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).

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).

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. 1.1A, 1.2A, 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.

Together the clavicles and scapulae, otherwise know as collar bone and shoulder blades, form the shoulder girdle (Figs. 1.1, 1.2, 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).

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.

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.