Normal Anatomy on chest X-ray

NORMAL  ANATOMY ON CHEST X-RAY

 The normal roentgen anatomy of the as seen on chest radiographs can be described in following headings.

Trachea

   Trachea is straight tube, midline in the upper part and deviates slightly to the right around the aortic knuckle. It shortens and deviates more to right on expiration. Its caliber is even with decreasing translucency as it is traced caudally. On plain chest radiograph the upper limits of coronal diameters in adults are 21 mm ( in females) and 25 mm (in males). The right tracheal margin ( Right paratracheal stripe ) can be traced down to the right main bronchus. It is 4 mm or less in thickness and measured above the azygos vein. The left paratracheal line is rarely visualized. After the age of 40 years, calcification of the cartilage rings of the trachea is a common finding. The enlarged azygos vein, which lies in the angle between the right main bronchus and trachea, may be normally seen as a round opacity in the tracheobronchial angle in the supine chest film.

Tracheobronchial Division

   The trachea divides into right and left main bronchus usually at D5 or D6 level in adults. The left main bronchus is longer and has more acute angle with trachea as compared to right main bronchus.

     The right main bronchus divides into upper lobe bronchus and bronchus intermedius. The upper lobe bronchus divides into apical, posterior and anterior segment bronchi. The bronchus intermedius divides into middle and lower lobe bronchi. Middle lobe bronchus has medial and lateral branches. The lower lobe bronchus has five branches; each for superior, anterior, lateral, posterior and medial basal segments of lower lobe. Absence of middle lobe on left side modifies the bronchial division on left side. The left main bronchus divides into upper and lower lobe bronchi. The upper lobe bronchus has two divisions; the upper division divides into apico-posterior and anterior branches to supply upper lobe, The lower division supplies the lingula with superior and inferior branches. The lower lobe bronchus on left side divides similar to the right side except the absence of separate medial basal branch. Major tracheobronchial divisions 

major Tracheobronchial division

                                          

Figures 1.6 A and B: Diagrammatic representation major tracheobronchial division as seen on frontal (A) and lateral (B) Orientation: (1-apical, 2-posterior and 3-anterior segments of upper lobe; 4-lateral segment of middle lobe/superior lingula, 5-medial segment of middle lobe/inferior lingula, 6-superior, 7-medial basal, 8-anterior basal, 9-lateral basal and 10- posterior basal segments of lower lobe)

Lungs

The lungs are divided into three lobes on the right side and two lobes on the left side by the interlobar. The major (oblique) fissures on both sides are similar. It runs obliquely forwards and downwards (upper portion facing forward and laterally and the lower portion facing backward and medially), passing through the hilum. On a lateral view, it starts at the level of fourth or fifth thoracic vertebra to reach the diaphragm 5 cm behind the costophrenic angle on the left and just behind the angle on the right side. 

major fissure on lateral chest

                                 1. Minor fissure

                                 2. Major fissure

Line diagram showing the position of major fissure on lateral chest radiograph (Reproduced with permission)

     The right lung has an additional fissure, the minor (horizontal) fissure. It can be drawn on chest PA film from right hilum to the sixth rib in axillary line 

minor fissure on PA chest

 Line diagram showing the positing of minor fissure on PA chest radiograph ( Reproduced with permission)

It separates the middle lobe from right upper lobe. There are some accessory fissure. which are occasionally seen. The azygos lobe fissure, so called because it contains the azygos vein on right and hemiazygos vein on left within its lower margin, is commonly seen on the right side with an incidence on 0.4 parcent.²⁸ It appears as a hairline with slight lateral convexity running across the right upper zone to end in a comma like expansion (azygos vein) near the hilum. The azygos lobe is the area of the ling medial to the azygos fissure. The left sided horizontal fissure, similar to the minor fissure  on the right, separated the lingular from the other upper lobe segment. The superior accessory fissure separated the apical from the basal segment of the lower lobes. The inferior accessory fissure separates the medial from the other basal segment.

Bronchopulmonary Segment     

        Beonchopulmonary segment of individual lobes are basal on the subdivisions of the lung, Which is supplied by an integral and relatively constant segmental bronchus and blood vessels. The boundaries between various segments are complex and with the rare exception of accessory fissure, the segments are not divided by septae. Although many pathological process may predominate in one segment or another, these usually never confirms precisely to whole of just one segment since collateral air drift occur across segmental boundaries. However, information of segmental involvement in disease process is particulary important to surgeons since these segments can be removed separately. These bronchopulmonary segments are designated as per the divisions of segmental bronchi. There is lot of overlap of bronchopulmpnary segments on a PA view of chest but they project separately on a lateral view. Their approximate location as seen on frontal and lateral radiographs is illustrated

  

Upper and middle lobe/lingula on PA projection

Lower lobe on PA projection

Right lung on lateral projection

Left lung on lateral projection

Line diagram showing approximate locations of various bronchopulmonary segments. A. upper and middle lobe/lingula on PA projection, B. lower lobe on PA projection, C. Right lung on lateral projection, D. Left lung on lateral projection (key same as figure 1.6)

   The radiographic density of the two lungs is symmetrical on a well-taken PA film. If the patient is rotated, the hemithorax closer to the film appear more radiodense. Both PA and lateral views are necessary to localise in one or more of the pulmonary segment. Since the normal bronchi are not visualised in the peripheral lung fields, it is difficult to make out the boundary of different pulmonary segment on plain radiograph of the chest.

Hilum and Pulmonary Vasculature   

The structures contributing to the formation of the hilum are the pulmonary arteries and their main branches, upper lobe pulmonary veins, the major bronchi and lymph glands. Of all the structures in the hilum, only the pulmonary arteries and upper lobe veins significantly contribute to the hilar shadows on a plain radiograph. Normal lymph nodes are not seen. The left hilum is usually 0.5 to 2 cm higher than the right . Both hila are of equal density and size with a concave lateral border on PA film.

    The diameter of the normal descending branch of right pulmonary artery is between 10-16 mm in males and 9-15 mm in female. The course of the pulmonary vessels can be described by dividing them into three zones depending upon their positions in the lunges, i.e. hilar, mid lung and peripheral. Mid lung vessels extend from hilum apto 2 cm from the chest wall. Peripheral vessels are present in other 2 cm of the lung fields and these are rarely seen on a normal chest radiograph. The pulmonary veins have fever branches and are straighter. The distinction between intrapulmonary arteries and veins is difficult and seldom useful so that they are collectively referred to us pulmonary vasculature. The pulmonary vessels taper radiographs; the upper zone vessels are comparatively narrower than lower zone vessels because of the effect of gravity. The bronchial vessels are normally not seen on chest radiograph.

Pleura  

Normal pleura is not visible on chest radiograph . The mediastinal surface of the pleura can occasionally be demonstrated near midline in a well-penetrated chest radiograph.

Mediastinum

It is a space lying between two lungs. It is bounded by sternum anteriorly, dorsal spine posteriorly and pleural sacs on both sides. The borders of the hearts and mediastinum are clearly defined except where the heart is in contact with the left hemidiaphragm. The bracheocephalic (innominate) vessels superior vena cava and right atrium from the right madiastinal border. Rarely a dilated aorta may also contribute. The left border is formed by left subclavian artery, aortic knuckle, left atrial appendage and left ventricle.

    The radiological division of the mediastinum can be ascetrained on a lateral chest radiograph by two imaginary lines 

Radiological divisions of the mediastinum

 Line diagram showing radiological divisions of the mediastinum (Reproduced with permission)

The first line is drawn from the diaphragm upward along the posterior border of heart and anterior border of the trachea into the neck. A second line is drawn connecting a point on each thoracic vertebra, 1 cm behind their anterior border. The anterior mediastinum is in front of the first line, the middle mediastinum is between the two lines and the posterior mediastinum is behind the second line.

  The anterior mediastinum contains thymus, heart with pericardium, great vessels and occasionally, aberrant thyroid. Middle mediastinum contains trachea and oesophagun. Nerve roots and descending thoracic aorta are the main contents of posterior mediastinum. Normal lymph nodes and adipose tissue is seen in all divisions of mediastinum. Conventional PA and lateral views of the chest are the first radiological investigation in any suspected mediastinal abnormality. However, a lesion may not be detected if it is not large enough to cause contour abnormality in the lung-mediastinum intrephase.

     In neonates and young children the normal thymus is seen as a triangular sail shaped structure with well-defined borders, sometimes wavy in outline. Its borders project from one or both sides of the mediastinum.

Mediastinal Lines and Interfaces

     As the two lungs approximate anteriorly, four layers of pleura and anterior mediastinum separate them forming a septum called as anterior junctional line. On PA film this line is oriented from upper right to lower left of the sternum. Similarly, posterior junctional line is produced by the posterior approximation of the lungs behind the oesophagus and anterior to spine. On PA film, the postrior junctional line usually projects through the air columm of trachea. Adjacent to the vertebral bodies runs the para spinal lines. Azygoesophageal recess is formed by contact of right lower lobe with esophagus and azygos vein. The recess is frequently identified on a well-penetrated PA film as an interface that extends from the diaphragm below to the azygos arch above. Typically, it is seen as a continuous arch concave to the right. It may be straight in young adults.²⁸ The paraspinal lines are usually 1 to 2 mm wide on PA film.

Heart

   Normally two thirds of the cardiac shadow lies to the left of the midline and one-third to the right. In normal individuals the transverse diameter of the heart on PA film is usually in the range of 11.5 to 15.5 cm. It is less than 11.5 cm in about 5 percent of people and only rarely exceeds 15.5 cm in heavy, stocky individuals. Assessment of cardiac size by determining cardiothoracic ratio is more usefil. Cardiothoracic ratio of 50 percent is accepted widely as the upper limit of normal, however, it exceeds 50 percent in at last 10 percent of normal individuals.²⁹ The cardiothoracic ratio may be upto 60 percent in neonates.³⁰

Diaphragm

   In most individuals it has a smooth dome shape. The peak or the hoghest point of the dome is medial. Flattening of the dome can be measured on PA view by dropping a perpendicular from mid point of the dome to the line connecting costophrenic and cardiophrenic angles of the same side. The distance is normally greater than 1.5 cm. In approximately 90 percent of normal individuals, the right hemidiaphragm is 1.5 to 2.5 cm higher than the left. In rest, either the domes are at the same level. The discrepancy in the level of the diaphragms is related to the position of the cardiac apex and not to the position of the liver. A  difference greater than 3 cm in the levels of the two hemidiaphragms is significant.

CONCLUSION

   Chest radiography still remains the first investigation in the diagnosis of various chest diseases. Knowledge of normal anatomy has utmost importance in proper diagnosis of disease process on chest X-ray. Conventional radiobraph may have technical limitation in some situations like critically ill patients in ICU; however, recent advances in electronics and computer technology have resulted in development of digital imaging which improves diagnostic quality of chest imaging.