Technical Aspects of Chest Radiography

Technical aspects of Chest Radiography:

Centring: 

If the film is well centred, the medial end of the clavicles are equidistant from the vertebral spinous processes at the T4/5 level. Small degrees of rotation distort the mediastinal borders, and the lung nearest the film appears less translucent. Thoracic deformities, especially a scoliosis negate the value of conventional centering. The orientation of the aortic arch, gastric bubble and heart should be determined to confirm normal situs and that the side markers are correct.

Suggested scheme for viewing the PA film:

1.       Request form : Name, age, date, sex, clinical information

2.       Technical : Adequate inspiration, centring, patient position/rotation, side markers, exposure/adequate penetration, collimation

3.       Trachea : Position, outline

4.       Heart and mediastinum : Size, shape, displacement

5.       Diaphragms : Outline, shape, relative position

6.       Pleural spaces : Position of horizontal fissure, costophrenic and cardiophrenic angles

7.       Lungs : Local, generalized abnormality, comparison of the translucency and vascular markings of the lungs

8.       Hidden areas : Apices, posterior sulcus, mediastinum, hila, bones

9.       Hila : Density, position, shape

10.   Below diaphragms: Gas shadoes, calcification

11.   Soft tissues : Mastectomy, gas, densities, etc.

12.   Bones : Destructive lesions etc.

Penetration:

With a low KV film the vertebral bodies and disc spaces should be just visible down to the T8/9 level through the cardiac shadow. Underpenetration increases the likelihood of missing an abnormality overlain by another structure. Overpenetration results in loss of visibility of a low density lesions such as early consolidation, although a bright light may reveal the abnormality.

Degree of inspiration: On full inspiration, the anterior ends of the diaphragm although the degree of inspiration achieved varies with patient build. On expiration the heart shadows is larger and there is basal opacity due to crowding of the normal vascular markings. Pulmonary diseases such as fibrosing alveolitis are associated with reduced pulmonary compliance, which may result in reduced inflation with elevation of the diaphragms.

The trachea:

The trachea should be examined for narrowing, displacement and intraluminal lesions. It is midline in its upper one part, then deviates slightly to the right around the aortic knuckle. On expiration, deviation to the right becomes marked. In addition there is shorerring on expiration so that an endotracheal tube situated just above the carina on inspiration may occlude the main bronchus on expiration.

Its caliber should be even, with translucency of the tracheal air column decreasing caudally. Normal maximum coronal diameter is 25 mm for males and 21 mm for females. The right tracheal margin where the trachea is in contact with the lung, can be traced from the clavicles down to the right main bronchus. This border is the right paratracheal stripe and is seen in 60% of patients, normally measuring less than 5 mm. widening of the stripe occurs most commonly with mediastinal lymphadenopathy but also with tracheal malignancy, mediastinal tumours, mediastinitis and pleural effusions. A left paratracheal line is not visualized because the left border of the trachea lies adjacent to the great vessels and not the lung.

The azygos vein lies in the angle between the right main bronchus and trachea. On the erect film it should be less than 10 mm in diameter. Its size decreases with the Valsalva manoeuvre and on inspiration. Enlargement occurs in the supine position but also with enlarged subcarinal nodes, pregnancy, portal hypertension, IVC, and SVC obstruction, right heart failure and constrictive pericarditis.

Widening of the carina occurs on inspiration. The normal angle is 60-75⁰. Pathological causes of widening include an enlarged left atrium and enlarged carinal nodes.

The mediastinum and heart:

The central dense shadow seen on the PA chest film comprises the mediastinum, heart, spine and sternum. With good centering, two thirds of the cardiac shadow lies to the left midline and one-third to the right, although this is quite variable in normal subjects. The transverse cardiac diameter (normal for females less than 14.5 cm and for males less than 15.5 cm) and the cardiothoracic ratio are assessed. The normal cardiothoracic ratio is less than 50% on a PA film. Measurement in isolation is of less value than when previous figures are available. An increase in excess of 1.5 cm in the transverse diameter on comparable serial films is significant. However, the heart shadow is enlarged with a short FFD, on expiration, in the supine and AP projections and when the diaphragms are elevated. The normal AP value is less than 60%.

All borders of the heart and mediastinum are clearly defined except where the heart sits on the left hemidiaphragm. The right superior mediastinal shadow is formed by the SVC and innominate vessels, a dilated aorta may contribute to this border. On the left side the superior mediastinal border is less sharp. It is formed by the subclavian artery above the aortic knuckle.

Various junction lines may be visualized. These are formed by the pleura being outlined by the adjacent air-filled lung. The anterior junction line is formed by the lungs meeting anterior to the ascending aorta. It is only 1 mm thick and overlying the tracheal transluceny, runs downward from below the suprasternal notch, slightly curving from right to left. The posterior junction line, where the lungs meet posteriorly behind the oesophagus, is a straight or curved line convex to the left some 2 mm wide and extending from the lung apices to the aortic knuckle or below. The azygo-oesophageal interface is the shape of an inverted hockey stick and runs from the diaphragm on the left midline up and to the right extending to the tracheobronchial angle where the aygos vein drains into the IVC. The curved pleuro-oesophageal stripe, formed by the lung and right wall of the oesophagus, extends from the lung apex to the azygos but is only visualized if the oesophagus contains air. The left wall of the oesophagus is not normally seen.

In young women, the pulmonary trunk is frequently very prominent.

In babies and young children, the normal thymus is a triangular sail-shaped structure with well-defined borders projecting from one or both sides of the mediastinum. Both borders may be wavy in outline, the ‘wave sign of Mulvey’, as a consequence of indentation by the costal cartilages. The right border is straighter than the left, which may be rounded. Thymic size decreases on inspiration and in response to stress and illness. The thymus is absent in DiGeorge’s syndrome. Enlargement may occur following recovery from an illness. A large thymus is more commonly seen in boys.

Adjacent to the vertebral bodies run the paraspinal lines. On the left this is normally less than 10 mm wide; on the right less than 3 mm. The left paraspinal line is wider due to the descending thoracic aorta. Enlargement  occurs with osteophytes, a tortuous aorta, vertebral, and adjacent soft-tissue masses, a paravertebral haematoma, and a dilated azygous system.

A search should be made for abnormal densities, fluid levels, mediastinal emphysema and calcification. Spinal abnormalities may accompany mediastinal masses; for example, hemivertebrae are associated with neuroenteric cysts.

The diaphragm:

In most patients the right hemidiaphragm is higher than the left. This is due to the heart depressing the left side and not to the liver pushing up the right hemidiaphragm: in dextrocardia with normal abdominal situs the right hemidiaphragm is the lowest. The hemidiaphragms may lie at the same level, and in a small percentage of the population the left side is the higher: Felson (1973) reports an incidence of 3%. This is more likely to occur if the stomach or splenic flexure is distended with gas. A difference greater than 3 cm in height is considered significant.

On inspiration the domes of the diaphragms are at the level of the sixth rib anteriorly and at or below the tenth rib posteriorly. In the supine position, the diaphragm is higher. Both domes have gently curbes which steepen toward the posterior angles. The upper borders are clearly seen except on the left side where the heart is in contact with the diaphragm, and in the cardiophrenic angles when there are prominent fat pads. Otherwise loss of outline indicates that the adjacent tissue does not contain air, for example in consolidation or pleural disease.

Free intraperitoneal gas outlines the undersurface of the diaphragm and shows it to be normally 2-3 mm thick.

The fissures: 

The main fissures: 

These fissures separate the lobes of the lung but are usually incomplete allowing collateral air drift to occur between adjacent lobes. They are visualized when the x-ray beam is tangential. The horizontal fissure is seen, often incompletely on the PA film running from the hilum to the region of the sixth rib in the axillary line, and may be straight or have a slight downward curve. Occasionally it has a double appearance.

All fissures are clearly seen on the lateral film. The horizontal fissure runs anteriorly and often slightly downward. Both oblique fissures commence posteriorly at the level of T4 or T5, passing through the hilum. The left is steeper and finishes 5 cm behind the anterior costophrenic  angle, whereas the right ends just behind the angle.

Accessory fissures:

The azygos fissure is comma shaped with a triangular base peripherally and is nearly always right sided. It forms in the apex of the lung and consists of paired folds of parietal and visceral pleura plus the azygos vein which has failed to migrate normally. Enlargement occurs in the supine position. At postmortem the incidence is 1% but radiologically it is 0.4%.When left sided, the fissure contains an accessory hemi-azygos vein.

The superior accessory fissure separates the apical from the basal segments of the lower lobes. It is commoner on the right side and has an incidence of 5% at postmortem. On the PA film it resembles the horizontal fissure but on the lateral film it can be differentiated as it runs posteriorly from the hilum.

The inferior accessory fissure appears as an oblique line running cranially from the cardiophrenic angle toward the hilum and separating the medial basal from the other basal segments. It is commoner on the right side and has an incidence of 5-8% on the chest film.

The left sided horizontal fissure separates the lingual from the other upper lobe segments. This is rare but in one study was found in 8% of postmortem specimens.

The costophrenic angles:

The normal costophrenic  angles are acute and well-defined but become obliterated when the diaphragms are flat. Frequently the cardiophrenic angles contain low-density ill-defined opacity caused by fat pads.

The lungs:

By comparing the lungs, areas of abnormal translucency or uneven distribution of lung markings are more easily detected. The size of the upper and lower zone vessels is assessed.

An abnormal opacity should be closely studied to ensure that it is not a composite opacity formed by superimposed normal structures such as vessels, bones or costal cartilage. The extent and location of the opacity is determined and specific features such as calcification or cavitation noted. A general survey is made to look for further. A general survey is made to look for further lesions and displacement of the normal landmarks.

The hidden areas:

The apices:

On the PA film the apices arc partially obscured by ribs, costal cartilage, clavicles and soft tissues. Visualization is very limited on the lateral view.

Mediastinum and hila:

Central lesions may be obscured by these structures or appear as a superimposed density. The abnormality is usually detectable on the lateral film.

Diaphragms:

The posterior and lateral basal segments of the lower lobes and the posterior sulcus are partially obscured by the downward curve of the posterior diaphragm. Visualization is further diminished if the film is not taken on full inspiration.

Bones:

Costal cartilage or bone may obscure a lung lesion. In addition, determining whether a density is pulmonary or bony when overlying a rib may be difficult; AP, expiratory and oblique films may be helpful and preclude the need to proceed to CT.

The hila:

In 97% of subjects the left hilum is higher than the right and in 3% they are at the same level. The hila should be of equal density and similar size with clearly defined concave lateral borders where the superior pulmonary vein meets the basal pulmonary artery.  However, there is a wide range of normal appearances. Any opacity which is not obviously vascular must be regarded with a high index of suspicion and investigated further. Old films for comparison are helpful in this situation.

Of all the structures in the hilum only the pulmonary arteries and upper lobe veins contribute significantly to the hilar shadows on the plain radiograph. Normal lymph nodes are not seen. Air can be identified within the proximal bronchi but normal bronchial wall are only seen end on. The anterior segment bronchus of the upper lobe is seen as a ring adjacent to the upper hilum and is seen on the right side in 45% of cases and the left side in 50%. Normally, there is less than 5 mm of soft tissue lateral to this bronchus. Thickening of the soft tissues suggests the presence of abnormal malignancy such as malignancy.

The inferior pulmonary ligament:

This is a double layer of pleura extending caudally from the lower margin of the inferior pulmonary vein in the hilum as a sheet which may or may not be attached to the diaphragm and which attaches the lower lobe to the mediastinum. It is rarely identified on a simple radiograph but is frequently seen at CT.

The pulmonary vessels:

The left pulmonary artery lies above the left main bronchus before passing posteriorly, whereas on the right side the artery is anterior to the bronchus resulting in the right hilum being the lower. Hilar size is very variable. The maximum diameter of the descending branch of the pulmonary artery measured 1 cm medial and 1 cm lateral to the hilar point is 16 mm for males and 15 mm for females.

The upper lobe veins lies lateral to the arteries, which are separated from the mediastinum by approximately 1 cm of lung tissue. At the first intercostal space the normal vessels should not exceed 3 mm in diameter. The lower lobe vessles are larger than those of the upper lobes in the erect position, perfusion and aeration of the upper zones being reduced. In the supine position, the vessels equalize. In the right paracardiac region, the vessels are invariably prominent.

The peripheral lung markings are mainly vascular, veins and arteries having no distinguishing characteristics. There should be an even distribution throughout the lung fields.

Centrally the arteries and veins have different features. The arteries accompany the bronchi, lying posterosuperior, whereas veins do not follow the bronchi but drain via the interlobular septa eventually forming superior and basal veins which converge on the left atrium.

This confluence of veins may be seen as a rounded structure to the right of midline superimposed on the heart, sometimes, simulating an enlarged left atrium. It is visible in 5% of PA films according to Felson. Pulmonary veins have fewer branches than arteries and are straighter, larger and less well-defined.

The bronchial vessels:

These are normally not visualized on the plain chest film. They arise from the ventral surface of the descending aorta at the T5/6 level. Their anatomy is variable. Usually there are two branches on the left and one on the right which often shares a common origin with an intercostal artery. On entering the hila the bronchial arteries accompany the bronchi. The veins drain into the pulmonary veins and to a lesser extent the azygos system.

Enlarged bronchial arteries appear as multiple small nodules around the hilum and as short lines in the proximal lung fields. Enlargement may occur with cyanotic heart disease, and focal enlargement with a local pulmonary lesion. Occasionally enlarged arteries indent the oesophagus.

Causes of enlarged bronchial arteries:

a.       General-cyanotic congenital heart disease, e.g. pulmonary atresia, severe Fallot’s tetralogy

b.      Local bronchiectasis, bronchial carcinoma

The pulmonary segments and bronchi:

The pulmonary segments are served by segmental bronchi and arteries but unlike the lobes are not separated by pleura. Normal bronchi are not visualized in the peripheral lung fields.

The right main bronchus is shorter, steeper and wider than the left, bifurcating earlier. The upper lobe bronchus arises 2.5 cm below the carina and is higher than the left upper lobe bronchus which arises after 5 cm. The bronchi divide between six and 20 times before becoming bronchioles with the terminal bronchioles measuring  0.2 mm in diameter. Each receives two to three respiratory bronchioles which connect with between two and 11 alveolar ducts. Each duct receives between 2 to 6 alveolar sacs which are connected to alveoli. The acinus, generally considered to be the functioning lung unit, is that portion of the lung arising from the terminal bronchiole. When filled with fluid,  it is seen on a radiograph as a 5-6 mm shadow, and this comprises the basic unit seen in acinar (alveolar/air space) shadowing.

The primary lobule arises from the last respiratory bronchiole. The secondary lobule is between 1.0 and 2.5 cm in size and is the smallest discrete unit of lung tissue surrounded by connective tissue septa. When thickened these septa become Kerley B lines.

Other connections exist between the air spaces allowing collateral air drift. These are the pores of kohn, 3-13 µm in size, which connect the alveoli, and the canals of Lambery (30 µm) which exist between bronchioles and alveoli.

The lymphatic system:

The lymphatics remove interstitial fluid and foreign particles. They run in the interlobular septa, connecting with subpleural lymphatics and draining via the deep lymphatics to the hilum, with valves controlling the direction of flow. Normal lymphatics are not seen but thickening of the lymphatics and surrounding connective tissue produces Kerley lines, which may be transient or persistent. Thickened connective tissues are the main contributors to the substance of these lines.

The lymph nodes:

The intrapulmonary lymphatics drain directly to the bronchopulmonary nodes and this group is the first to be involved by spread from a tumour. A small number of intrapulmonary nodes are present and can occasionally be seen at CT but never on the plain film. The node groups and their drainage are well described. Extensive intercommunications exist between the groups but the pattern of nodal involvement can sometimes indicate the site of the primary tumour. Mediastinal nodes may be involved by tumours both above and below the diaphragm.

1.       The anterior mediastinal nodes in the region of the aortic arch drain the thymus and right heart.

2.       The intrapulmonary nodes lie along the main bronchi.

3.       The middle mediastinal nodes drain the lungs, bronchi, left heart, the lower trachea and visceral pleura. There are four groups:

a.       Bronchopulmonary (hilar) nodes which enlarged appear as lobulated hilar masses.

b.      Carinal nodes

c.       Tracheobronchial nodes which lie adjacent to the azygos vein on the right side and near the recurrent laryngeal nerve on the left side.

d.      Paratracheal nodes are more numerous on the right side. There is significant cross drainage from left to right.

4.       The posterior mediastinal nodes drain the posterior diaphragm and lower oesophagus. They lie around the lower descending aorta and oesophagus

5.       The parietal nodes consist of anterior and posterior groups situated behind the sternum and posteriorly in the intercostal region draining the soft tissues and parietal pleura.

Below the diaphragm:

The lower lobes extend below the diaphragmatic outlines on the PA film. An erect chest film is preferred to an erect abdominal film for the diagnosis of a pneumoperitoneum. A search should be made for other abnormal gas shadows such as dilated bowel, abscesses, a displaced gastric bubble and intramural gas as well as calcified lesions. Interposition of colon between live and diaphragm Chilaiditi’s syndrome is a common and often transient finding particularly in the aged, the obvious haustral pattern distinguishing it from free gas. Subdiaphramatic fat in the obese may be confused with free gas on a single film.

Soft tissues:

A general survey of the soft tissues includes the chest wall, shoulders, and lower neck. It is important to confirm the presence or absence of breast shadows. The breat may partially obscure the lung bases. Nipple shadows are variable in position, often asymmetrical, and frequently only one shadow is seen. Care is necessary to avoid misinterpretation as a neoplasm or vice-versa. Nipple shadows are often well-defined laterally and may have a lucent halo. Repeat films with nipple markers are necessary if there is any doubt. Skin folds are often seen running vertically, particularly in the old and in babies. When overlying the lungs they can be confused with a pneumothorax. However, a skin fold if followed usually extends outside the lung field. The anterior axillary fold is a curving linear shadow extending from the axilla onto the lung fields and frequently causing ill-defined shadowing which must be differentiated from consolidation.

At the apices the opacity of the sternocleidomastoid muscles curving down and slightly outward may simuate a cavity or bulla. The floor of the supraclavicular fossa often resembles a fluid level. A deep sternoclavicular fossa, commonly present in the elderly, appears as a translucency overlying the trachea and simulating a gas-filled diverticulum.

Subpleural thickening seen peripherally is often due to subpleural fat or prominent intercostal muscles rather than to pleural pathology.

Companion shadows are formed by the soft tissues adjacent to bony structures, are 2-3 mm thick, and are frequently seen running parallel to the upper borders of the clavicles and the inferior borders of the lower ribs.

 

Apical pleural thickening, “the apical cap” has a reported incidence of 7% and occurs most commonly on the left side.

The bones:

All the bones should be surveyed. On occasions identification of an abnormality in association with pulmonary pathology may help to narrow the differential diagnosis. Sometimes a normal bony structure appears to be a lung lesion and further films such as oblique, lateral, inspiratory and expiratory or CT may be necessary.

The sternum:

The ossification centers are very variable in number, shape, position and growth rate. Usually there are single centers in the manubrium and xiphoid with three or four centers in the body. Parasternal ossicles and in infants, the ossification centers may be confused with lung masses.

The clavicles:

The rhomboid fossa is an irregular notch at the site of attachment of the costoclavicular ligament. It lies up to 3 cm from the medial end of the clavicle inferiorly and has a well-corticated margin. It is unilateral in 6% of cases and should not be mistaken for a destructive lesion. Superior companion shadows are a usual finding. The medial epiphyses fuse at 25 years and on occasions may appear as lung nodules.

The scapulae:

On the lateral film, the inferior angle overlies the lungs and can simulate a lung mass. The spine of the scapula on the PA film casts a linear shadow which at first glance may seem to be pleural.

The ribs:

Companion shadows are common on the upper ribs. Pathological rib notching as seen with aortic coarctation should not be confused with the normal notch on the inferior surface just lateral to the tubercle. The contours of the ribs are evaluated for destruction. However, the inferior borders of the middle and lower ribs are usually indistinct.

The first costal cartilage calcifies early and is often very dense, partly obscuring the upper zone. Costal cartilage calcification is rare before the age of 20. Central homogeneous or spotty calcification occurs in females whereas there is curvilinear marginal calcification in males. On the lateral film, the anterior end of the rib with its cartilage lying behind the sternum should not be confused with a mass.

The spine:

Routine evaluation is made for bone and disc destruction and spinal deformity. A scoliosis often results in apparent mediastinal widening and oblique films may be necessary to fully visualize both lung fields. The ends of the transverse processes on the PA film may look like a lung nodule.

In the neonate the vertebral bodies have a sandwich appearance due to large venous sinuses. Residual grooves may persist in the adult.

Viewing the lateral film:

Routinely the left side is adjacent to the film because more of the left lung than the right is obscured on the PA view, but if there is a specific lesion the side of interest is positioned adjacent to the film. A routine similar to that used for the PA film should be employed.

The clear spaces:

There are two clear spaces; these correspond to the sites where the lungs meet behind the sternum and the heart. Loss of translucency  of these areas indicate local pathology. Obliteration of the retrosternal space occurs with anteriormediastinal masses such as a thymomas, aneurysms of the ascending aorta and nodal masses. Normally this space is less than 3 cm deep maximum, widening occurs with emphysema.

Vertebral translucency:

The vertebral bodies become progressively more translucent caudally. Loss of this translucency may be the only sign of posterior basal consolidation.

Diaphragm outline:

Both diaphragms are visible through out their length except the left anteriorly where it merges with the heart. A small segment of the right hemidiaphragm is effaced by the IVC. The posterior costophrenic angles are acute and small amounts of pleural fluid may be detected by blunting of these angles.

The fissures:

The left greater fissure is steeper than the right and terminates 5 cm behind the anterior cardiophrenic angle. Loculated interlobar effusions are well shown and displacement or thickening of the fissures should be noted.

The trachea:

This passes down in a slightly posterior direction to the T6/7 level of the spine. It is partly overlapped by the scapulae and axillary folds. Anterior to the carina lies the right pulmonary artery. The left pulmonary artery is posterior and superior, and the veins are inferior. The venous confluence creates a bulge on the posterior cardiac border.

The normal posterior tracheal wall is invariably visible and measures less than 5 mm. this measurement includes both tracheal and oesophageal walls plus the pleura. Widening may occur with disease of all these structures. A branch of the aorta seen end-on may appear as a nodule overlying the trachea and above the aortic arch. The right upper lobe bronchus is seen end-on as a circular structure overlying the lower trachea. Lying inferiorly is the left upper lobe bronchus seen end-on with its artery superiorly and vein inferiorly.

Opacity seen in the region of the anterior cardiophrenic angle is thought to be due to mediastinalcut and the interface between the two lungs.

The sternum:

This should be studied carefully in known cases of malignancy or when there is a history of trauma.