For recognition of the abnormal it is essential the full knowledge of normal anatomy. An infinite number of normal variants are known. Of more significance are those variations that may simulate disease in imaging examinations.

The distinction between a normal anatomic variation and a congenital anomaly is an arbitrary one. In this work, the inclusion criterion was the absence of signs and symptoms directly related to the focused variant.

Chest wall

Developmental Deformities of the Ribs

There are various types of congenital anomalies and deformities of the ribs, including developmental fusion of two or more ribs (figure 1), articulation or bridge formation between two ribs, and bifid rib (forked rib). These morphologic anomalies and anatomic variants occur in 0.15%–0.31% of the population, have a female predilection, and occur more frequently on the right side than on the left side. These anomalies are usually of little or no clinical significance; however, one should be familiar with them to distinguish them from true rib diseases.

Fig.: Fusion of two right posterior ribs (3-4).

Cervical Rib

A cervical rib is a supernumerary or accessory rib arising from the seventh cervical vertebra (figure 2). This abnormality occurs in approximately 0.5% of the population and is more common in females than in males.

Fig.: Left cervical rib arising from the seventh cervical vertebra.

Although a cervical rib is usually asymptomatic, it is the most important anatomic rib variant from a clinical standpoint because it can cause thoracic outlet syndrome by compression of the brachial plexus or subclavian vessels (figure 3). This syndrome is usually associated with pain in the hand when the arm is elevated, difference in pulse intensity between the two arms when the affected extremity is in a certain position, and Raynaud phenomenon. Angiography is helpful in the diagnosis and essential in the evaluation of this syndrome in patients with a cervical rib.

Bronchi

A displaced bronchus is defined as one not originating from its normal higher order airway. If a normal bronchus also supplies the affected lung, the anomalous bronchus is defined as supernumerary.

There are numerous variations of lobar or segmental bronchial subdivisions. Most of these variations are seen in the upper lobes. In the right upper lobe, the typical trifurcation of segmental bronchi is seen in only 30% of cases; various types of bifurcation are seen in the other 70%.

The term tracheal bronchus encompasses a variety of bronchial anomalies originating from the trachea or main bronchus and directed to the upper lobe territory (figure 4). A true tracheal bronchus is any bronchus originating from the trachea, usually within 2 cm of the carina and up to 6 cm from the carina (figure 5).

Fig.: Coronal MPR shows a traqueal bronchus.

The displaced type is more frequent than the supernumerary type and this variant is more prevalent on the right side. When the entire upper lobe bronchus is displaced on the trachea, it is also called a “pig bronchus”.

Although these aberrant bronchi are usually asymptomatic, respiratory distress may occur if drainage is impaired or in association with other abnormalities.

An accessory cardiac bronchus was defined by Brock in 1946 as a “supernumerary bronchus arising from the inner wall of the right main bronchus or intermediate bronchus opposite to the origin of the right upper lobe bronchus.” In normal anatomy, the only bronchus arising from the medial wall of the right main bronchus is the medial basilar segmental bronchus.

Cough or hemoptysis can result from superinfection, aspergilloma, or tumor.

Lung and fissures

Knowledge of the anatomy and normal variants of the major fissures is essential for recognizing their variable imaging appearances as well as related abnormalities.

Fig.: Normal fissures. Coronal MPR of CT chest scan shows the two oblique fissures (yellow arrows) and the right horizontal fissure (blue arrow).

Accessory fissures

They usually occur at the boundaries between bronchopulmonary segments. Most common are the inferior accessory fissure (figure 7), which demarcates the medial basal segment; the superior accessory fissure, which demarcates the superior segment; and the left minor fissure, which demarcates the lingula.

Fig.: Right inferior accessory fissure. Thin line (arrow) extends from diaphragm obliquely upward toward hilum. The inferior accessory fissure surrounds the medial basal segment of the lower lobe. In some cases, the separation continues medially and superiorly most of the way to the hilum.

Anatomically, an accessory fissure is a cleft of varying depth lined by visceral pleura. Radiographically it appears as a thin white line, resembling the major or minor fissure, except for location. The line can be mistaken for an interlobar fissure, for a scar, for the wall of a bulla, or for a pleural line made visible by pneumothorax. Besides a thin line, the other usual radiographic manifestation of an accessory fissure occurs when the fissure acts as a barrier to the spread of infection, creating a sharply marginated pneumonia that can be mistaken for atelectasis or consolidation of a whole lobe, or even for a mediastinal mass. The part of lung that is separated off by an accessory fissure has been termed an accessory lobe. In reality, the arrangement of bronchopulmonary segments and the bronchial and arterial anatomy are all normal: only the fissure is supernumerary.

Incomplete Fissures

The major fissures as well as the minor fissures are often incomplete, and the lung parenchyma of the adjacent pulmonary lobes fuses. More than 95% of incomplete interlobar fissures are associated with bronchovascular structures traversing the fused segments. Most commonly the structure is a pulmonary vein (figure 8), and less commonly it is a pulmonary artery or bronchus, which may also influence the clinical course of various lung diseases.

Fig.: Minor (horizontal) fissure incomplete. The lung parenchyma of the adjacent pulmonary lobes is fused.

The degree of incompleteness ranges from nearly complete absence to nearly complete presence of the fissure. When part of the fissure is absent, it is always toward the mediastinal side of the fissure.

There are several clinical implications of an incomplete major fissure (figure 9). When there is an incomplete fissure, the lung parenchyma of adjacent lobes fuse and can allow the spread of disease and collateral air drift. Disease processes, such as pneumonia, can spread between adjacent lobes via the Kohn pores and the canals of Lambert. Collateral air drift can be clinically important in the setting of lobar bronchial obstruction. The air drift can allow continued aeration of the lobe with the obstructed bronchus via the adjacent unobstructed lobe across an incomplete major fissure. Incomplete major fissure also affects the distribution pattern of pleural effusion.

The Azygos Lobe

Incomplete medial migration of the right posterior cardinal vein, precursor of the azygos vein, gives rise to an azygos lobe of the right lung in 0.4%-1% of the population. This lobe is easily identified on CT scans of the thorax (figure 10).

Fig.: CT scan shows displaced azygos vein (arrow) and separating azygos lobe from right upper lobe.

The azygos vein is more lateral than usual as it ascends along the vertebral column and rises farther superiorly before arching anteriorly to enter the SVC or right brachiocephahic vein. The laterally positioned azygos arch allows intrusion of lung farther into the pretracheal and retrotracheal mediastinum than is found in normal patients (figure 11). The azygos fissure represents four layers of pleura interposed between the azygos lobe medially and the remainder of the right lung laterally.

Systemic vessels

When the process of arch development and regression does not follow the normal sequence, a variety of anomalies may result (figure 12).

Fig.: Thoracic DSA. Uncommon aortic arch anomaly. Two supra-aortic trunks: common origin of the common carotid arteries and common origin of the subclavian arteries.

Common anomalies include right aortic arch, “bovine arch” with the left common carotid artery origin arising from the innominate artery (figure 13) (figure 14), separate origins of right carotid and subclavian arteries, common origin of the left common and subclavian artery, origin of left vertebral artery from the aorta, and unilateral or bilateral separate origins of internal and external arteries.

Fig.: MR angiography - VRT. “Bovine arch” with the left common carotid artery origin arising from the innominate artery

The aberrant right subclavian artery (with left aortic arch) is the most common congenital aortic arch abnormality; it occurs in about 0.5-2% of the population (figure 15). The right subclavian artery is the last of the major arteries to arise from the aortic arch (figure 16).

Fig.: MR angiography - VRT. Aberrant right subclavian artery with left aortic arch

A diverticulum-like outpouching, the kommerell diverticulum, may be seen at the site of origin of the aberrant vessel.

Although this disorder is usually asymptomatic, dysphagia has been reported and has been called dysphagia lusoria.

The right-sided aortic arch occurs in approximately 0,05 - 0,2% of the population and appears as a “mass” in the right paratracheal region (figure 17) (figure 18) (figure 19).

Fig.: Barium swallow study (previous figure magnified). The oesophagus appears left deviated by the right-sided aortic arch.

Abnormalities of the right aortic arch are classified into three types, depending on the order of the branching vessels from the arch.

Right aortic arch with aberrant left subclavian artery is the most common type (figure 20); it is often asymptomatic and is associated with a low prevalence (5% to 12%) of congenital heart disease. The great arteries arise from the arch in the following order: left common carotid artery, right common carotid artery, right subclavian artery and left subclavian artery.

Fig.: Scheme. Right aortic arch with aberrant left subclavian artery is the most common type.

Fig.: The most frequent form of vascular ring with a right aortic arch and an aberrant origin of the left subclavian artery from a retroesophageal diverticulum (diverticulum of Kommerell) is present (arrows). The left subclavian artery is the last branch of the aortic arch (distal to the right subclavian artery).

Mirror-image branching (figure 21) is associated with a 98% prevalence of severe congenital heart disease and is usually diagnosed in early childhood.

Fig.: Scheme. Right aortic arch with mirror-image branching.

In the third type, which is extremely mare, the origin of the left subclavian artery is isolated.

The cervical aortic arch is a rare abnormality which simulates a posterior mediastinal mass or an apical mass. The aortic arch extends cephalad in the mediastinum anterior to the brachiocephalic vein, usually to the level of the subclavian arteries. The aortic arch is right sided in approximately two-thirds of the cases.

Signs and symptoms at presentation most commonly include a pulsatile neck mass followed by upper airway obstruction or dysphagia.

Anomalies of the systemic thoracic veins usually are asymptomatic but may be associated with other more serious cardiovascular abnormalities.

A persistent left superior vena cava can simulate a mass in the left mediastinum at the level of the aortic arch. This anomaly represents persistence of the embryonic left antenor cardinal vein.

A persistent left superior vena cava is found in approximately 1-3% of the general population. Is usually an isolated finding but it can be associated with congenital heart disease (figure 22), usually an atrial septal defect. In a majority of cases (82%–90% of patients), the right superior vena cava is present but is smaller than usual (figure 23).

Fig.: Coronal MPR reconstruction shows “both” the superior vena cava. There is intense enhancement of the “right superior vena cava” because contrast agent was injected into the right arm.

The persistent left superior vena cava most commonly courses lateral to the aortic arch (figure 24) and anterior to the left hilum and drains to the coronary sinus (which is physiologically insignificant). The coronary sinus in such patients is typically enlarged. In a small minority of patients, the anomalous vessel drains to the left atrium, creating a right-to-left shunt. The SVC and coronary sinus may enhance intensely if contrast agent is injected into the left arm.

The differential diagnosis includes anomalous pulmonary venous drainage (APVC) from the left upper lobe. Several findings are useful for distinguishing these conditions. In partial APVC of the left upper lobe, the intraparenchymal left upper lobe vessels can be shown to connect to the vertical vein. In persistent left SVC, two vessels (the normal left superior pulmonary vein and persistent left SVC) are identified anterior to the left main bronchus, the coronary sinus may be enlarged and the left brachiocephalic vein or right SVC may be small or absent.

Interrupted intrahepatic inferior vena cava with azygos or hemiazygos continuation is an interesting cause of enlargement of not only the azygos vein but also other structures that simulate mediastinal masses (figure 25a). The prevalence of this anomaly is about 0.6%.

Fig.: Azygos continuation of the inferior vena cava. a) Contrast-enhanced CT shows the dilated azygos vein (arrow) lateral to the descending aorta. b) Dilated azygos vein (*). c) Another patient with azygos continuation of the inferior vena cava (*), associated with mild enlargement of the hemiazigos vein (arrow).

Failure to develop a communication between the right subcardinal and hepatic venous system during embryogenesis has been postulated to cause interruption of the hepatic segment of the IVC, resulting in loss of continuity with the prerenal IVC (figure 25b) (figure 25c). The atrophy of the right subcardinal vein results in shunting of venous blood from the lower abdomen, pelvis, and lower extremities into the azygos or hemiazygos venous systems.

Although previously associated with congenital cardiovascular anomalies, asplenia, and polysplenia syndromes, this anomaly has been increasingly identified in asymptomatic patients.

Other IVC congenital anomalies that manifest as enlarged azygos and hemiazygos veins include absence of the infrarenal IVC and duplication of the IVC with azygos or hemiazygos continuation.

Preoperative knowledge of the anatomy may be important in planning cardiopulmonary bypass and to avoid difficulties in catheterizing the heart.

The lower eight posterior intercostal veins drain into the azygos vein on the right and the accessory hemiazygos and hemiazygos veins on the left. The first intercostal space is drained by the supreme intercostal vein, which drains into an ipsilateral vertebral or brachiocephalic vein. The second, third, and possibly fourth veins drain into a common vessel, the superior intercostal vein, which drains into the brachiocephalic veins. The left superior intercostal vein usually receives the accessory hemiazygos vein. This is sometimes termed the aortic nipple due to a focal lateral contour opacity seen on anterior radiographs of the aortic arch (figure 26).

Fig.: Chest radiograph (a) shows focal contour opacity of the aortic arch. Axial superior CT image (b) shows the left superior intercostal vein (arrow; green arrow in c) extending around the aortic arch. The azygos vein (yellow arrow in c) and hemiazygos vein (arrow in d) are also seen.

Pulmonary vessels

Pulmonary Artery Sling

Pulmonary artery sling, also known as anomalous origin of the left pulmonary artery, is a condition in which the left main pulmonary artery arises from the proximal right main pulmonary artery and courses to the left hemithorax. As the anomalous left pulmonary artery crosses to the left hemithorax, it courses between the trachea and esophagus, resulting in a “sling” around the distal trachea.

There are two types of pulmonary artery slings:

- In a type 1 pulmonary artery sling, the carina is normally located at the T4 through T5 vertebral levels and there is a characteristic compression of the posterior aspect of the trachea, right main stem bronchus, and anterior aspect of the esophagus.

-In a type 2 pulmonary artery sling, the carina is low in location, at the T6 level, and is associated with congenital long-segment tracheal stenosis, a horizontal course of the main bronchi (ie, T-shaped carina), a bridging bronchus, and several other congenital anomalies.

Patients with pulmonary artery sling typically present during the neonatal period with respiratory symptoms such as stridor, apneic spells, and hypoxia. Sometimes this entity can be diagnosed in adults (such as in the case shown in figure 27).

Fig.: Pulmonary Artery Sling in an asymptomatic 70 year-old woman. CT demonstrates the left pulmonary artery (arrow) arising from the right pulmonary artery, before crossing behind the trachea to reach the left lung.

Imaging findings of pulmonary artery sling depend on its type, as well as on other associated congenital anomalies.

Pulmonary Vein Anatomic Variants

Pulmonary vein anatomy is more variable than pulmonary artery anatomy, and developmental anomalies are common. A common or conjoined vein occurs when superior and inferior veins combine proximal to the left atrium, resulting in only one atriopulmonary venous junction on the involved side, whereas supernumerary or accessory pulmonary veins are extraveins with independent atriopulmonary venous junctions separate from the superior and inferior pulmonary veins.

Common anomalies include a common left or right pulmonary vein in 2.4– 25% of individuals. Supernumerary or accessory veins are also frequently seen. The most common is a separate right middle pulmonary vein (figure 28), which drains the middle lobe. This is estimated to occur in 1.6–19% of individuals. Accessory veins typically have a narrower atriopulmonary venous junction than do the superior and inferior pulmonary veins. The occurrence of more than three pulmonary veins on the same side has been described rarely in the literature. In most such instances, one or more of the pulmonary veins manifested an abnormal return.

Fig.: Coronal MIP reconstruction (a), posterior three-dimensional reconstruction (b) and VR image endocardial view (c) from nongated multi–detector row CT performed show an accessory right middle lobe pulmonary vein (arrows).

Radio-frequency catheter ablation of the distal pulmonary veins and posterior left atrium is increasingly being used to treat recurrent or refractory atrial fibrillation that resists pharmacologic therapy or cardioversion. Radiologists must be familiar with anatomic variants of the left atrium and distal pulmonary veins and understand the importance of these variants to the referring cardiac interventional electrophysiologist.

Several anomalous pulmonary and systemic connections exist, and drainage can be partial or total. They are also often associated with obstruction of pulmonary venous flow and abnormal shunting of blood, which lead to cyanosis and to be associated with congenital cardiac abnormalities, especially atrial septal defect.