4 The thorax

The thoracic cage

The roof of the thoracic cage is formed by the suprapleural membrane and the diaphragm is its floor. The walls are made of the skeleton and attached muscles. The bones involved are: (i) 12 thoracic vertebrae (see section on vertebral column); (ii) 12 ribs and their costal cartilages and (iii) the sternum.

The ribs

There are 12 pairs of ribs – 7 true, 3 false and 2 floating. Occasionally a normal subject has only 11 pairs.

Atypical ribs

Radiology pearl

Reflecting the position of neurovascular structures in a groove along the inferior surface of the rib, needle puncture to access the pleural space should be through soft tissues immediately above the rib.

Costal cartilages

These are the unossified anterior ends of the ribs. They slope upwards to the sternum, where they form synovial sternochondral joints (except the first, which forms a primary cartilaginous joint with the sternum). The joint between the rib and costal cartilage is a primary cartilaginous joint. The costal cartilages of the first seven ribs articulate with the sternum. The eighth to tenth ribs articulate with the costal cartilages of the ribs above. The eleventh and twelfth costal cartilages have pointed ends and end in the muscles of the abdominal wall.

The intercostal space and vessels

This is bridged by the muscles – the external, internal and innermost intercostal muscles. The neurovascular bundle lies between the internal and innermost muscle layers.

Intercostal veins

• Veins accompany arteries to internal thoracic and musculophrenic veins.

Radiology pearl

Rib notching of the inferior margin or costal groove of the rib is most frequently a marker of coarctation and reflects an attempt to circumvent the stenotic arterial segment through a collateral pathway routed through the posterior intercostal arteries. As the upper posterior intercostal arteries to ribs 1 and 2 arise from the costocervical trunk off the subclavian artery, notching is confined to ribs 3 to 11. This reflects the aortic origins of the intercostal arteries to these ribs.

Muscles of the thoracic cage (see Figs 4.45, 4.46)

The external, internal and innermost intercostal muscles occupy the spaces between the ribs. Subcostal muscles on the deep surface of the lower ribs span two or three ribs, with fibres that run in the same direction as the internal and the innermost intercostal muscles. The innermost intercostals and the subcostals separate the intercostal neurovascular bundles from the pleura. The transverse thoracic muscle arises on the deep surface of the sternum and adjacent lower costal cartilages and passes superolaterally to the deep surface of the anterior ribs.

Superficial to the ribs three muscle groups posteriorly, the costal levators, attach the ribs to the vertebrae: from the transverse processes of the vertebrae to the posterior part of the rib below, serratus posterior superior; from the spinous processes of the vertebrae superolaterally to the posterior ribs and serratus posterior inferior; from the spinous processes inferolaterally to the posterior aspect of the lower ribs.

Other muscles have attachments to the thoracic cage and can be seen on axial cross-sectional imaging, including the pectoral muscles anteriorly, the serratus anterior and the teres major and subscapularis laterally and posteriorly, and the rhomboids, the erector spinae and trapezius posteriorly.

Radiological features of the thoracic cage (Fig. 4.3)

• Normal variants: ribs may fuse or bifurcate; they may be splayed or hypoplastic. Variation is commoner in the upper ribs, especially the first.

• On a PA chest radiograph the subcostal groove may appear as a fine line below the rib, especially posteriorly. It may also appear as shallow indentation posteriorly near the neck (not to be mistaken for rib notching).

Figure 4.3 • Ribs. (A) Subcostal groove clearly visible below the second left rib (arrowheads). Note also the scalene tubercle on the first rib (arrow).

(B) Prominence on upper surface of the second rib due to the insertion of part of scalenus anterior muscle.

Radiology pearl

The subcostal groove appearing as a fine line below a rib should not be mistaken for a pneumothorax.

• A prominence is often seen on the second rib on a chest radiograph, which is usually symmetrical right and left. This is due to insertion of part of the scalenus anterior muscle.

• The costal cartilages, especially the first, may calcify or ossify from early adulthood. In males the calcification is usually in marginal bands, and in females in a central tongue.

• Cervical ribs: these are bony or fibrous bands between C7 and the first rib and are found in 1–2% of subjects. Of these, 50% are bilateral and often they are asymmetrical. A fibrous band is likely to be present if the anterior tubercle of the transverse process of C₇ is prominent on a radiograph. The brachial plexus is likely to be prefixed (that is, arise from C₄–C₈ rather than C₅–T₁ if the cervical rib is well developed). This situation is less likely to cause neurological symptoms.

Radiology pearl

Cervical ribs may be distinguished from the first rib by the orientation of the transverse process – that of C₇ points downward, whereas that of T₁ is horizontal or points upward.

• Lumbar ribs: the transverse process (which is the costal element of the lumbar vertebra) may fail to fuse with the vertebral body and retain a synovial joint with the neural arch. They are symptomless.

• Ossification of the ribs: a bony centre arises at the angle of the rib in the eighth fetal week. Secondary centres occur at the head and at the tubercle at 15 years of age and fuse at 25 years of age.

The sternum (Fig. 4.4A)

The sternum has:

• a manubrium opposite T₃ and T₄ that articulates with the clavicle and with one-and-a-half costal cartilages;

• a sternal angle, which is a secondary cartilaginous joint and lies opposite T_4/5 disc space;

• a body opposite T₅–T₉, made up of four stenebrae that articulate with five-and-a-half costal cartilages; and

• a xiphoid process, which remains cartilaginous well into adult life.

Figure 4.4 • The sternum. (A) Coronal reformatted CT image of the sternum and (B) MRI of the sternum.

Radiology pearl

The sternal angle, being a secondary cartilaginous articulation or synchondrosis between the manubrium and body of the sternum, may become inflamed as part of enthesitis in seronegative arthropathies such as Reiter’s syndrome, psoriatic arthropathies and ankylosing spondylitis.

Radiological features of the sternum

Plain films

On a PA chest radiograph the manubrial borders may simulate mediastinal widening. The remainder of the sternum is not seen. Oblique views are necessary to project the sternum away from the heart. These project it over the lungs, whose markings may cause confusing shadows. Lateral views are also helpful.

Radiology pearl

Variation in sternal configuration include: (i) depression of the lower end, known as pectus excavatum; and (ii) prominence of the midportion, known as pectus carinatum.

Ossification of the sternum

Bony centres for the manubrium and stenebrae appear from above downwards from the fifth to the ninth fetal months. Between 15 and 25 years of age stenebrae fuse from below upwards. The xiphoid process fuses with the body at 40 years of age and the body and manubrium fuse in old age, if at all.

Computed tomography and MRI of the sternum (Fig. 4.4B, C)

On CT images of the thorax the manubrium is usually angled with respect to the gantry, and this may cause cortical lack of sharpness. The body of the sternum is usually perpendicular to the beam and well demarcated. At MRI, the sternum is readily visualized in the coronal plane employing a surface coil placed immediately over the anterior chest wall. MRI is the optimum method to visualize the sternum.

Radiology pearl

Reflecting the presence of vascular red marrow within the sternum through adulthood, it is commonly seeded by bloodborne bony metastatic disease, is a common site for myelomatous deposits and is a site employed for marrow biopsy

The diaphragm (Fig. 4.5)

The diaphragm forms the highly convex floor of the thoracic cage. It arises from vertebral, costal and sternal origins and from the central tendon.

The vertebral part arises from the crura and arcuate ligaments. The right crus is attached to the bodies and discs of L₁–L₃ vertebrae. The smaller left crus arises from the vertebral body and disc of L₁ and L₂ vertebrae.

The medial arcuate ligament is a thickening of the fascia over the psoas muscle from the body of L₂ to the transverse process of L₁ lumbar vertebrae. The lateral arcuate ligament is a thickening of the fascia over quadratus lumborum from the transverse process of L₁ vertebrae to the twelfth rib. The median arcuate ligament is the fibrous medial part of both crura behind which the aorta passes; no diaphragmatic muscle arises from this.

The costal part of the diaphragm arises in slips from the lower six costal cartilages.

The sternal part of the diaphragm arises in two small slips of muscle from the posterior surface of the xiphisternum.

The central tendon is, in fact, not central but closer to the sternum. Its midpart is fused with the pericardium and its right and left posterior parts extend towards the paravertebral gutters.

Blood supply to the diaphragm

The diaphragm is supplied from its abdominal surface by the inferior phrenic arteries from the abdominal aorta. The costal margins are supplied by the intercostal arteries.

Nerve supply to the diaphragm

Right and left phrenic nerves from C₃–C₅ roots provide the motor supply of the diaphragm. Sensory impulses from the central part of the diaphragm pass with the phrenic nerves, and those from the peripheral part with the intercostal nerves.

Radiological features of the diaphragm (Fig. 4.5)

PA chest radiograph (Fig. 4.6)

The highest point of the right dome is at the sixth intercostal space anteriorly (ranging from the fourth to seventh ribs); The right dome is higher than the left by 2 cm but the left may be higher than the right in the normal subject, especially with swallowed gas in the colon.

Figure 4.6 • PA chest radiograph.

1 Posterior junctional line

2 Anterior junctional line

3 Azygo-oesophageal line

4 Lateral wall of descending aorta

5 Aortic knuckle

6 Aortopulmonary window

7 Pulmonary trunk/left pulmonary artery

8 Left superior pulmonary vein

9 Left inferior pulmonary artery

10 Left hilar point

11 Right superior pulmonary vein

12 Interlobar artery

13 Right hilar point

14 Trachea

15 Right main bronchus

16 Left main bronchus

17 Azygos vein and position of azygos node

18 Position of left atrial appendage

19 Left ventricle

20 Right atrium

21 Inferior aspect of left brachiocephalic vein

22 Medial end of right clavicle

23 Right lateral aspect of manubrium sterni

24 Spinous process of T₁

25 Superior surface of clavicle

26 Companion shadow of clavicle

27 Medial aspect of right scapula

28 Coracoid process of right scapula

29 Dome of right hemidiaphragm

30 Dome of left hemidiaphragm

31 Stomach bubble

32 Gas in splenic flexure of colon

Radiology pearl

It is more accurate to count anterior rather than posterior ribs, as the diaphragmatic dome is nearer to the anterior ribs and the film, and is therefore less subject to distortion by slight angulation of the patient or the beam.

The range of movement of the diaphragm with respiration is as follows:

• quiet respiration: 1 cm; and

• deep inspiration/expiration: 4 cm (wide range of normal).

In each case the left hemidiaphragm moves more than the right.

The variation of the diaphragm with posture is as follows:

• supine: higher; and

• lateral decubitus: dome on the dependent side is higher.

Radiology pearl

In dextrocardia, even if the liver is on the right side, the left dome of the diaphragm tends to be higher

Radiology pearl

Partial reduplication of the diaphragm – known as accessory hemidiaphragm – may occur. This is much commoner on the right side.

Lateral chest radiograph (Fig. 4.7)

The following anatomical details help identify the domes of the diaphragm:

• The heart shadow obliterates part of the left dome.

• The inferior vena cava may be seen piercing the right dome.

• Air within the gastric fundus lies under the left dome.

There is apparent thickness of the diaphragm on radiographs:

• With the pleura and peritoneum when there is air in the peritoneum: 2–3 mm thick.

• With the pleura and fundal wall of stomach: 5–8 mm thick.

Curvature of the dome

The perpendicular height of the dome of the diaphragm from a line between costophrenic and cardiophrenic angles is 1.5 cm.

Ultrasound

The diaphragm is readily imaged by ultrasound, using the liver or spleen as an acoustic window. It is seen as an echogenic line outlining the upper surface of these organs. The diaphragmatic interdigitations may occasionally be pronounced to give the spurious impression of an echogenic mass on the surface of the liver.

Computed tomography

The diaphragm is not usually visible as a structure discrete from the liver or other abdominal organs, unless there is a lot of fat on its abdominal aspect.

The costal origins may be prominent with deep inspirations. The crura are usually visible on the anterior surface of the upper lumbar vertebrae. In young, muscular subjects the crura may be very thick or even nodular; however, their tubular nature and changes with respiration serve to distinguish these from lymph nodes. The right crus extends more inferiorly than the left.

The retrocrural space contains fat, the azygos and hemiazygos veins, the thoracic duct and lymph nodes, and should not be more than 6 mm wide.

Radiology pearl

On axial CT scans of the upper abdomen in healthy subjects, the abdominal viscera, particularly the gastric fundus and liver, are separated from the posterior abdominal wall by intact posterior diaphragm. Following diaphragmatic rupture, the gastric fundus or liver fall to lie immediately against the posterior abdominal wall, the so called ‘dependent visceral sign’.

Magnetic resonance imaging (see Fig. 4.5)

This technique yields excellent sagittal and coronal images of the diaphragm as a thin muscular septum of intermediate signal intensity. The crura are elegantly displayed on coronal images.

The pleura (Figs 4.8, 4.9)

The pleura is a serous membrane that: (i) covers the lung (i.e. the visceral pleura); and (ii) lines the thoracic cavity and mediastinum (i.e. the parietal pleura). Parts of the pleura are named according to site, for example costal, diaphragmatic, mediastinal and apical.

Figure 4.8 • Pleura: (A) anterior view; (B) posterior view.

Figure 4.9 • (A) PA chest radiograph; (B) axial high-resolution CT scan of lungs which shows fissures as fine white lines (arrows).

1 Posterior junction line (arrows)

2 Azygo-oesophageal line

The visceral and parietal layers are continuous with each other anterior and posterior to the lung root, but below the hilum the two layers hang down in a loose fold called the pulmonary ligament. This may extend to the diaphragm or have a free inferior border, and allows descent of the lung root in respiration and also distension of the pulmonary veins (note that these lie inferiorly in the lung root).

The visceral pleura extends into interlobar and accessory fissures. At rest the parietal pleura extends deeper into the costophrenic and costomediastinal recesses than do the lungs and visceral pleura (see Table 4.1 for lower limits of lungs and pleura).

Table 4.1 Lower limits of lung and pleura at rest

	Visceral pleura and lung	Parietal pleura
Anterior	Sixth costal cartilage	Seventh costal cartilage
Midaxillary line	Eighth rib	Tenth rib
Posterior T₁₀	T₁₀	T₁₂

The parietal pleura is supplied by the systemic vessels. The visceral pleura receives arterial supply from both the bronchial and the pulmonary circulation.

Radiological features of the pleura

Plain films

On a chest radiograph the pleura is visible only if tangential to the beam and if fat or air is on each side of it. Thus the pleura is visible in a normal subject at:

• Fissures.

• Sites where parietal pleura lies on extrapleural fat:

seen just below the second rib and

extending vertically upwards from the costophrenic recess.

• Junction lines (see Figs 4.6, 4.9, 4.43, 4.44 and 4.48; see also section on mediastinal lines, p. 151):

Anterior junction line: anterior to the arch of the aorta the two lungs may come in contact with one another, separated only by four layers of pleura. This pleura is then seen as the anterior junction line on a PA chest radiograph.

Posterior junction line: if the lungs lie close to one another posteriorly, a posterior junction line is seen on a PA chest radiograph, extending vertically downwards from the apices (approximately T₁) for a variable distance. It disappears where the lungs envelop the aortic arch and may reform inferiorly. Where the junction lines are seen well, a mass between the lungs in that area can be excluded.

On axial CT the pleura cannot usually be distinguished from the thoracic wall or mediastinum unless it is thickened (see section on fissures, p. 125). The pulmonary ligaments can occasionally be seen extending below the inferior pulmonary vein caudally and posteriorly to the diaphragm. The right pulmonary ligament lies close to the inferior vena cava (IVC), whereas the left pulmonary ligament lies close to the oesophagus.

The trachea and bronchi (Figs 4.10–4.12)

The trachea

The trachea begins at the lower border of the cricoid cartilage at the level of C₆ vertebra. It extends to the carina at the level of the sternal angle (T₅ level, T₄ on inspiration and T₆ on expiration). The trachea is 15 cm long and 2 cm in diameter and is made up of 15–20 incomplete rings of cartilage that are bridged posteriorly by the trachealis muscle. The trachea is lined by ciliated columnar epithelium.

Figure 4.10 • Trachea and main bronchi: anterior relations.

Figure 4.11 • Diagrammatic representation of normal anatomy: situs solitus.

Figure 4.12 • Bronchial tree – main and segmental anatomy as seen on bronchography: (A) anterior view; (B) right lateral view; (C) left lateral view; (D) left oblique view.

The trachea in children is very pliable. It may be deviated to the right at almost 90° in a normal expiratory film. It only deviates to the left if the aortic arch is on the right side.

Blood supply of the trachea

The upper trachea is supplied by the inferior thyroid artery and the lower part is supplied by branches of the bronchial artery.

Venous drainage is to the inferior thyroid venous plexus.

Main bronchi (Figs 4.10–4.12)

Carina

This is the anteroposterior ridge at the junction of the main bronchi. It lies at T₅ vertebral level (T₄ on inspiration and T₆ on expiration) and at the level of the sternal angle. The carinal angle measures approximately 65° – that is, 20° to the right of the midline and 40° to the left. This angle is slightly larger in children. The carinal angle increases by 10° to 15° in recumbency.

Radiology pearl

Widening of the carinal angle on a PA chest radiograph above 65° is a marker of enlargement of the left atrium, which itself most frequently indicates mitral valve dysfunction, either stenosis or incompetence.

The right main bronchus (eparterial bronchus)

The right main bronchus lies at about 25° to the median plane. It is 2.5 cm long and 1.5 cm wide. It is thus wider, shorter and more vertical than the left main bronchus.

Left main bronchus (hyparterial bronchus)

The left main bronchus lies at 40° to the median plane. It is 5 cm long and 1.2 cm in diameter.

Relations

The relations of the left main bronchus are as follows:

• anterior: pulmonary trunk;

• posterior:

oesophagus

descending aorta; and

• superior:

aortic arch;

pulmonary artery.

The left main bronchus divides into upper- and lower-lobe bronchi within the lung. The upper-lobe divisions are similar to the right. The posterior and apical segmental bronchi usually have a common apicoposterior bronchus, which then subdivides. The lingular lobe bronchus comes off the upper-lobe bronchus and has superior and inferior divisions. The lower-lobe bronchus has apical, lateral, anterior and posterior basal segments but no medial basal segment.

The anatomy of the bronchial tree is shown diagrammatically in Figure 4.12. Naming basal bronchi laterally to medially is easier if the constant relationship of anterior, lateral and posterior (ALP) is remembered, with only the medial bronchus in the right lung changing its relative position.