The thorax - Elsevier · 2013-12-20 · THE THORAX 109 — Next three by the musculophrenic artery,...

THE THORACIC CAGE

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

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

The typical rib (Fig. 4.1)

A typical rib has a head, neck, tubercle and shaft.The head has two facets for articulation with vertebral

bodies, for example the sixth rib articulates with the bodiesof T5 and T6 vertebrae. These costovertebral joints are syn-ovial joints.

The neck of the rib is attached by a ligament to the trans-verse process of the vertebra above.

The tubercle has a facet medially for articulation with its own transverse process. This costotransverse joint is alsoa synovial joint. The tubercle also has a nonarticular partlaterally for ligament attachment.

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Chapter 4

The thorax

CHAPTER CONTENTS

The thoracic cage 107The diaphragm 111The pleura 114The trachea and bronchi 116The lungs 118The mediastinal divisions 124The heart 125The great vessels 133The oesophagus 136The thoracic duct and mediastinal lymphatics 139The thymus 139The azygos system 140Important nerves of the mediastinum 142The mediastinum on the chest radiograph 143Cross-sectional anatomy 145

Fig. 4.1 A typical rib.

NeckHead witharticular facets

Articulartubercle

For costalcartilage

Nonarticularpart of tubercle

Angle

Subcostalgroove

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ANATOMY FOR DIAGNOSTIC IMAGING108

The shaft has a posterior angle and a much less promi-nent anterior angle. It has a subcostal groove that is muchmore prominent posteriorly. This lodges the intercostal vessels and nerves.

Atypical ribs

First rib (Fig. 4.2)

This is the shortest, flattest and most curved rib. It articu-lates with T1 only. A tubercle on its inner border marks theattachment of the scalenus anterior muscle. It is grooved bythe subclavian vein anteriorly. Posteriorly, another groovemarks where the lowest trunk of the brachial plexus andthe subclavian artery – that is, the nerve trunk and not theartery – lies in contact with the bone.

Second rib

This is less curved and twice as long as the first rib. It has atubercle on its external (lower) border, which is often wellmarked on a chest radiograph, at the site of attachment ofthe second head of the scalenus anterior muscle.

Tenth rib

This differs from the typical ribs by having only one articu-lar facet on its head.

Eleventh rib

This also has only one articular facet on its head. It has notubercle for articulation with the transverse process.

Twelfth rib

This has only one articular facet on its head; it has no tubercle and no subcostal groove.

Costal cartilagesThese are the unossified anterior ends of the ribs. They slopeupwards to the sternum, where they form synovial sterno-chondral 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 withthe costal cartilages of the ribs above. The eleventh andtwelfth costal cartilages have pointed ends and end in themuscles of the abdominal wall.

The intercostal space and vessels

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

Intercostal arteriesPosterior ● Upper two spaces supplied by superior intercostal

arteries from the costocervical branch of the subclavian artery;

● Lower nine from the thoracic aorta.

Anterior ● Two branches to most intercostal spaces:

— Upper six spaces supplied by the internal thoracicbranch of the subclavian artery;

Fig. 4.2 (a) First rib; (b) structures crossing first rib.

A B

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THE THORAX 109

— Next three by the musculophrenic artery, thecontinuation of the internal thoracic artery.

The lower two spaces have no anterior intercostal artery.

Intercostal veinsPosterior (see Fig. 4.36)● First intercostal vein arches over the pleura to drain into

the brachiocephalic vein;● Second to fourth drain to a superior intercostal vein

which drains to the azygos vein on the right and to thebrachiocephalic vein on the left;

● Fifth to eleventh on the right drain to the azygos vein;● Left fifth to eighth drain to the accessory hemiazygos

vein and ninth to eleventh to the hemiazygos vein.

Anterior● Veins accompany arteries to internal thoracic and

musculophrenic veins.

Muscles of the thoracic cage (see Figs 4.42 and 4.43)

The external, internal and innermost intercostal musclesoccupy the spaces between the ribs. Subcostal muscles onthe 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 inter-costals and the subcostals separate the intercostal neuro-vascular bundles from the pleura. The transverse thoracicmuscle arises on the deep surface of the sternum and adja-cent lower costal cartilages and passes superolaterally tothe 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 posteriorpart of the rib below, serratus posterior superior; from the spinous processes of the vertebrae superolaterally to theposterior ribs and serratus posterior inferior; and from thespinous processes inferolaterally to the posterior aspect ofthe lower ribs.

Other muscles have attachments to the thoracic cage andcan be seen on axial cross-sectional imaging, including thepectoral muscles anteriorly, the serratus anterior and theteres major and subscapularis laterally and posteriorly, andthe 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 besplayed or hypoplastic. Variation is commoner in theupper ribs, especially the first.

● On a PA chest radiograph the subcostal groove mayappear as a fine line below the rib, especially posteriorly(not to be mistaken for a periosteal reaction orpneumothorax). It may also appear as shallowindentation posteriorly near the neck (not to bemistaken for rib notching).

● A prominence is often seen on the second rib on a chestradiograph, which is usually symmetrical right and left.This is due to insertion of part of the scalenus anteriormuscle.

● The costal cartilages, especially the first, may calcify orossify from early adulthood. In males the calcification isusually in marginal bands, and in females in a centraltongue.

● Cervical ribs: these are bony or fibrous bands betweenC7 and the first rib and are found in 1–2% of subjects. Of these, 50% are bilateral and often they areasymmetrical. They may be distinguished from the first rib by the orientation of the transverse process –that of C7 points downward, whereas that of T1 ishorizontal or points upward. A fibrous band is likely to be present if the anterior tubercle of the transverseprocess of C 7 is prominent on a radiograph. The brachialplexus 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 neurologicalsymptoms.

● Lumbar ribs: the transverse process (which is the costalelement of the lumbar vertebra), may fail to fuse withthe vertebral body and retain a synovial joint with theneural arch. They are symptomless.

● Ossification of the ribs: a bony centre arises at the angleof the rib in the eighth fetal week. Secondary centresoccur at the head and at the tubercle at 15 years of ageand fuse at 25 years of age.

The sternum (Fig. 4.4)

The sternum has:

● A manubrium opposite T3 and T4 which articulates withthe clavicle and with one-and-a-half costal cartilages;

● A sternal angle, which is a secondary cartilaginous jointand lies opposite T4/5 disc space;

● A body opposite T 5 –T9 , made up of four stenebraewhich articulate with five-and-a-half costal cartilages;and

● A xiphoid process, which remains cartilaginous wellinto adult life.

Radiological features of the sternum

Plain filmsOn a PA chest radiograph the manubrial borders may simu-late 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 thelungs, whose markings may cause confusing shadows.Lateral views are also helpful.

Variation in sternal configuration include: (i) depressionof 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 appearfrom above downwards from the fifth to the ninth fetalmonths. Between 15 and 25 years of age stenebrae fuse frombelow upwards. The xiphoid process fuses with the body at40 years of age and the body and manubrium fuse in oldage, if at all.

Computed tomography

On CT images of the thorax the manubrium is usuallyangled with respect to the gantry, and this may cause corti-cal lack of sharpness. The body of the sternum is usuallyperpendicular to the beam and well demarcated.

C

Fig. 4.4 The sternum.

Manubrium

Jugular notch

Facet for clavicle

First rib cartilage

Second rib cartilage

Third rib cartilage

Fourth rib cartilage

Fifth rib cartilageSixth rib cartilage

Seventh rib cartilage

Sternal angle

Body

Xiphoid process

A

B

Fig. 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 theinsertion of part of scalenus anterior muscle;

(c) cervical rib – a well developed bony cervical rib on the left side(arrows).


THE THORAX 111

THE DIAPHRAGM (Fig. 4.5)

The diaphragm forms the highly convex floor of the thoracic cage. It arises from vertebral, costal and sternalorigins 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 thevertebral body and disc of L1 and L2 vertebrae.

The medial arcuate ligament is a thickening of the fasciaover the psoas muscle from the body of L2 to the transverseprocess of L1 lumbar vertebrae. The lateral arcuate liga-ment is a thickening of the fascia over quadratus lumborum

A B

C

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1

Fig. 4.5 Diaphragm: (a) view from below showing origin and openings; (b) crura and arcuate ligaments; (courtesy of Professor J.B. Coakley);(c) crura of the diaphragm as seen on axial and (d) coronal MRI.

1. Right crus of diaphragm2. Left crus of diaphragm3. IVC

4. Aorta5. Vertebral body6. Spinal cord

7. Pedicle8. Lamina9. Psoas muscle



from the transverse process of L 1 vertebrae to the twelfthrib. The median arcuate ligament is the fibrous medial partof both crura behind which the aorta passes; no diaphrag-matic muscle arises from this.

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

The sternal part of the diaphragm arises in two small slipsof 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 andits right and left posterior parts extend towards the para-vertebral gutters.

Openings in the diaphragmThese are as follows:

● Aortic – at level T12 : in fact the aorta passes behind the median arcuate ligament rather than through thediaphragm. The thoracic duct and the azygos vein passwith the aorta;

● Oesophageal hiatus – at level T10 : this is to the left of themidline but is surrounded by fibres of the right crus.With the oesophagus it transmits the vagal trunks,branches of the left gastric artery, veins and lymphatics;

● Caval opening – at level T8 : transmits the inferior venacava, whose adventitial wall is fused with the centraltendon, and the right phrenic nerve;

● Behind the medial arcuate ligament – the sympathetictrunk;

● Behind the lateral arcuate ligament – the subcostalnerves and vessels; and

● Between sternal and costal origins – the superiorepigastric vessels.

Structures that pierce the diaphragmThe structures that pierce the diaphragm are as follows:

● Terminal branches of the left phrenic nerve pierce thecentral tendon;

● The greater, lesser and least splanchnic nerves, whichpierce each crus; and

● The lymph vessels between the abdomen and thorax,which pierce the diaphragm throughout, especiallyposteriorly.

Blood supply to the diaphragmThe diaphragm is supplied from its abdominal surface bythe inferior phrenic arteries from the abdominal aorta. Thecostal margins are supplied by the intercostal arteries.

Nerve supply to the diaphragmRight and left phrenic nerves from C 3–C5 roots provide themotor 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 intercostalnerves.

Radiological features of the diaphragm (see Fig. 4.5)

PA chest radiograph (Fig. 4.6)

The highest point of the right dome is at the sixth inter-costal space anteriorly (ranging from the fourth to seventhribs); it is more accurate to count anterior rather than posterior ribs, as the diaphragmatic dome is nearer to theanterior ribs and the film, and is therefore less subject todistortion by slight angulation of the patient or the beam.

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.

The range of movement of the diaphragm with respira-tion 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 thanthe right.

The variation of the diaphragm with posture is as follows:

● Supine: higher; and● Lateral decubitus: dome on the dependent side is higher.

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

Partial reduplication of the diaphragm – known as acces-sory hemidiaphragm – may occur. This is much commoneron the right side.

Lateral chest radiograph (Fig. 4.7)

The following anatomical details help identify the domes ofthe 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 theperitoneum: 2–3 mm thick; and

● With the pleura and fundal wall of stomach: 5–8 mmthick.

Curvature of the dome

The perpendicular height of the dome of the diaphragmfrom a line between costophrenic and cardiophrenic anglesis 1.5 cm.


THE THORAX 113

1

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1920

2122

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1. Posterior junctional line2. Anterior junctional line3. Azygo-oesophageal line4. Lateral wall of descending aorta5. Aortic knuckle6. Aortopulmonary window7. Pulmonary trunk/left pulmonary artery8. Left superior pulmonary vein9. Left inferior pulmonary artery

10. Left hilar point11. Right superior pulmonary vein

12. Interlobar artery13. Right hilar point14. Trachea15. Right main bronchus16. Left main bronchus17. Azygos vein and position of azygos node18. Position of left atrial appendage19. Left ventricle20. Right atrium21. Inferior aspect of left brachiocephalic vein22. Medial end of right clavicle

23. Right lateral aspect of manubrium sterni24. Spinous process of T 1

25. Superior surface of clavicle26. Companion shadow of clavicle27. Medial aspect of right scapula28. Coracoid process of right scapula29. Dome of right hemidiaphragm30. Dome of left hemidiaphragm31. Stomach bubble32. Gas in splenic flexure of colon

Fig. 4.6 PA chest radiograph.



Ultrasound

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

Computed tomography

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

The costal origins may be prominent with deep inspira-tions. The crura are usually visible on the anterior surface ofthe upper lumbar vertebrae. In young, muscular subjectsthe crura may be very thick or even nodular; however, theirtubular nature and changes with respiration serve to distin-guish these from lymph nodes. The right crus extends moreinferiorly than the left.

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

Magnetic resonance imaging (see Fig. 4.5)

This technique yields excellent sagittal and coronal imagesof the diaphragm as a thin muscular septum of intermedi-ate signal intensity. The crura are elegantly displayed oncoronal images.

THE PLEURA (Figs 4.8 and 4.9)

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

The visceral and parietal layers are continuous with eachother anterior and posterior to the lung root, but below thehilum the two layers hang down in a loose fold called thepulmonary ligament. This may extend to the diaphragm orhave a free inferior border, and allows descent of the lungroot in respiration and also distension of the pulmonaryveins (note that these lie inferiorly in the lung root).

The visceral pleura extends into interlobar and acces-sory fissures. At rest the parietal pleura extends deeperinto the costophrenic and costomediastinal recesses than dothe lungs and visceral pleura (see Table 4.1 for lower limitsof lungs and pleura).

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

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Fig. 4.7 Lateral chest radiograph.

1. Anterior wall of trachea2. Posterior tracheal stripe3. Scapulae4. Left lower-lobe bronchus5. Right lower-lobe bronchus6. Aorta (not well seen)7. Vertebral body of T 4

8. Anterior aspect of rightventricle

9. Pulmonary outflow tract10. Main pulmonary artery11. Right pulmonary artery12. Left pulmonary artery

13. Left atrium14. Left ventricle15. Inferior vena cava16. Horizontal (minor) fissure17. Oblique (major) fissure18. Sternum19. Manubriosternal joint20. Left hemidiaphragm21. Right hemidiaphragm22. Stomach bubble23. Lung projected anterior to

sternum in intercostal space24. Retrosternal airspace


THE THORAX 115

Fig. 4.8 Pleura: (a) anterior view; (b) posterior view.

A B

A

1

2

2

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Fig. 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

Table 4.1 Lower limits of lung and pleura at rest

Visceral pleura and lung Parietal pleura

Anterior 6th costal cartilage 7th costal cartilage

Mid-axillary line 8th rib 10th rib

Posterior T 10 T 10 T 12



Radiological features of the pleura

Plain films

On a chest radiograph the pleura is visible only if tangentialto the beam and if fat or air is on each side of it. Thus thepleura 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; and● Junction lines (see Figs 4.6, 4.9, 4.40, 4.41 and 4.45)

(see also section: Mediastinal lines):— Anterior junction line: anterior to the arch of the

aorta the two lungs may come in contact with oneanother, separated only by four layers of pleura. This pleura is then seen as the anterior junction lineon a PA chest radiograph;

— Posterior junction line: if the lungs lie close to oneanother posteriorly, a posterior junction line is seenon a PA chest radiograph, extending verticallydownwards from the apices (approximately T1 ) for a variable distance. It disappears where the lungsenvelop the aortic arch and may reform inferiorly.Where the junction lines are seen well, a massbetween the lungs in that area can be excluded.

Computed tomographyOn axial CT the pleura cannot usually be distinguishedfrom the thoracic wall or mediastinum unless it is thick-ened (see section on fissures, p. 119). The pulmonary liga-ments 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 venacava (IVC), whereas the left pulmonary ligament lies closeto the oesophagus.

THE TRACHEA AND BRONCHI (Figs 4.10–4.12)

The tracheaThe trachea begins at the lower border of the cricoid carti-lage at the level of C6 vertebra. It extends to the carina at the level of the sternal angle (T5 level, T4 on inspiration andT6 on expiration). The trachea is 15 cm long and 2 cm indiameter and is made up of 15–20 incomplete rings of carti-lage that are bridged posteriorly by the trachealis muscle.The trachea is lined by ciliated columnar epithelium.

The trachea in children is very pliable. It may be devi-ated to the right at almost 90º in a normal expiratory film. Itonly deviates to the left if the aortic arch is on the right side.

Relations of the trachea

Cervical (see Figs 1.34–1.36)The anterior relations are as follows:

● Anterior:— Isthmus of thyroid anterior to the second, third and

fourth rings— Inferior thyroid veins— Strap muscles: sternohyoid and sternothyroid;

● Posterior: oesophagus and recurrent laryngeal nerves;and

● Lateral: lobes of thyroid gland— Common carotid artery.

Fig. 4.10 Trachea and main bronchi: anterior relations.


THE THORAX 117

Thoracic (see Figs 4.7, 4.17, 4.42 and 4.43)The thoracic relations are as follows:

● Anterior:— Brachiocephalic and left common carotid arteries— Left brachiocephalic vein

● Posterior: oesophagus and left recurrent laryngeal nerve● Left lateral:

— Arch of the aorta— Left common carotid and left subclavian arteries; and

● Right lateral: right brachiocephalic artery— 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 arteryand the lower part is supplied by branches of the bronchialartery.

Venous drainage is to the inferior thyroid venous plexus.

Main bronchi (see Figs 4.10–4.12)

Carina

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

The right main bronchus (eparterial bronchus)

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

Relations

The relations of the right main bronchus are as follows:

● Anterior:— Superior vena cava— Right pulmonary artery;

● Posterior: azygos vein; and● Superior: arch of azygos vein.

The bronchus to the upper lobe arises almost immedi-ately after the tracheal bifurcation, entering the hilum of thelung separately and thereafter dividing into anterior, apicaland posterior bronchi. The right bronchus continues as thebronchus intermedius, which then divides into middle- andlower-lobe bronchi. The middle-lobe bronchus has medialand lateral divisions. The apical segment bronchus of thelower lobe comes off opposite the bronchus to the middlelobe. The lower-lobe bronchus divides into four basal seg-ment bronchi – posterior, lateral, anterior and medial.

Left main bronchus (hyparterial bronchus)The left main bronchus lies at 40º to the median plane. It is5 cm long and 1.2 cm in diameter.

RelationsThe 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 aresimilar to the right. The posterior and apical segmentalbronchi usually have a common apicoposterior bronchus,which then subdivides. The lingular lobe bronchus comesoff the upper-lobe bronchus and has superior and inferiordivisions. The lower-lobe bronchus has apical, lateral, anterior and posterior basal segments but no medial basalsegment.

The anatomy of the bronchial tree is shown diagram-matically in Figure 4.12. Naming basal bronchi laterally tomedially 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 relativeposition.

Blood supplyWhereas the lungs receive the entire output of the rightheart, their own nutritive supply arises from the bronchialarteries, branches of the thoracic aorta (see Aorta).

Fig. 4.11 Diagrammatic representation of normal anatomy: situs solitus.

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The bronchial veins drain on the right to the azygos sys-tem and on the left to the hemiazygos system.

Radiological features

The radiological features of the trachea and bronchi are dis-cussed in the following section on the lungs (see p. 122).

THE LUNGS (Fig. 4.13)

The lungs are described as having costal, mediastinal, apical and diaphragmatic surfaces. The right lung has threelobes and the left has two, with the lingula of the left upperlobe corresponding to the right middle lobe.

One terminal bronchiole with lung tissue forms an acinus which, together with vessels, lymphatics and nerves,

Fig. 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.

A B

C D

POSTERIOR ANTERIOR

POSTERIORANTERIOR


THE THORAX 119

forms the primary lobule. Three to five primary lobulesform a secondary lobule.

Interlobar fissures (see Fig. 4.8)

The depth of fissures varies from a superficial slit to com-plete separation of lobes.

The oblique (major) fissure

This is similar in both right and left lungs. It extends fromT4/T5 posteriorly to the diaphragm anteroinferiorly. The

left major fissure is more vertically orientated than theright. The fissures do not follow a straight plane from top tobottom but are undulating in their course. The medialaspect of each fissure passes through the hilum. The lateralaspect of each fissure is anterior to the medial aspect at thelevel of the hila and below. Above the hila, the relationshipchanges and the lateral aspect of the fissure is more poste-rior than the medial.

The transverse (minor) fissureThis separates the upper and middle lobes of the right lung.It runs horizontally from the hilum to the anterior and lat-eral surfaces of the right lung at the level of the fourth costalcartilage. Its posterior limit is the right oblique fissure,which it meets at the level of the sixth rib in the midaxillaryline. It is anatomically complete in only one-third of sub-jects and is absent in 10%.

Accessory fissures

The azygos fissure (Fig. 4.14)

This is in fact a downward invagination of the azygos veinthrough the apical portion of the right upper lobe. It there-fore has four pleural layers – two visceral and two parietal.The term ‘azygos lobe’ is inappropriate as there is no corre-sponding change in lobar architecture.

The superior accessory fissureThis separates the apical segment of the right lower lobefrom other basal segments. It lies parallel and inferior to the transverse fissure and passes posteriorly from the rightoblique fissure to the posterior surface of the lung.

The inferior accessory fissureThis separates the medial basal from other right lower-lobe segments. Called Twining’s line, it is seen in 30–50% of postmortem examinations but in only 8% of PA chestradiographs.

Left transverse fissureThis is found in 18% of postmortem specimens but is rarelyseen on chest radiographs.

Bronchopulmonary segments (see Figs 4.12 and 4.13)

Each lobe is subdivided into several bronchopulmonarysegments, each of which is supplied by a segmentalbronchus, artery and vein. Each segment takes its title fromthat of its supplying bronchus.

The anatomy of segmental bronchi is subject to variation,the most common being the origin of apical segmentalbronchi, especially the right, from the trachea.

A

Fig. 4.13 Bronchopulmonary segments as seen on PA chestradiograph: (a) upper lobes; (b) middle lobes; (c) lower lobes.

B

C



There is very little connection between segments exceptvia:

● The pores of Kohn: openings in the alveolar wallsconnecting adjacent alveolar lumens; and

● The canals of Lambert: connections between terminalbronchioles and adjacent alveoli.

These allow gas and fluid transfer between segments but not between lobes. Ventilation of a segment is thereforepossible when its segmental bronchus is occluded. This iscalled collateral air drift.

The pulmonary artery (Fig. 4.15; see also Fig. 4.44) (see also section on the great vessels)

The pulmonary trunk leaves the fibrous pericardium andbifurcates almost at once in the concavity of the aortic archanterior to the left main bronchus.

The right pulmonary artery is longer than the left. Itpasses across the midline below the carina and comes to lieanterior to the right main bronchus. It bifurcates while stillin the hilum of the right lung. An artery for the right upperlobe passes anterior to the right upper-lobe bronchus. Theinterlobar artery to the right middle and lower lobes passeswith the bronchus intermedius.

The left pulmonary artery spirals over the superioraspect of the left main bronchus to reach its posterior sur-face. It is attached to the concavity of the aortic arch by theligamentum arteriosum.

The pulmonary arteries further subdivide into segmen-tal arteries that travel with the segmental bronchi, for themost part on their posterolateral surface. The pulmonaryarteries supply only the alveoli (cf. the bronchial arteries,which supply the bronchi).

The pulmonary veins (Fig. 4.16)

These do not follow the bronchial pattern but tend to run inintersegmental septa. Two veins pass to each hilum – fromlung tissue above and below each oblique fissure. Theseenter the mediastinum slightly below and anterior to the pul-monary arteries. On the right side the veins from the lobesmay remain separate, so that three veins leave the right lungand enter the left atrium. On the left side the two pulmonaryveins may unite and enter the left atrium as a single vessel.

The bronchial arteriesThe bronchial arteries supply the bronchi, the visceralpleura and the connective tissue of the lungs. They arise

A

3

1

2

B

1 3

Fig. 4.14 Azygos fissure: (a) PA chest radiograph; (b) CT thorax.

Fig. 4.15 Pulmonary arteries.

1. Azygos fissure 2. Azygos vein 3. Azygos ‘lobe’


THE THORAX 121

from the thoracic aorta in 90% of subjects, at T5 or T6 verte-bral level in 80% of cases.

There are usually one right and two left bronchial arteries. The left bronchial artery arises at upper border ofT5 and the lower arises from the aorta just below the leftmain bronchus. When a second bronchial artery occurs onthe right side it often arises from the third intercostal artery.

Bronchial arteries may also arise from the subclavianartery or from its internal thoracic branch.

Tissues supplied by bronchial arteries drain to pul-monary veins or bronchial veins.

Bronchial veinsBronchial veins form two distinct systems. The deep veinsform a network of veins around the pulmonary interstitiumand communicate freely with the pulmonary veins. Theyalso form a bronchial vein trunk that drains to the pul-monary system. The superficial bronchial veins drain to the azygos vein on the right side and to the accessory hemi-azygos vein on the left side.

Lymphatics (Fig. 4.17)

Mediastinal lymph nodes that drain the lung are namedaccording to their position:

● Pulmonary nodes within the lung substance;● Bronchopulmonary nodes at the hilum;● Carinal nodes below the hilum;● Tracheobronchial nodes above the tracheobronchial

junction; and● Right and left paratracheal nodes on either side of the

trachea.

The lymph vessels of the lung are in superficial and deepplexuses. The superficial plexus beneath the pleura drainsaround the surface of the lungs and the margins of the

6

1

3

2

5

4

Fig. 4.16 Pulmonary veins: DSA of pulmonaryveins draining into left atrium. This angiogramwas achieved by passing a venous catheter from the right atrium through a 'probe-patent'foramen ovale into the left atrium.

1. Right upper pulmonary vein2. Left upper pulmonary vein3. Catheter4. Right inferior pulmonary vein5. Left inferior pulmonary vein6. Left atrium

Fig. 4.17 Lymph nodes related to the trachea and main bronchi.



fissures to converge at the hila and the bronchopulmonarynodes. The deep channels drain with the pulmonary vesselstowards the hila. There are few connections between the superficial and deep plexuses except at the hila. Thebronchopulmonary nodes drain to the tracheobronchialnodes and the paratracheal nodes and thence to the bronchomediastinal trunks.

Lung roots (Fig. 4.18)

The roots of the lungs are formed by the structures thatenter and emerge at the hila. They lie at vertebral levelsT5–T7 . The right lung root lies below the arch of the azygosvein and posterior to the superior vena cava and the rightatrium. The left lung root lies below the arch of the aortaand anterior to the descending aorta.

Radiological features of the lung and bronchial tree

PA chest radiograph (see Fig. 4.6)

Fissures Fissures are only seen if tangential to the beam,and because of curvatures in three dimensions they are seldom seen in their entire length. The transverse fissure of the right lung is seen in 80% of dissection specimens but only 50% of chest radiographs. The azygos fissure,although found in 1% of postmortem subjects, is seen in0.4% of radiographs with the ‘teardrop’ shadow of the azygos vein in its lower end. The superior accessory fissureis seen in 5% of chest radiographs and, when present, is at a lower level than the transverse fissure. The inferioraccessory fissure is seen in 8% of chest X-rays (CXR), 20% of high-resolution CT scans and 30–50% of anatomical

specimens. The left transverse fissure is seen in less than 2% of CXR, 9% of high-resolution CT scans and 18% ofanatomical specimens.

Trachea The trachea is seen as a midline translucencywith a slight inclination to the right in its lower half. Itslumen is 1.5–2 cm in diameter.

The right paratracheal stripe (normally < 3 mm) is formedby the right wall of the trachea and the pleura, outlined on both sides by air. The left side of the trachea is not seenseparately from the mediastinal shadows.

A smooth indentation on the trachea is commonly seenjust above the bifurcation on the left side. This is caused bythe arch of the aorta.

Gross AP or side-to-side (usually to the right side, owingto the aortic arch) displacement may be normal in a child,especially on an expiratory radiograph.

Bronchi The bronchi contribute very little to the lungmarkings seen on plain films. The proximal bronchi may,however, be seen if outlined by the lungs.

On the frontal chest radiograph the posterior segmentalbronchi of the left upper lobe and those of the apical segments of both lower lobes are often seen end-on. Theircorresponding pulmonary arteries may be seen accompa-nying the bronchi as circular densities of about the samesize. The right pair is higher than the left as the upper-lobebronchus comes off at a higher level on the right.

The right main bronchus is orientated more verticallythan the left.

The bronchus intermedius can be identified to the rightof the cardiac shadow and medial to the density of theinterlobar artery. The lower-lobe bronchus on the left is notnormally seen, as it lies behind the heart shadow.

Fig. 4.18 Lung roots: (a) right lung root; (b) left lung root.

A B


THE THORAX 123

Bronchopulmonary segments These are depicted inFigure 4.13.

Pulmonary vasculature The vessels account for most ofthe lung markings.

The pulmonary trunk forms part of the left border of theheart. The interlobar artery is seen lateral to the bronchusintermedius on the right. This should not measure morethan 16 mm midway down its visible length.

Bronchi and arteries are seen together radiating out fromthe hila. Pulmonary veins enter the mediastinum lowerthan the arteries as they converge toward the posterioraspect of the left atrium. This means that:

● In the upper zone the veins are inferolateral to thearteries; and

● In the lower zone the veins are almost horizontal andthe arteries almost vertical.

The inferior pulmonary veins drain anteriorly and supe-riorly from the lung bases to the left atrium. Owing to theirmore horizontal course, if seen end-on on the chest filmthey may simulate a pulmonary mass.

The hilar point is where the upper-lobe vein crosses the descending pulmonary artery. The right hilar point isprojected over the sixth posterior interspace and is 1 cmlower than the left.

The hilar angle is the angle between the vessels at thehilar point – normally 120º.

Situs inversus The identification of situs inversus (seeFig. 4.11) is helped by the appearance of the main bronchion high-voltage chest radiographs or tomography. Themorphological right lung, whether on the right or the leftside of the thorax, is trilobed, has a short main bronchus,and has its bronchus above its pulmonary artery.

Lymphatics Normal lymph nodes do not form dis-cernible densities on plain radiographs.

Lateral chest radiograph (see Fig. 4.7)

Fissures The left oblique fissure is more vertical andreaches the diaphragm more posteriorly than that on theright. The left oblique fissure is at an angle of 60º to the hor-izontal, and the right at an angle of 50º. The anterior ends ofthe oblique fissures are 2–3 cm behind the anterior chestwall. Occasionally the upper end of the oblique fissure mayreach T3 . As the fissures are undulating, they may appear tobe doubled in parts and are often not seen in entirety. Forthe same reason the posterior part of the transverse fissuremay occasionally seem to cross the right oblique fissure.Where there is a superior accessory fissure, this extendsposteriorly from the oblique fissure.

Trachea On a lateral chest radiograph (see Fig. 4.7) thetrachea is seen to enter the thorax midway between the sternum and the vertebrae. Owing to some posterior incli-nation it ends closer to the vertebrae.

The posterior paratracheal stripe is formed by the posterior wall of the trachea and the pleura and is visible if the lung passes behind the trachea.

The tracheo-oesophageal stripe is formed by the poste-rior wall of the trachea and the anterior wall of the oesoph-agus. It is visible if there is air in the oesophagus.

Bronchi On a lateral chest radiograph (see Fig. 4.7) theleft main bronchus, being more horizontal, is seen as a cir-cular structure. The right main bronchus is more verticaland is therefore tubular on this view. The left pulmonaryartery is seen as a comma-shaped density passing back-wards over the left main bronchus.

The posterior wall of the right main bronchus and itsdivision into upper-lobe bronchus and bronchus inter-medius are often visible as a thin stripe as they are in con-tact with the lung. On the left side, however, the pulmonaryartery and its branches are posterior to the bronchus andthus obscure the view of its posterior wall.

The upper-lobe bronchi may be seen as two roundedradiolucencies projected over the lower end of the trachea,with the right being 1–2 cm higher than the left. The rightupper-lobe bronchus is seen about half the time and the leftin about 75% of cases. The left has more sharply definedmargins as the pulmonary artery lies superiorly and thesuperior pulmonary vein lies inferiorly.

The lower-lobe bronchi may be seen running in an inferoposterior course in many cases. The left lower-lobebronchus is the more posterior and has a curved configura-tion in its anterior aspect, merging with the orifice of theupper-lobe bronchus.

Vasculature The pulmonary arteries may be seen at thehilum forming a conglomerate density with the pulmonaryveins. The right pulmonary artery is seen end-on and isoval in appearance. The left pulmonary artery is comma-shaped as it arches over the left main bronchus. They mayeach measure up to 3 cm in diameter.

The confluence of the pulmonary veins is seen below theoval density of the right pulmonary artery on the lateralfilm. The superior veins are more anterior than the inferiorpulmonary veins.

Pulmonary angiography

A catheter is inserted via the femoral vein to the inferiorvena cava (IVC), right heart and pulmonary trunk. The pul-monary trunk inclines posteriorly as it leaves the heart andis best seen when the beam is angled at 45º to the anteriorchest wall. The pulmonary arteries and branches are seen asdescribed above. Pulmonary veins are seen in the venousphase.

Bronchial angiography

This is used in the diagnosis and embolic treatment ofhaemoptysis. The bronchial arteries are catheterized from



the thoracic aorta if they arise from it. Owing to the variableorigin of the bronchial arteries it may be necessary tocatheterize the subclavian, internal thoracic or intercostalarteries.

As the spinal arteries also arise from these vessels (cf. Chapter 3) there is a risk of spinal ischaemia using thisprocedure.

Computed tomography (see Figs 4.40 and 4.42–4.44)

Fissures On conventional CT fissures are less visible thanon plain radiographs. They are seen as regions of relativeavascularity on the outer cortex of the lobe, where taperingvessels are less visible. Discrete lines are only seen if thevertical axis of the fissure is perpendicular to plane of theCT slice, which sometimes occurs in parts of the oblique fissure but not in the transverse fissure. On high-resolutionCT fissures are seen as sharp lines.

Bronchi The bronchi may be seen depending upon theirsize and orientation. Narrow slices improve visualization.

The horizontally orientated bronchi, such as the anteriorsegment bronchus of the upper lobes, the superior segmen-tal bronchi of the lower lobes and the proximal part of themiddle-lobe bronchus, may be seen as tubular structures.

The vertically orientated bronchi, such as the mainbronchi, bronchus intermedius, lower-lobe bronchi and apical segmental bronchi, may be seen as circular air-filledstructures.

The posterior wall of the right main bronchus and itsdivisions into upper-lobe bronchus and bronchus inter-medius should be outlined by lung as it invaginates into theazygo-oesophageal recess.

Occasionally, a pulmonary vein may pass behind thebronchus intermedius on its way to the left atrium, simu-lating a small mass, which is usually less than 1 cm indiameter.

The posterior walls of left main and upper-lobe bronchiare usually outlined by lung. Below the hilum, lung tissuemay also be seen in contact with the posterior wall of thelower-lobe bronchus.

Vasculature The vessels account for most of the lungmarkings seen on CT. The relationships of the pulmonaryarteries and veins to the bronchi are best seen at hilar level.The right pulmonary artery is anterior to the right bronchus,and the right superior pulmonary vein may be seen anteriorto this. The left pulmonary artery is seen anterior to the leftmain bronchus, and above it on a higher section. The lower-lobe artery is seen posterolateral to the lower-lobe bronchus.The left superior pulmonary vein is separated from thelower-lobe artery at hilar level by the left bronchus.

Magnetic resonance imaging

Because the lungs are of very low proton density and move with respiration they are poorly seen by this method.

Third-order pulmonary arteries are visible as are pulmonaryveins close to the hilum. Segmental bronchi and fissures arenot seen. The axial anatomy as seen is the same as that ofCT. MR angiography is increasingly being used, however,to image the pulmonary arteries non-invasively.

Isotope ventilation-perfusion scanning

A ventilation scan outlines the trachea and main bronchi inaddition to the lungs. A gap is seen owing to the media-stinum and a cardiac notch is seen in the anterior border ofthe left lung.

Perfusion scanning may show differential isotope distri-bution from apex to diaphragm, owing to variations inblood flow associated with this posture.

THE MEDIASTINAL DIVISIONS (Fig. 4.19)

The mediastinum is the space between the lungs and their pleura. It is arbitrarily divided into superior, middle,anterior and posterior sections. These divisions are notanatomical. They are used to describe the location of patho-logical processes. The superior mediastinum is above a linedrawn from the lower border of T4 to the sternal angle.Below this line are anterior, middle and posterior compart-ments. The middle mediastinum is occupied by the heartand its vessels. The anterior mediastinum is between theanterior part of the heart and the sternum. The posteriormediastinum is between the posterior part of the heart andthe thoracic spine, extending down behind the posteriorpart of the diaphragm as it slopes inferiorly.

The superior mediastinum contains the:

● Aortic arch and branches;● Brachiocephalic veins and superior vena cava;

Fig. 4.19 Mediastinal divisions. (Courtesy of Professor J.B. Coakley.)


THE THORAX 125

● Trachea;● Oesophagus;● Thoracic duct;● Lymph nodes; and● Nerves.

The anterior mediastinum contains the:

● Thymus;● Mammary vessels; and● Lymph nodes.

The posterior mediastinum contains the:

● Descending aorta;● Oesophagus;● Azygos venous system;● Thoracic duct; and● Para-aortic, oesophageal and paraspinal nodes.

The middle mediastinum contains the:

● Heart and pericardium:● Nerves;● Lymph nodes; and● Great vessels.

THE HEART

Gross anatomy and orientation (Figs 4.20–4.22; also Figs 4.6 and 4.7)

The heart is pyramidal in shape and lies obliquely in thechest. Its square-shaped base points posteriorly and theelongated apex to the left and inferiorly. The left atrium

forms the base or posterior part, with the superior and inferior pulmonary veins draining into its four corners. Theright atrium forms the right border, with superior and infe-rior venae cavae draining into its upper and lower parts.The apex and left border are formed by the left ventricle.The right ventricle forms the anterior part. The inferior(diaphragmatic) part of the heart is formed by both ventri-cles anteriorly and a small part of right atrium posteriorlywhere the IVC enters this chamber.

The oblique orientation of the heart causes the ventriclesto lie anterior and inferior to the atria. The heart is alsorotated in a clockwise fashion about its axis, so that theright atrium and ventricle are at a slightly higher level thantheir left counterparts. The interatrial and interventricularsepta are said to lie in the left anterior oblique plane. Thismeans that the long axis of the septa runs anteriorly to theleft. The tricuspid and mitral valves, which separate theright and left atria and ventricles respectively, are roughlyvertically oriented. The plane of the valves is also inclinedinferiorly and to the left. This means that the transverse axisof the pair of valves runs to the right and anteriorly, andthey are said to lie in the right anterior oblique plane.

Pericardium (Fig. 4.23)

This is a closed sac consisting of parietal and visceral layersthat enclose a potential space which contains 20–25 mL ofserous fluid. It is draped over the heart and great vessels.The visceral layer adheres to the myocardium and is alsoknown as the epicardium. The parietal layer is free, exceptinferiorly, where it is bound to the central tendon of thediaphragm, and superiorly where it fuses with the coveringof the great vessels. The pericardial reflections, which arereally the boundaries of the closed sac, are found posteriorly

Fig. 4.20 Heart: anterior view.Fig. 4.21 Heart and mediastinum viewed from right. (Courtesy ofProfessor J.B. Coakley.)

Vertebral column

Oesophagus

Trachea with rightvagus nerveRight phrenic nerve

Right brachiocephalicvein and superior venacavaManubrium sterniBody of sternum

Pulmonary trunk

Right ventricle

Right atriumInferiorvena cava

T12

T1

T4

Diaphragm

Arch of azygousvein

Root of right lung



around the IVC and pulmonary veins where the spacebetween the veins forms the oblique sinus of the peri-cardium. The serous pericardial layers extend anterosuperi-orly over the superior vena cava (SVC) and separately overthe aorta and pulmonary artery. The space between theaorta and pulmonary artery is known as the transversesinus of pericardium.

The pericardium extends superiorly for 2–3 cm over theascending aorta and over the pulmonary artery almost toits bifurcation. It also extends for a short distance over thevenae cavae and pulmonary veins. Some fat is presentbetween the epicardium and myocardium. This increaseswith age. Fat is also present between the pericardium andmediastinal pleura, and it may be extensive in the anteriorand lateral cardiophrenic angles, where it is known as thepericardial fat pad.

Radiological features of the pericardium

CT and MRI (Fig. 4.23)

The pericardium may be visible on CT as a thin dense lineseparated from the myocardium by a thin layer of epi-cardial fat. Pericardial sinuses and recesses with a little normal fluid may be visible around and between the greatvessels on CT or MRI and should not be misinterpreted as lymphadenopathy or other mediastinal disease. Thesuperior pericardial recess in particular may extend almostto the right paratracheal region.

Cardiac chambers and valves (Figs 4.20–4.22 and 4.24)

Right atrium

This has a smooth posterior wall into which the great veinsdrain. The coronary sinus drains into the posterior wallbetween the orifice of the IVC and the tricuspid valve. Theanterior wall has muscular ridges that are continuous withthe muscular ridges of the atrial appendage. The interatrialseptum bears an oval depression on its lower part, knownas the fossa ovalis. This represents the closed foramen ovale through which oxygenated blood from the maternalcirculation reached the left side of the heart in the fetus. Araised limbus surrounds the fossa ovalis. The right atrialappendage is roughly triangular and projects upward andforward and to the left; it is the only part of the right atriumto contribute to the cardiac outline on the lateral view. Theinner wall of the atrial appendage is ridged by musculipectinati with a vertical ridge, the crista terminalis, separat-ing it from the smooth-walled parts of the right atrium.

Right ventricle

This chamber is roughly triangular and flattened from frontto back as the left ventricle bulges into it. The lower half ofthe right ventricle normally touches the lower part of thesternum on the lateral view. Viewed from the side it has amuscular inflow tract and a smooth outflow tract, sepa-rated by a muscular prominence or conus also known as the

Fig. 4.22 Heart and mediastinum viewed from left. (Courtesy of Professor J.B. Coakley.)


THE THORAX 127

infundibulum. The outflow tract inclines superiorly, to the left and posterior to the pulmonary valve. The entranceto the right ventricle is the tricuspid valve. This has threeleaflets or cusps, each attached to the papillary muscles ofthe ventricular wall by several tendinous cords – the cordaetendinae. The pulmonary valve has three semilunar cusps –right and left anterior and a posterior cusp. It faces to the left and slightly posteriorly. It is the most anterior andsuperior of all the cardiac valves.

Left atrium

This is square-shaped and smooth-walled and forms theupper posterior part of the heart on the lateral view. It receives the four pulmonary veins in its upper part (Fig. 4.16). It has a long, narrow, trabeculated appendagethat projects anteriorly on the left side of the pulmonarytrunk, overlapping its origin. This is embedded in fat and isnot seen on the frontal view unless enlarged.

A

1 1 12

34

56

B

3

4

5

6

2

1

C

1

23

4Fig. 4.23 Pericardium: (a) CT scan showing pericardium.

1. Pericardium2. Right ventricle3. Left ventricle

4. Interventricular septum5. Aorta6. Oesophagus

(b) CT; and (c) MRI showing pericardial recesses.

1. Aorta2. Superior aortic recess

of the transverse sinus3. Superior vena cava

4. Right pulmonary artery5. Aorta6. Oblique sinus



Left ventricle

This is a thick-walled finely trabeculated cavity that isshaped like an elongated cone, being roughly circular incross-section. It forms the lower half of the posterior part ofthe heart on the lateral view. The mitral valve separates itfrom the left atrium. This valve has two cusps – anteriorand posterior – whose free margins are attached to the ven-tricular wall by chordae tendinae. Instead of a muscularconus as on the right, the larger anterior cusp of the mitralvalve separates inflow and outflow tracts, and blood flowsover both its surfaces. The mitral and aortic valves are infibrous continuity. The aortic valve has three semilunarcusps – anterior, and right and left posterior. Above eachcusp is a localized dilatation or sinus. These are known asthe sinuses of Valsalva.

The right coronary artery arises from the anterior sinusand this is also known as the right coronary sinus. The leftcoronary artery arises from the left posterior sinus – alsoknown as the left coronary sinus. No artery arises from theright posterior sinus, so this is also called the non-coronarysinus (Fig. 4.25). The ventricles are separated by the inter-ventricular septum, which is mostly thick and muscular. Ithas a short membranous part at the top and bulges into the

right ventricle, causing this to have a flattened appearancefrom front to back, and giving the left ventricle a circularshape in cross-section.

A

4

1 3

27

69

8

10 11

1213

5

B

14

9 4

8

6

6

1

7

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3

Fig. 4.24 (a) CT scan of thorax: axial section through heart; (b) oblique sagittal MRI through long axis of heart.

1. Right ventricle2. Left ventricle3. Interventricular septum4. Pericardium5. Atrioventricular groove6. Mitral valve7. Papillary muscle

8. Right atrium9. Left atrium

10. Right pulmonary vein11. Oesophagus12. Descending aorta13. Azygos vein14. SVC

Fig. 4.25 Relationship of aortic and pulmonary valves and origin ofright and left pulmonary arteries. RP, right posterior; LP, left posterior;A, anterior; Post, posterior; RA, right anterior; LA, left anterior.

Aortic

Right

coronary

artery

Posterior

Left

coronary

artery

Pulmonary

Anterior

RP LP

A

Post

LARA


THE THORAX 129

The coronary arteries and veins (Figs 4.25–4.28)

Right coronary artery (Figs 4.26 and 4.28)

The right coronary artery (RCA) supplies the right ventricleand inferior wall of the left ventricle. It arises from the anterior (right) sinus of Valsalva and passes to the rightbetween the pulmonary trunk and the right atrium todescend in the right atrioventricular groove as the marginalartery. On the inferior surface of the heart it anastomoseswith the left coronary artery in the region of the posteriorinterventricular groove.

BranchesThe branches of the right coronary artery are as follows:

● Conus artery to the pulmonary outflow tract;● Atrial and ventricular branches;● Branch to sinoatrial node, which curves anticlockwise

around the SVC to reach the sinoatrial node;● Acute marginal branches, which run anteriorly from the

RCA to supply the right ventricle;● Branch to atrioventricular node, which arises from the

RCA as it forms a characteristic loop at the region whereit continues as the posterior interventricular artery; and

● Posterior interventricular artery, which runs anteriorlyfrom the terminal part of the RCA in the posteriorinterventricular groove, and supplies the inferiorsurface of the left ventricle and the posterior two-thirdsof the interventricular septum.

Left coronary artery (Figs 4.27 and 4.28)

The left coronary artery (LCA) arises from the left posteriorsinus of Valsalva and supplies the remainder of the left ven-tricle. It arises as the left main coronary artery and passesbehind and to the left of the pulmonary trunk to reach theleft part of the atrioventricular groove. It bifurcates earlyinto the left circumflex artery, which continues laterally inthe atrioventricular groove to anastomose with the rightcoronary artery, and the anterior descending artery, whichdescends in the interventricular groove.

Branches of the anterior descending arteryThese are as follows:

● Septal branches;● Diagonal branches that run over the anterolateral

wall of the left ventricle supplying it; and● A branch to the right ventricle (occasionally).

1

2

34

5

6

A

1 2

3

4

5

B

Fig. 4.26 Right coronary angiogram: (a) left anterior oblique projection; (b) right anterior oblique projection. The AV nodal branch is not seen.

(a) (b)

1. Sinus artery2. Right coronary artery3. Conus branch

4. Right ventricular branch5. Posterolateral branch6. Posterior descending artery

1. Right coronary artery2. Right ventricular branch3. Conus branch

4. Posterolateral branch5. Posterior descending artery



Branches of the left circumflex arteryThese are as follows:

● Obtuse marginal branches, which supply the lateral wall of the left ventricle; and

● Atrial branches.

In general, the RCA supplies the right ventricle and theinferior part of the left ventricle. The left coronary artery

supplies the remainder of the left ventricle. The interven-tricular septum is supplied by the left coronary anteriorlyand the right coronary artery posteriorly. The atria have avariable supply. In more than 50% of cases the sinoatrialnode is supplied by the right coronary artery, and in 90% ofcases the right coronary artery supplies the atrioventricularnode. Coronary dominance is determined by the vessel thatsupplies the inferior and lateral walls of the left ventricle.

1 2

3 4 56

78

9

A

1

2

3

34

2

5

67

89

B

Fig. 4.27 Left coronary angiogram: (a) right anterior oblique projection; (b) left lateral projection.

(a) (b)

1. Left main coronary artery2. Left anterior descending

artery (LAD)3. Septal branch4. First diagonal branch

1. Left main coronaryartery seen end on

2. Circumflex artery3. Obtuse marginals4. Posterolateral branch

5. Left anterior descending artery (LAD)6. Septal branch7. First diagonal branch8. Second diagonal branch9. Continuation of LAD

5. Second diagonal branch6. Continuation of LAD7. Circumflex artery8. First obtuse marginal branch9. Second obtuse marginal branch

Fig. 4.28 The coronary arteries.


THE THORAX 131

In right dominance (the usual situation) the right coronaryartery gives rise to the posterior interventricular branchand continues around in the atrioventricular groove, givingbranches to the posterolateral wall of the left ventricle. Inleft-dominant situations the right coronary artery is short,and the left circumflex supplies the posterolateral wall ofthe left ventricle and gives off the posterior descendingartery.

The veins of the heart (Fig. 4.29)

Venous drainage of the heart is mainly (approximately60%) via veins that accompany the coronary arteries andwhich drain via the coronary sinus. The coronary sinus liesin the posterior atrioventricular groove and drains into theposterior wall of the right atrium to the left of the orifice ofthe IVC. Its tributaries are:

● The great cardiac vein, which ascends in the anteriorinterventricular groove and then runs to the left in theatrioventricular groove to become the coronary sinus;

● The middle cardiac vein, which ascends in the posteriorinterventricular groove;

● The small cardiac vein, which accompanies themarginal branches of the RCA on the inferior surface of the heart and then runs posteriorly in the rightatrioventricular groove to enter the right side of thecoronary sinus; and

● The left posterior ventricular vein, which accompaniesthe obtuse marginals of the left coronary artery, runningup the posterior aspect of the left ventricle to drain intothe coronary sinus.

● The anterior cardiac veins drain much of the anteriorsurface of the heart and drain into the anterior wall ofthe right atrium directly. Several small veins, the venaecordis minimae, drain directly into the cardiacchambers.

The conducting system of the heartThe sinoatrial node, which is just to the right of the orificeof the SVC, initiates the cardiac impulse. The impulsespreads through the musculature of the right atrium to theatrioventricular node, which lies in the interatrial septum.It is then conducted to the ventricles via the atrioventricu-lar bundle of His (a specialized muscle that can conductthe electrical impulse). The bundle of His divides into rightand left branches in the upper part of the interventricularseptum which activate the ventricular contraction.

Radiology of the heart

Chest radiography

The cardiac contour is seen on the frontal and lateral chestfilm. Posteroanterior films are preferred to anteroposteriorones as the heart, being anterior, is closer to the film and isnot magnified to the same extent as with anteroposteriorfilms. The cardiothoracic ratio, usually less than 50%, maybe up to 55% in Asian and Afro-Caribbean subjects and upto 60% in infants. (For a description of the cardiac contour,see the section on the mediastinal contour, p. 143.)

The coronary arteries may be calcified in normal people.The position of the valves may be deduced on PA and

lateral films in relation to the cardiac outline or the sternumand ribs (Fig. 4.30). The aortic and mitral valves are themost important to recognize, as they are most often affectedby disease. On a PA chest radiograph the valves lie close toa line from the left atrium to the lowest point of the rightheart border. On a lateral view the pulmonary and aorticvalves lie just above, and the mitral and aortic valves justbelow, a line drawn from T5 to the apex of the heart. Thelowest part of the aortic valve is very close to the anteriorpart of the mitral valve, where they are anatomically infibrous continuity.

Fig. 4.29 The coronary veins.



Fluoroscopy

The heart and its valves may be assessed by fluoroscopy. If the valves are calcified (usually pathologically) they maybe distinguished by their characteristic motion as well astheir location. The aortic valve has a to-and-fro motion in

the plane of the ascending aorta, that is, upwards, back-wards and to the right. The mitral valve has a circular aswell as a to-and-fro motion in the left anterior oblique planeof the left atrium and ventricle.

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Fig. 4.30 Position of cardiac valves: (a) as seen on AP chestradiograph; (b) as seen on lateral CXR; (c) AP CXR; (d) lateral CXR. P, pulmonary; A, aortic; M, mitral; T, tricuspid.

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THE THORAX 133

EchocardiographyTwo-dimensional echocardiography uses ultrasound toimage the heart. A subcostal or intracostal window may be used and images may be obtained in any plane.Longitudinal images through the outflow tracts are usuallyobtained, as well as cross-sectional images through thevalves and chambers. Ultrasound is probably the bestmodality for imaging the internal anatomy of the heart, thewalls, chambers and valves. The movement of the wallsand valves may also be assessed dynamically throughoutthe cardiac cycle. Transoesophageal echocardiographyallows much closer inspection of the heart because of theclose apposition of the left atrium to the anterior wall of thedistal oesophagus, without intervening air or lung.

Angiocardiography

This technique involves the injection of contrast directlyinto the heart chambers via a pigtail catheter, which is usu-ally introduced through the femoral artery or vein for theleft and right chambers, respectively. The chambers are rec-ognized by their position and characteristic configuration.

Coronary angiography

Coronary angiography involves selective catheterization of the coronary arteries. A small volume of contrast isinjected and images may be obtained in lateral, anterioroblique and AP projections. There is individual variation inthe branches of the coronary arteries, which are demon-strated from case to case. This is due both to anatomicalvariation and technical factors. The major arteries aredemonstrated in Figures 4.26 and 4.27.

Nuclear medicine

Nuclear medicine studies are used mainly for functionalassessment of the heart, which in clinical practice is oftenmore important than the demonstration of the anatomy.Thallium-201 (201Th) and technetium-99m (99mTc)-labelledMIBI (2-methoxy isobutyl isonitrile) are taken up by normally perfused myocardium, and images obtained bygamma camera show the heart. The use of SPECT (singlephoton emission CT) allows images to be constructed inany plane – usually with three sets of images – along theshort cardiac axis (at right-angles to the long axis of theheart), and along the vertical and horizontal long axes. It also improves the target:background ratio, as neitherradiopharmaceutical agent is taken up exclusively by themyocardium. Other functional information on ventricularfilling, ejection fraction and so on may be obtained byblood-pool imaging using 99mTc-labelled red blood cells andelectrocardiogram (ECG)-gated acquisition of data.

Computed tomography (see Fig. 4.24)

CT scanning shows the heart and vessels in cross-section.The pericardium may be identified between epicardial and mediastinal fat. Dynamic scans obtained during intravenous infusion of contrast demonstrate the cardiacchambers and vessels to greater advantage. ECG gatingallows images to be acquired during the same part of thecardiac cycle, thus reducing motion artefacts and providingbetter images.

Magnetic resonance imaging

The applications for MRI in cardiac radiology are steadilyincreasing. Acquisition of images is gated to the ECG toovercome motion artefact, and faster scan times haveimproved image quality. The cardiac chambers, valves andmajor vessels may be imaged in any plane to give informa-tion previously only obtainable with cardioangiography,and with the added advantage of demonstrating the softtissues. The pericardium is shown as a dark line 1–2 mmthick.

THE GREAT VESSELS(see Figs 4.10, 4.20, 4.22, 4.40 and 4.42–4.44)

The aorta (see Figs 4.10, 4.31 and 4.32)

The aortic arch

The ascending aorta begins at the aortic valve at the level of the lower border of the third costal cartilage. It ascends to the right, arching over the pulmonary trunk to lie behind the upper border of the second right costal cartilage. The first few centimetres of the ascending aorta and the pulmonary trunk are enclosed in a commonsheath of pericardium. At its origin it lies behind the outflow tract of the right ventricle and the pulmonarytrunk, and the right atrial appendage overlaps it. It ascendsanteriorly and to the right, passing over the right pul-monary artery and right main bronchus. The right lung and sternum are anterior. The coronary arteries arise fromaortic sinuses – three localized dilatations above the cuspsof the aortic valve.

The arch of the aorta passes posteriorly and from right toleft. It passes anterior to the trachea and arches over the leftmainstem bronchus and pulmonary artery to come to lie tothe left of the body of T4 . Anterior and to the left of the archare the pleura and left lung. On its right side, from front toback, are the trachea, oesophagus, thoracic duct and bodyof T4 . Its inferior aspect is connected to the ligamentumarteriosum, the fibrous remnant of the ductus arteriosus.Superiorly are the three branches of the arch that arecrossed anteriorly by the left brachiocephalic vein. The left



superior intercostal vein runs down to the brachiocephalicvein, passing anterior to the aorta and occasionally causinga small bulge on the arch, which is visible on the chest radiograph. The branches of the arch of the aorta are thebrachiocephalic, the left common carotid and the left subclavian arteries. The brachiocephalic and left commoncarotid arteries ascend on either side of the trachea in a V shape to come to lie behind the sternoclavicularjoints, at which point the brachiocephalic bifurcates into the right common carotid and subclavian arteries. Fromthis point the common carotids ascend symmetrically intothe neck.

Variation is common in the branches of the aortic arch,such that the ‘normal’ pattern as described above is onlyseen in 65% of subjects.

● In 5% of subjects the left vertebral artery arises directlyfrom the arch of the aorta, between the origins of the left common carotid and left subclavian arteries.

● 2.7% have a common origin of the left common carotidand subclavian as a left brachiocephalic artery.

● In 2.5% the left common carotid arises from thebrachiocephalic artery.

● In 0.5% an aberrant right subclavian artery arises distalto the left subclavian artery and passes to the right,posterior to the oesophagus.

● Rare: right common carotid and subclavian arteriesarising separately.

● Very rarely the common carotid is absent so that theinternal and external carotid arteries arise separatelyfrom the aortic arch on one or both sides.

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Fig. 4.31 Aortograms: (a) DSA; (b) MR angiogram.

1. Brachiocephalic artery2. Left common carotid3. Left subclavian artery

4. Ascending aorta5. Descending aorta6. Angiography catheter

7. Anterior cusp of aortic valve8. Left posterior cusp of

aortic valve

9. Left coronary artery10. Left atrium11. Left pulmonary artery


THE THORAX 135

● The following other arteries may also arise from theaortic arch:— One or both bronchial arteries— Thyroidea ima artery— Inferior thyroid artery— Internal thoracic artery

● The origins of the brachiocephalic and the left common carotid arteries may also vary. If the vesselsarise earlier than normal the left common carotid has to pass anterior to the trachea to assume its position left of the aorta. If the vessels arise more distally thebrachiocephalic artery passes anterior to the trachea toget to its right side. Either arrangement may lead toanterior compression of the trachea, which may causesymptoms especially in the infant, whose anteriormediastinum is crowded by the thymus.

The aortic isthmus is the junction of the arch of the aortaand the descending aorta. This area is relatively fixed and is thus prone to injury with the shearing forces of blunttrauma.

The descending aorta

The descending aorta passes inferiorly through the poste-rior mediastinum to the left of the spinal column. It passesposterior to the diaphragm at the level of T12. On its left side are the pleura and left lung. Posteriorly are the verte-bral column and the hemiazygos veins. At its highest pointthe oesophagus lies to its right. The descending aorta thenlies behind the oesophagus as the latter passes anteriorly,and the right lung and pleura lie to its right in contact withit. As it descends it passes behind the left main bronchusand pulmonary artery, the left atrium, the oesophagus andthe posterior part of the diaphragm. To its right are the thoracic duct and the azygos vein.

The branches of the descending aorta are as follows:

● Nine pairs of posterior intercostal arteries and a pair ofsubcostal arteries arise from the posterior aspect of thedescending aorta to run in the neurovascular grooves of the third to twelfth ribs. These anastomose with theanterior intercostal arteries, which are branches of theinternal thoracic aorta. The intercostal arteries give riseto radicular and radiculomedullary arteries to the spinalcord and its nerve roots;

● Two to three bronchial arteries, the origins of which arevariable. The right bronchial artery usually arises fromthe third posterior intercostal artery and the two leftbronchial arteries from the aorta itself. The upper leftusually arises opposite T5 and the lower left bronchialartery below the left main bronchus;

● Four to five oesophageal branches arise from the frontof the aorta. These ramify on the oesophagus, forming a network with oesophageal branches of the inferior

thyroid artery and ascending branches of the leftphrenic and left gastric arteries;

● Mediastinal branches;● Phrenic branches to the upper part of the posterior

diaphragm (which receives most of its blood supply by inferior phrenic arteries that arise from theabdominal artery; and

● Pericardial branches to the posterior pericardium.

Subclavian artery (see Fig. 1.42)

The right subclavian artery arises from the bifurcation of the brachiocephalic trunk behind the right sterno-clavicular joint. The left subclavian artery arises from the arch of the aorta in front of the trachea at the level of the T3/T4 disc space. It ascends on the left of the trachea to behind the left sternoclavicular joint. From this point,both arteries have a similar course. The scalenus anteriormuscle (which passes from the transverse processes of the upper cervical vertebrae to the inner border of the first rib) divides the artery into three parts. The first partarches over the apex of the lung and lies deeply in the neck. The second part passes laterally behind the scalenusanterior muscle, which separates it from the subclavianvein. The third part passes to the lateral border of the firstrib, from where it becomes the axillary artery (see Chapter 1for branches).

Pulmonary arteries (see Figs 4.15 and 4.20) (see also pulmonary arteries in the lung)

The pulmonary trunk begins at the pulmonary valve and is approximately 5 cm long. At first it lies anterior to theaorta; it then passes posteriorly and to its left to lie in the concavity of the arch, where it bifurcates into right andleft main pulmonary arteries. The entire pulmonary trunkis covered by the pericardium in a common sheath with theascending aorta.

The right and left atrial appendages and the right andleft coronary arteries surround the base of the pulmonarytrunk. Anterior and to the left it is in contact with the leftlung and pleura.

The right pulmonary artery runs horizontally and to theright, passing behind the ascending aorta and SVC in frontof the right main bronchus. It is crossed anteriorly by theright superior pulmonary vein as this drains to the leftatrium.

The left pulmonary artery runs to the left, anterior to theleft main bronchus and arching over this structure as itgives off the upper-lobe bronchus. It has a slightly higherposition in the chest and is slightly shorter and smaller thanthe right pulmonary artery. It is crossed anteriorly by theleft superior pulmonary vein. It is attached to the concavityof the aortic arch by the ligamentum arteriosum.



Great veins (see Figs 4.10 and 1.44)

The brachiocephalic veins are formed by the union of theinternal jugular and subclavian veins at either side, behindthe medial end of the clavicles. On the right, the brachio-cephalic vein runs inferiorly behind the right border of the manubrium, anterolateral to the brachiocephalic artery. The left brachiocephalic vein is longer. It descends obliquelybehind the manubrium, crossing the origins of the left com-mon carotid and subclavian arteries. It joints with the rightbrachiocephalic vein to form the superior vena cava behindthe junction of the first right costal cartilage with themanubrium.

The superior vena cava runs inferiorly behind the rightborder of the manubrium to enter the right atrium at thelevel of the third costal cartilage. Its only tributary is theazygos vein, which enters its posterior aspect just above the upper limit of its covering sheath of pericardium.

Tributaries of the brachiocephalic veins

These are as follows:

● Internal thoracic (mammary) veins, which drain into the inferior aspect of the brachiocephalic veins;

● Inferior thyroid veins, which arise in the thyroid glandand form a plexus anterior to the trachea. From here, left and right veins drain into the correspondingbrachiocephalic vein, close to their confluence; and

● Left superior intercostal vein, which drains the leftsecond and third posterior intercostal veins and passesobliquely down anterior to the aortic arch to drain intothe left brachiocephalic vein.

Radiology of the great vesselsThe great vessels may be imaged by two-dimensionalechocardiography, angiography, CT or MRI. Using MRI onecan image in any plane without the need for contrast. Asagittal oblique plane is particularly useful for imaging thethoracic aorta. With two-dimensional echocardiography theaortic root and sinuses, ascending and descending aortamay be visualized using an anterior approach. The arch ofthe aorta and its branches are occasionally well visualizedfrom a suprasternal approach.

THE OESOPHAGUS (Figs 4.32 and 4.33 and Figs 4.45–4.49)

This begins at the level of C5/C6 or the lower border of the cricoid cartilage as the continuation of the oropharynx(see also Chapter 1). Its upper limit is defined by the crico-pharyngeus muscle, which encircles it from front to back. Itdescends behind the trachea and thyroid, lying in front of

the lower cervical vertebrae. It then inclines slightly to theleft in the neck and upper mediastinum before returning to the midline at the level of T5 , from where it passes to the left again before sweeping forward to pass through thediaphragm. In the chest it passes behind the trachea, leftmain bronchus, left atrium and upper part of the left ventri-cle from above downward; it then passes behind the poste-rior sloping part of the diaphragm before traversing this atthe level of T10 . The oesophageal hiatus in the diaphragm issurrounded by a sling of fibres from the right crus of thediaphragm.

On its left side, where is found the origin of the left sub-clavian artery, it is grooved by the arch of the aorta. Belowthis level, its left side lies on left lung and pleural tissue. Onits right side it is crossed by the termination of the azygosvein at the level of T4 . Below this, the azygos vein liesbehind and to its right and it is in contact with right lung and pleura. Posteriorly are the thoracic vertebrae and thoracic duct, the azygos vein and tributaries, and theright posterior intercostal arteries as these cross the verte-bral column from the descending aorta. The descendingaorta lies to its left side initially. Then, as the oesophaguspasses forwards and to the left, it becomes anterior to thisvessel in the mid thorax and anterior and to its left as itpasses through the diaphragm. In its terminal part in theabdomen it is retroperitoneal and grooves the posterioraspect of the liver. It enters the stomach at the oesophago-gastric junction.

Fig. 4.32 Posterior mediastinum. (Courtesy of Professor J.B. Coakley.)


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Fig. 4.33 Barium-swallow technique showing theoesophagus: (a) upper oesophagus and oropharynx,lateral view; (b) midoesophagus, right anterior obliqueview; (c) distal oesophagus; (d) distal oesophagus inanother patient with prominent Schatzki ring.

(a)1. Hyoid bone2. Cricoid cartilage3. Cricoid impression4. Cricopharyngeus muscle impression5. Postcricoid venous plexus6. Oesophagus7. Impression caused by osteophytes

(b)1. Impression of aortic arch2. Impression of left main bronchus

(c)1. Cardiac impression (mainly left atrium)2. A ring: upper limit of ampulla (vestibule)3. B ring (Schatzki ring): lower limit of ampulla4. Ampulla (oesophageal vestibule)5. Diaphragmatic hiatus6. Oesophageal mucosal folds7. Oesophagogastric junction (Z-line)8. Diaphragm9. Air in fundus of stomach

(d)1. Schatzki ring2. Gastro-oesophageal junction



Blood supplyThe blood supply is organized in thirds (with free anasto-mosis between each third) as follows:

● Branches of the inferior thyroid artery supply the upper third;

● Branches from the descending aorta supply the middle third; and

● Branches from the left gastric artery supply the lower third.

Venous drainage is also found in thirds: to the inferiorthyroid veins, the azygos system, and to the portal systemvia the left gastric vein. Thus there is communication of thesystemic and portal venous systems in the oesophagus.

Lymph drainageThis is via a rich paraoesophageal plexus to posterior medi-astinal nodes, draining from here to supraclavicular nodes.The lower part drains to left gastric and coeliac nodes.

Radiology of the oesophagus (Fig. 4.33)

Plain films

The right wall of the oesophagus and the azygos vein areoutlined by lung, which may be seen on the frontal film asthe azygo-oesophageal line. Above the level of the arch ofthe azygos (at T4 level) the right wall is sometimes seen as the pleuro-oesophageal line. If the oesophagus containsair, the posterior aspect of the anterior oesophageal wallmay be seen on the lateral film, behind the trachea. This isknown as the ‘posterior tracheal stripe’.

Barium studies

The main radiological method of assessing the oesophagusis the barium-swallow technique, where the oesophagus isoutlined by barium. Gas is usually swallowed in additionto give a double-contrast examination and to distend theoesophagus. The use of a paralysing agent such as intra-venous hyoscine butylbromide (Buscopan) stops intrinsicoesophageal contraction, allowing a better appreciation ofoesophageal anatomy.

On the frontal view, the oropharynx may be examined.The piriform fossae are outlined by barium and the epi-glottis and base of the tongue show as filling defects in themidline. The cervical oesophagus is seen to curve slightly to the left.

On the lateral view the tongue base and epiglottis areseen from the side, with the vallecula between. A posteriorindentation caused by the contraction of cricopharyngeusmuscle to initiate deglutition indicates the commencement

of the oesophagus proper. Just below this, on the anteriorwall of the oesophagus, a small impression may be made bya submucosal plexus of veins. The cervical oesophagus isseen to lie on the anterior surface of the vertebral bodies ofthe cervical spine.

In the chest, the oesophagus is best demonstrated withthe subject rotated slightly off lateral – usually in the rightanterior oblique position. Three major impressions are seenanteriorly. These are made by the aortic arch, the left mainbronchus and the left heart chambers (mainly left atrium)from above down. In this position the oesophagus can beseen to curve anteriorly in its distal part to enter the stomach.

On the frontal view, the oesophagus has a left-sidedindentation from the aortic arch. The left main bronchusalso makes an impression on its left side and may make alinear impression as it indents the anterior wall. The loweroesophagus curves to the left to enter the stomach. In elderly people the course of the oesophagus may be alteredby unfolding of the aorta.

The lower end of the oesophagus has a fusiform dila-tation just above the oesophagogastric junction. This is called the oesophageal vestibule. On barium examination,the upper part of the vestibule is defined by a transientlycontractile ring known as the ‘A ring’. The lower limit of the vestibule is defined by another transiently contractilering known as the ‘B ring’, ‘Schatzki ring’ or ‘transversemucosal fold’. The vestibule corresponds to the mano-metrically measurable zone of increased pressure that is felt to represent the lower oesophageal sphincter. The upperoesophageal sphincter is formed by cricopharyngeus. Inyoung people the vestibule may span the diaphragm. In this case only the upper ring may be identified radiolog-ically. As the oesophagus passes through the diaphragm thelatter makes an indentation on it.

The oesophageal mucosa is arranged in longitudinalfolds that are best seen when the oesophagus is not dis-tended. These measure approximately 3 mm in width. Thethicker folds of the stomach are seen distally, indicating theoesophagogastric junction. In normal people this may riseinto the thorax on swallowing.

Cross-sectional imaging

The oesophagus may also be imaged by CT and MRI. On cross-section its relationship to the other structures ofthe thorax is appreciated (see Figs 4.40 and 4.42–4.44). Itsvisualization is improved if it contains air. When collapsedit is seen as a narrow, thin-walled structure in the posteriormediastinum. Appreciation of the areas in which air-containing lung is adjacent to the oesophagus provides anunderstanding of the mediastinal lines seen on the frontalchest radiograph.


THE THORAX 139

THE THORACIC DUCT AND MEDIASTINALLYMPHATICS (see Figs 4.17 and 4.32)

The thoracic duct arises from a saccular lymphatic reservoircalled the ‘cisterna chyli’, which lies behind the right crus of the diaphragm, anterior to the bodies of L1 and L2 .Lymph from the abdomen and lower limbs passes fromhere into the thoracic duct, which runs superiorly into theposterior mediastinum, passing through the aortic openingin the diaphragm. The thoracic duct ascends anterior to thevertebral column, with the descending aorta to its left andthe azygos vein to its right. The right posterior intercostalarteries run across the vertebral column behind the duct,and the terminal parts of the hemiazygos and accessoryazygos veins cross behind it to enter the azygos vein. Itascends behind the oesophagus, coming to lie to the left ofthis at T5 and then running superiorly on its left side intothe neck. It then passes laterally behind the carotid vessels,and at the level of C7 the duct arches anteriorly over the leftlung, rising 3–4 cm above the clavicle. It then descends todrain into the left subclavian vein at the jugulosubclavianjunction. The thoracic duct has many valves in its last 5 cm.It receives lymph drainage from the left thorax as well asthe left side of the head and neck and the left subclaviantrunk, which drains the left arm, and the lymph from theabdomen and legs that drains through the cisterna chyli.

The right lymphatic duct has three converging trunks:

● Right jugular trunk drains the right side of the head and neck.

● Right subclavian trunk drains the right arm, the righthalf of the thoracoabdominal wall as far inferiorly as the umbilicus anteriorly and the iliac crest posteriorly,and including much of the right breast.

● Right bronchomediastinal trunk drains the hemithorax,including the right heart, the right lung and part of the liver.

The right lymphatic duct drains into the right jugulo-subclavian junction. It is about 1 cm long. Two semilunarvalves at its entry into the vein prevent the reflux of bloodinto the lymphatic channel. It is not unusual for the threeconverging trunks to enter into the jugulosubclavian junc-tion separately without the formation of any right lym-phatic duct.

The mediastinal lymph nodes contain numerouslymph-node groups, which may enlarge in disease states.The anterior mediastinal nodes comprise the internal mam-mary chain, accompanying the internal mammary vessels,and a preaortic group lying anterior to the ascending aorta.The tracheobronchial nodes lie centrally around the tracheaand its bifurcation. They comprise:

● Bronchopulmonary nodes at the hilum of the lungs;● Carinal nodes below the hilum;

● Tracheobronchial nodes above the tracheobronchialjunction;

● Right and left paratracheal nodes on either side of thetrachea;

● The azygos node in the right tracheobronchial angle;● Pretracheal nodes anterior to the trachea;● Aortopulmonary node between the concavity of the

aortic arch and the pulmonary artery.

The posterior mediastinal lymph nodes are found alongthe aorta and oesophagus and also anterior to and on either side of the spinal column. They take their name fromtheir location, for example paraspinal, para-aortic and soon. Normal nodes are not appreciated on plain chest radi-ographs. Nodes may be seen on CT; if they measure morethan 1 cm in diameter they are likely to be pathological.

THE THYMUS (Figs 4.34 and 4.35)

This organ is part of the lymphatic system and lies in the anterior mediastinum. It has two lobes, the left largerand higher than the right, that extend down as far as thefourth costal cartilage in infancy. It is a soft structure and is moulded into the sternum and ribs anteriorly, and thepericardium, great vessels and trachea posteriorly. It isextremely variable in size and shape, even in the same subject at different times. It may extend into the posteriormediastinum or be entirely below the level of the aorticarch. It is visible on the chest radiograph within 24 hours ofbirth, gradually involuting after 2 years of age. It is rarelyseen on chest radiographs after 8 years of age, but may beidentified into early adulthood on CT as a small triangularstructure in the anterior mediastinum, rarely measuringgreater than 1 cm in diameter.

Radiology of the thymus (Figs 4.34 and 4.35)

Chest radiograph

The thymus is visible on the chest radiograph as a media-stinal structure of soft tissue density. It may be focally ordiffusely quite large and mass like. Typically lung markingscan be seen through it; the edges are not sharp, but gradu-ally fade or are wavy because of overlying ribs. Its lowerwell defined horizontal border may protrude from themediastinal contour on the right or the left as a ‘sail sign’. It typically has clear lung lateral to it, or clear lung can beseen lateral to it if the patient is oblique enough. A normalthymus does not deviate a normal structure. It shouldnever, for example, cause deviation of the trachea. The thymus appears larger on expiration than on inspirationradiographs. Its anterior location can be seen on a lateralradiograph.



CT and MRI

The normal thymus can be seen on CT or MRI long after it is no longer visible on a radiograph. On CT it is homo-geneous, with a density similar to that of muscle. On T1-and T2-weighted MRI the thymus has a signal greater thanthat of muscle.

Ultrasound

Using high-frequency high-resolution ultrasound the inter-nal architecture of the thymus can be seen. One can see thevascular septa dividing parenchymal lobules, which havean echogenic medulla centrally and a relatively hypoechoiccortex.

THE AZYGOS SYSTEM (Figs 4.21, 4.32, 4.36, 4.37, 4.43 and 4.44)

These veins are found in the posterior mediastinum. Thesystem consists of the azygos vein on the right and the hemi-azygos and accessory hemiazygos azygos veins on the left.

The azygos vein commences variably anterior to the L2

vertebra, either as a branch of the inferior vena cava or as a confluence of the right ascending lumbar vein and rightsubcostal vein. It ascends to the right of the aorta and thoracic duct, anteriorly to the bodies of T12 –T5 and theright posterior intercostal arteries, which cross behind it.The right lung and pleura are to its right. It usually passesbehind the right crus of the diaphragm to enter the thorax(see Fig. 4.4). At the level of T4 the azygos vein arches

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Fig. 4.34 Variable appearance of normal thymus on chest radiographs of young children: (a) sail-like; (b) like consolidation of right upper lobe;(c) like mediastinal mass; (d) like cardiomegaly.


THE THORAX 141

anteriorly, passing over the hilum of the right lung with thetrachea and oesophagus medially from the posterior side tothe anterior side. The azygos vein drains into the superiorvena cava just above the upper limit of the pericardium,behind the third costal cartilage.

The azygos vein receives all but the first intercostal veinon the right. The second, third and fourth intercostal veinsusually drain via a common channel – the right superiorintercostal vein. The hemiazygos and accessory azygosveins drain into the azygos vein at midthoracic level. Theright bronchial veins drain into the azygos vein near its ter-mination. It also receives tributaries from the oesophagus,pericardium and mediastinum.

The hemiazygos vein commences in a similar manner to the azygos vein on the left side of the aorta. Its origin maycommunicate with the left renal vein, and it ascends anteriorto the vertebral column to the midthoracic level, then passesbehind the aorta to drain into the azygos vein. It receives theleft ascending lumbar and lowest four posterior intercostalveins, as well as mediastinal and oesophageal veins.

The accessory hemiazygos vein receives the fourth to the eighth posterior intercostal veins. (The first to third posterior intercostal veins drain into the left brachio-cephalic vein.) It may also receive the left bronchial veins. It descends on the left side of the vertebral column to the midthoracic level, then crosses to the right behind theaorta to drain into the azygos vein. The hemiazygos andaccessory hemiazygos veins may drain into the azygos vein separately or via a common trunk.

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Fig. 4.35 Coronal MRI of thymus: (a) T1-weighted; (b) T2-weighted.

1. Left lobe of thymus2. Right lobe of thymus

3. Left ventricle4. Ascending aorta

5. Brachiocephalic vein6. Right subclavian vein

7. Left subclavian vein

Fig. 4.36 Azygos system. (Courtesy of Professor J.B. Coakley.)



IMPORTANT NERVES OF THE MEDIASTINUM

The vagus nerves are the tenth cranial nerves and passthrough the neck in the carotid sheath. Composed of motorand sensory fibres, the vagus nerve enters the superiormediastinum posterior to the internal jugular vein and bra-chiocephalic veins. It passes behind the main bronchi, form-ing a posterior pulmonary plexus; branches then pass to theoesophagus, forming anterior and posterior oesophagealplexuses, which continue into the abdomen through theoesophageal hiatus as anterior and posterior vagal trunks.The recurrent laryngeal nerves arise from the vagi. On theright the recurrent laryngeal nerve winds around the sub-clavian artery, on the left it winds around the aortic arch.

The phrenic nerves arise from the third to fifth cervicalnerves and are the motor supply to the diaphragm. Theyenter the chest anterior to the subclavian artery and deep tothe vein. The right phrenic nerve runs down in front of thesuperior vena cava, over the right heart border and inferiorvena cava to reach the diaphragm. The left phrenic nerveruns down over the aortic arch and left superior intercostalvein, in front of the left hilum, and runs on the left heartborder to reach the diaphragm.

The sympathetic trunk runs inferiorly in the posteriormediastinum in the paraspinal gutters, coming to lie infront of the bodies of T11 and T12 before passing through the diaphragm behind the medial arcuate ligament to continue as the lumbar sympathetic trunk. It is distributed

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Fig. 4.37 Axial CT thorax with contrast to show azygos and hemiazygos system of veins: (a) contrast densely opacifies the arch of theazygos vein: (b) accessory hemiazygos vein drains to azygos system (arrows) at T8 level; (c) at a slightly lower level the hemiazygos vein also drains to the azygos vein; (d) below this both the hemiazygos and azygos veins are visible.

1. Arch of azygos vein 2. Accessory hemiazygos vein 3. Azygos vein 4. Hemiazygos vein


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to the thoracic viscera and forms the splanchnic nerves tothe abdominal viscera.

THE MEDIASTINUM ON THE CHEST RADIOGRAPH

Mediastinal contour on the frontal chestradiograph (see Fig. 4.6)

The heart and great vessels form a characteristic contour on the frontal chest radiograph. The right side of the mediastinal contour is formed from above downward bythe brachiocephalic vein and the SVC. The SVC forms ashallow angle with the right atrium, which forms the rightheart border. The terminal part of the IVC may be seen just medial to the cardiophrenic angle, which is usuallysharp. Occasionally a fibrofatty pad displaces the rightpleura laterally, obscuring the cardiophrenic angle.

The left side of the mediastinal contour is formed by thecomposite shadow of the subclavian vessels superiorly. Theartery is lower and actually forms the contour. This fadesout laterally and is usually indistinct. Below this the aorticprominence is termed the ‘aortic knuckle’ or ‘knob’. This isformed by the posterior part of the arch. It may be indistinctin young people and very prominent in older people, espe-cially if there is aortic unfolding. Sometimes a small ‘nipple’may be seen projecting from the aortic knuckle. This iscaused by the left superior intercostal vein as it crosses theaorta to drain into the left brachiocephalic vein (Fig. 4.38).In older people the left side of the descending aorta may bevisible descending from the aortic knuckle. Below the aorticknuckle is an air space called the aortopulmonary window.Failure to identify this clear space indicates pathology.

Below the aortopulmonary window is the main pulmonaryartery, which has a straight upper border, and below this isthe left ventricle. The left atrial appendage lies embeddedin fat below the pulmonary artery but is not contour form-ing unless enlarged. The right cardiophrenic angle is not assharp as the left. In deep inspiration air-filled lung may beseen under the apex of the left ventricle. Occasionally, a fatpad is present in the left cardiophrenic angle.

The pulmonary arteries and veins form the densities ofthe hila on the frontal chest radiograph (see radiologicalfeatures of the lung, p. 122).

Mediastinal contours on the lateral chestradiograph (see Fig. 4.7)

The heart shadow lies behind the lower third of the ster-num. The anterior border is formed by the right ventricleand outflow tract. Higher up the lungs are in contact witheach other behind the sternum and in front of the ascendingaorta, forming the retrosternal air space. The posterior con-tour of the heart shadow is formed by the left atrium aboveand the left ventricle below. The IVC may be identified as a triangular structure crossing the diaphragm to enter theright atrium, which lies in a more anterior plane than theleft atrium.

The aortaThe aorta may be invisible in young people but is usuallyseen, at least in part, in middle-aged subjects. The ascend-ing aorta is indistinct. The arch curves evenly from front to back and the descending aorta is seen anterior to the ver-tebral column. Its walls should be parallel. In older people,unfolding may cause it to overlie the vertebral bodies.

Mediastinal lines (Figs 4.6, 4.7, 4.9, 4.39 and 4.40)

Wherever air-filled lung outlines a linear soft-tissue struc-ture, the difference in density is detected by the plain radi-ograph as a line. If air outlines two sides of a thin structurethe soft-tissue density is seen as a stripe. This relationshipmay be appreciated on CT scans.

Right paratracheal stripe The lung is in contact with the trachea from the level of theclavicles to the azygos vein in the right tracheobronchialangle. As the trachea is air-filled the right tracheal wall isseen as a stripe, which should not measure more than 3 mm(the left tracheal wall is separated from lung by the aortaand great vessels and is not seen).

Posterior junction line (Fig. 4.9)

This is formed by the apposition of the two lungs posteri-orly. It extends from well above the clavicles verticallydownwards to the arch of the aorta. The aortic arch

Fig. 4.38 PA chest radiograph showing an ‘aortic nipple’ (arrows)caused by the left superior intercostal vein as it passes anterior tothe arch of the aorta.



Fig. 4.39 PA chest radiograph showingthe azygo-oesophageal line (arrows).

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Fig. 4.40 CT scan of thorax: upperT4 level showing mediastinal lines.

1. Anterior junction line2. Air-filled trachea3. Oesophagus4. Right paratracheal stripe5. Azygo-oesophageal stripe 6. Superior vena cava7. Aortic arch 8. Right paraspinal line9. Left paraspinal line

10. Azygos vein


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separates the lungs and the line disappears. It may reformbelow the arch. The line represents the four layers of pleurabetween the posterior parts of the lungs seen from the front.

Anterior junction line

This line is formed by the apposition of the lungs anteriorly.It begins below the clavicles and runs inferiorly and to theleft. Its oblique course is because of the differing anteriorextent of the two lungs. It always ends at the right ventricu-lar outflow tract.

Pleuro-oesophageal line

This is formed by the right lung outlining the right wall ofthe oesophagus above the level of the azygos vein. If theoesophagus happens to be distended with air, the pleuro-oesophageal stripe is seen. The thickness of this representsthe thickness of the oesophageal wall and pleura andshould not exceed 2 mm.

Azygo-oesophageal line (Fig. 4.39)

The azygos vein is closely related to the right posterolateralaspect of the oesophagus in the posterior mediastinum. The right lung abuts the two structures forming the azygo-oesophageal line, tucking in behind them in the azygo-oesophageal recess. On the frontal chest film the line curvesto the right as the azygos vein crosses to gain the right tracheobronchial angle before draining into the SVC.

Paraspinal lines

These are outlined by lung apposing the spinal column. Theright line is sharper than the left as the descending aorta

tends to reflect the pleura off the thoracic spine on the left, making the line more indistinct. The distance from thelateral border of the spine to the line may vary with bodyhabitus, and may be 1 cm on the left in obese people withparaspinal fat deposits. Widening of the paravertebral softtissue on the right is always abnormal. Widening of the paravertebral soft tissue beyond the transverse process isalways abnormal.

Aortopulmonary mediastinal stripe

This is the reflected pleura from the aortic arch to the pulmonary trunk and left pulmonary artery. It defines thelateral boundary of the aortopulmonary window.

Posterior tracheal stripe

This is seen on the lateral view and represents the posteriorwall of the trachea outlined on either side by air. It meas-ures 2–3 mm. If the collapsed oesophagus is apposed to theposterior trachea the stripe may measure 1 cm. If there is airin the oesophagus the stripe represents posterior trachealand anterior oesophageal walls.

CROSS-SECTIONAL ANATOMY(Refer to Figs 4.41 and 4.45–4.49 for this section)

Level T3 (Fig. 4.41)

This is the superior mediastinal level. The trachea is seen in the midline, with the great vessels anteriorly and theoesophagus behind. The brachiocephalic veins are anteriorand lateral to the arteries and unite to form the superiorvena cava at about this level. The left brachiocephalic veinis seen passing anteriorly to the branches of the aortic arch.

Fig. 4.41 Cross-sectional anatomy: level T3 .

Manubrium



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Fig. 4.42 CT scan of thorax: level T3 .

Fig. 4.43 CT scan of thorax: level T4 .

1. Body of T4

2. Thecal sac and spinal cord3. Arch of aorta4. Superior vena cava5. Azygos vein6. Trachea7. Oesophagus8. Erector spinae muscle9. Trapezius muscle

10. Infraspinatus muscle11. Subscapularis muscle12. Pectoralis minor muscle13. Pectoralis major muscle

1. Body of T3

2. Body of manubrium sterni3. Right brachiocephalic vein4. Left brachiocephalic vein5. Right subclavian artery6. Right common carotid artery7. Left common carotid artery8. Left subclavian artery9. Trachea

10. Oesophagus11. Right scapula

12. Spine of scapula13. Erector spinae muscle14. Trapezius muscle15. Supraspinatus muscle16. Infraspinatus muscle17. Subscapularis muscle18. Deltoid muscle19. Fat in axilla20. Pectoralis minor muscle21. Pectoralis major muscle22. Subcutaneous fat


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Level T4 (see Fig. 4.45)

This plane passes through the lower border of T4 and thesternal angle. Above this plane is the superior mediastinum.At this level is the lower part of the aortic arch to the left of the trachea, and the arch of the azygos vein to the

right, crossing the right hilum to drain into the posterioraspect of the superior vena cava. The oblique fissure,although demonstrated on these figures, is not always seenon CT as a line.

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Fig. 4.44 CT scan of thorax: lower T5 level.

1. Body of T5

2. Sternum3. Spinous process of T5

4. Ascending aorta5. Descending aorta6. Superior vena cava7. Main pulmonary trunk8. Right pulmonary artery9. Left pulmonary artery

10. Carina11. Left main bronchus12. Oesophagus13. Azygos vein14. Azygo-oesophageal recess15. Mediastinal fat





This plane is at, or just below, the tracheal bifurcation. The main pulmonary artery is seen dividing into right pul-monary artery, which runs anteriorly to its bronchus, andleft pulmonary artery, which arches superiorly out of theplane to pass over the left bronchus. The descending lower-lobe artery is seen lying posterolateral to the bronchus.

The superior pulmonary veins are seen anteriorly. On the right this is anterior to the interlobar artery. On the left it is separated from the lower-lobe artery by the leftbronchus.

The ascending aorta is seen in cross-section, lying in amore posterior plane than the origin of the main pulmonaryartery.

Anterior to this is the junction of the anterior pleurae ofthe lungs, which forms the anterior junction line on thefrontal chest radiograph.

The oesophagus, azygos veins and descending aorta areseen in the posterior mediastinum. The azygo-oesophagealrecess may be seen.


This section passes through the upper part of the heart. Theright atrial appendage may be seen overlapping the originof the aorta. The coronary arteries come off the aorta at thislevel. The right ventricular outflow tract is seen anterior tothe origin of the aorta. The relationship of the oesophagusto the left atrium is seen.




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This section passes through the heart and shows the relationship of the chambers to each other. The left ven-tricular inflow and outflow tracts are separated from each other by the anterior leaflet of the mitral valve. Theaortic sinuses are seen as localized bulges. Note that theright heart border is formed by the right atrium, the anterior border is formed by the right ventricle, the left border by the left ventricle and the posterior border by theleft atrium.


At this level, part of the abdominal cavity is also seen as thedome of the diaphragm reaches the level of approximatelyT9 . The oesophagus traverses the diaphragm at this level, in a relatively anterior position, through the oesophagealhiatus, and joins the stomach at the oesophagogastric junc-tion. The fundus of the stomach and part of the spleen andliver are seen. The inferior vena cava is situated slightlyanteriorly; on higher cuts it enters the right atrium. Thedescending aorta with azygos and hemiazygos veins on itsright and left are posterior.




The thorax - Elsevier · 2013-12-20 · THE THORAX 109 — Next three by the musculophrenic artery,...

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Transcript of The thorax - Elsevier · 2013-12-20 · THE THORAX 109 — Next three by the musculophrenic artery,...