The Core Curriculum: Cardiopulmonary Imaging, 1st Edition (2004)

Chapter 1. Basic Thoracic Anatomy and Physiology

An understanding of thoracic imaging requires knowledge of the anatomy being imaged, as described in this chapter, as well as the imaging techniques applied to the thorax, covered in Chapter 2Table 1.1 lists the major anatomic structures within the thorax that are discussed. The anatomic illustrations are presented as either line drawings or as computed tomography (CT) renderings of thoracic anatomy generated from 16-row multidetector CT examinations of the thorax at thin collimation.

Chest Wall

The thoracic contents are bounded by the chest wall, providing both the shape of the thorax and protection for the intrathoracic contents (Table 1.2). The skin, subcutaneous tissues, and muscles that surround the rib cage and shoulder girdle appear radiographically indistinguishable from each other, whereas on CT the skin, fat, and muscles are recognized by their difference in attenuation. The hard endoskeleton of the thorax is comprised primarily of the ribs, sternum, and spine anchored to the shoulder girdle by the clavicles, scapulae, and surrounding muscles (Fig. 1.1).

Ribs

The obliquely oriented ribs are slanted anteriorly in a downward direction laterally to medially (Figs. 1.2A and 1.3A), whereas the posterior ribs are slanted in an upward direction from laterally to medially (Figs. 1.2B and 1.3B) (1). On the inferior inner surface is a costal groove, in which an intercostal artery, vein, and nerve run. Posteriorly, the ribs articulate with the spine, and anteriorly the costochondral cartilages (first to seventh) are attached to the sternum and a cartilaginous bridge (eighth to twelfth). Anteriorly, the costochondral cartilages are radiolucent in childhood through young adulthood, gradually calcifying with advancing age. There are gender-specific patterns by which this cartilage calcifies. In women the central portion calcifies first, whereas in men the upper and lower borders calcify first (2). The first costochondral cartilage is often particularly bulky and can mimic an underlying pulmonary nodule, particularly when asymmetric. The right ribs can be differentiated from the left ribs on a lateral radiograph using the “big-rib sign” (Fig. 1.3B) (3). By convention, lateral images are taken with the left side against the radiographic plate; the right ribs are therefore further from the radiographic plate and appear to be magnified or bigger because of divergence of the x-ray beam. The most common supernumerary or accessory rib is a cervical rib arising from the seventh cervical vertebra as an enlarged costal element. They occur in approximately 0.5% to 1.5% of the population, are usually bilateral, are more common in women, and may be a cause of thoracic outlet syndrome (1,4,5). Occasionally, ribs may be congenitally fused or bifid in appearance.

 

Usually, the thorax is wider in transverse dimension than in the anteroposterior dimension. A “barrel-shaped” chest refers to a thorax that is equally wide in both dimensions and is a sequela of chronic obstructive lung disease, such as emphysema. Along the inferior border of the first and second ribs, 1 to 5 mm thick “companion shadows” are seen on radiographs in approximately one-third of individuals and are due to adjacent fat (6,7); they should not be mistaken for a pneumothorax (Chapter 17).

Table 1.1: Thoracic Anatomy Overview

Chest wall
Diaphragm
Mediastinum
Pleura
Airway
Lungs
Pericardium
Heart
Aorta and arterial structures
Vena cava and venous structures
Lymphatic system

Table 1.2: The Chest Wall

Nonosseous

Osseous

Skin

Ribs

Subcutaneous fat

Sternum

Muscles

Shoulder girdle
Spine

In women the central portion of the costochondral cartilage calcifies first; in men the upper and lower borders calcify first.

The “big-rib sign” on a lateral radiograph is used to identify the right ribs.

Figure 1.1 Normal thoracic endoskeleton. Three-dimensional computed tomography reconstructions in the (A) anterior and (B) posterior projections. Asterisk, scapulae; C, clavicles; M, manubrium; S, sternal body; V, vertebrae; X, xiphoid.

Figure 1.2 Ribs, sternum, and scapulae on (A) anterior and (B) posterior projections. The intercostal arteries (IA) are seen running immediately inferior to the ribs. Note the downward slant of the anterior ribs from the lateral to medial direction. The partially calcified costochondral cartilages (CC) extend from the anterior rib ends to the sternum. The second costochondral cartilage extends to the sternomanubrial angle (CC#2). The seventh costochondral cartilage (CC#7) extends medially to the xiphoid process. The internal mammary arteries (IMA) and veins (IMV) parallel the lateral borders of the sternum. Note the upward slant of the posterior ribs from the lateral to medial direction. Asterisk, scapulae; M, manubrium; S, sternal body.

Figure 1.3 Normal (A) posteroanterior and (B) lateral chest radiographs demonstrate the position of the anterior and posterior ribs. Thedashed vertical line separates the anterior and middle mediastinum, and the solid vertical line separates the middle and posterior mediastinum. Note the “big rib” sign on the lateral radiograph (larger more posterior appearing ribs are the right ribs [RR] versus the smaller left ribs [LR]). SC, scapulae; A, anterior mediastinum; AE, azygoesophageal recess; APW, aortopulmonary window; C, clavicles; M, middle mediastinum; P, posterior mediastinum; RPT, right paratracheal stripe; S, sternum; SP, spinous processes; v, vertebral bodies; Asterisk, azygos vein.

On axial CT images, only a portion of each rib is seen because of their oblique orientation. Counting the ribs on CT can be done using three anatomic landmarks: clavicular heads (rib 1), sternal angle (rib 2), and xiphoid process (rib 7). The first rib can be found behind the medial third of the clavicle (8). This is the method we generally use, acknowledging this is tedious when counting down to the lower most ribs. The second costochondral cartilage anteriorly is located at the sternal angle; this method can also be used on a lateral radiograph. Finally, the seventh costochondral cartilage can be found at the xiphoid process (1); this landmark is less consistent (Fig. 1.2A).

Counting the ribs on CT can be done using three landmarks, from which successive ribs can be counted in the posterior direction.

Sternum

The sternum forms the midline anterior portion of the thoracic cage and is made up of the manubrium (the thickest portion; articulates with the clavicles), the sternal body, and the xiphoid process (Figs. 1.1A1.2A, and 1.4). The sternal notch, also known as the jugular notch, is at the upper manubrium border; palpating this area reveals the trachea. The sternal angle is formed by the manubrium and sternal body and is also known as the angle of Louis. On a chest radiograph it is at the T-4 vertebral body level.The xiphisternal articulation is at the T-9 vertebral body level. Seven costochondral cartilages articulate with the sternum. Deformities of the sternum occur in 1 in 300 people and are usually of little clinical consequence, other than being aesthetically unattractive; when severe, respiratory symptoms may occur (9). The most common is pectus excavatum, also known as “funnel chest”; it represents depression of the sternum from its normal location, easily recognizable on lateral radiographs. On a frontal radiograph it may cause blurring of the right heart border, mimicking a right middle lobe process, with displacement of the heart to the left, and exaggerated anterior angulation of the ribs. Pectus carinatum or “pigeon breast” represents abnormal protrusion of the sternum; the middle and lower sternum are most commonly involved (9,10). There may be an association with congenital heart disease in up to 10% of individuals. A sternal foramen is an incidental focal defect in ossification or failure of union of embryonic elements that form the sternum, readily visible on CT as a hole in the sternum. Although the sternum is difficult to evaluate on chest radiographs, particularly on the frontal views, it can be evaluated in great detail with CT.

On a posteroanterior radiograph, the sternal angle is at the T-4 vertebral body level.

Pectus excavatum creates an ill-defined right heart border on a posteroanterior radiograph.

Figure 1.4 Midline sagittal reconstruction demonstrates the vertically oriented esophagus (E) anterior to the descending thoracic aorta (DA). CA, celiac axis; D, diaphragm; LSA, left subclavian artery; M, manubrium; NF, neural foramina; S, sternum; SMA, superior mesenteric artery; SP, spinous processes; V, vertebra by thoracic number.

Shoulder Girdle

Together the clavicles and scapulae, otherwise know as collar bone and shoulder blades, form the shoulder girdle (Figs. 1.11.2, and 1.3). The clavicle is a double-curved bone that medially articulates with the sternum and laterally with the acromion of the scapula; both are synovial joints. Numerous ligaments anchor the clavicle as well, including the costoclavicular and sternoclavicular ligaments medially and the acromioclavicular and coracoclavicular ligaments laterally. The rhomboid fossa is a notch that is occasionally seen radiographically on the undersurface of the medial clavicle 1 to 2 cm from the sternal articulation and is the location from which the costoclavicular ligament arises. The interspace between the first and second ribs anteriorly can be palpated immediately below the medial clavicle.

The scapulae are thin flat bones with a thick ridge or spine projecting posteriorly. The costal surface of the scapula faces the ribs and is concave, containing the subscapularis muscle, whereas the dorsal surface is convex and contains the supraspinatus and infraspinatus muscles above and below the spinal ridge, respectively. Laterally the spine continues into the acromion. The superolateral aspect of the scapula is concave, forming the glenoid fossa. Superiorly there is a small scapular notch. Sprengel’s deformity, or high-riding scapula, is an uncommon congenital abnormality (11).

Spine

There are 12 thoracic vertebrae (Figs. 1.1B1.3B, and 1.4). There is less variation in the number of thoracic vertebral bodies than in the lumbosacral spine. Each vertebra is made up anteriorly of a body or corpus designed for weight-bearing, connected posteriorly to the vertebral arches that are made up of the pedicles and laminae. The cross-sectional anatomy of a thoracic vertebra is illustrated in Fig. 1.5. Progressively lower thoracic vertebrae are larger than the one above it. The bodies of adjacent vertebra articulate with each other through a cartilaginous joint containing a fibrocartilaginous intervertebral disc. The corpus and arches surround the vertebral foramen, which contains the spinal cord. The primary function of the arches is to protect the spinal cord. The spinous processes project posteriorly and inferiorly from the arches. The transverse processes project laterally from the arches. The superior and inferior articular processes project upwardly and downwardly from the laminae, respectively, to form the synovial facet joints. Spinous processes and lamina of adjacent vertebra articulate across fibrous joints. Each vertebra articulates with a pair of ribs; the rib head articulates with the upper aspect of vertebra of the same number, the tubercle with the inferior aspect. At many levels the rib head also articulates with the rib at the next lower segmental level. The space between the pedicles of two adjacent vertebra form the intervertebral foramen, through which the intercostal nerves exit the spinal canal (Fig. 1.4). The upper four thoracic vertebrae may have features of cervical vertebra, the middle four are typically thoracic, and the lower four may have features of lumbar vertebrae. There is normally kyphosis to the thoracic spine. A loss of thoracic kyphosis results in a straight thoracic spine. This “straight-spine” or “straight-back” syndrome may include thoracic scoliosis, pectus excavatum, and is associated with mitral valve prolapse (12,13).

The 12 thoracic vertebral bodies form the normal kyphosis of the posterior chest wall.

Diaphragm

The diaphragm is a dome-shaped convex-upward musculotendinous structure that separates the thoracic and abdominal cavities; it is divided into three portions. The sternal portion is the smallest and arises from the xiphoid process. The costal portion is the largest, arising from the seventh through twelfth ribs. The vertebral (a.k.a. lumbar) portion arises from the lateral aspects of the L1-4 vertebrae in the form of two crura; the right crus is thicker and longer than the left. Thefibrous component of each portion extends centrally to form the central tendon. The right hemidiaphragm is usually half an interspace higher than the left, with its upper border at the fifth anterior rib through the sixth to seventh anterior rib interspace level on the posteroanterior radiograph (4,14). There are two possible explanations: The liver below the hemidiaphragm on the right elevates it or the heart above the left hemidiaphragm pushes it down.

The muscular diaphragm plays a major role in inspiration by moving caudally, generating negative intrathoracic pressure and therefore airflow into the lungs.

Three openings in the diaphragm allow passage of structures between the thorax and abdomen. The foramen for the inferior vena cava is located in the central tendon at the T8-9 intervertebral disc level to the right of midline. The midline esophageal hiatus arises within fibers of the right crus, contains the esophagus and vagus nerve, and is located at the T-10 vertebral body level. The aortic hiatus is located at the T12-L1 intervertebral disc level just to the left of midline and contains the aorta, azygos and hemiazygos veins, and thoracic duct. It actually lies behind the crura of the diaphragm where they join. A tendinous arch joins the crura together anterior to the hiatus, known as the median arcuate ligament. Where the diaphragm overlies the psoas muscles and quadratus lumborum, there is a thickening in the fascia that forms the medial and lateral arcuate ligaments, respectively. In attaching to the diaphragm, they seal the abdominal contents below the diaphragm. The diaphragm is inseparable from the abdominal structures on radiographs. On CT it is visible as a soft tissue attenuation band-like structure (Fig. 1.4).

The common congenital hernias of the diaphragm arise from failure of portions of the diaphragm to fuse. Ninety percent of these areBochdalek hernias, which arise posterolaterally due to failure of the costal and vertebral portions of the diaphragm to fuse; these are more common on the left than the right because of the protective effect of the liver posteriorly and are usually diagnosed in neonates and young children. Although an acute hernia can be life threatening in adults because of bowel strangulation, small incidental Bochdalek hernias are common on CT, particularly in patients with long-standing obstructive lung disease (15). Morgagni hernias arise anteromedially from failure of the sternal and costal portions to fuse; these are more common on the right.

Mediastinum

The mediastinum is bounded laterally by the mediastinal pleura, inferiorly by the diaphragm, and superiorly by the thoracic inlet. It is divided into compartments. Anatomists, surgeons, and radiologists often use different definitions of these compartments. The definitions used here are based on the classification of Dr. Benjamin Felson. Many have said that the specification of “compartment” is artificial today, because the use of cross-sectional imaging makes discrete localization of abnormalities possible. However, the compartments convey information on anatomic localization, imply differential diagnoses, and are very applicable when interpreting chest radiographs. The easiest way to describe these compartments is by drawing two vertical lines on a lateral radiograph (Fig. 1.3B). The anterior mediastinum is everything anterior to the line drawn along the anterior aspect of the trachea, continuing inferiorly along the posterior heart border. Theposterior mediastinum is everything posterior to the line drawn one-third of the vertebral body width behind the anterior border of the spine. Everything between these two lines is the middle mediastinum. Many classifications (but not this one) place the heart in the middle mediastinum. The aortopulmonary window is that part of the mediastinum that is between the main pulmonary artery and the aortic arch and is normally concave to flat on the posteroanterior radiograph (Figs. 1.3A and 1.5). The left recurrent laryngeal nerve runs in the aortopulmonary window; hence, masses in this location may be associated with left vocal cord paralysis and hoarseness. The contents of the mediastinal compartments are listed in Table 1.3. The borders of the cardiopericardial silhouette and mediastinum are described in Chapter 3.

The left recurrent laryngeal nerve runs in the aortopulmonary window; new hoarseness or vocal cord paralysis may be due to a mass in this location.

Esophagus

The esophagus is a vertically oriented structure that runs the length of the middle mediastinum, anterior to the spine, just to the left of midline, posterior to the airway, and to the right of the descending thoracic aorta (Figs. 1.4 and 1.5). The esophagus is divided into thirds. The proximal 5 cm long cervical esophagus begins at the cricoid cartilage at the C-6 vertebral body level and ends at the thoracic inlet. The middle third of the esophagus begins at the thoracic inlet and ends at the tracheal carina, whereas the lower third extends from the carina to the gastric fundus at the gastroesophageal junction, located at the T-11 vertebral body level (16). The reflection of the right mediastinal pleura against the vertically oriented esophagus and azygos vein forms the azygoesophageal recess below the level of the azygos arch (Fig. 1.3A). Smooth indentations are noted on the esophagus at the level of the aortic arch and the left main bronchus during an esophagram. The esophagus is a muscular organ made up of striated muscle proximally and smooth muscle distally, the primary function of which is the propulsion of food. The resting state of the esophagus is collapsed, becoming distended transiently when liquid or solid material is swallowed. The esophageal wall has mucosal, submucosal, and muscular layers, but unlike the stomach and intestines does not possess an adventitia or covering serosal layer. This permits the spread of esophageal malignancy directly into the mediastinum.

Figure 1.5 Axial computed tomography image through the thymus gland (T) and the aortopulmonary window (APW). The thymus gland drapes over the ascending aorta and aortopulmonary window. In this 52 year old it is almost completely of fat attenuation. A, azygos vein; AA, ascending aorta; DA, descending aorta; E, esophagus; R, ribs; S, sternal body; TS, transverse sinus of the pericardium; Asterisk, scapulae.

Displacement of the azygoesophageal recess to the right in the subcarinal region indicates a subcarinal mass, such as a bronchogenic cyst or lymph node mass. Displacement just above the diaphragm is most commonly due to a hiatal hernia.

Unlike the rest of the gastrointestinal tract, the esophagus has no outer adventitial layer to impede the spread of malignancy.

Table 1.3: Mediastinal Compartments and Their Contents

Anterior

Middle

Posterior

Thymus

Airway

Nerve roots

Heart

Aortic arch
Esophagus
SVC
Aygos/Hemiazygos veins
Thoracic duct

Descending aorta

Lymph nodes

Lymph nodes
Phrenic/vagus nerves

Lymph nodes

Thymus

The thymus is a bilobed gland located in the superior portion of the anterior mediastinum behind the manubrium, anterior to the trachea, aortic arch, great vessels, and the brachiocephalic veins, extending caudally onto the surface of the pericardium (Fig. 1.5). It is comprised predominantly of lymphocytes and has a connective tissue capsule. T-lymphocytes are produced in the thymus gland and migrate to other lymphoid organs in the body for maturation. This quadrilateral shaped structure is readily visible on the chest radiographs of children, reaching its largest size at age 2 (16). Over time the thymus involutes and becomes replaced with fat; more than one-half of individuals over 40 years of age have a completely fatty thymus on CT (17). On CT, the soft tissue attenuation of the thymus can be located in all individuals under 30 years of age, but in only 17% of adults over 49 years of age. By 60 years of age the thymus gland weighs less than 50% of what it weighed before 19 years of age (18). With thymic hyperplasia the thymus may be abnormally enlarged. Rebound thymic hyperplasia often occurs in children and young adults after chemotherapy or corticosteroid treatment. In contrast, follicular or lymphoid thymic hyperplasia occurs with conditions such as myasthenia gravis.

The bilobed quadrilaterally shaped thymus reaches peak size at age 2, gradually involuting with fatty replacement through the fifth decade of life.

Pleura and Fissures

The pleura is comprised of a single cell layer of mesothelial cells, with submesothelial connective tissue containing a rich network of lymphatics. The lungs are enclosed within the pleural space, as if the lungs were a fist that was pushed into a balloon until the opposing sides of the balloon meet, with the balloon (pleura) almost completely surrounding the fist except at the wrist, analogous to the hilum of the lung. The parietal pleura lines the thoracic cage, and the visceral pleura is adherent to the lungs. Parietal pleura is named according to that portion of the thoracic cage that is adhered by the surrounding endothoracic fascia, hence cervical pleura over the apex of the lung, costal pleura along the ribs, diaphragmatic, and mediastinal pleura, respectively. The parietal pleura and visceral pleura are inseparable at the hila of the lungs. At the upper border of the hila they are directly contiguous. At the lower border of the hila there is a redundancy where the visceral and parietal meet, known as the inferior pulmonary ligament. This runs caudally from the inferior pulmonary vein through the lungs as the inter-sublobular septum, ending on the surface of the diaphragm. It is the thickness of two layers of pleura, anchors the lung to the diaphragm, and is a potential pleural space in which fluid or air may appear loculated (19). A small amount of fluid within the pleural space provides lubrication during respiration, as the parietal pleura moves with the chest wall, whereas the visceral pleura moves with the lung. The caudal extent of the parietal pleura is much greater than the visceral pleura, with the costodiaphragmatic recesses extending caudally by several interspaces or approximately 5 cm in quiet respiration.

Parietal and visceral pleura blend at the hilum of each lung; redundancy inferior to this creates the inferior pulmonary ligament.

The caudal edge of the pleural space is several centimeters lower than the edge of the lung, particularly posteriorly.

The fissures are invaginations of visceral pleura into the lungs, dividing the lungs into lobes (Figs. 1.6 and 1.7). The normal fissures include a major (or oblique) fissure that runs obliquely through each lung and separates the upper and lower lobes and a dome-shaped minor (or horizontal) fissure that runs horizontally through in the right lung at the level of the fourth anterior rib and separates the right upper and middle lobes. On radiographs, the major fissures begin posteriorly at the T-5 vertebral level and run caudally and anteriorly downward, touching the diaphragm a few centimeters behind the anterior chest wall. On radiographs, the pleural surfaces are not visible, except for the fissures. On thick-section CT the fissures appear as avascular planes. With thin collimation CT the fissures themselves are seen, as is the parietal pleura, particularly if there is abundant extrapleural fat. Although we think of fissures as complete structures that prevent the spread of disease such as pneumonia from one lobe to another, the use of thin collimation CT routinely reveals that in 83% of right lungs and 50% of left lungs the fissures are incomplete (20).

Pleural fissures are commonly incomplete.

Accessory fissures are extra fissures that can be seen on approximately 10% of chest radiographs, 20% of conventional CTs, and with higher frequency on thinner multidetector CT acquisitions. Detail on specific accessory fissures, including the azygos fissure, the inferior accessory fissure, the superior accessory fissure, and other accessory fissures is given in Chapter 17 (21,22,23). There are many more accessory fissures seen on gross pathologic examination and with thin-section CT, most of which are unnamed (24).

Figure 1.6 Sagittal computed tomography reconstructions demonstrate the pleural fissures. A. Oblique course of the left major fissure (arrows). B. Oblique course of the right major (arrows) is slightly less vertical than the left; horizontal course of the minor fissure (arrowheads). LLL, left lower lobe; LUL, left upper lobe; RLL, right lower lobe; RML, right middle lobe; RUL, right upper lobe.

Figure 1.7 Coronal computed tomography reconstructions demonstrate the pleural fissures. A. Anterior: the caudal extent of the major fissures (MF) is anterior. CM, cardiopericardial silhouette. B. Middle: minor (MIN) and major fissures on the right. C. Posterior: the cephalad extent of the major fissures (MF) is highest posteriorly. LLL, left lower lobe; LUL, left upper lobe; RLL, right lower lobe; RUL, right upper lobe.

Airway

The airway conducts air from the larynx to and from the alveoli of the lungs for the purpose of gas exchange with blood in the pulmonary capillary circulation. The trachea and large bronchi are rigid and noncollapsible because of the presence of cartilaginous c-shaped rings or plates and are thick enough so that there is no gas exchange across their walls, their primary function being to conduct air. They are lined by pseudostratified ciliated columnar epithelium that facilitates the clearing of small particulate matter from the airways in an upward direction toward the oropharynx. Glands within the subepithelial connective tissue secrete mucus. Bronchioles do not contain cartilage in their walls and are of two types. Terminal (a.k.a. membranous) bronchioles measure approximately 0.6 mm in diameter and conduct air like the trachea and bronchi. The smaller respiratory bronchioles have alveoli arising from their walls, and both conduct air as well as exchange gas (25).

Trachea and Main Bronchi

Airway anatomy is illustrated in Fig. 1.8. The trachea extends from the larynx at approximately the C-6 vertebral level to the T-5 level where it bifurcates at the carina (26). It extends above and below the sternal notch, above which it can be directly palpated. The trachea is generally a midline structure, normally deviating to the right at the level of the aortic arch and with a gradual slant from the anterior to posterior direction as it extends caudally from the larynx. This rightward slant explains why aspirated material more readily reaches the right lung than the left. The trachea contains 16 to 20 c-shaped cartilage rings that are horizontal and parallel to each other, that prevent airway collapse during expiration. Posteriorly, the membranous (noncartilaginous) wall may be convex outward, flat, or convex inward; the latter usually indicates expiration on imaging studies. The right main bronchus has six to eight cartilage rings and is shorter and more horizontal than the left main bronchus, arising at a 25-degree angle. The left main bronchus is 4 to 5 cm long, contains 9 to 12 cartilage rings, and is more oblique in orientation, arising at a 45-degree angle (26). The right main bronchus is the eparterial bronchus, that is, the bronchus sits above the right pulmonary artery, whereas the left main bronchus is the hyparterial bronchus, positioned below the left pulmonary artery. The right main bronchus divides into the right upper lobe bronchus and the 3 to 4 cm long bronchus intermedius, the latter then dividing into the right middle and right lower lobe bronchi. The left main bronchus divides into the left upper lobe bronchus, lingular bronchus, and left lower lobe bronchus. Additional details on tracheal and main bronchus diameter and length and common variants of anatomy, such as the tracheal or “pig” bronchus, the “bridging” bronchus, and the “paracardiac” bronchus, can be found in Chapter 16 (27).

On a posteroanterior radiograph, the sternal angle is at the T-4 vertebral body level.

A concave posterior membranous wall of the trachea on CT is usually an indication that the image was obtained during expiration.

The right main bronchus is eparterial. The left main bronchus is hyparterial.

Figure 1.8 Coronal minimum intensity projection of the central airways. Asterisk, carina; AA, aortic arch; BI, bronchus intermedius; L, lingula; LLL, left lower lobe bronchus; LM, left main bronchus; LUL, left upper lobe bronchus; RLL, right lower lobe bronchus; RM, right main bronchus; RML, right middle lobe bronchus; RUL, right upper lobe bronchus; T, trachea.

The right paratracheal stripe is formed by the reflection of mediastinal pleura against the right wall of the trachea (Fig. 1.3A). The esophagus lies immediately posterior to the trachea and slightly to the left; with disease such as esophageal cancer, care must be taken to exclude airway invasion when the tumor is in direct contiguity with the airway. Small mucous pseudotumors are estimated to appear in the central tracheobronchial tree on CT in approximately 2% of examinations; with coughing and repeat image these should be cleared (28).

Intrapulmonary Bronchi

Lobar bronchi divide into the segmental bronchi, as listed in Table 16.1. The lobar bronchi are 1 to 2 cm in length; the lingular bronchus is 2 to 3 cm long. The azygos vein arches over the right upper lobe bronchus from the posterior to anterior direction; on a posteroanterior radiograph it is seen end-on as a biconvex structure 1 cm or smaller in size in the tracheobronchial angle (Fig. 1.3A). Of note, the right middle lobe bronchus arises anterolaterally at the same level that the right lower lobe superior segment bronchus arises and heads directly posteriorly. In the lower lobes the superior segments arise first, followed by a basilar stem that gives rise to the segmental bronchi, as listed in Table 1.4. The basilar bronchi arise in the order they are listed; as seen on CT they arise counterclockwise in the right lower lobe and clockwise in the left lower lobe. The medial and anterior segments are generally anterior to the inferior pulmonary veins and the lateral and posterior segments posterior to the inferior pulmonary vein. Although most bronchi run obliquely or perpendicular to the CT axial sections and are therefore seen in cross-section, bronchi such as the lingular bronchus and its segmental bronchi and the segmental bronchi of the right middle lobe run in the axial plane of CT and are seen in the axial CT plane. Ten generations of bronchi divide in a generally dichotomous manner, decreasing in diameter from 12.2 mm centrally to 1.3 mm peripherally (29).

“M.A.L.P.” is the branching order of the lower lobe segmental bronchi in a clockwise direction in the left lower lobe and the counterclockwise direction in the right lower lobe. M, medial; A, anterior; L, lateral; P, posterior.

Table 1.4: Pulmonary Lobar and Segmental Bronchi

Lobar

Segmental

Right upper lobe

Anterior Posterior   Apical

Right middle lobe

Medial   Lateral

Right lower lobe Superior

Medial   Anterior   Lateral   Posterior

Left upper lobe

Anterior   Apicoposterior

Lingula Superior Inferior

Left lower lobe Superior

Anteromedial   Lateral   Posterior

Lungs

Distal to the respiratory bronchioles are approximately four generations of alveolar ducts lined with alveoli, the last divisions of which measure approximately 0.4 mm (29). The primary function of the lungs is gas exchange, and the largest part of the lungs is made up of the gas within the 300 million alveoli found in the lungs, with a total surface area of 140 m2 (30). As estimated in rats, the alveolar portion of the lung comprises approximately 87% of total lung volume, whereas the nonalveolar portion of the lungs, including bronchioles and larger airways, comprises the remaining 13%. Of the alveolar volume, only 6% is comprised of tissue versus 13% of the nonalveolar lung; the remainder is gas (31). The large percentage of lung volume comprised of air, with relatively little tissue, explains why the lungs are lucent on radiographs and have a very low attenuation number on CT. Normal lung attenuation on inspiratory CT ranges from approximately -800 to -850 Hounsfield units (HU). There is an attenuation gradient from the anterior nondependent lung to the posterior dependent lung that may vary between 40 and 60 HU, the more dependent lung having the higher attenuation value. With increasing expiration, lung attenuation increases as the air component of the lung decreases. Between 90% and 10% of vital capacity, attenuation increases by approximately 160 HU (32). Physically, the right lung is larger than the left because of the presence of the heart on the left side.

Normal lung attenuation on inspiratory CT ranges from approximately -800 to -850 HU.

Gas exchange occurs across the alveolar walls. The common wall of adjacent alveoli is called the interalveolar septum. Alveolar walls contain a rich capillary network, making the lungs a very vascular structure and explaining why atelectatic lung enhances intensely with intravenous contrast on CT. Inhaled air and capillaries are separated by epithelium, interstitium, and vascular endothelium for a distance of approximately 0.2 μm (30). Oxygen and carbon dioxide are exchanged through the alveolar walls, into and out of the circulating red blood cells. At any one time, red blood cell volume in the capillaries alone is 0.2 L. Over 90% of the alveolar surface is lined by type I pneumocytes. The remainder is made up of the type II pneumocytes that produce surfactant. Type I cells are much larger than type II cells; in total there are twice as many type II cells than type I cells. Surfactant is made up of phospholipids, the predominant one being dipalmitoyl phosphatidylcholine. The primary property of surfactant is to reduce alveolar surface tension, which prevents alveolar collapse with expiration and allows for ease of reexpansion with subsequent inspiration.

The lungs are very vascular structures and enhance intensely with intravenous contrast when they are collapsed and emptied of air.

The connective tissue network of the lungs is divided into two compartments. The central or axial compartment surrounds the bronchovascular bundles and extends centrally from the hila to the opening of the alveoli. The peripheral or septal interstitium arises from the visceral pleural connective tissue and includes the interlobular septa, known better as “Kerley B” lines on chest radiographs in the setting of interstitial edema. These septa contain pulmonary veins and lymphatics. Details of the secondary pulmonary lobule anatomy can be found in Chapter 14 and Fig. 14.26.

The central/axial interstitium surrounds the bronchovascular bundles. The peripheral interstitium includes the interlobular septa.

Another major function of the lungs is immune surveillance and the clearance of particulate matter inhaled with air (33). This function is served both by the ciliated airway lining and alveolar macrophages; the latter are found in much greater abundance in the lungs of smokers than nonsmokers (34). Approximately 9% of the cells in the lungs are macrophages. Most inhaled particulate matter is either quickly phagocytosed by macrophages or expelled through the airway within 24 hours (35).

Pericardium

The pericardium is to the heart what the pleura is to the lungs. The pericardial space surrounds the heart and normally contains 20 to 25 mL of serous fluid for lubrication during cardiac motion (36). Visceral pericardium is attached to the myocardium by fat-containing subepicardial connective tissue, whereas the parietal pericardium is a thick fibrous layer that blends with the adventitia of great vessels as they enter and exit the heart. Like the pleura, the pericardium is lined by mesothelial cells. Sternopericardial ligaments connect the heart anteriorly to the xiphisternal junction and manubrium. The upper border of the pericardial sac is at approximately the sternal angle where pericardium blends with the aorta, pulmonary arteries, and superior vena cava. Caudally, the parietal pericardium is adherent to the central tendon of the diaphragm. The normal “potential” capacity of the pericardial sac is approximately 300 mL. Rapid accumulations of greater than this amount of fluid impairs cardiac function, largely by reduced systemic venous return and compression of the lower pressure right-sided cardiac chambers. Larger volumes of fluid may be physiologically adjusted for when accumulation occurs slowly. Congenital partial or complete absence of the pericardium is rare, resulting in communication of the pericardial and pleural spaces; when focal it is most common over the left atrial appendage and left atrium, which can herniate through the defect. Most such defects are incidental.

The cephalad border of the pericardium is at the level of the sternal angle.

The normal pericardium measures 1 to 2 mm on cross-sectional imaging, such as CT and magnetic resonance imaging (MRI) (Fig. 1.9) (37). Pericardium is most visible anterior to the right ventricle on CT and MRI where there is adjacent fat. Several pericardial recesses are readily visible with cross-sectional imaging (38,39,40). The transverse sinus of the pericardium is posterior to the ascending aorta and main pulmonary artery, seen on imaging as a fluid-filled, usually curved structure immediately posterior the ascending aorta (Fig. 1.5). It can be confused for a lymph node or an aortic dissection flap. High-riding recesses can extend into the right paratracheal area and anterior to the aortic arch in the prevascular space, and should not be confused for underlying pathology.

Normal pericardium measures 1 to 2 mm in thickness on CT and MRI.

Figure 1.9 Axial computed tomography four-chamber image of the heart. Note the thin precardium (arrowheads) seen anteriorly. DA, descending thoracic aorta; gCV, great cardiac vein; LA, left atrium; LAD, left anterior descending coronary artery; LV, left ventricle; LVot, left ventricular outflow tract; LCX, left circumflex coronary artery; MB, moderator band; RA, right atrium; RCA, right coronary artery; RV, right ventricle; arrows, mitral valve.

Heart

Cardiac anatomy is often not covered or is only minimally covered in thoracic radiology texts. However, the heart is an important part of thoracic radiology, whether it is the heart “shadow” in the middle of every chest radiograph or the detailed anatomic or functional information available from MRI and CT. Cardiac imaging evaluates not only anatomy, the basis of radiologic interpretation, but also cardiac function, perfusion, metabolism, and tissue characterization.

The human heart is a four-chamber conical structure predominantly located within the left hemithorax (Figs. 1.101.111.12, and 1.13). The heart has an apex at the inferolateral margin of the left ventricle and a base posteriorly at the left atrium. Nonoxygenated blood from the superior and inferior vena cavae (systemic venous return) and the coronary sinus (the cardiac venous return) passes into right atrium, through the tricuspid valve into the right ventricle, and then through the pulmonic valve into the pulmonary artery, which carries the blood into the lungs for gas exchange. The openings of the inferior vena cava and coronary sinus into the right atrium are adjacent to each other on the inferior margin of the heart and contain valves known as the eustachian and thebesian valves, respectively. Oxygenated blood exits the lungs through the pulmonary veins into the left atrium, passes through the mitral valve into the left ventricle, after which it passes through the aortic valve into the ascending thoracic aorta (41). There are usually four pulmonary veins that drain into the left atrium, each receiving three to five major venous tributaries from the lungs (Fig. 1.10). The four pulmonary veins are the right superior and inferior pulmonary veins and the left superior and inferior pulmonary veins. Sometimes two veins may join together before draining into the left atrium, such as the left superior and inferior pulmonary veins. In other cases there is an extra vein draining directly into the left atrium, such as an additional right middle lobe vein. Anomalous pulmonary venous return creates a left-to-right shunt and is discussed in Chapter 19.

Cardiac chambers, heart borders and changes with chamber enlargement are discussed in Chapters 3 and 20Figures 1.9 and 1.11A are axial CT images demonstrating the cardiac chambers. The right atrial appendage lies anterior and superior to the right atrium. The right ventricle is the most anterior chamber, residing immediately behind the sternum. As such, it is prone to contusion when blunt force is applied to the anterior chest wall during trauma, such as a steering wheel injury during a motor vehicle accident or a crush injury while playing football. The right ventricle is heavily trabeculated. A distinct structure seen on cross-sectional imaging within the right ventricular apex is the moderator band. The left atrium forms most of the posterior cardiac border and the right atrium most of the right heart border. The wall of the left ventricle is thicker and more muscular than the right ventricle. The cardiac chambers are lined internally by endocardium, beneath which the myocardium made up of cardiac muscle fibers resides. The left atrial appendage lies anterior and superior to the left atrium and is longer and narrower than the right atrial appendage (Fig. 1.14). The atrial septum between the right and left atrium is thin and translucent; it contains a depression on the right atrial side known as the fossa ovalis, a remnant of the foramen ovale through which blood passed from right atrium to left atrium before birth. The ventricular septum separates the right and left ventricles and is thicker and more muscular. The aortic, tricuspid, and pulmonic valves are normally three-leaflet valves, whereas the mitral valve is a two-leaflet valve. Papillary muscles in the right and left ventricles are connected by chordae tendineae to the tricuspid and mitral valves, respectively, as illustrated in Fig. 1.11A. The papillary muscles prevent prolapse of the valves into their respective atria during ventricular contraction during systole (41,42).

The right ventricle is the most anterior cardiac chamber, forms most of the anterior heart border on a posteroanterior radiograph, and is prone to contusion during blunt force trauma to the anterior chest wall.

The right atrium forms most of the right heart border on a posteroanterior radiograph, whereas the left atrium forms most of the posterior heart border on a lateral radiograph.

Figure 1.10 Shaded surface display of the left atrium and pulmonary veins in the posterior projection from electrocardiogram-gated 16-row multidetector computed tomography. A, aorta; LA, left atrium; LI, left inferior pulmonary vein; LPA, left pulmonary artery; LS, left superior pulmonary vein; LV, left ventricle; RI, right inferior pulmonary vein; RPA, right pulmonary artery; RS, right superior pulmonary vein.

Figure 1.11 Axial and coronal cardiac images from electrocardiogram-gated 16-row multidetector computed tomography (CT) of the heart. In contrast to the non–electrocardiogram-gated image in Fig. 1.9, note the greater detail in both the axial plane and the coronal reconstruction, the latter possible due to the isotropic voxels of 16-row MDCT. A. Axial CT image illustrates the four cardiac chambers. Note the mitral valve (MV) with leaflets in the open position, because this image represents ventricular diastole. Chorda tendina (CT) is seen attaching to a papillary muscle (PM). DA, descending aorta; LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle; SP, spinous process; TP, transverse process; V, vertebral body. B. Coronal CT image. AscAo, ascending aorta; C, clavicle; LAX, left axillary artery; LCCA, left common cartoid artery; MPA, main pulmonary artery; RBR, right brachiocephalic vein; RIJ, right internal jugular vein; RINN, right innominate artery; RSV, right subclavian vein; SVC, superior vena cava.

Figure 1.12 Cardiac surface anatomy from electrocardiogram-gated 16-row multidetector computed tomography in the (A) anterior projection and (B) long-axis projection. Note the right coronary artery (black arrowheads) in the right atrioventricular groove and left anterior descending coronary artery (white arrowheads) that runs in the interventricular groove; both are epicardial vessels. AscAo, ascending aorta; D, first diagonal; LCX, left circumflex coronary artery; MPA, main pulmonary artery; RA, right atrium; RV, right ventricle; SVC, superior vena cava; small arrows indicate the pulmonic valve.

Figure 1.13 Cardiac surface anatomy and the coronary arteries. A. Projection along the interventricular groove demonstrates the right (RV) and left ventricles (LV) with the left anterior descending coronary artery (LAD) in the interventricular groove and the left circumflex coronary artery (LCX) in the more posterior left atrioventricular groove. B. Left main (LM) coronary artery as it arises from the left coronary cusp of the ascending aorta (AscAo). Left circumflex (LCX) and left anterior descending coronary arteries (LAD) on a projection from above the heart. C. View of the inferior surface of the heart demonstrates the inferior most aspect of the right coronary artery, as well as the coronary sinus (CS) and inferior vena cava (IVC) draining into the inferior aspect of the right atrium. RAA, right atrial appendage.

Figure 1.14 Atrial appendages on axial computed tomography. AscAo, ascending aorta; DA, descending aorta; IMA, internal mammary artery; IMV, internal mammary vein; LA, left atrium; LAA, left atrial appendage; LD, latissimus dorsi; PA, pulmonary artery; RAA, right atrial appendage; SA, serratus anterior; TP, trapezius.

The heart is supplied by two coronary arteries that arise from the aortic root (Figs. 1.12 and 1.13). The right coronary artery arises from the right anterolateral aspect of the aortic root and runs anteriorly and inferiorly in the right atrioventricular groove. In addition to supplying the right atrium, right ventricle, and atrial septum, it usually supplies the sinoatrial and atrioventricular nodes. As it extends under the inferior surface of the heart, it becomes the posterior descending coronary artery. The left main coronary artery arises from the left lateral aspect of the aortic root, is 0.5 to 1.5 cm long, and quickly bifurcates into the left anterior descending coronary artery and the left circumflex coronary artery. The left main coronary artery supplies the left atrium, left ventricle, and the ventricular septum. The left anterior descending coronary artery runs left anterolaterally into the interventricular groove, whereas the left circumflex coronary artery runs posterolaterally and inferiorly in the left atrioventricular groove. Acute marginal branches arise from the right coronary artery, diagonal arteries from the left anterior descending coronary artery, and obtuse marginal branches from the left circumflex coronary artery. Anomalous coronary artery origins are discussed in Chapter 19. All these coronary arteries are epicardial, as they reside on the surface of the heart. Septal and muscular perforating arteries arise from the epicardial arteries and dive into the cardiac wall (42). Most cardiac blood supply occurs during diastole. Cardiac venous drainage parallels the arterial blood supply, with epicardial veins paralleling the epicardial arteries, that together drain into the coronary sinus on the inferior/posterior aspect of the heart, adjacent to the left circumflex coronary artery. The gold standard method for evaluating the coronary arteries is direct catheterization and contrast injection. Both CT and MRI are used to map the pulmonary veins and left atrium and cardiac veins for electrophysiologic ablation procedures and planning for biventricular pacemaker placement in which a lead is placed into the coronary sinus (43). CT and MRI are also making strides in the mapping and evaluation of the coronary arteries as well.

The coronary arteries are epicardial, that is they lie on the surface of the heart.

Central and cardiac pressure measurements vary throughout the cardiac cycle. Alterations in cardiac physiology and how that relates to the development of pulmonary edema are described in Chapter 10.

Aorta and Arteries

The thoracic aorta is divided into the ascending aorta, the aortic arch, and the descending thoracic aorta. At the root of the aorta there are three aortic sinuses, right, left, and noncoronary, the first two giving rise to their respective coronary arteries. The aorta exits the left ventricle at the aortic valve, approximately at the third left costochondral cartilage in the parasternal region. It then extends anteriorly to the right and in the cephalad direction for approximately 5 cm, ending in the aortic arch. The ascending aortic diameter is approximately 3 cm; greater than 4 cm is considered abnormal. The aortic arch gives rise to the great vessels in the following order: right innominate artery, left common carotid artery, left subclavian artery (Figs. 1.15 and 1.16). The right innominate artery is approximately 4 to 5 cm long and gives rise to the right subclavian and right common carotid arteries. Common variants include the bovine aortic arch, in which the left common carotid artery arises from the right innominate artery (Figs. 1.15) and direct origin of the left vertebral artery from the aortic arch, rather than from the left subclavian artery (44,45). Common congenital anomalies of the aortic arch are described in Chapter 19. On a radiograph, the junction of ascending aorta and aortic arch is at the right half of the sternal angle, where the aorta emerges from the pericardial sac. The arch continues cephalad, before turning posterior and leftward behind the lower half of the manubrium. The undersurface of the aortic arch is at the level of the sternal angle. The ligamentum arteriosum is the remnant of the ductus arteriosus that closes at birth, having permitted the passage of blood from the main pulmonary artery to the aorta during fetal development. In adults it is an approximately 1.5-cm long fibrous band. On radiographs, the aortic knob is formed by the posterior leftward course of the aortic arch.

On a posteroanterior radiograph, the aortic valve is at the third costochondral cartilage level in the left parasternal region.

On a posteroanterior radiograph, the undersurface of the aortic arch is at the level of the sternal angle. The aortic knob is formed by the posterior aspect of the aortic arch.

At approximately the T4-5 intervertebral disc level, the aortic arch becomes the descending thoracic aorta. The latter gives rise to intercostal arteries and to three bronchial arteries (two left and one right) with variable origins near the T-5 level (16). At the aortic hiatus of the diaphragm, the descending thoracic aorta passes into the abdomen as the abdominal aorta. Uncommonly, an artery arising from the descending thoracic aorta supplies the blood flow to an isolated portion of lower lobe lung parenchyma, known as a sequestration. Intrapulmonary sequestrations are contained within the normal pleura of the lower lobe, whereas extrapulmonary sequestrations have their own pleural lining and are more commonly associated with additional congenital anomalies (46).

Figure 1.15 Thoracic aorta with bovine branching of the aortic arch. Note the left pulmonary artery extends posteriorly from the main pulmonary artery as a “mini-arch” under the aortic arch. A. Anterior projection. B. Oblique sagittal projection. 1, right innominate artery; 2, left common carotid artery; 3, left subclavian artery; ARCH, aortic arch; Asc, ascending aorta; DA, descending aorta; MPA, main pulmonary artery.

Figure 1.16 Great vessels and chest wall at the level of the upper mediastinum on axial computed tomography image. Asterisk, scapula; 1, right innominate artery; 2, left common carotid artery; 3, left subclavian artery; L, left brachiocephalic vein; M, manubrium; PM, pectoralis major; PMi, pectoralis minor; R, right brachiocephalic vein; SS, supraspinatus; SubS, subscapularis; TP, trapezius.

Other arterial structures of note include the internal mammary arteries that arise from the subclavian arteries and run vertically in the right and left parasternal regions (Figs. 1.2A and 1.14). The subclavian arteries become the axillary arteries when they course over the first anterior ribs at the lateral margin of the thoracic cage (Fig. 1.11B). The lateral thoracic arteries arise from the proximal axillary arteries and descend vertically in the anterolateral chest wall. The inferior thyroidal arteries are now commonly seen with fast thin-section CT arising from the thyrocervical trunk off of the subclavian artery in the superior mediastinum above the thoracic inlet.

Veins

The veins parallel the arteries described above. The internal jugular veins parallel the common carotid arteries. The subclavian veins parallel the subclavian arteries. The internal jugular and subclavian veins join to form the brachiocephalic veins behind the medial heads of the clavicles. They join together to form the valveless superior vena cava behind the right first costochondral cartilage (Fig. 1.11B). The valveless brachiocephalic veins are 2.5 cm long on the right and 6 cm long on the left, the greater length of the left vein because of its oblique downward course across the mediastinum to the right-sided superior vena cava (16). The superior vena cava is 7.5 cm long, forms the upper right border of the mediastinum, and drains into the right atrium. The lower portion of the superior vena cava is contained within the pericardial sac and is immediately anterior to the right main bronchus. Uncommonly, individuals may have both left- and right-sided vena cavae; an isolated left superior vena cava is rare. When present, the left-sided vena cava usually still communicates with the right-sided superior vena cava across a left brachiocephalic vein. Left-sided superior venae cavae usually drain directly into the coronary sinus of the heart (47).

The internal jugular and subclavian veins join to form the brachiocephalic veins behind the medial heads of the clavicles.

A left superior vena cava is usually a dual system accompanied by a right superior vena cava; rarely, it is isolated.

The azygos vein courses vertically along the right anterior aspect of the thoracic spine from the abdomen up to the superior vena cava, where it enters posteriorly at the T-4 vertebral body level (Figs. 1.17 and 1.18). It is formed at the diaphragm by the ascending lumbar vein and the right subcostal vein. Unlike the superior vena cava and brachiocephalic veins, the azygos vein does contain valves. The hemiazygos vein similarly runs vertically along the left anterior aspect of the thoracic spine, crossing rightward and draining into the azygos vein at the T-9 level. An interazygos vein often communicates between the azygos and hemiazygos veins. The intercostal veins, esophageal veins, and bronchial veins drain into the azygos/hemiazygos system (16).

Figure 1.17 Azygos vein anatomy on an oblique sagittal CT reconstruction viewed from the right. A, azygos vein; AA, azygos arch; H, hemiazygos vein; IA, interazygos vein; RPA, right pulmonary artery; DA, descending aorta.

Figure 1.18 Azygos vein runs vertically to the right of midline, anterior to the spine. Note the intercostal arteries arising from the descending aorta (DA). A, azygos vein; H, hemiazygous vein; IA, interazygos vein runs posterior to the aorta.

Lymphatic System

The large lymphatic structure that is responsible for most of the lymph return from the thorax, abdomen, pelvis, and lower extremities is thethoracic duct. It is the thoracic continuation of the abdominal cisterna chyli after it enters the thorax through the diaphragm at the aortic hiatus. The thoracic duct is a vertically oriented structure that runs anterior to the thoracic vertebral bodies to the right of the descending thoracic aorta. At the T6-7 level the duct deviates from midline toward the left, draining into the left internal jugular vein. The pleural layers contain an extensive network of lymphatics. Lymph flows medially from the visceral pleura to the hilar lymph nodes through the axial/bronchovascular and peripheral/interlobular septal interstitium of the lungs. Occasionally, small intrapulmonary lymph nodes are visible in the subpleural lung as 5 to 12 mm, smoothly marginated, round or ovoid, soft tissue attenuation nodules on CT; these are not uncommon in smokers (48).

The thoracic duct runs anterior to the spine, toward the right side of the descending aorta below the T6-7 level, and to the left side above that level.

Small intrapulmonary lymph nodes may appear as 5- to 12-mm, noncalcified, smooth, round or ovoid nodules in the subpleural lung and are common in smokers.

Normal lymph nodes are not visible on chest radiographs but are seen as round to ovoid soft tissue attenuation structures on CT, sometimes with a fatty hilum. By convention, lymph nodes 1 cm and larger in short axis are considered abnormally enlarged and in the setting of malignancy suspicious for tumor spread. Short axis is used because it is more consistent than long axis. However, we know that tumor cells may reside in lymph nodes less than 1 cm in size, and nodes larger than 1 cm may be hyperplastic. Most tracheobronchial mediastinal nodes and pulmonary lymph nodes are classified by the American Thoracic Society Classification scheme (Fig. 1.19). Although the most common use of this scheme is for the staging of lung cancer, it can be used for the description of lymph nodes due to any disease process. Other lymph node groups visible with thoracic imaging include internal mammary lymph nodes adjacent to the arteries and veins of the same name, in a parasternal location; they drain the breast tissue and anterior chest wall. Cardiophrenic lymph nodes are found adjacent to the heart at the level of the xiphoid. Anterior mediastinal lymph nodes are found anterior to the superior vena cava, left innominate artery, and ascending aorta behind the sternum. Nodes are also located along the esophagus and descending aorta, termed paraesophageal and paraaortic nodes, respectively.

Figure 1.19 Lymph node mapping scheme for lung cancer. A. Drawing. B. Table defining node stations. (Courtesy of Dr. JP Ko.)

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