatus, regardless of whether there is a fight-sided or left-sided aortic arch. The aorta only enters the abdomen through a fight-sided aortic hiatus in situs inversus. In general, the diameter of the aorta must be estimated on plain film because only the lateral border of the aorta may be visualized in the frontal projection. When the lateral border of the ascending aorta extends beyond the vascular pedicle of the right hilum or the shadow of the superior vena cava, abnormal enlargement of the ascending aorta should be considered. However, widening of the mediastinum in not uncommonly caused by mediasfinal masses. Careful examination of the film or awareness of a relevant medical history will direct the observer to consider alternative diagnoses (Fig 3). On the lateral film, observation of increased curvature in the middle third of the right heart border, or of a forward bulge in the anterior border of the aorta, would further confirm abnormal enlargement of the ascending aorta. The aortic arch diameter 2 is equal to the sum of the distances between the midline and right-most ascending aortic contour and left-most aortic shadow, measured from above the base of the heart.
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Fig 3. Mediastinal hematoma mimicking an aortic aneurysm is shown, (A) Anteroposterior chest film in a 50-year-old woman with a widened mediastinum. There is a sharply defined curvilinear density (small arrows) extending from the level of the right hilum to above the clavicle, apparently displacing the trachea toward the left. The left-sided aortic arch is obscurred. (B) Anteroposterior chest film obtained 3 days earlier shows a malplaced central venous catheter (small arrows). Multiple attempts at line placement resulted in vein perforation and mediastinal hemorrhage.
In normal individuals, the diameter of the aortic arch should not exceed 4 cm. However, the ascending aorta--for which there are no "normal" v a l u e s - i s most frequently enlarged in disease. The sinotubular junction (the junction between the aorta and the sinuses of valsalva) is a site of possible aneurysmal dilatation in individuals with Marfan's syndrome and other connective tissue diseases. Peripheral, discrete calcification of the aorta is usually secondary to degenerative intimal change, primarily related to atherosclerosis. Careful attention must be paid to the apparent distance between the outer wall of the aorta and intimal calcifications. When this distance is greater than 1 cm, a diagnosis of aortic dissection may be made. 3 However, it is important to consider the patient's position when evaluating the distance between the aortic intima and outer wall because slight differences in obliquity may have a significant effect on the appearance of the wall thickness between intimal calcifications and the outer aortic silhouette. CLASSIFICATION OF AORTIC DISEASES
Aortic disease may be diagnosed on plain-film examination by identification of changes in the aorta, which manifest themselves as changes in the heart borders formed by the aorta itself. Unless the abnormal portion of the aorta forms a portion of the
heart border, it will not be identified. Thus, other than direct demonstration of the presence of calcification, or presence of an abnormality great enough to significantly displace or alter the appearance of a heart border-forming structure, only changes in the aorta that are heart border-forming are identified on plain-film examination. A scheme for identification and classification of aortic disease on plain-film examination may be based on whether the contour of the ascending aorta, aortic arch, and proximal descending aorta (the heart border forming portions of the thoracic aorta) are altered, that is, increased or decreased in caliber. Using this general scheme, one may begin the process of differential diagnosis by correlation of the nature of the abnormality (increase or decrease in caliber) with the location of the abnormality (reflecting the portion of aorta that is diseased). CONDITIONS CAUSING INCREASED AORTIC ARCH CALIBER Aortic Dissection
Aortic dissection is most commonly caused by systemic hypertension. The likelihood of dissection is directly related to the severity of the systemic blood pressure. 4 Although only about 70% of patients with aortic dissection have elevated systemic blood pressure, aortic dissection is often their first serious illness. The dissecting channel, or false
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lumen, is nearly always in the outer half of the aortic wall. Thus, the outer aortic wall is very thin. The partition between true and false lumen is usually about three times as thick as the outer wall. Therefore, the outer wall frequently ruptures; extravasation of blood outside the aorta is more common than rerupture of the inner wall into the aortic true lumen. 5 Most aortic dissections originate in the descending aorta immediately distal to the origin of the left subclavian artery (aortic isthmus) (Fig 4) or approximately 1 cm superior to the sinotubular junction in the ascending aorta. When the dissection involves the aortic arch, approximately 80% to 90% of these patients will have an abnormal appearing aortic arch. 6 Abnormal chest radiograph findings suggestive of aortic dissection include an abnormally wide aortic arch just distal to the left subclavian artery, and intimal aortic calcifications separated by more than 6 mm 3,7 to 10 mm s4° from the outer contour of the arch shadow. It is important to recognize that calcifications within the ascending aorta may be projected medially to the outer contour of the distal arch, and that calcification of mural thrombus or inflammatory thickening of the aorta wall, may produce a falsepositive diagnosis. However, the most important factor in diagnosing aortic dissection remains a high index of suspicion, 3 especially in patients with
acute onset of severe chest pain associated with clinically significant asymmetric peripheral pulses or blood pressure measurements, because the plain film may appear entirely normal, u Any subtle or questionable finding becomes significant when a change in appearance from a previous examination is demonstrated.
Systemic Hypertension On the frontal chest film (Fig 5), an increase in the caliber of the ascending aorta may be secondary to aortic tortuosity or may represent a true increase in aortic caliber. Tortuosity of the aorta may be secondary to systemic hypertension or to longstanding atherosclerosis. On the lateral chest film, retrosternal filling may be the only clue to the presence of a dilated or tortuous aorta because the changes may not be readily apparent on the frontal examination if the predominant direction of the aortic curvature is anteriorly into the mediastinum.
Aortic Stenosis A true increase in aortic caliber (Fig 6) may be seen with valvar aortic stenosis, which can cause focal poststenotic dilation of the proximal ascending aorta. ~2 Aortic stenosis is most frequently secondary to degeneration of a congenital bicuspid valve (seen in 0.5 to 2% of the population). ~3
Ao
Fig 4. Acute aortic dissection in a 62-year-old hypertensive man complaining of acute onset of upper back pain is shown. (A) Posteroanterior chest film obtained immediately after surgery (note left upper rib fracture, small black arrow) for aortic dissection. The left ventricular portion of the left heart border (white arrows} is increased in curvature. The ascending aortic contour extends beyond the right hilum. The aortic arch {Ao) and proximal descending aorta (long arrows) are dilated, causing widening of the mediastinum. (B) Posteroanterior examination obtained 6 months earlier shows that the left ventricular portion of the left heart border as well as the ascending aorta have not changed in appearance since that time, However, the caliber of the aortic arch (Ao) and size of descending aorta (short arrows} were very much smaller at that time.
PLAIN-FILM EVALUATION OF THE THORACIC AORTA
Fig 5. A 62-year-old man with systemic hypertension is shown. Greater than half the perimeter of the aortic arch (Ao} is clearly identified. Although overall heart size is normal, the left ventricular portion of the left heart border is increased in caliber, resulting in a "left ventricular configuration." The contour of the ascending aorta (arrows) extends beyond the right hilum.
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individuals with rheumatic aortic valvular disease, the aortic valve cusps become infiltrated with fibrous tissue and retract, which prevents their closure and commonly leads to aortic regurgitation. 16 The same fibrotic process may also restrict opening of the aortic valve as a result of commissural fusion, thus causing mixed aortic stenosis and regurgitation. Infective endocarditis destroys the valve or leaves vegetations, which may interfere with proper coaptation of the cusps. In addition, trauma may cause the ascending aorta to tear. The aorta then may extend inferiorly to the valve plane, causing prolapse of the cusps from loss of commissural support. Aortic regurgitation also may be seen in approximately 7% to 20% of individuals with bicuspid aortic valves. 16 Due to the excessive length of the conjoined cusp, the free edge of this cusp lies lower than the smaller cusp. During diastole, the improper apposition of the cusps thus leads to aortic regurgitation. Aortoannular ectasia and Marfan's syndrome cause dilation of the aortic root (Fig 7). Marfan's syndrome is a generalized disorder of connective tissue involving skeletal, ocular, and cardiovascular systems. Cardiovascular manifestations include dilation, aneurysm, and dissection of the aorta from
Although less common, aortic stenosis may be secondary to rheumatic heart disease or to degeneration of a tricuspid aortic valve in patients over the age of 65 years. 14The abnormal valvar architecture induces turbulent flow, which ultimately damages the leaflets and causes fibrosis, calcification, and narrowing of the aortic orifice. Thus, calcification of the aortic valve in the presence of focal dilation of the ascending aorta is highly suggestive of aortic stenosis. If infective endocarditis develops on these valves, aortic regurgitation may occur. Combined aortic and mitral valvar calcifications are typically found in rheumatic heart disease.
Aortic Regurgitation Aortic regurgitation and aortic dissection may produce a more generalized dilation of the ascending aorta compared with aortic stenosis. Aortic regurgitation can be seen with primary disease of the aortic valve, with that of the aorta, or with both. Rheumatic heart disease had accounted for nearly 50% of cases of aortic regurgitation. 15 The disease is still common but is declining in frequency. In
Fig 6. Posteroanterior film of the woman with aortic stenosis in Figure 1 is shown. Although overall heart size is not increased, the contour of the left ventricular portion of the left heart border is increased in curvature. The ascending aorta (small arrows) extends over the right hilum. Notice that the portion of the perimeter of the aortic arch (Ao) visible in this figure is less than that visualized in Figure 5.
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Fig 7. A 4g-year-old man with exercise-induced dyspnea is shown. (A) Posteroanterior chest film shows enlargement of the left ventricular portion of the left heart border, but little evidence of increased curvature of the ascending aorta. (B) Anteroposterior aortogram shows dilatation of the ascending aorta (Ao). Opacification of the left ventricle (LV) indicates aortic regurgitation.
cystic medial necrosis (Fig 8). 17 Aortic regurgitation may result from aortic dilation or myxomatous degeneration of the aortic valve, is Cardiovascular disease is the most common cause of death in patients with the Marfan syndrome. Death is usually premature. 19,2°Roberts 19 personally examined
18 patients whom he divided into three groups: Group I included 12 of 18, with fusiform aneurysms of the ascending aorta; all 12 had aortic regurgitation, and 6 of 12 had associated mitral regurgitation. In each case, the ascending aortic aneurysm involved the sinus portion and proximal
Fig 8. A 32-year-old patient with Ntarfan's syndrome is shown. [A) Posteroanterior chest film obtained at the time of acute onset of tearing back pain. The aortic arch (Ao) and descending aorta (arrow) are dilated, causing widening of the superior mediastinum. (B) Previous posteroanterior chest film obtained 2 years earlier shows that the aortic arch and proximal descending aorta were abnormal, but the mediastinum was much narrower.
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tubular portion of the ascending aorta. Group 2 included 3 of 18 patients with dissection of the entire aorta. Before dissection, the ascending aorta was normal in caliber. Aortic regurgitation was seen in only one of these patients, after their dissection. Group 3 included 2 of 18 patients without apparent aortic disease and marked mitral regurgitation. Aortic regurgitation can initiate a vicious cycle. When the ascending aorta and aortic annulus become dilated, the aortic leaflets become stretched apart leading to aortic regurgitation. In turn, the aortic regurgitation causes the left ventricle to dilate secondary to an increased volume load. Left ventricular stroke volume is then increased as the left ventricle moves along the Frank-Starling curve. This increased stroke volume leads to further poststenotic dilatation of the aorta, and ultimately causes increased aortic regurgitation.
Atherosclerotic Aneurysm In general, a diffuse increase in the caliber of the aorta is commonly associated with atherosclerosis
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or systemic hypertension. These disease processes often cause the aorta to appear prominent but not necessarily dilated to pathological size. Focal dilation of the aorta may result from aortic dissection, post-traumatic pseudoaneurysm, 21 or focal atherosclerotic anenrysms. Historically, aneurysms of the thoracic aorta were mostly secondary to inflammatory disease, most notably syphilis (Fig 9)22,23 Today, atherosclerosis is the most common cause of thoracic aneurysms (Fig 10) 24,25 and tends to spare the ascending aorta. 26 Thus, most atherosclerotic thoracic aortic aneurysms involve the descending aorta (Fig 11). Approximately 50% of atherosclerotic aneurysms involve the aortic arch. 27 Generally, these aneurysms project into the left chest on frontal films and typically arise immediately distal to the origin of the left subclavian artery. They ordinarily appear as fusiform enlargement of the aortic contour. Usually focal, these aneurysms can elongate and extend into the abdominal aorta. The most common complication is acute rupture, which can be catastrophic. As a general rule, descending thoracic
Ao
Fig 9. A 54-year-old man with syphillis is shown. (A) Posteroanterior radiograph shows marked dilatation of the ascending aorta (arrows) and aortic arch (Ao). (B) Lateral view shows filling of the retrosternal clear space by the dilated ascending aorta. Typical intimal calcification (arrows) of inflammatory aortic disease outlines the course of the ascending aorta.
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Fig 10. Atherosclerotic thoracic aortic aneurysm is shown. (A) Posteroanterior film of a 74-year-old man without acute clinical complaints. The increased caliber of the aortic arch (Ao) and proximal descending aorta (arrows) cause widening of the mediastinum. (B) Same patient, lateral view. The normal course and curvature of the ascending aorta (short arrows) is in marked contrast to the profound widening and tortuous course of the descending aorta.
Fig 11, Atherosclerotic descending thoracic aortic aneurysm mimicking left atrial enlargement in a 54-year-old woman is shown. (A) Posteroanterior film shows a large double density superimposed on the middle of the heart, The aortic arch (Ao) and proximal descending aorta (small arrows) are dilated. (B) Lateral view shows the high, posterior location of the mass. The anterior border of the mass (arrows) is outlined by lung and displaces the carina (long arrow) anteriorly.
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aneurysms rupture into the adjacent left pleural space, whereas arch aneurysms tend to rupture into the contiguous mediastinum.
Pseudocoarctation of the Aorta Pseudocoarctation is the term used to describe a lesion of the aorta that may have the radiological appearance of a coarctation but does not possess a significant pressure gradient across the lesion. However, in coarctation of the aorta, the aortic arch is decreased in expected caliber (vida infra). Rather, in pseudocoarctation, the contour of the aortic arch is elongated (Fig 12) and appears to extend cephalad, above the clavicle into the pulmonary apex, and buckles anteriorly at the level of the aortic isthmus? 8 Because there is no pressure gradient across the aorta, collateral flow is not present and rib notching is not seen.
False Aneurysm of the Aorta A false aneurysm (pseudoaneurysm) is a focal aortic dilatation caused by perforation of the intima
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and media, creating an encapsulated periadventitial hematoma. False aneurysms are differentiated from true aneurysms by the absence of all layers of the aorta in the pseudoaneurysm wall. They are most commonly caused by trauma, but may be complications of surgery or catheter arteriography. Inflammatory insult to the aortic wall may result in a mycotic aneurysm. Such aneurysms are typically associated with a history of sepsis, septic embolism, or bacterial endocarditis. Traumatic aortic rupture is not uncommon. Aortic interruption in the supravalvular ascending aorta are almost immediately fatal. The most common location is in the aortic isthmus in the region of the ligamentum arteriosum (Fig 13). In a review of 125 cases of traumatic rupture of the aorta, 56 (45%) occurred in this area. Nearly half of these patients survived their initial insult. 29 Isthmus rupture is probably caused by horizontal decelleration, most commonly seen in automobile accidents. Among those cases that survive the initial traumatic episode, spontaneous healing may occur, resulting in
Fig 12. An asymptomatic 63-year-old man is shown. (A) Posteroanterior radiograph shows a sharply defined, peripherally calcified mass extending from the region of the aortic arch into the pulmonary apex (arrow). (B) Left anterior oblique view from an ascending aortogram. The catheter enters from the right subclavian artery and passes through a dilated innominate artery. The mid-aortic arch (Ao) is elongated. The distal arch (arrowheads) takes a hairpin turn before forming the descending aorta.
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Fig 13, This 26-year-old man was in a motor vehicle accident 2 years previously. (A) Posteroanterior radiograph shows a sharply defined soft-tissue density in the superior mediastinum. (B) Image from an anteroposterior ascending aortogram shows a Iobulated contrast containing pseudoaneurysm extrinsically compressing the proximal descending aorta. In this patient with a double aortic arch, the right subclavian (double small arrows) and right common carotid (double long arrows) arise from the superior right-sided arch (RAo) and the left common carotid (single long arrow) and left subclavian (single short arrow) arteries arise from the smaller, left-sided aortic arch (LAo).
late development of a saccular paeudoaneurysm. 3°,~ Such pseudoaneurysms may calcify or enlarge over time. CONDITIONS CAUSING DECREASED AORTIC ARCH CALIBER
Coarctation of the Aorta In patients with coarctation of the aorta, the aortic arch segment is focally decreased in caliber (Fig 14). The focal coarctation presents as a concave (to the right) notch in the proximal descending aorta, generally in the region of the aortic isthmus (the portion between the left subclavian artery and the ductus/ligamentum arteriosum). The descending aortic shadow may usually be followed from below, in a cephalad direction just to the left of the spine, continuous with the aortic arch shadow. In patients with coarctation, the shadow of
the descending aorta is interrupted caudal to its continuity with the aortic arch. In many individuals with coarctation, associated poststenotic dilatation of the aorta occurs just distal to this apparent interruption in the aortic shadow. It is this configuration that has been described as the "figure three sign." Rib notching is a reliable sign of retrograde collateral flow to the post-coarctation aorta by dilated intercostal arteries. Unilateral, left-sided rib notching suggests an anomalous right subclavian artery. Unilateral, right-sided rib notching is seen with coarctations that are proximal to the left subclavian artery. Coarctation of the aorta is frequently associated with bicuspid aortic valve, 32 predisposing them to aortic stenosis and bacterial endocarditis, mitral valve abnormalities, 33 and other aortic arch anomalies (A.L. Schaffer, K. Ellis, personal communication, 1993).
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Right Aortic Arch
Fig 14. A 25-year-old woman with differential pulses between upper and lower extremities. The left ventrieular portion of the lower left heart border is increased in curvature, The aortic arch (Ao) is inconspicuous. Left subclavian artery dilatation is seen as a sharply defined density ascending from the aortic arch (small arrows). Note the break in the shadow of the descending aorta (arrow) just inferior to the aortic arch at the site of the aortic narrowing. Irregular erosions (arrowheads) on the underside of the left and right fourth ribs (rib notching) are seen. (Reprinted with permission. 43)
Right-sided aortic arches may be classified 34-36 into anterior or posterior arches, depending on their relationship to the esophagus. The distinction between the two arches is important. Anterior arches are almost uniformly associated with congenital heart disease (Fig 15). Mirror-image branching is frequently seen in anterior right aortic arches and, in this instance, commonly associated with congenital heart disease. 37 The order of branching of the great vessels most commonly is as follows: left brachiocephalic artery, fight carotid artery, and right subclavian artery. On the lateral chest radiograph, there is no anterior bowing of the tracheal air column because the aorta is anterior to the trachea and esophagus. In anterior right aortic arches, the left subclavian artery is also anterior to the trachea and thus the ductus arteriosum, which connects the left subclavian artery and the left pulmonary artery; it does not form a vascular ring. The aortic arch is found to the right of the trachea, causing asymmetric widening of the superior mediastinum by rightward displacement of the superior vena cava. On lateral chest examination, the trachea is not displaced. Approximately 18% 38 to 34% 39 of anterior
Fig 15. A 32-year-old man with an isolated right aortic arch. (A) Posteroanterior examination shows the aortic arch (Ao) on the right displacing the tracheal air column toward the left, (B) Lateral examination shows no anterior displacement of the trachea (arrowheads). (Reprinted with permission. 43)
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right aortic arches are associated with tetralogy of Fallot, 33% 40 to 60% 37 are associated with truncus arteriosus and 4.9% 4t to 6.7% 37 are associated with D-transposition of the great arteries. Plain-film observations of the superior mediastinum, pulmonary artery segment, and pulmonary vasculature may be combined to differentiate these various lesions. For example, tetralogy of Fallot has a small pulmonary artery segment with decreased pulmonary vascularity. In D-transposition, the pulmonary artery segment is not a left heart border-forming structure but, instead, is posterior to the aorta causing a narrow mediastinal shadow on frontal radiographs and a wide mediastinal shadow on lateral chest radiograph. Furthermore, in D-transposition and persistent truncus arteriosus, the pulmonary artery segment is not found on the left heart border, and there is increased pulmonary vascularity. Posterior aortic arches are generally associated with a normal heart (Fig 16). Posterior arches are present in approximately 0.1% of the general population. 3%42 On the lateral chest radiograph,
Fig 16. A 78-year-old man without cardiovascular complaints is shown. (A) Posteroanterior chest radiograph shows the convexity of the right aortic arch (Ao) displacing the trachea to the left (arrows}. (B) In lateral view, the trachea is bowed anteriorly by the calcified (arrows) arch (Ao). (Reprinted with permission. 43}
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anterior bowing of the tracheal air column provides a useful clue that a posterior arch may be present. Patients with posterior, fight-sided aortic arches are commonly asymptomatic and have an aberrant left subclavian artery. If the ductus arteriosum connects this aberrant, retroesophageal left subclavian artery to the anterior left pulmonary artery, then a vascular ring is formed. These patients have a slight risk of developing symptomatic tracheal and esophageal compression if the ductus persists. Double Aortic Arch
Although the double aortic arches are rare (Fig 13), it is the most common form of symptomatic vascular ring. It results from persistence of the embryonic right and left aortic arches or their remnants. The ascending aorta generally divides into a larger and more superior fight arch, which circles to the right of the trachea and esophagus. This right arch then joins the smaller, or atretic, and more inferior left arch posterior to the trachea and esophagus. These arches then fuse to form a single descending aorta. The bilateral paratracheal masses
PLAIN-FILM EVALUATION OF THE THORACIC AORTA
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s e e n o n t h e f r o n t a l c h e s t film m a y b e difficult to d i s t i n g u i s h f r o m a r i g h t - s i d e d aortic a r c h w i t h a n a b e r r a n t left s u b c l a v i a n artery a r i s i n g f r o m a large diverticulum. Bilateral and posterior compression o f t h e b a r i u m - f i l l e d e s o p h a g u s at t h e l e v e l o f t h e aortic a r c h e s is d i a g n o s t i c o f this c o n d i t i o n .
U n l i k e c o a r c t a t i o n o f the aorta, t h e r e is n o c o n n e c t i o n b e t w e e n t h e a s c e n d i n g a n d d e s c e n d i n g aorta. W h e n t h e c o n v e x s h a d o w o f the aortic arch lies to t h e r i g h t o f t h e t r a c h e a l air c o l u m n , d e v i a t i n g the t r a c h e a a n d e s o p h a g u s to t h e left at a p p r o x i m a t e l y the l e v e l o f t h e t h o r a c i c inlet, a r i g h t - s i d e d aortic arch is present. T h e d e s c e n d i n g t h o r a c i c aortic s i l h o u e t t e is p r e s e n t o n t h e r i g h t side o f t h e
Aortic Interruption A o r t i c a r c h i n t e r r u p t i o n is a rare c o n g e n i t a l a n o m a l y a n d p r i m a r i l y is d i a g n o s e d in n e o n a t e s .
m e d i a s t i n u m . I n f e r i o r l y in the thorax, a b o v e the d i a p h r a g m , t h e a o r t a c o u r s e s to t h e left to e n t e r t h e a b d o m e n v i a a n o r m a l l e f t - s i d e d aortic hiatus.
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