The lungs and mediastinum

Normal anatomy and variations

 

Chest wall

The chest wall is made up of the thoracic skeleton and the surrounding soft tissues. The shape of the thorax varies considerably, from the very tall, thin individuals, on whom a superficial glance at the frontal view may lead one to suspect hyperinflation, to short individuals with a large anteroposterior diameter and an almost barrel-shaped chest in the lateral view. The clavicles appear symmetrically on either side of the mid-line. This is an important observation, as asymmetry indicates inadvertent oblique positioning. In such slightly oblique films, the ascending aorta and the manubrium sterni may emerge from the medastinal shadow and give rise to misinterpretations. The medial epiphysis of the clavicle does not ossify before adulthood, and the epiphysis should not be confused with a lung opacity. There is a hollow (rhomboid fossa) on the lower side of the medial end of the clavicle. This can sometimes be so deep and obvious that it is confused with a pathological erosion.

Figure 13. Bilateral cervical ribs (arrows).

It is important to be familiar with the appearance of the ribs, and to be able to follow the rib from behind - where they are horizontal or slightly sloping - forwards to where they turn medially and caudally. Medially, they become cartilaginous and are then not visible on the radiograph unless calcified. Calcification may be distributed in patches, and be confused with calcifications of the kidney or gallbladder by inexperienced observers. The arrangement and direction of the longest axis of the calcifications are usually such that it is normally not necessary to take oblique pictures to verify (Fig. 12).

Anomalies of the ribs in the form of bridge formation between two adjacent ribs or Y -shaped division of a rib in the front (bifid rib) may occur and be confused with a lung opacity. Cervical ribs usually articulate with the 7th cervical vertebra (Fig. 13). They appear as tap-shaped outgrowths, which point caudally over the apex of the lung.

Frontal views of the thorax are exposed with the back of the hand placed on the iliac crest and the elbows pushed forward in order to laterally rotate the shadow of the scapula. With insufficient rotation, the lower corner of the scapula will be projected over the lateral segments of the lung, and may be confused with a pleural lesion.

Of the soft tissue structures of the chest wall, breasts produce the most obvious shadows. Large breast shadows may cause insufficient exposure of the underlying parenchyma. After unilateral resection of a breast, the underlying lung area will be more translucent, and this should not be misinterpreted as a pathological condition. In the absence of one breast shadow, the ribs and lungs should be scrutinized for metastases.

Figure 14. Drawing of the pleura on the right side, showing the course of the pleura on surface of lung. Only the visceral pleura passes into a normal fissure. The parietal pleura follows the chest wall, except when the course of the azygos vein is abnormal. Both layers of pleura then pass into the fissure, which makes this fissure prominent.

The nipple of the breast may be confused with a round basal shadow in the lung. This is generally no problem as both nipples are seen symmetrically on either side of the midline in the frontal view, while no shadow at the corresponding level is seen in the lateral view. The lateral border of the shadow of the nipples is often more distinct than the medial border. Other soft tissue structures that can be identified in frontal views are the lower part of the stemocleidomastoid and pectoral muscles. Misinterpretations may occur when the pectoral muscles have been removed operatively or are atrophic (for example after poliomyelitis), and the underlying lung may be regarded as emphysematous, or it is thought that the contralateral lung has increased opacity.

Pleura

The parietal pleura lines the inner side of the thoracic cavity and the lateral side of the mediastinum without passing into the normal lung fissures (Fig. 14). Around the structures in the hilar region, it crosses to cover the medial surface of the lung and continues as visceral pleura on the surface of the lobes of the lung. The pleural cavity is located between the two layers of the pleura, and normally surrounds the whole lung expect the hilar region. In some cases, a fold of the pleura extends from the hilar region to the diaphragm, forming a ligament that obstructs direct continuity medially between the front and back of the pleural space. This ligament, the pulmonary ligament, may form a small tent-shaped triangular opacity extending from the hilum toward the dome of the diaphragm, which may be confused radiologically with adhesion between

Figure 15. The rounded, smooth bordered prominence in the right tracheobronchial angle (v.a) is caused by a dilated azygos vein. The patient had occlusion of the inferior vena cava and collateral circulation to the azygos vein.

the two layers of pleura. Unlike the parietal pleura, the visceral pleura follows the surface of the lobes of the lungs into the fissures.

The fissure between the upper and lower lobe (major interlobar fissure) is found bilaterally. On the right side, an additional fissure, the minor interlobar fissure, between the middle and upper lobes is also present. This is the only fissure visible in the frontal plane, while the fissures between the upper and lower lobes are projected over each other in lateral views. The CT scan shows the major interlobar fissure on both sides.

The fissures may be incomplete, and accessory lobes and fissures may occur. A fissure consists of two layers of pleura with a potential deft between them.

A special anomaly found in almost 1% of the population is the azygos lobe (Figs. 12, 14). This is due to an abnormal course of the azygos vein. Normally, this vein runs up along the vertebral column and turns forwards in the mediastinum to open into the anteriorly situated superior vena cava at the level of the tracheo-bronchial angle (Fig. 15). When the anomaly is present, the veins run more laterally and curves forwards. It makes a deep groove in the right apex. On the medial side of the groove, one gets the impression of an extra lobe of the lung, and its fissure is especially prominent because it consists of four layers of pleura, since the vein lies outside the pleura throughout its whole course (Fig. 14). At the transition between the diaphragm and the chest wall, the layers of the pleura form a very acute angle, the costophrenic angle. Posteriorly, it extends in a caudal direction; this anatomic feature may explain that a metal

 

Figure 16. Lobes and segments of the lungs.

dip, lost during thoracotomy may appear on a frontal radiograph of the abdomen just above the upper pole of the kidney.

Lungs

The lungs are divided into lobes, the lobes into segments, which in turn consist of still smaller units, the so-called secondary lobules. The right lung has three, and the left two lobes (Fig. 16). The right lower lo be has five segments, the superior (apical) and four basal (anterior, posterior, medial, and lateral) segments. The left lower lobe lacks the medial segment.

Both upper lobes have three segments. The right has posterior, apical, and anterior segments, while the left has apico-posterior, anterior, and lingular segments. The latter extends caudally and anteriorly, corresponding

Figure 17. Visible lines between secondary lobules in a patient with interstitial edema (Kerley's B lines).

to the position of the middle lobe on the right side.

The bronchial tree divides into bronchial branches to the lobes, and these divide again forming a segmental bronchus to each segment. The lingular bronchus originates from the left upper lobe bronchus (Fig. 16), in agreement with the fact that the lingula is a segment of the upper lobe. Infectious processes, for example lobar pneumonia, respect the anatomical boundaries between the lobes of the lung, but there are no correspondingly distinct anatomical boundaries between the segments. The superior segments of the lower lobes extend in a cranial direction posteriorly, so that an opacity, which appears level with the manubrium sterni in a frontal view, is not necessarily located in the upper lobe. Lateral views are required to localize findings to the individual segments.

Secondary lobules are 1-2.5 cm large, and although their shape may vary, they often appear to be pyramid-shaped with their base towards the pleura, in the centre of the pyramid there is a terminal bronchiole that supplies three to five acini with branches. Branches of the pulmonary and bronchial arteries run along the bronchioles. The veins run in the interlobular septa between the individual lobules together with lymph vessels and strands of connective tissue. Neither the secondary lobules nor the interlobular septa are seen in radiographs of normal patients. In patients with venous congestion or lymphatic obstruction with accompanying interstitial edema, the septa become visible as thin, one or two centimetre long stripes perpendicular to the pleura, at the base of the lungs. These are the Kerley B lines (Fig. 17), which may be due to venous congestion, lymphatic obstruction, or in some cases of pneumoconiosis.

Normal aerated bronchi are not visible peripheral to the hilar area unless the x-ray beam is parallel to them. They then sometimes appear as ring-shaped structures.

The normal lung markings are chiefly due to blood in the arteries and veins that ramify out towards the lung surface. The lung markings are therefore pronounced when the vessels are distended. In the upright position, the upper thirds of the lungs are relatively poorly perfused because of low. pressure in the pulmonary arteries. The vessels in the upper third therefore contain less blood than the vessels in the lower third, and they seem to be narrower. This will change when lying down or in the case of failure of the left side of the heart with pulmonary congestion. An increase in breadth and opacity of the vessel shadows in the upper segments of the lungs may therefore be an important observation.

When considering lateral views, it is useful to note that there are normally few soft tissue structures and therefore radiolucency of the film behind the upper part of the sternum, where the mediastinum may consist of a very thin tissue lamella. Increased opacity here may be due to expansive processes, a wide ascending aorta, or enlargement of the heart.

The soft tissue structures in the axillae and shoulder region make the upper part of the thoracic column appear to be radiodense in lateral pictures. Normally, radiolucency over the thoracic vertebral column in a lateral view should gradually increase caudally. An opacity posteriorly in the lower lobes can often be discovered because the radiolucency over the lower part of the vertebral column diminishes (the column becomes lighter).

Mediastinum

The mediastinum is the area lying between the medial parts of the right and left pleura. The mediastinum is mobile from side to side, and observation of mediastinal displacement and movement provides important diagnostic information about the nature of the disease process. Normally, the lower part of the right mediastinal outline is made by the right atrium. When this is not the case, but rather this outline is made by the vertebral column, it is a sign of displacement of the heart and lower part of the mediastinum to the left.

Above the hilar region, the right mediastinal outline is made by the superior vena cava and the brachiocephalic vessels (Fig. 18). Above the left hilum, the upper mediastinal outline is made by the arch of the aorta and subclavian vessels. A right-sided aortic arch forms a prominence of the upper right mediastinal outline. This may be misinterpreted as an expansive process unless the absence of a normal aortic arch is recognized. In patients whose thorax is very deep (and those with emphysema), the anterior, upper mediastinum consists of a tissue lamella so narrow that the lungs almost meet in the mid-line. When this lamella is oriented sagitally (parallel to the x-ray beam) it forms a linear opacity in the mid-line (anterior junction line) on the frontal film. Further posteriorly, the mediastinum has narrow areas, for example under the arch of the azygos vein, where the pleural cavity approximates the oesophaus (the azygooesohpageal recess).

In the tracheo-bronchial angle on the right side, a circular shadow represents the arch of the azygos vein, lying parallel to the x-ray beam (in the same way as the aortic arch on the left side). In the presence of venous obstruction and collateral circulation through the azygos system, the diameter of the azygos vein may increase. Close to the azygos arch, there is an important lymph node, the azygos node. Like the other lymph nodes in the mediastinum and hilum, this is only visible in standard radiographs when it is enlarged. The azygos node is the primary lymph node in the spread of lung cancer from the right lung and from the left lower lobe. The corresponding node for spread from the left upper lobe lies to the left in the mediastinum, under the arch of the aorta, with the left pulmonary artery below it. The node lies close to the obliterated ductus arteriosus, and is therefore called the" ductus node". The area in which it lies is called the" aortopulmonary window". In this area, relatively large expansive processes may escape attention in standard chest radiographs, while they are easily discovered by CT scan. CT scans disclose normal mediastinal lymph nodes when there is sufficient mediastinal fat so that the organs do not all coalesce. The mediastinal lymph nodes are divided into the anterior, middle, and posterior nodes, with a series of subgroups. If lymph node enlargement is suspected, one should systematically inspect the fat spaces in front of and lateral to the trachea, in front of the carina, below the carina (in the angle between the left and right main bronchi), and in the aortopulmonary window. The normal size of the mediastinal lymph nodes varies from one region to another, and there is no set limit where a larger lymph nodes are pathological and all smaller ones are normal.

Figure 18. Diagrammatic drawing of a chest x-ray, PA and lateral views.

The posterior part of the mediastinal shadow is made up of the thoracic column and the paravertebral soft tissues. In a correctly exposed frontal view of the thorax, it should be just possible to see the thoracic vertebrae. A well penetrated view shows them better, and also shows two accompanying soft tissue shadows along the left side (Fig. 19). The shadow furthest to the left is the outline of the descending aorta. In older people this will bulge slightly, with lateral convexity. The paravertebral soft tissue stripe lies between this and the vertebral column, and represents the medial part of the pleura behind the aorta. This is most often only seen clearly on the left side, and has a straight course along the vertebral column. A number of pathological conditions may lead to a spindle-shaped swelling of the paravertebral soft tissue shadow.

The hilar shadows are chiefly made up of the pulmonary arteries and veins (Fig. 18). The hilar shadow on the left side is a little higher than on the right. Doubt can sometimes arise as to whether the hilar shadows are enlarged or not, and in these cases it is useful to study the lateral views as well.

Figure 19. Drawing of the mediastinal/paravertebral soft tissue stripes. PA and axial projection.

Figure 20. The crurae of the diaphragm se en from the front, and as axial sections at three levels. A = aorta, R. C. = right crus, L. C. = left crus

Figure 21. Significance of good inspiration when taking chest x-rays a) First film. Large heart and basal opacities? b) New film after instruction on inspiration. Normal findings. a

b

Diaphragm

The diaphragm originates from the lumbar vertebrae posteriorly with its two crurae (Fig. 20), from a ligament (arcus tendinosus) between the vertebral column and the lowest ribs, from the ribs at the back and laterally, and from the sternum in front. The fibres course upward in an even curve towards the central tendinous part, which has openings for the esophagus, vena cava and aorta.

The level of the right dome of the diaphragm is usually somewhat higher than the left, while the left has greatest mobility, about five to six cm. On full inspiration, the upper part of the right dome of the diaphragm is near the tip of the sixth rib at the front, and between the 10th and 11th rib at the back.

It is important that the patients understand that they must breathe in and hold their breath. What appear to be basal opacifites in the lung can vanish on a repeat film obtained with full inspiration, shown by the position of the domes of the diaphragm (Fig. 21 a, b). The outline of the lower side of the diaphragm is only shown when there is free intraperitoneal air providing contrast. The complete outline of the upper side of the diaphragm is seen against the aerated lung tissue, except in the area where the heart shadow lies in direct contact.

Figure 22. Normal retrocrural space (R). The space consists of the posterior, inferior part of the mediastinum and contains fat, vessels, nerves, and lymph nodes. Aorta (A). Diaphragm (arrows).

 

The posterior parts of the domes of the diaphragm are only seen in a lateral view. In lateral films of normal individuals, the outline of the right dome of the diaphragm can be followed forward to the chest wall, white the left is obliterated in front because of contact with the heart shadow (Fig. 18). The gastric air bubble may also help to identify the left from the right diaphragmatic dome in the lateral view. A staircase-shaped diaphragm in the lateral projection may be a normal variant. In such cases the frontal view may show two outlines at different levels.

The crurae originate from the anterior part of the lumbar column as far down as L3 on the right side and L2 on the left. In a CT section of the abdomen, the crurae may be se en as oval prevertebral structures, which can be confused with enlarged lymph nodes. In a CT section further cranially, the upper parts of the crurae form a bowed structure over the aorta at the thoracolumbar transition. In such sections, the other abdominal organs are seen in front of the diaphragm, and the retrocrural space, which is the very lowest part of the chest cavity, is seen behind it (Fig. 22). This space may contain pathologically enlarged lymph nodes that can only be demonstrated by CT scan (or MRI).

Figure 23. Bulging of the right mediastinal outline caused by kyphoscoliosis. Note the short distance between the ribs on the left, large distance on the right side.

 

Alf Kolbenstvedt, Arnulf Skjennald and Charles B. Higgins