1

What are the principles of chest radiography (Figure 1)?

A chest radiograph (CXR) is obtained when X-rays pass from the source (the X-ray machine), through the patient, to reach a photo-stimulable phosphur plate. Frontal radiographs are obtained in the following projections:

a)

posteroanterior (PA) erect view – where the X-ray beam passes from posterior through the patient to the plate that is placed anteriorly. It is usually obtained in the radiology department with the patient standing and the X-ray tube is aimed horizontally, at a distance of 1.8m from the film;

b)

anteroposterior (AP) erect view – where the X-ray beam passes from anterior to posterior and the X-ray source is closer to the film, at a distance of about 1m. Depending on the position of the patient, the X-ray tube is angled so that the beam is approximately perpendicular to the chest. All portable films are taken in an AP projection;

c)

anteroposterior (AP) supine view – which is taken if the patient is too unwell to sit.

There are fundamental differences between PA and AP views, including:

a)

AP views magnify the anterior structures, so that the heart and hila appear enlarged and less sharp;

b)

the scapulae are visible on AP views, thus obscuring pathology in the upper lobes;

c)

portable AP radiographs emit lower energy and, hence, the X-ray beam is less able to penetrate and define dense structures, including the mediastinum, bones and the area behind the heart;

d)

AP supine views demonstrate congested lungs due to vascular redistribution in the recumbent position.

Lateral CXRs are not obtained routinely but may be performed to clarify the presence or absence of a possible abnormality on the frontal view and are also useful for confirming the position of permanent pacemaker wires.

Lateral CXRs are described according to the hemithorax closest to the film. Hence, a left lateral projection (Figure 2) represents the X-ray beam passing from the right to the left and the left hemithorax being closest to the film.

Bearing in mind the problems with magnification described above, the side showing possible pathology should always be positioned closest to the film on a lateral CXR.

2

What are the important technical features to assess on a chest radiograph?

Correct patient – which should be cross-checked by at least three identifiers, including patient name, date of birth and medical record number. The correct date of the examination must also be confirmed.

Annotation with a side marker – which should be done when the CXR is taken. The cardiac apex and aortic arch can usually be used to confirm the left-hand side of the patient but is not always the case, such as in patients with dextrocardia, mesocardia or a right-sided aortic arch.

Complete coverage – which should include the lungs and chest wall in their entirety, including the apices and costophrenic angles.

Depth of inspiration (Figure 3) – which, if adequate, should demonstrate 5-7 anterior ribs or 10 posterior ribs projected above the dome of the diaphragm. Expiratory radiographs can cause apparent cardiomegaly and pulmonary congestion, and obscure more of the lower lobes by the diaphragm.

Non-rotated film (Figure 4) – where the patient is properly centred and the clavicles are equidistant from the spinous processes. Rotated films give the appearance of differential lucency between the hemithoraces, which may simulate pathology, such as pleural effusions. A rotated film also distorts the mediastinal and hilar contours.

Depth of penetration – which is assessed by ensuring that the thoracic vertebrae are visible to the level of T5. An underpenetrated film causes the mediastinum to appear too white and may obscure pathology behind the heart and diaphragm, whereas an overpenetrated film causes the lungs to appear too black and may obscure lung pathology and pneumothoraces.

Absence of artifacts (Figure 5) – including clothing, jewellery and long hair.

Projection – which is usually a standard departmental PA film, unless otherwise stated as portable or supine, which are AP projections.

3

What are the important anatomical features to assess on a frontal chest radiograph?

Airway:

a)

trachea – which should be assessed for filling defects that may indicate a tumour;

b)

carina – which lies at approximately the level of the T5 vertebra;

c)

main bronchi – where the left is longer and more horizontal than the right.

Breathing – lung fields (Figure 6):

a)

right upper lobe (RUL) – which extends from the lung apex and is bounded by the horizontal fissure anteriorly, oblique fissure posteriorly and superior mediastinum medially;

b)

right middle lobe (RML) – which is small, triangular in shape, lies anteriorly and abuts the right heart border. It is bounded superiorly by the horizontal fissure and posteriorly by the oblique fissure;

c)

left upper lobe (LUL) – which extends from the lung apex to the diaphragm anteriorly;

d)

lingula – which can be thought of as the left-sided equivalent of the middle lobe, lying anteriorly and abutting the left heart border;

e)

right and left lower lobes (RLL/LLL) – which lie behind the oblique fissure and extend from the level of T5. They are bounded by the diaphragm inferiorly. The lower lobe is superimposed over both the upper lobe and middle lobe or lingula on a frontal radiograph.

Circulation – including the mediastinum and hila (Figure 7):

a)

right-sided vascular structures – which are venous and include the right brachiocephalic vein (RBCV), superior vena cava (SVC), right atrium (RA) and inferior vena cava (IVC);

b)

azygos vein – which can be seen as an oval structure in the right tracheobronchial angle and should be no greater than 10mm in diameter;

c)

left-sided mediastinal contours – which are formed by arterial structures and cardiac chambers, including the left subclavian artery (LSA), aortic arch and left ventricle (LV);

d)

left atrial appendage (LAA) – which forms part of the left-sided mediastinal contours only if it is enlarged and lies below the level of the left main bronchus;

e)

aortopulmonary (AP) window – which represents the indentation between the aortic arch and pulmonary artery. A convex AP window profile would suggest lymphadenopathy;

f)

main pulmonary artery – which lies above the left main bronchus.

g)

hila (Figure 8) – which should both be of similar density. The left hilum is usually higher than the right and should not be lower; it may be at the same level. Change in the position of the hila may indicate lobar collapse and an increase in density may indicate an overlying mass;

h)

right paratracheal stripe (Figure 9) – which is a vertical line that extends from the level of the clavicles to the azygos vein and should measure no more than 4mm in thickness. If widened, it may indicate lymphadenopathy.

Diaphragm – where gas under the diaphragm, as well as the position of the two hemidiaphragms, should be identified. The dome of the right hemidiaphragm normally intersects the anterior 6th rib and is slightly higher in women and older subjects. In normal patients, the left hemidiaphragm is only very rarely higher than the right, and then usually only by 1cm. If the dome of the right hemidiaphragm lies at or below the right anterior 7th rib, this may indicate hyperinflation. If there is associated diaphragmatic flattening, this is much more specific for airflow obstruction (Figure 10).
Eventration of the diaphragm is due to incomplete muscularisation. It most commonly occurs in the anteromedial portion of the right hemidiaphragm, which is seen as a smooth hump. Total eventration may occur and is more common on the left, with elevation of the whole diaphragm, and mimics diaphragmatic paralysis.

External structures – which includes the bony skeleton and should be inspected for fractures and for abnormal lucent or sclerotic areas that may indicate bone metastases:

a)

ribs – where at least ten posterior and six anterior ribs should be easily identified projected over the lungs on a well-inspired frontal radiograph. The lower ribs are projected over the upper abdomen and are not always well visualised;

b)

clavicles – which should be visualised in their entirety, along with part or all of the shoulder girdle;

c)

vertebrae – which should be visualised to at least the level of T5 on a well-penetrated frontal radiograph;

d)

sternum – which is poorly visualised on a frontal radiograph, although occasionally the manubrium is visible.

There are several normal variants which may simulate pathology, including:

a)

subcostal line – which is a fine line that is sometimes seen below the ribs posteriorly (Figure 11) and should not be mistaken for a pneumothorax, as lung markings are visible distal to it;

b)

costal cartilage calcification – which is a common finding, particularly with increasing age. In females, it tends to occur centrally, adjacent to the sternum, whereas in males, it occurs peripherally, adjacent to the costochondral junctions. The first costochondral junctions may calcify asymmetrically or develop osteophytes, which may simulate a pulmonary nodule. A repeat radiograph, using a different projection (such as AP or an apical view) can help to distinguish costochondral calcification from pulmonary pathology (Figure 12);

c)

pectus excavatum – which may simulate the appearance of middle lobe consolidation. There are, however, characteristic associated features of pectus excavatum on a frontal radiograph (Figure 13), including:

i)

horizontal orientation of the posterior ribs;

ii)

vertical orientation of the anterior ribs;

iii)

leftward displacement of the heart with straightening of the left heart border;

iv)

loss of clarity of the right heart border, simulating middle lobe consolidation.

Review areas, which are regions where pathology is commonly missed, including:

a)

lung apices (Figure 14);

b)

hila;

c)

area behind the heart;

d)

costophrenic angles – where blunting usually indicates a pleural effusion or pleural thickening;

e)

beneath the diaphragm – where lung nodules in the lower lobes may lie below the level of the diaphragmatic dome (Figure 15).

4

What are the important anatomical features to assess on a lateral chest radiograph?

Airway:

a)

trachea – which is air-filled and hence can be visualised throughout its length. The carina, however, cannot be seen on a lateral CXR;

b)

posterior tracheal stripe – which is formed by the posterior wall of the trachea and the anterior wall of the oesophagus. It is seen in >50% of adults and can be of variable thickness, as the oesophagus can either be filled with air or collapsed. The thickness of the stripe should therefore be interpreted with caution;

c)

retrotracheal space (Figure 16) – which can be difficult to spot on a frontal radiograph. On a lateral CXR, the retrotracheal space is bounded by the:

i)

posterior wall of the trachea (anteriorly);

ii)

anterior surface of the spine (posteriorly);

iii)

thoracic inlet (superiorly);

iv)

aortic arch (inferiorly).

Breathing:

a)

lungs – where a lateral CXR can help to determine the location of a mass identified on a frontal CXR. The lungs overlying the spine should become progressively darker towards the lung bases. Masses in this region may be missed on a frontal radiograph (Figure 17);

i)

oblique fissure – which separates the upper lobe (and middle lobe on the right) from the lower lobe. It runs obliquely downward from posterior to anterior, approximately at the level of T5 to just behind the anterior costophrenic angle (Figure 18);

ii)

horizontal fissure – which separates the upper and middle lobe on the right. Its position is variable, intersecting the lateral chest wall between the anterior portion of the 2nd to 6th ribs, most usually at the level of the 4th anterior rib;

b)

retrosternal space – which is the area between the sternum and anterior cardiac surface (Figure 16). A normal sized heart is only in contact with the lower third of the sternum, whereas an enlarged right ventricle or mediastinal mass may obliterate this space;

c)

retrocardiac space – which is bordered by the posterior aspect of the heart and the anterior aspect of the spine.

Circulation – where the left and right pulmonary arteries can be identified in close proximity to the left upper lobe bronchus (Figure 19). These shadows become enlarged when there is hilar lymphadenopathy.

Diaphragm – where the hemidiaphragm closest to the film is less magnified. The left hemidiaphragm is usually lower than the right, silhouetted anteriorly by the heart and can be identified by the presence of the stomach bubble (Figure 18).

External structures – including:

a)

sternum;

b)

vertebrae;

c)

ribs – where the ribs furthest away from the film are magnified.

5

What are the chest radiographic features of a pleural effusion?

The appearance of a pleural effusion depends on the position of the patient (erect versus supine) and on whether the effusion is free-flowing or not.

Erect patient, free-flowing fluid (Figure 20):

a)

uniformly increased density in the lower zone, which obscures the diaphragm and lateral costophrenic angle;

b)

meniscus-like density extending for a variable distance cranially, often with a hazy increased opacity above the meniscus, due to fluid tracking anteriorly and posteriorly;

c)

fluid tracking along the fissures:

i)

fluid in the oblique fissure creates a hazy curvilinear interface laterally;

ii)

fluid in the horizontal fissure is seen as a thin triangular opacity on the frontal radiograph;

d)

contralateral shift when the effusion is large (Figure 21). If the mediastinum is central in the presence of a large pleural effusion, underlying lung collapse (usually secondary to a proximal bronchogenic carcinoma) should be suspected. The mediastinum is often frozen in a pleural effusion secondary to mesothelioma but associated rib crowding may provide a clue as to the underlying diagnosis (Figure 22).

Supine patient, free-flowing effusion (Figure 23):

a)

diffuse increased opacification of the hemithorax, which may only affect the lower zones when small;

b)

lack of clarity of the hemidiaphragm;

c)

blunting of the costophrenic angle;

d)

apical cap;

e)

widening of the paraspinal lines and thickening of the horizontal fissure.