Mediastinal vascular anomalies are important developmental variants. They can result in a wide range of symptoms and sometimes are diagnosed incidentally. The recognition of these anomalies is important not only for radiologists, but also for surgeons to avoid inappropriate treatment. The chest radiograph, CT, and MRI each may contribute to establishing a diagnosis.

Aorta and branches: Congenital anomalies

Aberrant right subclavian artery --Aberrant right subclavian artery (ARSA) occurs in 0.5% of individuals. It is due to involution of the entire right fourth aortic arch. Thus, the right seventh intersegmental artery, which would normally become confluent with the right fourth arch (innominate artery), persists in its attachment to the distal descending aorta. ¹ The ARSA is the last branch of the aortic arch. It arises from its medial wall, traversing the mediastinum from left to right in a retroesophageal location towards the right axilla. Other reported courses, such as passage between the esophagus and trachea and anterior to the trachea, are rare or nonexistent.

ARSA usually is asymptomatic. Pediatric patients may present with respiratory compromise due to tracheal compression. Swallowing difficulty, termed dysphagia lusoria, occasionally occurs because the aberrant vessel abuts the esophagus. ARSA is usually an incidental finding on chest radiography. On the frontal chest radiograph, the vessel may manifest as an interface that courses obliquely upward toward the left, or may appear as a right paratracheal mass (figure 1). On the lateral film, ARSA appears as a retrotracheal density that produces anterior bowing of the trachea. Chest CT shows a four artery sign--right common carotid artery, left common carotid artery, left subclavian artery, and aberrant right subclavian artery in order of origination. CT is useful in showing the precise vessel size, mural thrombus, and relationship of ARSA to other mediastinal structures. On CT, the vessel is well-visualized as it passes behind the esophagus. MRI shows the relationship of the various mediastinal vessels and other structures. Spin-echo and gradient echo transaxial and coronal or sagittal imaging provide optimal delineation of ARSA. Transaxial imaging just below the arch demonstrates the origin of the aberrant vessel and its course posterior to the esophagus (figure 1). Cine MRI in the same plane helps to evaluate the amount of constriction the vessel exerts on the esophagus and trachea during the cardiac cycle. Contrast-enhanced MR angiography is a newer, promising technique.

One variant of ARSA is a diverticulum of Kommerell, ² which is dilatation of the vessel origin representing the remnant of the persistent aortic arch. An atherosclerotic aneurysm (figure 1) can also form at the point of attachment of ARSA to the descending aorta. Both lesions manifest as a superior mediastinal mass. The recognition of ARSA is important to surgeons because of its influence on the course of right recurrent laryngeal nerve to the larynx. Moreover, cross clamping of the aorta proximal to the left subclavian artery may impair circulation to the brain stem and cause infarction.

Right aortic arch --Right aortic arch is the most common congenital anomaly of aortic arch, occurring in 1% of the population. The abnormality is due to the involution of the embryonic left aortic arch. There are two major types of right arch. Mirror image right arch (left innominate artery, right carotid artery, right subclavian artery) is almost invariably associated with congenital cardiac anomalies, particularly tetralogy of Fallot and truncus arteriosus. The second type is right aortic arch associated with aberrant left subclavian artery. In both varieties of right arch, the upper descending aorta is usually right-sided. More distally, the aorta crosses from right to left in the lower mediastinum.

Right aortic arch in the adult is often asymptomatic and is discovered incidentally during head, neck, or vascular surgery or noted on a routine chest radiograph as mediastinal density. Chest radiography reveals a right paratracheal density, with leftward deviation of the trachea. Contrast-enhanced CT and MRI are valuable to identify the right aortic arch and evaluate the branches of the arch at sequential levels to determine the type of aortic arch (figure 2).

Double aortic arch --Double aortic arch represents 1% to 3% of all congenital heart disease. It is due to failure of regression of the right dorsal aorta forming a vascular ring that surrounds the trachea and esophagus. ³ The ascending aorta splits into right and left components. The right arch, which is usually higher and larger than the left, passes to the right of the trachea and esophagus, crosses behind these structures, and rejoins the left arch, which occupies a relatively normal position. The left arch may be atretic. Each arch gives rise to its respective subclavian and common carotid arteries; no innominate artery is present. This gives a symmetric appearance to great vessels in the supra-aortic mediastinum that is highly suggestive of the diagnosis. Double aortic arch descends most commonly on the left.

When both arches persist, the esophagus and trachea are completely encircled and often compressed, leading to severe respiratory and feeding difficulties. ⁴ Any tubes that are introduced (endotracheal, nasogastric) can produce pressure necrosis and resultant fistula. ⁵ Contrast-enhanced CT or MRI demonstrates the vascular nature of bilateral arches with symmetrical appearance of their respective carotid and subclavian arteries. Either technique is useful to evaluate the caliber and position of the arches (figure 3).

Coarctation of aorta --Aortic coarctation is a localized infolding of the media of the aorta distal to the origin of the left subclavian artery opposite to, or in the vicinity of, the ductal ligament, causing an eccentric narrowing. The abnormality accounts for 8% of congenital anomalies in children and 6% in adults.

Bicuspid aortic valve, patent ductus arteriosus, and ventricular septal defect are the most common congenital heart diseases associated with aortic coarctation. Coarctation is the most frequent cardiovascular lesion that occurs with Turner's syndrome. Patients may be asymptomatic or present with headache, epistaxis, cold extremities, and claudication.

Collateral circulation develops through the intercostal artery and branches of the subclavian arteries. Bilateral rib notching is produced on the inferior margin of ribs because the internal mammary artery is a major source of collaterals. This finding manifests after 8 years of age. Unilateral right rib notching indicates origin of the left subclavian artery distal to the coarctation. If rib notching is limited to the left side, an aberrant right subclavian artery is most likely. Dilatation of intercostal artery acting as a collateral pathway may cause retrosternal notching. The "3" sign may be present on chest radiography. The upper curve of the "3" is due to the aortic arch or enlarged left subclavian artery and the lower curve is caused by post stenotic dilatation of the descending aorta beyond the coarctation, which is responsible for the notch between two curves. The cardiac silhouette is normal or may show evidence of left ventricular hypertrophy.

MRI is an excellent noninvasive imaging modality for diagnosing and monitoring patients with coarctation of the aorta. Spin-echo or gradient-echo MRI in the left anterior oblique plane through the middle of the ascending and descending aorta results in an image that "unfolds" the aorta in a question mark configuration. This view is excellent to assess the site of coarctation, hypoplasia of the aortic isthmus, and postcoarctation dilatation of the descending aorta. The coronal view demonstrates enlarged intercostal collateral arteries. Contrast-enhanced 3D MR angiography is also valuable for this purpose. Cine MRI helps in assessing the degree of coarctation based on the length and the width of signal void jet. ⁶ Phase mapping is used to measure the gradient across the coarctation as determined by the modified Bernoulli equation. Phase mapping is also valuable to evaluate the aortic valve if aortic stenosis is present due to a bicuspid valve. Its use in identifying reversal of flow in intercostal arteries helps to document collateral flow to the descending aorta.

Pseudocoarctation of the aorta is a congenital anomaly in which the aorta is kinked at the level of ligamentum arteriosum. There is no significant pressure gradient across the kink in aortic pseudocoarctation. On CT, the abnormality is visualized as a kinked aortic arch and proximal descending aorta with a narrow lumen. The aortic arch is higher than normal and descends in an abnormal anterior position in front of the spine. At a level near the carina it angles posteriorly, forming a second arch and more inferiorly assumes a normal position anterolateral to the spine.

Aorta and branches: Acquired aortic dissection

Aortic dissection is caused by a tear in the intima with rupture of blood into the media or by bleeding in the media with secondary rupture into the lumen. The most common cause of aortic dissection is hypertension. According to the Stanford classification, aortic dissection is categorized into two types: Stanford Type A dissection involves the ascending aorta; Stanford Type B dissection involves only the de-scending aorta.

The classic finding of the intimal flap separating the true lumen from the false lumen is well-demonstrated on both CT and MRI. Blood flow study (phase mapping) with MRI helps in evaluating flow characteristics. For Type A dissections, signal loss on MR images emanating from the aortic valve and extending into the left ventricle during diastole indicates aortic insufficiency. Changes in the aortic wall, such as thickening, that suggest intramural dissection or a penetrating ulcer are also demonstrated on MRI. These patients should be monitored closely because of the risk of progression to frank aortic dissection. Postoperative MRI is useful in assessing complications and for evaluation of the course of the disease.

Aortic aneurysm

An aortic diameter >4 cm is defined as an aortic aneurysm. Atherosclerosis is the most common cause. The proximal descending aorta is the most frequent location.

Aneurysms are fusiform or saccular. They may cause symptoms by impinging on the esophagus, airway, or vascular structures. In a true aortic aneurysm, the aneurysm contains all layers of the aortic wall. False aneurysm results from a breach in the wall of aorta, such that only the adventitia or surrounding fibrous tissue contain the blood. Pseudoaneurysms are formed after trauma or a surgical complication. Risk factors for rupture of aneurysm are: size >6 cm, progressive expansion, or symptoms of chest or back pain.

CT and MRI reveal the relationship of the aneurysm to other vascular structures. Evaluation of the aneurysm in different imaging planes can be done directly with MRI or with reformatted images on CT. On CT, contrast is usually required to distinguish clot from flow. On MRI, clot in the aneurysmal segment can be identified on spin-echo images by its higher signal intensity within the aortic lumen. Both CT and MRI demonstrate the neck of a saccular aneurysm or pseudoaneurysm.

Pulmonary artery and branches

The most common cause of enlargement of the pulmonary artery in adults is pulmonary arterial hypertension. Both main pulmonary arteries dilate. The chest radiograph reveals enlarged central pulmonary arteries with distal tapering. CT and MRI show the size of pulmonary arteries more precisely (figure 4). In pulmonary arterial hypertension, spin-echo MRI demonstrates increased intraluminal signal during systole. Cine MRI shows loss of normal systolic to diastolic signal variations in the pulmonary artery during the cardiac cycle. This reflects a decrease in systolic velocity and damped flow pulse in the pulmonary artery (figure 4C).

Pulmonary valve stenosis can cause enlargement of the main and left pulmonary artery with a normal right pulmonary artery, due to the posterior vector of the stenotic jet. CT and MRI show a characteristic appearance of the branch pulmonary arteries, with the left being much larger than the right (figure 4).

Pulmonary artery sling is a congenital anomaly in which the left pulmonary artery arises from the right pulmonary artery rightward of its usual position. The anomalous vessel courses leftward between the trachea and esophagus to reach its normal position, thereby causing anterior indentation on the esophagus and compression of the posterior aspect of trachea (figure 5). Tracheal anomalies such as complete rings and tracheomalacia are associated findings and may lead to stridor. Contrast-enhanced CT and MRI show the origin and course of this anomalous vessel and its effects on adjacent structures.

Anomalies of intrathoracic veins: Systemic veins

Persistent left superior vena cava --Lack of involution of the left anterior and common cardinal vein results in a persistent left superior vena cava (SVC). This anomaly is a variant in 0.3% of otherwise healthy individuals and 4.4% of those with associated congenital heart disease. The left SVC drains both the left internal jugular and left subclavian vein. The vessel courses lateral to the aortic arch and anterior to left hilum to terminate in the coronary sinus. If associated with congenital heart disease, it may drain into the left atrium.

This anomaly is usually discovered during cardiac catheterization, surgical repair of congenital heart disease, during insertion of central lines, ⁷ or incidentally during imaging. Patients with congenital heart disease are at risk of paradoxical embolism because of accompanying lesions (ASD, unroofed coronary sinus, and direct communication of vein to left atrium).

On chest radiography there is widening of left superior mediastinum (figure 6), but the usual method of recognition is from an apparently misplaced central venous catheter into a normal left SVC. ⁸ Other radiographic findings include a wide aortic shadow with a venous crescent extending from left border of the aortic arch to the middle of left clavicle. CT or MRI reveal the enhancing vessel in the left mediastinum anterolateral or lateral to the aortic arch and its branches (figure 6). Sequential contiguous sections show the tubular structure of the vessel. The left brachiocephalic vein is often absent. The right SVC is smaller than normal or may be absent.

Azygous continuation of the inferior vena cava--Azygous continuation of inferior vena cava (IVC) is characterized by interruption of the intrahepatic part of IVC and diversion of blood more posteriorly to a large azygous vein that ascends along the right margin of the vertebral bodies and empties into the SVC. The anomaly is caused by persistence of the embryologic right supracardinal vein and lack of development of the suprarenal part of subcardinal vein. Systemic flow beyond this point is accommodated by the dilated azygous and hemiazygous veins. The azygous vein receives blood from the right intercostal veins; intercostal veins of the lower left hemithorax supply the herniazygous vein. Between the eighth and eleventh thoracic vertebrae, the hemiazygous vein crosses obliquely to join the azygous vein. This anomaly can occur as a part of heterotaxy syndrome, particularly polysplenia.

The azygous arch is enlarged on the chest radiograph, which often produces a right paratracheal density. The IVC shadow may be absent. Dilated ayzgous and hemiazygous veins are seen to the right or left of the aorta, respectively, as they course toward their respective SVC. ⁹ CT demonstrates the dilated azygous vessel adjacent to the descending aorta and absence of the intrahepatic inferior vena cava. MRI provides an accurate diagnosis of venous anomalies because of its multiplanar capability (figure 7). Contrast enhancement is not necessary.

Anomalies of intrathoracic veins: Pulmonary veins

Total anomalous pulmonary venous return (TAPVR) is a congenital cardiac defect in which the pulmonary veins do not empty into the left atrium but drain anomalously into the right atrium or into a systemic vein. The entity constitutes 2% of all congenital cardiac defects. It is categorized according to the location of drainage. ¹⁰ In the supracardiac variety of TAPVR, the vertical vein drains into left brachiocephalic vein. In the cardiac variety, the anomalous vessel empties into right atrium or coronary sinus. In the subdiaphragmatic variety, the venous drainage is usually into the portal vein. This last variety is associated with obstruction to venous flow and early development of pulmonary venous congestion. This anomaly usually manifests in infancy and is associated with an atrial shunt. TAPVR is more common in asplenia syndrome.

Chest radiography in unobstructed TAPVR demonstrates increased pulmonary vascular markings because of significantly increased blood flow. A "Snowman" or "figure-of-eight" shape is characteristic of the cardiac silhouette in supracardiac variety and consists of the enlarged vertical vein, left brachiocephalic vein, and left SVC atop the heart (figure 8). Volume overloading leads to right heart enlargement.

When pulmonary venous return is obstructed (subdiaphragmatic variety) there is a hazy reticulogranular appearance of lungs, fanning out from hilar area, obscuring the cardiac borders, and leading to an appearance similar to retained lung fluid or hyaline membrane disease. A decreased volume of blood returns to the heart due to pulmonary venous obstruction so the cardiac chambers do not dilate.

Partial anomalous pulmonary venous return (PAPVR) occurs in isolation or in association with cardiac or pulmonary defects. The most common form of PAPVR is an anomalous right upper lobe vein entering the SVC or the right atrium. A sinus venous type atrial septal defect is frequently associated with this abnormality. A less common form of PAPVR is the right pulmonary vein draining into the IVC. A characteristic "scimitar" appearance on a chest radiograph is demonstrated as the vessel descends towards the right lung base. This abnormality is associated with hypogenetic right lung.

Anomalous left pulmonary vein with PAPVR is not uncommon. In this condition, the anomalous vessel descends lateral to the aortic arch. Spin-echo MRI demonstrates the course of anomalous vessel and gradient-echo images are useful to distinguish from the surrounding lung tissue or adjacent vessels (figure 9). MRI helps in identifying an associated atrial septal defect with PAPVR.

Conclusion

Many mediastinal vascular anomalies are readily diagnosed on plain film. For those anomalies that remain undiagnosed or require further clarification, either CT or MRI may prove valuable. AR