Chapter30 Aorta

Thoracic Aorta

Chapter 30 n

KATHLEEN JACOBS, MICHAEL J. HOROWITZ, and SETH KLIGERMAN

Aortic Root Anatomy and Variants Aortic Valve Ascending Aorta Aortic Arch Anatomy and Variants Left Arch Variants Right Arch Variants Double Aortic Arch Cervical Aortic Arch Interrupted Aortic Arch Circumflex Aorta Descending Thoracic Aorta Aortic Coarctation Pseudocoarctation Atheroma Aneurysm Sinus of Valsalva Aneurysm

Ascending Thoracic Aorta and Aortic Arch Aneurysms Descending Thoracic Aortic Aneurysms Acute Aortic Syndrome Aortic Dissection Intramural Hematoma Penetrating Atherosclerotic Ulcer Aortic Pseudoaneurysm Aortic Fistulas Acute Traumatic Aortic Injury Postoperative Aorta Complications Thoracic Endovascular Aortic Repair Aortitis Aortic Tumors Conclusion

The thoracic aorta is a tubular, candy-cane–shaped structure that connects the left ventricle to the systemic circulation. It extends from the level of the aortic valve to the diaphragmatic hiatus where it transitions to the abdominal aorta, approxi- mately at the level of T12. The thoracic aorta is anatomically divided into the aortic root, ascending aorta, transverse arch, and descending thoracic aorta (Fig. 30.1).

posteriorly between the right and left atria. Above the sinuses of Valsalva is the sinotubular junction, which is an anatomic waist between the sinuses of Valsalva and tubular ascending aorta. Dimensions of the aorta vary with age, gender, and body size in adults. The aorta is generally largest in diameter at the sinuses of Valsalva and progressively tapers distally. Reported normal diameter of the aortic root is 3.5 to 3.72 cm in females and 3.63 to 3.91 cm in males on CT, measured orthogonal to the aorta. Aortic Valve The aortic valve serves as both a physical and hemodynamic boundary between the left ventricle and aorta. The normal aor- tic valve is composed of three leaflets/cusps which insert into the annular ring and coapt and form a trileaflet valve plane just inferior to the sinuses of Valsalva (Fig. 30.2). The contact points between valve leaflets are termed the valve commissures, which are best visualized during end diastole when the aortic valve is closed. Congenital anomalies of the aortic valve are not uncom- mon and include unicuspid, bicuspid, or quadricuspid valve morphologies. Bicuspid aortic valve is the most common con- genital cardiovascular anomaly with a prevalence of 0.5% to 2%. There are two major morphologic types of bicuspid aortic valve. A true bicuspid valve with completely separate and sym- metric valves without a fused raphe is less frequent, compris- ing approximately 7% of bicuspid aortic valves (Fig. 30.3).

Aortic Root Anatomy and Variants

The aortic root extends from the aortic annular ring to the sinotubular junction. The aortic valve annulus is a fibrous oval ring where the leaflets of the aortic valve attach and extend superiorly toward the sinuses of Valsalva. The aortic annulus is coupled to the mitral annulus via aortomitral fibrous tissue, which is a defining feature of the left ventricle. This is in con- tradistinction to the pulmonary valve, which is supported by the muscular right ventricular outflow tract. Superior to the annulus are the sinuses of Valsalva which are three anatomic bulges of the aorta (Fig. 30.2). The three leaflets of the aortic valve form the valve plane at the level of the sinuses. The coronary artery ostia arise from the sinuses of Valsalva above the valve plane but below the sinotubular junction. The sinuses are named based on their respective cor- onary artery. The right coronary artery (RCA) arises from the right sinus of Valsalva which is directed anteriorly, and the left main coronary artery arises from the leftward facing left sinus of Valsalva. The noncoronary sinus is usually directed

763

764

Section V: Cardiac Radiology

aneurysm formation are also associated with a bicuspid aortic valve. Due to the increased risk of rupture compared to the general population, guidelines recommend the repair of these aneurysms when they measure between 4.5 and 5 cm diame- ter versus 5.5 cm in the general population. Another common association with a bicuspid aortic valve is aortic coarctation, which is discussed below. Unicuspid aortic valve has a reported incidence of 0.02%. Unicuspid aortic valve is defined by a single opening/commissure (i.e., unicommissural) usually in the left poste- rior position and has similar associations as bicuspid aortic valve (Fig. 30.4). Quadricuspid aortic valve has a clover-leaf morphology, is extremely rare, and more typically associ- ated with early-onset regurgitation as opposed to stenosis (Fig. 30.5). Ascending Aorta The ascending aorta extends from the sinotubular junction to the origin of the right brachiocephalic artery (Fig. 30.1). The normal ascending aorta arises posterior and to the right of the main pulmonary artery (Fig. 30.6). CT or MR evaluation of the aortic root and ascending aorta should use ECG gating to minimize cardiac motion artifact. It is important to reduce cardiac motion for multi- ple reasons including improved visualization of the valve/ root anatomy, accurate measurement of the aorta in assess- ment for aneurysm, and to prevent false-positive diagnosis of aortic dissection. Gating technique (prospective vs. retrospec- tive) will also vary depending on the indication of the study. For instance, evaluation of valvular function or dysfunction requires retrospective gating, while basic anatomic evaluation can be performed with prospective gating.

Left Common Carotid

Left Subclavian

Branchio - cephalic

Ascending Aorta

Sinuses of Valsalva

Aortic Root

Descending Aorta

Figure 30.1.  3D Volume-Rendered Reformat of the Thoracic Aorta. Aortic root extends from the aortic valve annulus ( dashed line ) to the sinotubular junction ( solid line ) to the origin of the brachiocephalic artery. Normal three-vessel branching pattern of the aortic arch.

In 93% of cases, there is visible fusion between two leaflets or cusps. The fusion point between them is termed a raphe and appears as a dysmorphic, partially formed commissure below the valve plane. Of bicuspid valves with a raphe, fusion between the right and left coronary cusps is most common (70%) (Fig. 30.3), followed by fusion of the right and non- coronary cusps (28%), and fusion of the left and noncoronary cusps (1.4%). Early development of aortic stenosis is a common com- plication in patients with a bicuspid aortic valve secondary to myxoid degeneration. This occurs in patients from 30 to 50 years, in contrast to senile aortic valve degeneration which occurs in patients 80 to 90 years old. Aortopathy and

Aortic Arch Anatomy and Variants

The aortic arch is a transverse segment from which the great vessels arise. The normal aortic arch is left sided and courses

Aorta

RA

R

L N

RA

LV

RV

LA

A

B

Figure 30.2.  Coronal view through the aortic root ( A ), left coronary artery ostium indicated by black arrow . Orthogonal cross section through the sinuses of Valsalva ( black line ) produces a transverse view of the right ( R ), left ( L ), and noncoronary ( N ) cusps ( B ). Note that the normal non- coronary cusp is directed toward the interatrial septum between the left atrium ( LA ) and right atrium ( RA ). RV, right ventricle; LV, left ventricle.

765

Chapter 30: Thoracic Aorta

B

Figure 30.3.  MR GRE image through the aortic valve during systole in a 23 year-old woman with Turners syndrome shows a true bicuspid aortic valve with two separate leaflets and a cen- tral fish mouth ( arrow , A ). There is no fused raphe. CT image through the aortic valve in a 51-year-old man with shortness of breath shows a thickened and partially calcified bicuspid aortic valve ( red arrow ) with fusion of the right and left coronary cusps. Calcified and thickened soft tissue is present inferior to the valve plane ( white arrow ) indicating the fused raphe ( B ). Oblique coro- nal MR GRE in this patient demonstrates flow acceleration across bicuspid aortic valve ( red arrow , C ).

C

A

B

Figure 30.4.  Unicuspid Aortic Valve. CT image transverse to the aortic valve demonstrates a single, eccentric opening/commissure ( arrow , A ), indicating an unicuspid valve. There is an associated aneurysmal dilation of the ascending aorta measuring up to 5.4 cm on coronal-oblique reformat ( double-headed arrow, B ).

766

Section V: Cardiac Radiology

*

*

1 2

RA

4 3

LA

Figure 30.7.  Subclavian Steal. Coronal MIP CT demonstrates narrowing of the proximal left subclavian artery secondary to large noncalcified atherosclerotic plaque ( arrow ). Normal origins of the vertebral arteries (*) from the ipsilateral subclavian arteries. Patient presented with diminished left upper extremity pulses.

Figure 30.5.  Quadricuspid Aortic Valve. Gradient echo MR image, transverse view of the aortic valve demonstrates the clover-leaf mor- phology of a quadricuspid valve ( arrow ) with four valve leaflets (1–4). LA, left atrium; RA, right atrium.

in subclavian steal syndrome if there is hemodynamically sig- nificant obstruction. Findings on cross-sectional imaging of proximal subclavian narrowing with suggestive clinical his- tory including symptoms of limb ischemia, differential arm pressures, or vertebrobasilar insufficiency should prompt con- cern for subclavian steal syndrome (Fig. 30.7). The aortic isthmus is a physiologic narrowing of the aortic arch between the left subclavian artery origin and ligamentum arteriosum, the embryologic remnant of the ductus arteriosus (Fig. 30.8). Focal prominence of the aorta at the ligamentum arteriosum is a normal variant, termed a “ductus diverticu- lum” or “ductus bump” and should not be confused for an aneurysm or pseudoaneurysm. Distal to the ligamentum arte- riosum, the aorta continues as the descending thoracic aorta

above the pulmonary arteries toward the left (Fig. 30.6). Usually the aortic arch gives rise to three great vessels, occurring in 74% to 80% of the population (Fig. 30.1). The first vessel to branch from the aorta is the right brachioce- phalic or innominate artery which bifurcates into the right common carotid artery and subclavian artery a few centime- ters from its origin. The left common carotid artery and left subclavian artery are the second and third vessels to branch from the aortic arch, respectively. The vertebral arteries normally arise from the proximal aspect of the subclavian arteries. Narrowing of the subcla- vian arteries proximal to the origin of the vertebral arteries, whether degenerative, inflammatory, or iatrogenic, can result

Trachea

Ao

PA

A.Ao

PA

D.Ao

A

B

Figure 30.6.  Normal Ascending Aorta and Left Arch. Axial CT image demonstrates ascending aorta ( A. Ao ) is located slightly posterior and to the right of the main pulmonary artery ( PA ) ( A ). Descending thoracic aorta ( D. Ao ) is to the left of the spine. PA chest radiograph demonstrates a normal left-sided aortic arch to the left of the trachea ( arrow ) and spine, positioned above the main pulmonary artery contour ( B ).

767

Chapter 30: Thoracic Aorta

L common carotid

*

L subclavian

*

Brachiocephalic

Figure 30.9.  Two-Vessel Arch. 3D volume-rendered image of the aortic arch shows common origin ( * ) of the brachiocephalic artery and left common carotid artery.

Figure 30.8.  Sagittal MIP CT demonstrates focal narrowing of the distal aortic arch corresponding to the aortic isthmus ( black arrow ), just distal to the left subclavian artery takeoff (*). Note the focal prom- inence of the aorta ( dashed line ) at the origin of the ligamentum arte- riosum ( white arrow ) which extends toward the left pulmonary artery. This is often referred to as a “ductus bump” or “ductus diverticulum.”

diverticulum of Kommerell and vascular ring, an aberrant right subclavian artery is typically asymptomatic, but about 10% of patients can have dysphagia (aka “dysphagia luso- ria”) secondary to extrinsic compression of the esophagus. Right Arch Variants Right aortic arch has a prevalence of 0.05%. On frontal pro- jection chest radiograph, the normal indentation of the left aortic arch on the left aspect of the trachea is absent, replaced

abutting the left aspect of the thoracic spine and becoming the abdominal aorta at the diaphragmatic hiatus.

Left Arch Variants Left aortic arch variants are common incidental findings usu- ally of little clinical significance. Two-vessel aortic arch is characterized by common origin of the right brachiocephalic and left common carotid arteries and occurs in 13% to 20% of the population (Fig. 30.9). Although this is often termed a “bovine arch,” this is a misnomer as a true bovine arch has only a single vessel from the aortic arch. A four-vessel arch in which the left vertebral artery has an independent origin from the aortic arch occurs in 5% to 6% of the population. In this case, the left vertebral artery originates between the left com- mon carotid and subclavian arteries (Fig. 30.10). Left aortic arch with aberrant right subclavian artery has a prevalence of 0.5% to 2%. Instead of the normal origin from the right brachiocephalic artery, the right subclavian artery arises distal to the left subclavian artery from the distal aortic arch and travels through the mediastinum behind the esophagus to supply the right upper extremity. On esopha- gram, the aberrant subclavian artery indents the posterior aspect of the esophagus (Fig. 30.11). In approximately 15% of cases, the aberrant right subclavian artery is associated with an aneurysm at its origin, termed a diverticulum of Kommerell. The diverticulum of Kommerell is an embryologic remnant of the dorsal aortic arch and can cause compressive symptoms on the esophagus if large. However, in most instances of an aberrant right subclavian artery, this diverticulum is absent or small. Additionally, this configuration does not form a vascu- lar ring in the vast majority of instances. Only in the setting of a very rare right ligamentum arteriosum, which is a fibrous remnant of the ductus arteriosum, does a vascular ring occur with an aberrant right subclavian artery. In the absence of a

2 3 4

1

Figure 30.10.  Four-Vessel Arch. Oblique sagittal MIP CT demon- strates separate origin of the left vertebral artery ( 3 ) between the left common carotid artery ( 2 ) and left subclavian artery ( 4 ). The brachio- cephalic artery ( 1 ) is the first branch off the arch.

768

Section V: Cardiac Radiology

A

Figure 30.11.  Left Arch and Aberrant Right Subclavian Artery. Axial ( A ) and sagittal CT ( B ) demonstrate the right subclavian artery arising from the distal arch ( arrow ) which indents the posterior esophagus cor- responding to the aberrant right subclavian artery. This is a common ana- tomic variant that leads to no symptoms in the vast majority of patients.

B

by a rounded soft tissue structure abutting and indenting the inferior aspect of the right trachea (Fig. 30.12). Right aortic arch can have variable arch branching pat- terns, but the most common are aberrant left subclavian artery and mirror-image branching. In right arch with aber- rant left subclavian artery, the first branch from the aortic arch is the left common carotid artery, followed by the right carotid artery, right subclavian artery, and the aberrant left subclavian artery. The left subclavian artery passes posterior

*

Figure 30.13.  Right Arch and Aberrant Left Subclavian Artery. Axial CT scan in a patient with dysphagia shows a right aortic arch (*). The first vessel off the arch is the left common carotid artery ( black arrowhead ). The right common carotid and right subclavian arteries, which are the second and third vessels off the arch, respectively, are not seen on this image. The last vessel off the arch is the aberrant left subclavian artery ( yellow arrow ). This vessel courses posterior to the esophagus ( white arrow ) and trachea via a large 3.5-cm diverticulum of Kommerell ( black arrow ) which compresses the esophagus and slightly narrows the trachea. A left ligamentum arteriosum, which is not usually visualized, forms a vascular ring that can cause symptoms.

Figure 30.12.  Right Arch. PA chest radiograph with a right aortic arch ( arrows ) indenting the right lateral aspect of the distal trachea.

769

Chapter 30: Thoracic Aorta

*

C

*

A

B

Figure 30.14.  Right Arch and Mirror-Image Branching. A : Axial CTA images in a patient with repaired tetralogy of Fallot demonstrate a right aortic arch ( * ). B : A 20-mm coronal MIP image shows the right arch (*). The first branch off the right arch is the left brachiocephalic artery ( black arrow ), which divides into the left subclavian ( white arrow ) and left com- mon carotid ( yellow arrow ) arteries. The next branch off the arch is the right common carotid artery ( black arrowhead ) and the last branch is the right subclavian artery ( red arrows ). C : Axial image above the arch shows the left subclavian ( white arrow ), left common carotid ( yellow arrow ), right common carotid ( black arrowhead ), and right subclavian ( red arrow ) arteries. There is no retroaortic subclavian artery.

C

to the esophagus, often with an associated diverticulum of Kommerell (Fig. 30.13). This is most commonly associated with left-sided ligamentum arteriosum which forms a vascular ring that can cause symptoms due to compression. However, the ligamentum itself is usually not visualized on imaging. In right aortic arch with mirror-image branching, the first branch is the left brachiocephalic artery which divides into the left common carotid and subclavian arteries, followed by the right common carotid artery and right subclavian artery (Fig. 30.14). If an aberrant subclavian artery is present, there cannot be mirror-image branching. Congenital heart disease, especially tetralogy of Fallot, is commonly seen with right arch and mirror-image branching. Right aortic arch with iso- lated arch vessels is extremely rare and associated with con- genital heart disease. Isolation indicates that the vessel arises from the pulmonary artery rather than the aorta.

Double Aortic Arch

Right aortic arch with aberrant left subclavian artery and dou- ble aortic arch represent the two most common vascular rings. Double aortic arch results from persistence of both right and left embryologic aortic arches. The common carotid and sub- clavian arteries arise from their ipsilateral arch, resulting in a four-vessel branching pattern. On frontal projection radiogra- phy, double aortic arch will present as bilateral indentations on the lower trachea (Fig. 30.15). Posterior indentation of the esophagus may be seen on esophagram, similar to that seen in cases of aberrant subclavian arteries (Fig. 30.16). On axial CT or MR images, there is symmetric, four-vessel branching at the thoracic inlet in contrast to right or left arch variants, which results in asymmetric vessel branching.

770

Section V: Cardiac Radiology

*

A

B

C

Figure 30.15.  Double Aortic Arch. PA chest radiograph in an adult ( A ) with mild dysphagia demonstrates two bilateral indentations on the lower trachea (*), a slightly larger and more superior right indentation ( red arrow ) and slightly smaller and more inferior left indentation ( white arrow ). Coronal CT image ( B ) shows that the indentations represent a larger and more superior right aortic arch ( red arrow ) and smaller and more inferior left aortic arch ( white arrow ). Axial MIP image ( C ) shows the double aortic arch. The right arch is larger than the left arch, which is common.

The left arch is typically hypoplastic and located inferior to the dominant right arch (Fig. 30.15), with a left-sided descend- ing thoracic aorta and ductus arteriosus. Since the right and left arches encircle the trachea and esophagus, patients present in childhood with findings of airway compromise, includ- ing wheezing and stridor (Fig. 30.16). Double aortic arch is uncommonly associated with congenital heart disease. Cervical Aortic Arch Cervical aortic arch is extremely rare, with case reports describ- ing a high location of the aortic arch above the level of the clav- icle (Fig. 30.17). It is most often associated with a right arch, although a left cervical arch can occur. While often presenting

as an asymptomatic pulsatile mass in the neck or supraclavicu- lar region, it can be associated with other aortic abnormalities, aneurysm formation, and congenital heart disease.

Interrupted Aortic Arch Interrupted aortic arch occurs in 2 of every 100,000 births, characterized by discontinuity of the aortic arch in which there is complete absence or a fibrous remnant of the inter- rupted segment. There are three main types of interrupted aortic arch (A, B, and C) depending on location of inter- ruption. Type A interruption occurs distal to the left subcla- vian takeoff at the isthmus, type B between the left common carotid and subclavian origins, and type C between the right

B

A

Figure 30.16.  Double Aortic Arch. Axial MIP image in a 1-month-old baby with severe stridor and vomiting ( A ) shows a double aortic arch creating a vascular ring and causing compression of the trachea ( black arrow ). Additionally, lateral view from an esophagram ( B ) shows marked compression of the posterior wall of the esophagus ( black arrow ).

771

Chapter 30: Thoracic Aorta

Figure 30.18.  Type B Interrupted Aortic Arch. 3D VR shows a type B interrupted aorta arch with the ascending aorta ( dashed yellow arrow ) giving rise to the right brachiocephalic artery ( yellow arrow- head ) and left common carotid artery (LCCA, white arrowhead ). The aortic arch is absent, or interrupted, after the origin of the LCCA ( white arrow ). The left subclavian artery ( dashed white arrow ) arises from the descending thoracic aorta which received flow through a large patent ductus arteriosus ( yellow arrow ).

brachiocephalic and left common carotid origins. Type B is most common (50% to 60%) and is associated with VSD, bicuspid aortic valve, and left ventricular outflow tract anom- alies (Fig. 30.18). All types require a patent ductus arteriosus for survival. Methods of surgical repair are similar to those of aortic coarctation, described below. Please see “Postoperative Aorta” section for further discussion. Circumflex Aorta Circumflex aorta is an extremely rare anomaly which can occur with either a left- or right-sided aortic arch. The aortic arch travels posteriorly as usual but crosses the midline behind the esophagus, above the tracheal carina at the level of the distal arch/descending thoracic aorta, and continues distally Figure 30.17.  Cervical Arch With Aberrant Left Subclavian Artery. Coronal oblique MIP CT image shows the ascending aorta extending high into the right supraclavicular region ( red arrows ) with a right- sided cervical arch ( yellow arrow ). Similar to other right arches with an aberrant subclavian artery, the first vessel of the aorta is the left common carotid artery ( yellow arrowheads ) followed by the right common carotid artery ( white arrow ) and right subclavian artery (not visualized). The last branch off the aorta is the aberrant left subcla- vian artery ( black arrow ), the origin of which is not visualized in this view. (Courtesy of David Godwin, MD.)

contralateral to the aortic arch side (Fig. 30.19). A vascular ring can be present depending on the location of the ductus arteriosus.

Descending Thoracic Aorta The descending thoracic aorta begins after the ligamentum arteriosum and transitions to the abdominal aorta after pass- ing through the diaphragmatic hiatus. The descending tho- racic aorta gives rise to multiple systemic vessels, including the intercostal and bronchial arteries. Aortic Coarctation Aortic coarctation is defined as focal narrowing of the aorta adjacent to the ductus arteriosus (i.e., juxtaductal) and often occurs with varying degrees of aortic arch hypoplasia. In very rare instances, it can involve the abdominal aorta. Aor- tic coarctation is a relatively common anomaly, representing

*

Figure 30.19.  Circumflex Aortic Arch. Sequential axial MR GRE images demonstrate a right-sided arch (*) which passes behind the esophagus before continuing as a left-sided descending thoracic aorta.

772

Section V: Cardiac Radiology

*

*

A

B

C

Figure 30.20.  Aortic Coarctation. Sagittal CT ( A ), sagittal CT MIP ( B ), and coronal MIP ( C ). Focal narrowing of the proximal descending thoracic aorta corresponds to a postductal aortic coarctation ( white arrow ). Large collateral intercostal ( B , dashed black arrows ) and internal mammary arteries ( C , *) are present.

blood flow occurs via adjacent internal mammary and inter- costal arteries which become enlarged (Fig. 30.20). Although classically there is differential blood pressure and asymmetric pulses between the right and left upper extremities (in the con- text of preductal coarctation) or between the upper and lower extremities (postductal coarctation), blood pressure between upper and lower extremities can potentially equalize in the setting of very extensive collateral formation. Radiographic findings of aortic coarctation may only be apparent in severe cases. Indentation of the distal aortic arch with pre- and poststenotic dilation results in a “figure-of-3” sign on chest radiograph. Hypertrophied intercostal arteries result in bilateral central rib notching, involving the posterior fourth through eighth ribs (Fig. 30.21). Although CT best

approximately 6% to 8% of all congenital heart disease. There is a strong association with bicuspid aortic valve, which occurs in up to 75% of coarctation cases, and Turner syn- drome. The etiology remains unclear but a common pathogen- esis with bicuspid aortic valve has been proposed, including abnormalities of neural crest tissue migration, decreased in utero blood flow, and aortopathy with cystic medial necrosis. There are two main types of aortic coarctation: preduc- tal and postductal. Preductal coarctation tends to be more severe, involving a longer segment. It commonly presents in infancy, with systemic hypoperfusion following closure of the ductus arteriosus. Postductal coarctation usually presents in adulthood with hypertension and signs of left heart failure. To bypass the area of aortic narrowing, collateral systemic

Figure 30.21.  Aortic Coarctation. Coned down frontal chest radiograph in a patient with aortic coarctation demonstrates the “ figure-of-3 ” sign with indentation of the 3 corresponding to the area of aortic narrowing. Inferior rib notching is also present due to hypertrophied intercostal collateral arteries ( arrows ).

773

Chapter 30: Thoracic Aorta

up to 30% of patients. Treatment options in these patients include endovascular stenting, surgical resection, aortoplasty, and bypass grafting.

Pseudocoarctation Congenital elongation with prominent kinking of the aorta at the aortic isthmus can mimic the appearance of coarctation and is termed pseudocoarctation (Fig. 30.23). Pseudocoarcta- tion lacks the hemodynamic changes of true coarctation, such as a significant pressure gradient and arterial collateral forma- tion. Although usually asymptomatic, pseudocoarctation can be associated with hypertension and aortic aneurysm. Like coarctations, it is associated with a bicuspid aortic valve. On chest radiograph, the superior mediastinum may appear wid- ened with a superiorly positioned aortic arch (Fig. 30.23A). Atheroma The pathogenesis and consequences of atherosclerotic dis- ease are more thoroughly discussed in the section of this text dedicated to coronary artery disease; however, it warrants mentioning here as it is so highly prevalent in the aorta. Risk factors for the development of aortic atherosclerotic disease include advanced age, heredity, hypertension, diabetes, smok- ing, hyperlipidemia, sedentary lifestyle, and endothelial dys- function. Atheroma formation is a cyclical process that starts with lipoprotein phagocytosis by macrophages, which are then incorporated into the subintima of the aortic wall. Intra- cellular processes within the macrophage lead to the forma- tion of “foam cells.” Eventually, the macrophages die, with a resultant influx of additional white blood cells and fibro- blasts. The result of this cycle is an intramural mass consist- ing of the inner extracellular lipid core with an outer layer of inflammatory cells and connective tissue that can narrow the arterial lumen. Similar to coronary artery plaques, non- calcified or mixed plaques with a thin fibrous cap and a large necrotic core are more likely to rupture and are termed vul- nerable plaques. As plaques age and calcify, they generally become less prone to rupture. Atherosclerotic disease involving the thoracic aorta is a common finding on chest radiograph and should follow the

Figure 30.22.  Aortic Coarctation. Four-dimensional phase-contrast, sagittal MPR demonstrates flow acceleration, indicated by

red color flow ( yellow arrow ) across the coarctation in the proximal thoracic aortic; pressure gradient quantified as 31 mm Hg.

demonstrates anatomy, MRI and echocardiography allow quantitative evaluation of severity, including pressure gradient and flow acceleration across the coarctation (Fig. 30.22). Aortic coarctation is treated with surgical repair, which typically involves resection of the narrowed segment and pri- mary anastomosis or interposition graft. Additional repair techniques include subclavian flap or prosthetic patch aor- toplasty to augment the coarcted segment, extra-anatomic bypass grafting, and endovascular balloon dilation. Interven- tion is recommended when the coarctation pressure gradient exceeds 20 mm Hg. Following surgical repair, there is a 72% to 74% reported 30-year survival. Postoperative complica- tions include aneurysm formation, rupture with pseudoan- eurysm formation, accelerated atherosclerotic disease, and increased cardiovascular morbidity. Restenosis can occur in

C

A

B

Figure 30.23.  Pseudocoarctation. PA chest radiograph ( A ) demonstrates a rounded density ( white arrow ) superior to the aortic arch ( black arrow ). On sagittal CT (B), the aortic arch and proximal descending thoracic aorta are elongated and folded on themselves ( white arrow ), pro- ducing focal kinking ( white arrow ) but without significant narrowing. 3D VR of same patient ( C ).

774

Section V: Cardiac Radiology

*

*

A

B

C

Figure 30.24.  Diffuse Thoracic Aortic Atherosclerotic Disease. Frontal ( A ) and lateral ( B ) chest radiographs in a 95-year-old man demonstrate multifocal calcified plaques ( arrowheads ) along the thoracic aorta. The patient has had prior coronary artery bypass grafting indicated by the surgical clips ( arrows ) and transcatheter aortic valve intervention (TAVI, *). Sagittal nonenhanced chest CT in a different patient ( C ), a 79-year- old man, demonstrates multifocal calcifications along the descending aorta.

course of the aorta (Fig. 30.24). On chest CT, aortic athero- sclerotic disease is a common finding and does not usually lead to direct hemodynamic compromise, given the large cali- ber of the aorta. However, given that atherosclerotic disease is a multifocal process, in the thorax there may be concomitant disease involving the subclavian or carotid arteries which can lead to hemodynamic compromise. In some patients with severe atherosclerotic disease, thick layers of diffuse, predominantly noncalcified atherosclerotic plaques can layer much of the thoracic and abdominal aorta, which has been termed as “complex atheroma” by some authors (Fig. 30.25). These complex atheromas, which are an indirect sign of previous plaque rupture, are independent risk factors for the development of future ischemic events and should be mentioned as they may change medical or surgical management. In certain areas, contrast can be seen extending between areas of complex plaque, toward the wall of the aorta, which some authors refer to as “plaque ulceration.” This appearance can mimic a penetrating atherosclerotic ulcer (PAU), which is

discussed in more detail below, but the distinction between the two is imperative. While plaque ulceration is an indirect sign of previous plaque rupture and can lead to thromboem- bolic events, a PAU is a sign of intimal disruption and lies in the “acute aortic syndrome” spectrum, which is also discussed below. In general, ulcerated plaque will not extend beyond the lumen of the aorta into the intima which is demarcated in areas by linear calcification due to atherosclerosis (Fig. 30.26). However, distinction between the two is not always easy, even among expert radiologists.

Aneurysm Sinus of Valsalva Aneurysm

Sinus of Valsalva aneurysms are abnormal dilations of the sinuses which can be congenital or acquired. Congenital aneu- rysm secondary to weakness in the fibroelastic elements is seen in connective tissue disorders such as Marfan, Ehlers–Danlos,

A

*

*

D

*

A

B

D

C

Figure 30.25.  Complex Atherosclerotic Plaque in a Patient With Aneurysmal Dilation of the Aorta. Frontal ( A ) and lateral ( B ) chest radiographs in a 75-year-old woman demonstrate aneurysmal dilation of the ascending aorta ( yellow arrow ), aortic arch ( black arrowheads ), and descending thoracic aorta ( black arrows ). CT angiography at the level of the left pulmonary artery ( C ) shows aneurysmal dilation of the ascending ( A ) and descending ( D ) thoracic aorta. At the level of the aortic arch ( D ), the aortic aneurysm measures up to 5.8 cm. Layering mural thrombus (*) is pres- ent along the arch and descending thoracic aorta and should not be confused with intramural hematoma.

775

Chapter 30: Thoracic Aorta

A

B

Figure 30.26.  Areas of Ulcerated Plaque in a 65-Year-Old Man. Parasagittal ( A ) and axial ( B ) CT images of the aorta show extensive layering, mixed but predominantly noncalcified plaque throughout the thoracic aorta ( white arrows ). In certain areas, contrast can be seen extending into the plaque ( yellow arrows ) but does not extend beyond the intima, which is demarcated by a thin calcification along the aortic wall ( white arrow- heads ). It is important to differentiate this ulcerated plaque from penetrating atherosclerotic ulcers, as they have different treatments.

Symptoms are nonspecific but are usually secondary to complication such as rupture, aortic regurgitation, or com- pression of adjacent cardiovascular structures. Rupture often occurs into a cardiac structure, most commonly the right ven- tricle and right atrium. This results in a left-to-right shunt with development of heart failure. Early surgical or endovascular repair is essential, as mean survival after rupture is 1 to 2 years.

and Loeys–Dietz syndromes (Fig. 30.27). Congenital aneu- rysms are also associated with bicuspid aortic valve and VSD. Acquired sinus of Valsalva aneurysms often represent pseudo- aneurysms and result from bacterial aortic valve endocarditis or aortic surgery. Dilation of the sinuses may be diffuse and cir- cumferential or eccentric, focally involving one of the coronary sinuses. The right sinus of Valsalva is involved in 70% of cases.

A

B

Figure 30.27.  Sinus of Valsalva Aneurysm. Coronal oblique CTA in a patient with chest pain and no significant past medical history shows a large aneurysm arising from the left sinus of Valsalva ( A , arrow ). Still image from a coronary angiography shows that the large sinus of Valsalva aneurysm stretches and narrows the left anterior descending coronary artery ( B , arrow ). The patient’s symptoms resolved after surgical repair.

776

Section V: Cardiac Radiology

aortitis. Giant cell arteritis (GCA), rheumatic fever, and relaps- ing polychondritis may result in ascending aortic aneurysms, while Takayasu arteritis may affect the ascending aorta, aortic arch, arch vessels, abdominal aorta, and/or pulmonary arteries. Infective aortitis may arise in the setting of bacterial endocardi- tis, with resultant aneurysm formation most often in the proxi- mal ascending aorta. Historically, syphilis was a more common cause of ascending aortitis and aneurysm; incidence of syphilitic aortitis has decreased, but a number of other organisms (includ- ing Streptococcus and Staphylococcus spp.) can be implicated in infective aortitis (“mycosis”) which may lead to formation of a “mycotic” aneurysm. Bicuspid valve is an independent risk factor for ascending TAA, unrelated to the presence of associ- ated aortic stenosis. Chest radiography may demonstrate dilation of the ascend- ing aorta or arch (Fig. 30.25), with or without associated cal- cifications. Technique is also an important consideration; the ascending aortic contour may be exaggerated if the patient is rotated to the right and this should not be confused for an aneurysm. There may be tracheal deviation and/or left upper lobe atelectasis in the setting of an aortic arch aneurysm. CT angiography (CTA) has replaced traditional catheter angi- ography as the mainstay of imaging and has the benefit of extraluminal evaluation, though treatment cannot be con- currently performed as with catheter angiography. CTA will demonstrate typically fusiform and concentric focal dilation of the ascending aorta or arch (Fig. 30.29) to greater than 4 cm. Patients with Marfan syndrome may demonstrate dilation of the main pulmonary artery in addition to annu- loaortic ectasia (Fig. 30.30). Aneurysms cause turbulent flow patterns with nonlaminar flow, often eventually resulting in discontinuous or circumferential mural thrombus formation (Fig. 30.25). Noncontrast imaging is not always routinely per- formed in follow-up imaging of known aneurysms, but some studies have suggested that focal crescentic hyperattenuation in mural thrombus may be a sign of impending rupture. Mul- tiplanar (MPR) and three-dimensional reformats can be eas- ily rendered, allowing for accurate orthogonal measurements and aiding with pre- and/or periprocedural planning and

Ascending Thoracic Aorta and Aortic Arch Aneurysms

Thoracic aortic aneurysms (TAAs), defined as aortic enlarge- ment to greater than 4 cm with preservation of vessel wall integrity—that is, without intimal disruption—may occur anywhere along the vessel; 50% occur in the ascending aorta (proximal to the right brachiocephalic artery), 10% in the aor- tic arch, and 40% in the descending aorta (distal to the left subclavian artery). There are a multitude of risk factors for the development of TAA, but by far the highest association is with atherosclerosis, which is seen in 70% of cases. Aneurysms sec- ondary to atherosclerotic disease are more commonly seen in the descending thoracic aorta but may occur anywhere along the thoracic and abdominal aorta (Fig. 30.28). Thus, imag- ing of the abdominal aorta is also indicated in these patients. Homocystinuria, Marfan syndrome, and other connective tissue disorders may result in dilation of the aortic annulus and prox- imal ascending aorta, termed annuloaortic ectasia. Ascending aortic aneurysm can also result from noninfective or infective

Figure 30.28.  Multifocal Thoracoabdominal Aortic Aneurysm. Sag- ittal CT image in a 69-year-old woman demonstrates multifocal aneu- rysms in the thoracic ( black arrows ) and infrarenal abdominal aorta ( white arrow ). Note layering mural thrombus in the abdominal aortic aneurysm.

Figure 30.29.  Ascending Thoracic Aortic Aneurysm. Axial CT in a 77-year-old woman demonstrates marked dilation of the ascending aorta measuring 7.7 cm consistent with aneurysm ( A ).

777

Chapter 30: Thoracic Aorta

A

A

A

B

C

Figure 30.30.  Marfan Syndrome With Aortic and Pulmonary Artery Involvement. Sagittal CT image ( A ) in a 59-year-old man demonstrates aortic annulus/root aneurysm ( A ), with characteristic “tulip” appearance of the dilated root. Axial oblique reformat through the sinuses of Val- salva ( B ) demonstrates aneurysmal dilation measuring 4.8 cm ( A ). Sagittal CT image ( C ) demonstrates aneurysmal dilation of the main pulmo- nary artery ( double-headed arrow ), which is one of the established criteria for the diagnosis of the syndrome.

navigation for interventionalists. These can be particularly helpful over time for risk stratification. MR and MR angiography (MRA), like CT, are also highly sensitive and specific for aneurysm, along with mural throm- bus and other associated complications. Cine sequences allow visualization of flow patterns, though further study is needed to determine the clinical usefulness of this information. MR may be the most appropriate modality in younger patients and/or those requiring more extensive follow-up imaging, given the lack of ionizing radiation; and non–contrast-enhanced imaging is likely adequate in patients with allergies or poor renal function. Ascending aortic aneurysms may be complicated by dis- section or rupture, and rupture is the leading cause of death in these patients. The risk of rupture increases with size, and ascending aortic aneurysms greater than 6 cm carry a risk of rupture of approximately 14%. Rupture carries a 97% to 100% mortality rate, if not emergently treated; the periop- erative mortality associated with repair is not insignificant but is considerably lower in elective compared to emergent repair (9% vs. 22%). For this reason, early diagnosis is cru- cial, and patients with known aneurysms are monitored with regular, serial imaging to evaluate for size and interval growth. Ascending thoracic aneurysm diameter greater than 5.5 cm and/or interval growth (greater than 0.5 cm in 6 months or 1 cm in 1 year) are/is indication(s) for intervention, either sur- gical or endovascular. In the setting of connective tissue dis- ease such as Marfan syndrome, there is a lower threshold for repair, usually greater than 5 cm. Descending Thoracic Aortic Aneurysms Aneurysms in the descending aorta are most commonly asso- ciated with atherosclerosis, though aneurysms secondary to other diseases including collagen vascular disease, infective aortitis, and autoimmune/inflammatory disease may occur in the descending aorta as well. Chest radiography may demonstrate dilation of the descending aortic contour (Fig. 30.25); any associated calci- fications will be displaced outward in a true aneurysm. CTA is highly sensitive and specific for aneurysm, with or without associated mural thrombus. MRA is also highly sensitive and specific for detection of aneurysms and their complications. Though hemodynamically unstable patients are unsuitable

for MR imaging, it may be more appropriate in the setting of routine surveillance imaging, particularly given the lack of ionizing radiation. Descending thoracic aortic aneurysms may be complicated by dissection, rupture, or fistula formation to the esophagus and/or airways; rupture is the leading cause of death. Risk fac- tors for rupture include age, size greater than 5 cm, hyperten- sion, smoking, and COPD. The rate of growth increases with aneurysm size and is estimated at 0.12 cm/yr for aneurysm diameter greater than 5.2 cm. Thus, aneurysms are monitored with serial imaging; size greater than 6.5 cm and/or interval growth (greater than 0.5 cm in 6 months or 1 cm in 1 year) are/is indication(s) for open surgical or endovascular treat- ment. Operative mortality is 5% to 12% with renal failure (5% to 13%) and spinal cord ischemia (4% to 30%, depend- ing on the extent of disease/repair), among the major postop- erative complications. As in the ascending aorta, aneurysms associated with connective tissue disease are managed more aggressively with earlier intervention. Acute Aortic Syndrome Acute aortic syndrome (AAS) includes aortic dissection, acute intramural hematoma (IMH), and PAU, which share the com- mon classical clinical presentation of excruciating chest pain that may radiate to the back. While these have traditionally been classified as distinct entities, mounting evidence suggests that they may rather represent variants or a spectrum of dis- ease, as described in more detail below. The three cannot be distinguished by clinical history or physical examination, and thus, imaging plays an integral role in diagnosis. Transesopha- geal echocardiography (TEE), multidetector CT (MDCT)/CTA, and MRA are all useful, highly sensitive, and specific. Owing to its ubiquity, rapid acquisition, and high accuracy, CTA is the most commonly employed modality in this setting, with a sensitivity and specificity of 100% and 98%, respectively, for thoracic aortic dissection. Careful consideration of imaging parameters and the manner of intravenous administration of contrast material for CTA is paramount. Noncontrast phase imaging is important to identify IMH, and timing of imaging relative to administration of the contrast material bolus is crit- ical for optimal imaging. Two commonly used methods for ensuring arterial phase imaging are the timing/test bolus and

778

Section V: Cardiac Radiology

LAD

RBCA

LCCA

LM

T

LSCA

RCA

F

F

F T

A

B

C

D

Figure 30.31.  Type A Aortic Dissection, RCA Occlusion, and Propagation Into the Right Brachiocephalic Artery. Axial maximum intensity pro- jection (MIP) CT image at the level of the left main coronary artery ( A ) demonstrates the proximal margin of the dissection flap ( arrow ) extending to the aortic root. The left main ( LM ) and left anterior descending ( LAD ) coronary arteries are patent. The dissection extends into the descending thoracic aorta with delineation of the flap ( arrow ), true ( T ), and false ( L ) lumens. Oblique coronal MIP image at the level of the right coronary artery ( B ) demonstrates the dissection flap ( black arrow ) extending into the ostium of the right coronary artery ( RCA ) leading to its occlusion ( white arrows ). Axial image at the level of the aortic arch ( C ) demonstrates the dissection flap ( arrow ), true ( T ), and false ( F ) lumens. Note inward displacement of intimal calcifications ( arrowhead ). Axial image at the level of the arch vessels ( D ) demonstrates propagation of the dissection flap into the right brachiocephalic artery ( RBCA, white arrow ). The left common carotid ( LCCA ) and subclavian ( LSCA ) arteries are supplied by the true lumen.

bolus tracking. With a test bolus, a small amount of contrast is administered and repeat axial images are obtained at a single level to assess the time to maximum opacification. Once this is determined, the full contrast bolus is administered, and imag- ing occurs with a scan delay as indicated by the timing bolus. With bolus tracking, a region of interest (ROI) is prescribed in the ascending aorta, the contrast bolus is administered, and when the Hounsfield Units in the ROI exceed a preset thresh- old, the scan is triggered. If there is concern for an ascending aortic dissection, prospective ECG gating should be employed to avoid false-positive diagnoses from motion or other artifacts at the aortic root. With gating and careful angiographic tim- ing, the coronary arteries may also be evaluated for dissection propagation in this setting.

Aortic Dissection Thoracic dissections are classified according to the Stanford sys- tem by their most proximal extent: type A dissections involve the ascending aorta (proximal to the innominate artery) and require immediate surgical management with stent–graft place- ment (Figs. 30.31 and 30.32). Type B dissections involve only the descending aorta (distal to the leftsubclavian artery) and are often managed medically unless there is evidence of end-organ ischemia or impending rupture, in which case surgical or endovascular stent grafting is indicated (Figs. 30.33 and 30.34). Dissections that involve the aortic arch but do not extend prox- imal to the innominate artery (Figs. 30.35 and 30.36) are rare,

T

T

F

F

A

B

Figure 30.32.  Type A Aortic Dissection With Occlusion of the Right Common Carotid Artery. Parasagittal CT reformat ( A ) demonstrates a type A dissection with very slow flow in the false lumen ( F ) compared to the true lumen ( T ). The dissection flap extends into and occludes the right common carotid artery ( white arrow ). Axial image from a head CT ( B ) shows relative hypoattenuation of nearly the entire right cerebral hemisphere ( white arrows ) due to right common carotid artery occlusion.

779

Chapter 30: Thoracic Aorta

F

T

F

T

T

F

F

A

B

Figure 30.33.  Type B Aortic Dissection With Impending Rupture. Parasagittal image from a CTA ( A ) shows the intimomedial flap ( yellow arrows ) extending to the level of the left subclavian artery ( red arrow ) but not involving the vessel or extending proximally into the aortic arch, consistent with a type B dissection. The true lumen ( T ) is smaller in size and shows increased contrast opacification compared to the false lumen due to the increased pressures within the false lumen. Axial image ( B ) shows the large size of the false lumen ( F ) compared to the true lumen ( T ). The maximum diameter of the aorta was 7.1 cm. Although most type B dissections are treated medically, this patient was treated surgically due to the large size of the aorta and the risk of subsequent aortic rupture.

and bicuspid aortic valve, cocaine and methamphetamine use, pregnancy, and aortitis. The altered microenvironment and function of the media predispose to the acute phase of dissec- tion when the intima is disrupted, with resultant blood flow from the true aortic lumen into the media and formation of a second, false lumen. The intimomedial tear most commonly occurs along the right lateral wall of the ascending aorta, 1 to 2 cm from the sinotubular junction, or in the proximal descend- ing aorta near the insertion of the ligamentum arteriosum, the sites of maximum shear stress. Once blood has entered the false lumen, it propagates longitudinally along the aortic wall, typ- ically in retrograde fashion; a second, reentrance tear allows blood to circulate through the false lumen. This process also induces a robust inflammatory response—the aortic wall is fri- able and fragile in the acute phase, with a higher risk of rapid expansion and/or rupture compared to the chronic setting (Figs. 30.33 and 30.34). Dissections involving the ascending

comprising approximately 7% of all dissections. They are not specifically classified by the traditional surgical systems and have not been entirely characterized in the medical or surgi- cal literature. For the purposes of reporting and to facilitate understanding among providers, they may be described as type B dissections with aortic arch involvement. Postoperative surveillance imaging in patients who undergo endovascular treatment is important to assess for the presence of endoleaks, discussed in more detail later in the chapter. The pathogenesis of aortic dissection is a complex process involving degeneration of the aortic media, a dynamic struc- ture that plays a vital role in regulating aortic compliance among other functions. This degeneration may be congenital, secondary to aberrant or defective protein production (e.g., in Marfan and Ehler–Danlos syndromes), or acquired, most com- monly secondary to chronic hypertension which causes medial degeneration. Other risk factors include Turner syndrome

FT

T

F

F

*

*

T

A

B

C

D

Figure 30.34.  Type B Aortic Dissection With Rupture. Axial CT in the angiographic ( A ) window/level demonstrates the intimomedial flap ( arrow ), with delineation of the true ( T ) and false ( F ) lumens. The large area of contrast extravasation extending posteromedially from the pos- terior aspect of the pseudoaneurysm ( arrowheads ) represents rupture. The soft tissue window/level ( B ) shows the mediastinal hematoma (*) to better advantage; the esophagus is obscured. Coronal ( C ) and sagittal ( D ) reformats can be easily rendered from the original dataset at the work- station; the sagittal view demonstrates partial thrombosis of the false lumen ( F ) in the mid-distal descending thoracic aorta.