The thorax is the portion of the trunk that lies between the lower neck and the upper abdomen. It is made up of the chest wall (the thoracic cage and associated soft tissues) and the visceral contents of the thoracic cavity. The diaphragm separates the thorax from the abdomen. The boundary between the lower neck and the thorax is less clearly demarcated.
The Thoracic Cage
The thoracic cage or bony thorax is bounded superiorly by the suprapleural membrane, a layer of fascia attached to the C7 transverse process, the inner margins of the first rib and first costal cartilage and the mediastinal pleura. The thoracic cage is made up of: (1) 12 thoracic vertebrae (see Chapter 3 ); (2) 12 ribs and their costal cartilages and (3) the sternum.
THE RIBS
There are usually 12 pairs of ribs (i.e., the ribs that articulate with the 12 thoracic vertebrae from T1 to T12); however, there is a variation in rib number (see variant anatomy in the ribs). The ribs can be classified into true ribs, false ribs and floating ribs based on their costal cartilages and articulations with the sternum (see costal cartilages paragraph). The posterior portions of the ribs have a relatively horizontal orientation, while the mid and anterior portions of the ribs course inferiorly. This manifests on chest radiograph as horizontal posterior ribs and more vertical/oblique anterior ribs. On cross-sectional imaging, portions of several adjacent ribs can be seen on a single axial image. The ribs also provide attachment for some of the muscles that make up the chest wall. The primary ossification centre for the body of the ribs appears at the eighth fetal week. Secondary ossification centres for the rib epiphyses (head and costal tubercle) become visible around puberty and only fully fuse around 20 years of age.
THE TYPICAL RIB ( Fig. 4.1 )
The third to ninth ribs are considered typical ribs. A typical rib has a head, neck and body with a costal tubercle posteriorly, separating the neck and the body of the rib (see Fig. 4.1 ). The anterior portion of a typical rib is cartilaginous while the posterior portion is bony and forms two separate articulations: the costovertebral and the costotransverse joints, both of which are synovial joints. The head of the typical rib has two facets for articulation with two adjacent vertebral bodies (the costovertebral joint); for example the head of the sixth rib articulates with the inferior portion of T5 and the superior portion of T6, spanning the T5/T6 intervertebral disc. The costal tubercle has a facet medially for articulation with its own transverse process; for example, the tubercle of the sixth rib articulates with the T6 transverse process. The costal tubercle also has a nonarticular part laterally for ligament attachment. The neck of the rib is attached by a ligament to the transverse process of the vertebra above without forming a joint. A typical rib possesses a costal groove anteroinferiorly to accommodate the intercostal neurovascular bundle. The first rib is relatively immobile. The second to twelfth ribs move like bucket handles up and down. In inspiration, accessory muscles of inspiration draw the ribs upwards, which serves to increase the anteroposterior (AP) diameter of the thorax. Increasing the volume of the thoracic cavity decreases the internal pressure and draws gas into it.
The costal groove can project as a fine line inferior to the rib, which can be confused with a pneumothorax.
As the heads of the typical ribs articulate with two adjacent vertebrae, it can be difficult to match the rib with its numerically corresponding vertebra. The costotransverse joint can be an easier way to determine with which vertebra a particular rib is matched, on both chest radiograph and on computed tomography (CT).
ATYPICAL RIBS
First Rib ( Fig. 4.2 )
This is the shortest, flattest and most curved rib, and it plays a vital role in protecting vital organs. It articulates with T1 only. Along with its costal tubercle posteriorly, the first rib possesses a scalene tubercle along its inner margin, which serves as the site of insertion of the scalenus anterior muscle; this may be visible on chest radiograph as a superior protuberance. The superior aspect of the first rib has a groove for the subclavian vein anteriorly and a groove for the lowest trunk of the brachial plexus posteriorly. The subclavian artery directly abuts the nerve trunk and not the rib. These two grooves are separated by the scalene tubercle along with the distal scalenus anterior muscle itself (see Fig. 4.2B and C ).
- 1.
First thoracic vertebra
- 2.
Brachial plexus (yellow)
- 3.
Scalenus anterior muscle
- 4.
Subclavian artery
- 5.
Subclavian vein
- 6.
Clavicle
- 7.
First costal cartilage
- 8.
Manubrium of the sternum
- 1.
Subclavian vein
- 2.
Scalenus anterior
- 3.
Subclavian artery
- 4.
Brachial plexus
Second Rib
This is less curved and twice as long as the first rib. Like the first rib, it has additional tubercles for the attachments of serratus anterior and scalenus posterior.
Tenth Rib
This differs from the typical ribs by having only one articular facet on its head.
Eleventh Rib
This also has only one articular facet on its head. It has no costal tubercle for articulation with the transverse process.
Twelfth Rib
This has only one articular facet on its head; it has no costal tubercle and no costal groove.
VARIANT ANATOMY IN THE RIBS
Cervical Rib ( Fig. 4.3 )
A supernumerary bony or fibrous rib articulates with C7 posteriorly and fuses with the first rib anteriorly. This occurs in approximately 0.5% of people and when present, is bilateral in up to 50% of these cases. Cervical ribs can predispose to thoracic outlet compression. It can be difficult to distinguish a cervical rib from an abnormally elongated C7 transverse process.
Short Rib
A short rib or hypoplastic rib occurs when one or more of ribs 1–10 is abnormally short and does not extend anteriorly to the sternum. This is likely due to premature closure of the physis. The rib often fuses with a rib above or below, simulating a partial rib resection. Short rib occurs in up to 16%.
Bifid Rib ( Fig. 4.4 )
Bifid or forked rib occurs when the anterior portion of a rib divides into two separate processes. This most commonly affects the fourth ribs. Its incidence is unknown.
- 1.
Costal cartilages
- 2.
Bifid rib
- 3.
Manubrium of the sternum
- 4.
Body of the sternum
- 5.
Medial end of the left clavicle
Absent Ribs
Absence of at least one rib is common, with up to 8% only having 11 ribs. While it can be associated with congenital disease, it is much more commonly a normal anatomic variation.
Lumbar Rib
This may represent a true accessory rib arising from L1, a nonfused transverse process or an elongated transverse process. This is seen in approximately 1% of the population. The presence of a lumbar rib is commonly associated with transitional spinal anatomy.
Due to the large variation in the number of ribs, the upper-most and lower-most ribs cannot be assumed to be arising from T1 and T12, respectively, when trying to determine vertebral level.
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.
As the first rib is closely associated with the subclavian vessels and is protected anteriorly by the overlying clavicle, a fracture of the first rib in trauma has a significant association with neurovascular injury and should not be considered a typical rib fracture.
COSTAL CARTILAGES (see Fig. 4.4 )
The costal cartilages are the cartilaginous anterior ends of the ribs which slope obliquely upwards to join with the sternum. The junctions between the bony and cartilaginous portions of a rib, the costochondral junctions, are primary cartilaginous joints/synchondroses for all 12 ribs. The costochondral junctions permit little or no movement. The unions between the medial ends of the costal cartilages and the sternum vary in nature. The first costal cartilage forms a synchondrosis with the manubrium which, along with its primary cartilaginous costochondral junction, explains the relative immobility of the first rib. The second to seventh sternocostal joints form synovial joints which permit mobility. The second costal cartilage articulates with the manubriosternal joint and the third to seventh costal cartilages articulate directly with the body of the sternum. The eighth to tenth costal cartilages attach to each other in series and then with the seventh costal cartilage to join the body of the sternum. This combination of the eighth to tenth costal cartilages creates the costal arches which form the upper boundaries of the anterior abdomen. The eleventh and twelfth costal cartilages are small in size with pointed ends that terminate in the muscles of the abdominal wall. The first to seventh ribs are considered true ribs as they articulate directly with the sternum; the eighth to tenth are considered false ribs as they articulate indirectly, and the eleventh and twelfth ribs floating ribs as there is no articulation with the sternum. Calcification of the costal cartilages is normal with advancing age, seen in 6% in the third decade of life and in 45% in the ninth decade of life. It affects the first costal cartilage earliest and most prominently.
Prominent calcification of the first costal cartilages, along with first sternocostal joint osteophytes, often simulates apical lung nodules on chest radiograph.
The Intercostal Space and Vessels
The spaces between consecutive ribs are known as the intercostal spaces and are designated by the rib above, for example, the first intercostal space lies between the first and second ribs and the eleventh intercostal space lies between the eleventh and twelfth ribs. Hence, there are 11 intercostal spaces bilaterally; the regions below the twelfth ribs are known as the subcostal regions. The intercostal spaces are filled by the intercostal muscles, described below. The neurovascular bundle lies between the internal and innermost muscle layers.
Intercostal Arteries
The first to ninth intercostal spaces receive dual arterial blood supply from anterior and posterior intercostal arteries which arise from different parent vessels. The tenth and eleventh intercostal spaces are supplied solely by posterior intercostal arteries. The intercostal arteries also help to supply the skin of the chest wall and the parietal pleura.
Posterior Intercostal Arteries
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The first and second intercostal spaces receive posterior supply from branches of a single superior or supreme intercostal artery, which in turn arises from the costocervical branch of the subclavian artery.
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The third to eleventh posterior intercostal arteries arise individually from the descending thoracic aorta and supply their corresponding intercostal space.
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As the descending thoracic aorta lies to the left of the thoracic spine, the right posterior intercostal arteries are longer than the left and they must course anterior to the vertebra before entering the posterior portion of the intercostal space.
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The posterior intercostal arteries can often be identified on CT.
Anterior Intercostal Arteries
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The first to sixth anterior intercostal arteries arise directly from the internal thoracic artery (a branch of the subclavian artery).
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The seventh to ninth anterior intercostal arteries arise from the musculophrenic artery, the continuation of the internal thoracic artery.
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The tenth and eleventh intercostal spaces have no anterior intercostal artery.
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The anterior intercostal arteries are generally not visible on CT, in contrast to the posterior intercostals.
Intercostal Veins
Most of the intercostal spaces are drained by paired anterior and posterior intercostal veins, similar to the intercostal arterial supply; however, the posterior intercostal venous drainage is anatomically unique.
Posterior
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The first posterior intercostal veins arch over the pleura to drain into the brachiocephalic veins as the supreme intercostal veins.
Fig. 4.47
Azygos system.
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The second to fourth intercostal veins unite to form the right and left superior intercostal veins. The right superior intercostal vein drains to the azygos vein. The left superior intercostal vein courses laterally and then posteriorly around the aortic arch to drain into the left brachiocephalic vein or the accessory hemiazygos vein. As it travels along the lateral aspect of the aortic arch from anterior to posterior, it can create a small, rounded opacity adjacent to the aortic knuckle on chest radiograph known as the aortic nipple. This normal variant is reportedly seen in 10% of posteroanterior (PA) chest radiographs and can be confused with pathology.
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The right fifth to eleventh posterior intercostal veins drain into the azygos vein.
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The left fifth to eighth posterior intercostal veins drain into the accessory hemiazygos vein and the ninth to eleventh into the hemiazygos vein.
Anterior
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The anterior intercostal veins mirror the anterior intercostal arteries draining to the internal thoracic and musculophrenic veins.
Reflecting the position of neurovascular structures in the subcostal groove immediately inferior to the rib, needle puncture into the pleural space should be through soft tissues immediately above the upper margin of the rib.
Notching of the inferior margins of the posterior ribs is most frequently seen in postductal coarctation of the aorta (congenital focal stenosis of the thoracic aorta just distal to the left subclavian artery). This rib notching reflects abnormally high pressures in the posterior intercostal arteries as blood flows retrogradely through these vessels in order to bypass the stenotic segment of aorta and drain into the descending thoracic aorta distal to the site of coarctation. The first and second posterior intercostal arteries cannot bypass a postductal coarctation in the aortic arch as they arise proximal to the coarct from the thyrocervical trunk (from the subclavian arteries), so rib notching is seen in the third to eleventh ribs, but not the first and second.
MUSCLES OF THE THORACIC CAGE
The intercostal muscles are made up of the external, internal and innermost intercostal muscles but they cannot be reliably distinguished from each other on imaging and appear as a single muscle group. The intercostal neurovascular bundle runs between the internal and innermost intercostal muscles. Their order from superior to inferior is vein, artery and then nerve (VAN). 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.
- 1.
Body of T3
- 2.
Body of manubrium sterni
- 3.
Right brachiocephalic vein
- 4.
Left brachiocephalic vein
- 5.
Right subclavian artery
- 6.
Right common carotid artery
- 7.
Left common carotid artery
- 8.
Left subclavian artery
- 9.
Trachea
- 10.
Oesophagus
- 11.
Right scapula
- 12.
Spine of scapula
- 13.
Erector spinae muscle
- 14.
Trapezius muscle
- 15.
Supraspinatus muscle
- 16.
Infraspinatus muscle
- 17.
Subscapularis muscle
- 18.
Deltoid muscle
- 19.
Fat in axilla
- 20.
Pectoralis minor muscle
- 21.
Pectoralis major muscle
- 22.
Subcutaneous fat
- 1.
Body of T4
- 2.
Thecal sac and spinal cord
- 3.
Arch of aorta
- 4.
Superior vena cava
- 5.
Azygos vein
- 6.
Trachea
- 7.
Oesophagus
- 8.
Erector spinae muscle
- 9.
Trapezius muscle
- 10.
Infraspinatus muscle
- 11.
Subscapularis muscle
- 12.
Pectoralis minor muscle
- 13.
Pectoralis major muscle
Other thoracic cage muscles that can be seen on cross-sectional imaging include the pectoral major and minor muscles anteriorly, the serratus anterior and the teres major laterally, the rhomboids, the erector spinae and trapezius posteriorly and the subclavius and levator scapulae superiorly. The rotator cuff muscles are also well delineated on CT and magnetic resonance imaging (MRI) but not considered part of the thoracic cage. Sternalis is a rare accessory muscle that can be identified on CT and occasionally confused with pathology. It runs craniocaudally along the superficial surface of the sternum just medially to the pectoralis major.
‘Companion shadow’ is a radiographic term used to denote the smooth, homogeneous, well-defined radiodense stripes that can parallel bones on radiographs. Companion shadows are created by the soft tissues that accompany bones. They are often seen along the superior borders of the clavicles, the medial borders of the scapulae and the inferior margins of the ribs. Rib companion shadows are created by the intercostal fat and soft tissues, primarily the intercostal muscles. Companion shadows can simulate pleural pathology.
THE STERNUM ( Fig. 4.5 )
The sternum measures up to 17 cm in length and is composed of three components: the manubrium, the body and the xiphoid process.
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The manubrium is a broad polygonal bone making up the upper sternum directly opposite the T3 and T4 vertebral bodies. Its superior notch is known as the suprasternal notch and is usually visible as a surface depression in the skin of the lower neck. Superolateral notches form the sternoclavicular joints, the most superior articulation of the sternum. A pair of notches immediately inferior to this are for articulation with the costal cartilage of the first ribs. The manubriosternal joint (also known as the sternal angle or the angle of Louis) is a secondary cartilaginous joint and lies opposite the T4/T5 intervertebral disc space. The second costal cartilage articulates with both the manubrium and the sternal body, centred at the manubriosternal joint. This joint permits little movement and is completely fused in 5%.
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The body of the sternum is a large flat bone opposite T5–T9. The sternal body articulates directly with the third to sixth costal cartilages. It has demifacets superiorly and inferiorly for partial articulation with the second costal cartilage (shared with the manubrium) and the seventh costal cartilage (shared with the xiphoid).
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The xiphoid process (also known as the xiphisternum) usually remains cartilaginous well into adult life but eventually ossifies. The xiphoid usually points inferiorly but can be angled anteriorly (where it can be palpated) or posteriorly.
The manubriosternal joint, being a secondary cartilaginous joint between the manubrium and body of the sternum, may become inflamed as part of axial seronegative arthropathies such as ankylosing spondylitis.
Ossification of the Sternum
The sternum is made up of six primary ossification centres (one for the manubrium, four for the sternal body and one for the xiphoid), also known as sternebrae. The sternebrae ossify from superior to inferior from 6 months of fetal life onwards with some not yet ossified at birth. Between 15 and 25 years of age, sternebrae fuse from inferior to superior. Incomplete fusion of these ossification centres can result in large sternal foramina (see Fig. 4.5C ) which are seen in 5% of the population. These are most commonly seen in the second to fourth sternebrae in the body of the sternum. The xiphoid process fuses with the body at 40 years of age and the body and manubrium fuse in old age, if at all. Accessory ossification centres above the manubrium; episternal ossicles (see Fig. 4.5D ), another common variant, are seen in 4%.
RADIOLOGICAL FEATURES OF THE STERNUM
Plain Films
On a frontal chest radiograph, the manubrial borders may simulate mediastinal widening. The remainder of the sternum is not seen. The sternum can be evaluated on lateral and oblique radiographs.
Variation in sternal configuration includes: (1) depression of the lower end, known as pectus excavatum and (2) prominence of the mid-portion, known as pectus carinatum. Pectus excavatum can be objectively assessed on CT using the Haller index (maximum transverse diameter/narrowest AP length of chest), usually <2. Pectus excavatum causes characteristic findings on chest radiograph including exaggeration of the vertical course of the anterior ribs (7-shaped ribs) and loss of the normal right heart border due to displacement of the heart to the left ( Fig. 4.6 ).
Reflecting the presence of vascular red marrow within the sternum through adulthood, it is commonly seeded by bloodborne infection or bony metastatic disease, and is a common site for myelomatous deposits.
The Diaphragm ( Fig. 4.7 )
The diaphragm is an anatomically complex, dome-shaped skeletal muscle separating the thorax from the abdomen. In radiology, it is frequently subdivided into the left and right hemidiaphragms but these are descriptive terms for imaging studies and anatomically the diaphragm is a single structure. Unlike typical skeletal muscles, the diaphragm has a large number of tendinous origins from bones and ligaments around the internal circumference of the lower chest. Rather than having a tendinous insertion into a bone, its numerous muscle fibres converge to insert centrally as a large central tendon within the substance of the muscle itself such that the periphery of the diaphragm is composed of muscle fibres and the centre is composed of tendon. This unique design allows it to pull its centre downwards in inspiration. As previously described, increasing the volume of the thoracic cavity decreases its internal pressure and results in air being drawn into the lungs. The central tendon is, in fact, not central but closer to the sternum.
- 1.
Right crus of diaphragm
- 2.
Left crus of diaphragm
- 3.
IVC
- 4.
Aorta
- 5.
Vertebral body
- 6.
Spinal cord
- 7.
Pedicle
- 8.
Lamina
- 9.
Psoas muscle
The multiple peripheral origins of the diaphragm can be grouped into:
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Anterior
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Sternal origin: Two small slips from the posterior surface of the xiphisternum make up the sternal origins of the diaphragm.
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Lateral
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Costal origin: The costal part of the diaphragm arises in multiple slips from the lower six costal cartilages.
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Posterior
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Vertebral origin: Two large tendons called the diaphragmatic crura (singular crus) arise from the left and right sides of the upper lumbar vertebrae. The right crus is longer than the left arising from the lateral aspects of the L1–L3 vertebral bodies. Superior fibres of the right crus encircle the oesophagus as it traverses the diaphragm. The smaller left crus arises from the vertebral body and disc of the L1 and L2 vertebrae. The crura blend with the anterior longitudinal ligament of the spine. Both crura form muscular fibres superiorly which course anteriorly towards the central tendon. These fibres make up a significant portion of the overall diaphragm (the crural portion of the diaphragm).
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Ligamentous origin: Medial and lateral arcuate ligaments also serve as posterior origins for the diaphragm. They are actually focal fascial thickenings rather than true ligaments. The medial arcuate ligaments are thickenings of the psoas major fascia that extend from the L1/L2 intervertebral discs to the transverse processes of the L1 lumbar vertebra. Medially, they are continuous with the adjacent diaphragmatic crura. The lateral arcuate ligaments extend from the transverse processes of the L1 vertebra to the twelfth ribs and are thickenings of the quadratus lumborum fascia. The median arcuate ligament, also a misnomer, is a single thickened fibrous arch connecting the anterior aspects of the right and left crura in the midline. No portion of the diaphragm arises from the median arcuate ligament.
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OPENINGS IN THE DIAPHRAGM
These are as follows:
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Aortic – at level T12: The aorta passes posterior to the diaphragm, between the right and left crura, immediately posterior to the median arcuate ligament rather than through a true opening in the diaphragm. The thoracic duct and the azygos vein pass with the aorta.
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Oesophageal hiatus – at level T10: this is to the left of the midline but is surrounded by fibres of the right crus. With the oesophagus it transmits the right and left vagus nerves, branches of the left gastric artery, veins and lymphatics. Hiatus hernia is a common finding and refers to upward herniation of abdominal contents through the oesophageal hiatus.
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Caval opening – at level T8: transmits the inferior vena cava (IVC), whose adventitial wall is fused with the central tendon, and the right phrenic nerve.
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Behind the medial arcuate ligament – the sympathetic trunk.
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Behind the lateral arcuate ligament – the subcostal nerves and vessels.
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Between sternal and costal origins – the superior epigastric vessels.
The median arcuate ligament lies immediately anterior to the aorta as it passes from the thorax into the abdomen and the inferior margin of the ligament often abuts the superior margin of the coeliac trunk. When low-lying or thickened, the median arcuate ligament can cause varying levels of stenosis of the proximal coeliac trunk. Median arcuate ligament syndrome is the occurrence of postprandial pain or other symptoms suggestive of coeliac ischaemia in the setting of coeliac stenosis due to compression by the median arcuate ligament. However, asymptomatic stenosis of the coeliac trunk due to the median arcuate ligament is common and this syndrome should not be diagnosed on imaging alone.
STRUCTURES THAT PIERCE THE DIAPHRAGM
The structures that pierce the diaphragm are as follows:
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Terminal branches of the left phrenic nerve pierce the central tendon.
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The greater, lesser and least splanchnic nerves pierce each crus.
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The lymph vessels between the abdomen and thorax pierce the diaphragm throughout, especially posteriorly.
EMBRYOLOGY OF THE DIAPHRAGM
The anterior half of the diaphragm arises from the continuous septum transversum, which ultimately forms the central tendon. The posterior half of the diaphragm forms from left and right pleuroparietal membranes which give rise to the majority of the muscular diaphragm. These membranes are separated by the dorsal oesophageal mesentery which gives rise to the openings in the diaphragm. The pleuroparietal membranes eventually fuse with the septum transversum anteriorly as well as contributions from the thoracic mesoderm posteriorly and posterolaterally, forming a continuous diaphragm. Failure of fusion of the pleuroparietal membranes results in Bochdalek hernias. A congenital fusion defect between the sternum and the septum transversum gives rise to Morgagni hernias. Small anterior and posterior diaphragmatic defects are a common finding at cross-sectional imaging.
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 to C5 roots provide the motor supply of the diaphragm. The phrenic nerves descend from the neck along the lateral aspects of the mediastinum and then course from medial to lateral along the central domes of the left and right hemidiaphragms. The phrenic nerves can be visualized on CT coursing along the diaphragms. Sensory impulses from the central part of the diaphragm pass with the phrenic nerves, and those from the peripheral part with the intercostal nerves.
While the diaphragm is one single structure, the left and right sides are innervated separately by the left and right phrenic nerves, respectively. Hence unilateral diaphragmatic paralysis can occur, and so use of the terms left and right hemidiaphragm is appropriate in the context of imaging interpretation. The highest central portion of a hemidiaphragm is often referred to as the dome of the hemidiaphragm.
RADIOLOGICAL FEATURES OF THE DIAPHRAGM (see Fig. 4.7 )
Frontal Chest Radiograph ( Fig. 4.8 )
The central tendon represents the highest portion of the diaphragm; therefore on frontal chest radiographs, the medial portion of the diaphragm is higher than the lateral portions, and the anterior portions are higher than the posterior. The dome of the hemidiaphragm should be at least 1.5 cm above a straight line from the medial cardiophrenic angle to the lateral costophrenic angle. The right hemidiaphragm is typically higher than the left by approximately 2 cm. The left hemidiaphragm can be higher than the right in the normal subject, especially with gas in the colon, but causes of abnormal diaphragmatic elevation should be considered in this case. Position of the diaphragm relative to the ribs on PA chest radiographs is often used to evaluate for hyperinflation of the lungs. Due to the inferior angulation of the anterior ribs, more of the posterior ribs will project above the diaphragm than anterior ribs. True position of the anterior ribs relative to the diaphragm may be more accurate than the posterior ribs, as the apex of the diaphragm is nearer to the anterior ribs and to the detector (on PA chest radiographs), and is therefore less subject to distortion by magnification or beam angulation. Typically, up to 10 posterior ribs and 6 anterior ribs project above the hemidiaphragms and an increase in this number can indicate hyperinflation. However, this should be interpreted with caution given the natural asymmetry of the left and right hemidiaphragm position. Additionally, the diaphragm can be significantly depressed in maximal inspiration in normal health, particularly in young athletic individuals. Loss of the normal convexity is a more accurate marker of hyperinflation; this is easier to judge on lateral chest radiographs.
Lateral Chest Radiograph ( Fig. 4.9 )
On lateral chest radiographs, the right and left hemidiaphragms appear as smooth interfaces between the inferior lungs and the upper abdomen. They are highest anteriorly and course inferiorly towards the posterior costophrenic recesses. It can be surprisingly difficult to distinguish the left from the right hemidiaphragm, but the following anatomical details can help to identify them:
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A portion of the left ventricle sits on the medial left hemidiaphragm. This will obscure the anterior 20%–30% of the left hemidiaphragm silhouette, whereas the dome of the right hemidiaphragm does not abut the heart and can usually be traced anteriorly to the anterior chest wall. This is typically the most reliable differentiator. As lateral chest radiographs are conventionally acquired with the left chest wall adjacent to the detector, the right hemithorax will be magnified by up to 10% due to its distance from the detector by the diverging X-ray beam. This causes the right posterior costophrenic angle to project more inferiorly and posteriorly than the left. The hemidiaphragm, which can be traced to the more posteroinferior costophrenic angle, is usually the right.
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The IVC may be seen piercing the right dome.
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Air within the gastric fundus lies under the left diaphragm. It is often more anterior than you might think.
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Anterior wall of the trachea
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Posterior tracheal stripe
- 3.
Scapulae
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Left lower lobe bronchus
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Right lower lobe bronchus
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Aorta (not well seen)
- 7.
Vertebral body of T4
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Anterior aspect of the right ventricle
- 9.
Pulmonary outflow tract
- 10.
Main pulmonary artery
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Right pulmonary artery
- 12.
Left pulmonary artery
- 13.
Left atrium
- 14.
Left ventricle
- 15.
Inferior vena cava
- 16.
Horizontal (minor) fissure
- 17.
Oblique (major) fissure
- 18.
Sternum
- 19.
Manubriosternal joint
- 20.
Left hemidiaphragm
- 21.
Right hemidiaphragm
- 22.
Stomach bubble
- 23.
Lung projected anterior to sternum in intercostal space
- 24.
Retrosternal airspace
Red line: Anterior wall of right ventricle
Light blue line: Right ventricular outflow tract
Yellow lines: Thoracic aorta
Purple lines: Left pulmonary artery
Orange circle: Right Pulmonary artery
Green line: Left atrium
Pink line: Left ventricle
Dark blue line: Inferior vena cava
Blue line: Left hemidiaphragm
Red line: Right hemidiaphragm
Blue lines: Posterior tracheal stripe
Yellow circle: Right upper lobe bronchus
Green circle: Left upper lobe bronchus
Purple line: Posterior wall of the bronchus intermedius
There is apparent thickness of the diaphragm on radiographs:
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With the pleura and peritoneum when there is air in the peritoneum: 2–3 mm thick.
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With the pleura and fundal wall of stomach: 5–8 mm thick.
The posterior costophrenic angle (visible on a lateral chest radiograph) lies more inferiorly than the lateral costophrenic angle (visible on frontal chest radiograph), making it the most dependent portion of the pleural space and the point where fluid will collect first in the standing position. Therefore lateral chest radiographs are considerably more sensitive than frontal in the assessment for small pleural effusions.
Curvature of the Dome
Similar to the frontal radiograph, the perpendicular height of the dome of the diaphragm from a line between the posterior costophrenic and the anterior cardiophrenic angles should be at least 1.5 cm. Flattening of the hemidiaphragm is typically the most sensitive assessment for hyperinflation.
Fluoroscopy
Continuous fluoroscopy can be used to assess normal diaphragmatic movement. The range of movement of the diaphragm with respiration is as follows:
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Quiet respiration: 1 cm
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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:
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Supine: higher
- ■
Lateral decubitus: dome on the dependent side is higher
In cases of suspected diaphragmatic paralysis (phrenic nerve palsy), a sniff test can be used. The patient is asked to take rapid inspirations through the nose (sniffs) while screening in both frontal and lateral positions. In normal motor function, the hemidiaphragms contract/move downwards and flatten out. When paralyzed, the affected hemidiaphragm can remain stable or move upwards (paradoxical motion). The reason for paradoxical upward movement is the accessory muscles of inspiration, which are still functional. These will still contract with sniffing, increasing the volume and decreasing the pressure within the hemithorax. As the affected hemidiaphragm is paralyzed, it can no longer resist the pressure drop and is drawn upwards.
Eventration is focal thinning of a portion of the diaphragm resulting in upward ballooning of a portion of the diaphragm. This is a common finding, most commonly seen in the anteromedial right hemidiaphragm. This can be confused with diaphragmatic paralysis as well as other pathologies.
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
Portions of the diaphragm, in particular the dome of the right hemidiaphragm adjacent to the liver, are frequently indistinguishable from the adjacent viscera. The costal and posterior origins of the diaphragm are usually well delineated. The costal origins frequently form well-defined slips rather than a continuous structure. When these slips are large, they can indent the lateral aspect of the liver, simulating a nodular liver contour. The right and left crura are usually visible on the anterior surface of the upper lumbar vertebrae and can be confused with lymph nodes. Defects in the posterior (ligamentous) portions of the diaphragms are common, seen in at least 1% of cases.
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 viscera sign’.
Magnetic Resonance Imaging (see Fig. 4.7 )
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 ( Fig. 4.10 )
The pleura is a serous membranous closed sac that has two components:
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The visceral pleura, the inner layer covering the lung and extending between the lobes of the lungs into the major, minor and accessory fissures.
- ■
The parietal pleura, the outer layer adherent to the inner thoracic wall and mediastinum. Parts of the pleura and pleural space are named according to site, for example costal, diaphragmatic, mediastinal and apical.
- 1.
Anterior Junction Line
- 1.
Anterior junction line (blue), four layers of pleura, 24.5%–57% of frontal chest radiographs.
The visceral pleura conforms to the contour of the lung and changes dynamically with inspiration/expiration. The position of the parietal pleura is fixed and 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). The parietal pleura is supplied by the systemic vessels. The visceral pleura receives arterial supply from both the bronchial and the pulmonary circulation.
| Visceral Pleura and Lung | Parietal Pleura | |
|---|---|---|
| Anterior | 6th costal cartilage | 7th costal cartilage |
| Midaxillary line | 8th rib | 10th rib |
| Posterior T10 | T10 | T12 |
INFERIOR PULMONARY LIGAMENTS
The lung roots/hila are surrounded by pleura, which is the continuation of the visceral pleura lining the medial surface of the lung and the parietal pleura lining the lateral aspect of the mediastinum. This pleura is closely adherent to the superior, anterior and lateral surfaces of the hilar structures but inferiorly the double layer of pleura continues caudally to the diaphragm as the inferior pulmonary ligaments. The inferior pulmonary ligaments contain lymphatic tissue including lymph nodes as well as connective tissue and the diaphragmatic courses of the phrenic nerves. They allow the lung root to move with respiration and also for distension of the pulmonary veins, which lie inferiorly in the lung root. The inferior pulmonary ligaments therefore divide the medial portion of the pleura space, below the level of the hila, into anterior and posterior compartments. The inferior pulmonary ligaments can usually be identified on CT.
The lower limits of the parietal pleura are more inferior than is intuitive, particularly when there is a shallow depth of inspiration and there is a large distance (up to 5 cm) between the inferior-most parietal pleural reflection and the lung. This is an important consideration when planning intercostal approaches to upper abdominal interventions as the parietal pleura will commonly be traversed. As long as the visceral pleura (lung) is not traversed, pneumothorax would not be expected but traversing the pleural space with a needle or catheter creates a temporary communication between the abdominal and pleural cavities, potentially contaminating the pleural space with infection/tumour.
The healthy pleural space is continuous throughout the hemithorax and so the position of air or fluid in the pleural space is related to patient position and gravity. In supine position, pleural air will collect along the anterior chest wall, predominantly at the anterior costophrenic recess. In standing position, pleural air will collect at the apex of the pleural space and laterally in the lateral decubitus position.
RADIOLOGICAL FEATURES OF THE PLEURA
Radiography
As with all structures, the pleura is only visible on a chest radiograph if a sufficient amount of it is tangential to the X-ray beam to attenuate the beam and if there are structures of a different density (non-soft tissue) adjacent to it. As the pleura is a very thin structure, it is often not visible on chest radiography and its position has to be inferred.
The pleura may be visible in a normal subject at:
- ■
Fissures that are tangential to the X-ray beam, which varies with frontal and lateral views (see interlobar fissures section).
- ■
Sites where the parietal pleura lies on prominent extrapleural fat.
- ■
Junction lines (or junctional lines): Formed by the apposition of the pleural layers ± intervening mediastinal fat where both lungs meet, these lines will not necessarily be visible on all normal chest X-rays (CXRs; see Figs. 4.8 and 4.11 ):
- ■
Anterior junction line: Formed by the lungs meeting anteromedially anterior to the arch of the aorta. This line courses obliquely across the upper two thirds of the sternum on a PA chest radiograph and should not extend above the manubriosternal joint.
- ■
Posterior junction line: Formed by the lungs meeting posteromedially, which is more superiorly located compared to the anterior junction line. This line may be seen above the clavicles, projected over the trachea to the level of the aortic arch.
Fig. 4.8
Posteroanterior chest radiograph. (A) Clear. (B) Annotated.
- 1.
Right brachiocephalic vessels (dark blue dotted line)
- 2.
Right paratracheal stripe (purple dotted line)
- 3.
Azygos vein (purple circle)
- 4.
Superior vena cava draining into right atrium (blue dotted line)
- 5.
Right hilar point (red)
- 6.
Interlobar artery (green dotted line)
- 7.
Confluence of the right pulmonary veins with the left atrium (orange dotted line right side of mediastinum)
- 8.
Right heart border (right atrium, red dotted line)
- 9.
Left paratracheal interface (commonly made up of the left subclavian artery, dark orange dotted line)
- 10.
Aortic knuckle (green dotted line)
- 11.
Aortopulmonary window (dark blue dotted line)
- 12.
Pulmonary trunk (yellow dotted line)
- 13.
Left atrial appendage (light orange dotted line left side of mediastinum)
- 14.
Left lower lobe pulmonary artery (turquoise dotted lines)
- 15.
Left heart border (left ventricle, purple dotted line left side of mediastinum)
- 16.
Trachea
- 17.
Left main bronchus
- 18.
Right main bronchus
- 19.
Lateral wall of the descending thoracic aorta
- 20.
Right hemidiaphragm
- 21.
Left hemidiaphragm
- 22.
Right clavicle
- 23.
Companion shadow of the right clavicle
- 24.
Coracoid process of the right scapula
Fig. 4.51
Computed tomography scan of the thorax: upper T4 level showing mediastinal lines.
- 1.
Anterior junction line
- 2.
Air-filled trachea
- 3.
Oesophagus
- 4.
Right paratracheal stripe
- 5.
Azygo-oesophageal stripe
- 6.
Superior vena cava
- 7.
Aortic arch
- 8.
Right paraspinal line
- 9.
Left paraspinal line
- 10.
Azygos vein
Fig. 4.52
Cross-sectional anatomy: level T3.
Fig. 4.56
Cross-sectional anatomy: level T4.
- ■
The anterior junction line, when visible, can serve as an important marker of midline shift in a tension pneumothorax as it puts up much less resistance to free air than the mediastinum.
While the pleura usually abuts the ribs on one side and the lung on the other side (both structures are of a different density to the soft tissue-density pleura), it cannot usually be discerned from the high-density bones. Prominent low-density fat in the extrapleural space, when abundant enough, can provide enough contrast to resolve the pleura. Similarly, when a pneumothorax is present, the air in the pleural space provides contrast, so the radiodense line that you can see is the visceral pleura rather than just the edge of the lung parenchyma. Identifying this thin white line is essential in distinguishing a pneumothorax from a skin fold.
Radiology parlance with regard to the pleura can be confusing. The use of the “pleural” to describe a lesion should be reserved for lesions that directly arise from or involve the pleura. The term ‘subpleural’ is a localizing term to describe something that is in close proximity to the pleura but without arising from it. “Peripheral” is the preferred term for such lesions however subpleural is commonly used. A lesion that does not arise from the pleura, but has contact with it, should be described as “pleura-based”.A glossary of terminology for thoracic imaging has been published by the Fleischner Society, and can be a helpful reference when interpreting thoracic imaging. If you think something is arising from the either the lung parenchyma or the pleura, say so! If you are unsure, say that you are unsure and just describe the location.
CT of the Pleura
As with radiographs, the pleura can also be very difficult to identify on CT and the pleura cannot usually be distinguished from the thoracic wall or mediastinum unless it is thickened. It will be reliably seen in the same locations as on radiographs (fissures, junction lines and adjacent to extrapleural fat). Due to CT’s ability to be reconstructed in any plane, all fissures will be visible on CT, regardless of orientation. The inferior pulmonary ligaments are also usually visible on CT extending below the inferior pulmonary vein caudally and posteriorly to the diaphragm. The right inferior pulmonary ligament lies close to the IVC, whereas the left lies close to the oesophagus.
The pleura usually enhances most vividly with contrast after a delay of 70 seconds (not the typical scanning delay of a routine CT thorax). A delayed protocol could be employed to try and highlight pleural pathology.
The Extrapleural Space
The extrapleural space is the space external to the pleura, located between the parietal pleura and the inner margin of the ribs. It usually contains, fat, lymphatic tissue including lymph nodes, blood vessels and the innermost intercostal muscles. Typically, it is difficult to visualize the extrapleural space at radiography and cross-sectional imaging unless distended with fat (common in obesity and corticosteroid treatment). It is not a common site of primary pathology and often overlooked, but is commonly involved by pathological process arising from the chest wall or pleura.
Prominent extrapleural fat simulates pleural thickening/pathology on chest radiographs; however it will be of a lower (fat) density.
The Trachea and Bronchi ( Figs. 4.12 and 4.13 )
THE TRACHEA
The trachea is the inferior continuation of the respiratory tract beyond the larynx. It begins at the lower border of the cricoid cartilage at the level of the C6 vertebra. It extends to the carina , the point where it divides into the left and right main bronchi, 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. Its inner layer consists of ciliated columnar epithelium which circumferentially lines its anterior, lateral and posterior walls. Fifteen to twenty thick, but incomplete rings of cartilage are present in the walls of the trachea outside the mucosa. These cartilage rings protect the anterior and lateral walls of the trachea (the cartilaginous trachea). The posterior wall of the trachea is devoid of cartilage and is lined by a longitudinal muscle called the trachealis instead. The posterior wall of the trachea is also known as the membranous trachea and can freely collapse inwards into the lumen of the trachea whereas the cartilaginous trachea is a solid fixed structure.
Relations of the Trachea
Cervical (see Figure 1.34 , Figure 1.35 )
The anterior relations are as follows:
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Anterior:
- ■
Isthmus of thyroid anterior to the second, third and fourth rings
- ■
Inferior thyroid veins
- ■
Strap muscles: sternohyoid and sternothyroid
- ■
- ■
Posterior:
- ■
Oesophagus (often left posterolateral at the 4 or 5 o’clock position)
- ■
- ■
Lateral:
- ■
Left and right lobes of the thyroid gland
- ■
Common carotid artery
- ■
Recurrent laryngeal nerves
- ■
Inferior thyroid arteries
- ■
Thoracic (see Fig. 4.9 )
The thoracic relations are as follows:
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Anterior:
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Brachiocephalic trunk and left common carotid arteries
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Anterior jugular veins
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Left brachiocephalic vein
- ■
Arch of the aorta
- ■
- ■
Posterior:
- ■
Oesophagus (again, usually left posterolateral)
- ■
Left recurrent laryngeal nerve
- ■
- ■
Left lateral:
- ■
Left common carotid and left subclavian arteries
- ■
Left recurrent laryngeal nerve
- ■
- ■
Right lateral:
- ■
Right innominate vein/superior vena cava (SVC)
- ■
Right vagus nerve
- ■
Arch of the azygos vein
- ■
Pleura (in direct contact unlike the other 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 arteries. Venous drainage is to the inferior thyroid venous plexus.
Tracheal deviation on a chest radiograph occurs with asymmetrical changes in volume between the hemithoraces. For example, in the case of volume loss (e.g. pulmonary fibrosis) the trachea may deviate towards the abnormality, whereas in pathologies with volume expansion (e.g. pneumothorax) the trachea will deviate away from the abnormality .
The dynamic nature of the carina is an important consideration in positioning of the tip of an endotracheal tube. A tube tip that projects close to the carina in inspiration may enter into one of the main bronchi in expiration. Some radiologists use 3 cm above the carina as the minimum safe distance for endotracheal tube tip position.
AIRWAYS ( Fig. 4.14 )
‘Airways’ is a collective term for the bronchi and bronchioles (the trachea can be included in this term but is often considered separately). Like the anatomy of the lung parenchyma, there is significant asymmetry between the airway anatomy of the left and right lungs. The airway of a right or a left lung follows a general blueprint but there is a large amount of variation from one person to the next. Airways undergo successive divisions into two or more smaller calibre airways with 23 divisions between the trachea and the alveoli. The airways branch in tandem with the pulmonary artery branches which lie immediately adjacent to them. Successive airway divisions can be referred to as successive ‘generations’ with the first, second and third generations representing the main bronchi, the lobar and the segmental bronchi based on the unit of lung they supply. All generations distal to the segmental bronchi can be termed the subsegmental airways. An airway is designated a bronchus (plural = bronchi) based on the presence of cartilage in its submucosa (this cartilage is C-shaped like in the trachea and serves to keep the airway patent). An airway lacking cartilage is referred to as a bronchiole (plural – bronchioles or bronchioli) and they maintain their patency through the elastic recoil of the surrounding parenchyma. Bronchioles begin around the sixth-generation airways. The diameter of an airway depends on its generation and the wall thickness depends on its diameter. CT can only resolve airways with a diameter of 2 mm and a wall thickness of 0.2–0.3 mm, so the vast majority of bronchioles cannot be seen on CT. These bronchioles are referred to as ‘small airways’.
Airways in the peripheral lungs, within 1 cm of the pleura, have undergone numerous divisions and should have a diameter and wall thickness that cannot be resolved with current CT resolution. Visualization of the lumen of an airway within 1 cm of the pleura is abnormal and can be termed bronchiolectasis.
Carina
The carina is the bifurcation of the trachea into the left and right main bronchi and typically lies at the level of the T4/T5 intervertebral disc (also the level of the manubriosternal joint) but moves significantly with breathing (T4 on inspiration and as low as T6 on expiration). The carinal angle measures approximately 65 degrees – that is, 20 degrees to the right of the midline and 40 degrees to the left. This asymmetry gives the right main bronchus a more vertical downward course and the left main bronchus a more horizontal course. This angle is slightly larger in children. The carinal angle increases by 10 degrees to 15 degrees in recumbency.
Widening of the carinal angle on a PA chest radiograph above 65 degrees can be a marker of enlargement of the left atrium and the carinal angle should be considered before calling a heart enlarged on chest radiograph.
RIGHT MAIN BRONCHUS
The right main bronchus courses from the mediastinum into the lung along with the pulmonary vessels and lymphatics but has already divided by the time it reaches the root of the lung (rounded opening in the medial aspect of the lung devoid of pleura/interstitium that transmits major structures). As the trachea and carina lie slightly to the right of midline, the right main bronchus is shorter and more vertically orientated than the left main bronchus, measuring 2.5 cm in length and forming an angle of about 25 degrees to the vertical median (left main bronchus: 5 cm long and 40-degree angle). It is also a wider bronchus measuring 1.5 cm in diameter versus 1.2 cm on the left. The course of the right main bronchus lies posteriorly and parallel to the right main pulmonary artery. This fact helps identify it on both lateral chest radiographs and sagittal CTs.
Relations of the Right Main Bronchus
The relations of the right main bronchus are as follows:
- ■
Anterior:
- ■
SVC
- ■
Right main pulmonary artery
- ■
- ■
Posterior:
- ■
Azygos vein
- ■
- ■
Superior:
- ■
Azygos arch
- ■
The right main bronchus gives rise to three lobar bronchi supplying the upper, middle and lower lobes of the right lung. The bronchus to the upper lobe is also known as the eparterial bronchus (epi – above). This arises from the superior aspect of the right main bronchus and it is the only one of the lobar bronchi on either side to branch above the level of a main pulmonary artery. This is important as it is a consistent marker of right-sided pulmonary anatomy. Determining whether a lung has the correct anatomy for the side of the body that it is on can be important in congenital anomalies. The right upper lobe bronchus arises almost immediately after the tracheal bifurcation and enters the hilum of the lung separately to the bronchus intermedius. The right upper lobe bronchus then trifurcates into the apical, anterior and posterior segmental branches. The short segment of combined bronchus distal to the branching of the right upper lobe bronchus and before the division into the right middle and lower lobar bronchi is called the bronchus intermedius. This airway courses almost directly inferiorly so it is identifiable on a lateral radiograph as it is tangential to the beam.
The lobar bronchi divide into segmental bronchi supplying the bronchopulmonary segments of each lobe along with a corresponding pulmonary arterial branch. There are 10 bronchopulmonary segments in total in the right lung.
Right upper lobe bronchus segmental divisions:
- ■
Apical
- ■
Posterior
- ■
Anterior
Right middle lobe bronchus segmental divisions:
- ■
Lateral
- ■
Medial
Right lower lobe bronchus segmental divisions:
- ■
Superior (this comes off opposite to the right middle lobe bronchus)
- ■
Medial
- ■
Anterior
- ■
Lateral
- ■
Posterior
LEFT MAIN BRONCHUS
The left main bronchus is twice as long as the right main bronchus (5 cm vs 2.5 cm), having to travel to the left lung from the right side of the mediastinum. It courses more horizontally than the left (40 degrees from vertical vs 25 degrees) and it is narrower (1.2 cm vs 1.5 cm). The left main bronchus also courses more posteriorly than the right main bronchus, which is more or less parallel to the horizontal in the axial plane. This explains its more posterior position than the right on a lateral chest radiograph. In contrast to the right main bronchus, the left main bronchus travels inferior to the left main pulmonary artery rather than posterior to it. The left main pulmonary artery runs from posterior to anterior, almost perpendicular to the left main bronchus. This pattern of the left main pulmonary artery running over the top of the left main bronchus distinguishes the left hilum from the right where the bronchus and artery run in parallel with the vessel anteriorly and the airway posteriorly.
Relations of the Left Main Bronchus
The relations of the left main bronchus are as follows:
- ■
Anterior: Pulmonary trunk
- ■
Posterior:
- ■
Oesophagus
- ■
Descending aorta
- ■
Superior:
- ■
Aortic arch
- ■
Left main pulmonary artery
The left main bronchus divides into the left upper and lower lobar bronchi within the lung (distal to the root of the left lung). The hyparterial bronchus (hyp – below) describes a segmental bronchus branching below the level of the main pulmonary artery. ‘Hyparterial’ could technically describe all segmental bronchi except the eparterial right upper lobe bronchus, but the term hyparterial bronchus conventionally refers to the left upper lobe bronchus to highlight the difference between left- and right-sided airway anatomy. Again, this can be helpful in cases of congenital anomalies. There are 8 left bronchopulmonary segments and corresponding bronchi in comparison to the right lung’s 10.
Left upper lobe bronchus segmental divisions:
- ■
Apicoposterior: The posterior and apical segmental bronchi usually have a common apicoposterior bronchus, which then subdivides into separate apical and posterior segmental branches.
- ■
Anterior.
- ■
Lingular bronchus comes off the upper lobe bronchus. This divides into:
- ■
Superior lingular segment
- ■
Inferior lingular segment.
- ■
Left lower lobe bronchus segmental divisions:
- ■
Superior
- ■
Anteromedial
- ■
Lateral
- ■
Posterior
The anatomy of the bronchial tree is shown diagrammatically in Fig. 4.14 . 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.
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