Chest XRay, chest radiograph, cardiothoracic ratio, survey of lung fields, bones and soft tissues, Rib notching, mediastinum, heart, cardiovascular silhouette, Interstitial opacities, Pulmonary nodules, septal lines, reticular pattern

Methods of imaging the respiratory tree, associated structures and regional anatomy include:

• plain PA chest radiogram and lateral radiogram
• chest radiography with fluoroscopic screening
• tomographic radiography
• computerised axial tomography (CAT) scanning
• magnetic resonance imaging (NMR) scanning
• pulmonary angiography or ventilation / perfusion scanning
• bronchography
• mediastinoscopy
• thoracoscopy

A chest radiograph should be examined in a systematic way:

• check the name on the film
• make an estimate of age and sex of patient if this information is not provided
• check the projection which should be posterior-anterior - PA
• the whole of the chest should be visible on the film, check for rotation and that there has been satsifactory penetration
• check chest expansion and the diaphragm - the right hemidiaphragm is 2 cm higher than the left
• check for normal lung markings - the horizontal fissure is visible in 60% of normal chest X-rays; it runs from the centre of the right hilum, laterally and horizontally, to meet the sixth rib in the mid-axilla.

The following are described in more detail in subsequent nodes:

• check the mediastinum
• assess the cardiothoracic ratio (only on the PA film)
• look at the pulmonary vessels
• lung fields
• bones
• soft tissues

If the chest radiograph appears normal then the following should be checked for:

• a mastectomy
• a small apical pneumothorax
• a fluid level below the heart, due to a hiatus hernia
• collapse of the right middle lobe and loss of clarity of the right heart border
• absence of the outline of the left diaphragm behind the heart (sail sign) as a result of left lower lobe collapse
• deviation of the trachea
• paratracheal lymphadenopathy
• air beneath the diaphragm
• dextrocardia with the film reversed
• always check the apices, hila, behind the heart and the costophrenic angles

cardiothoracic ratio

This is the transverse cardiac diameter (the horizontal distance between the most rightward and leftward borders of the heart seen on a postero-anterior (PA) chest radiograph) divided by the transverse chest diameter (measured from the inside rib margin at the widest point above the costophrenic angles on a PA chest film).

A cardiothoracic ratio of more than 50% is considered abnormal in an adult; more than 66% in a neonate. The cardiac diameter itself can also be measured and, in normal individuals, is less than 15.5 cm in males, and less than 14.5 cm in females. A change in diameter of greater than 1.5 cm between two X-rays is significant.

Possible causes of a ratio greater than 50% include:

• cardiac failure
• pericardial effusion
• left or right ventricular hypertrophy

survey of lung fields, bones and soft tissues

Lung fields:

• identify any obvious abnormality
• if none is seen then examine the lung fields systematically:
• compare both sides
• note the lung markings on the two sides and follow them to the lung edge
• look at the apices and the costophrenic angles

Bones:

• look for erosion, notching, fractures

Soft tissues:

• look for surgical emphysema, air in the soft tissues, mastectomy

Rib notching is a radiological finding, causes of which include:

• coarctation of the aorta
• neurofibromatosis secondary to hypertrophic nerves
• hypertrophic polyneuropathy

the mediastinum

Check that the:

• tracheal shadow is central
• mediastinum is of the normal width
• left hilum is at the same height or above (up to 2 cm) the right hilum

the heart

The heart and great vessels are apparent on a chest film because they are filled with blood that is relatively radiodense in comparison to the air-filled lungs which lie laterally. Standardly, posteroanterior (PA) films are taken so as to reduce artefactual magnification of the heart due to divergence of the x-rays.

On a PA film, the heart and great vessel borders are termed the cardiovascular silhouette. This should be inspected systematically for the presence and size of the expected structures - see submenu.

The sidedness of the aortic arch can be determined - even in a slightly rotated film - by looking which side of the trachea it lies. A right sided aortic arch would strongly suggest congential cyanotic heart disease.

The ventricles become rounded as they enlarge.

Examination of the lung vasculature of the lungs is important in analysis of the heart.

cardiovascular silhouette 

The cardiovascular silhouette is the radiodensity apparent on a standard PA chest radiograph due to the contrast of the blood-filled heart and great vessels against the surrounding air-filled lungs.

It has prominences on each border which occur in a regular sequence and represent cardiovascular structures:

• right border, from superior to inferior:
• superior vena cava
• right atrium
• inferior vena cava
• left border, from superior to inferior:
• arch of aorta - this prominence is termed the aortic knob
• pulmonary trunk
• left auricle
• left ventricle

Inferiorly, the heart shadow merges with that of the diaphragm.

The overall shape of the cardiovascular silhouette is dependent on the phase of breathing due to the attachments of fibrous pericardium to the diaphragm inferiorly. The pericardium is relatively fixed by the great vessels to the roots of the lungs superiorly, so descent of the diaphragm during inspiration tends to make the silhouette look thinner and longer. Hence, standardly chest films are taken in inspiration. Conversely, the normal variation within the population of the shape of the heart results both in extremes of very thin and broad silhouettes. The former are more common in the obese and pregnant ladies; the latter are more common in thin individuals.

Lateral radiographs reveal a radiolucent space posterior to the heart and anterior to the vertebral column; this is the retrocardiac space. It contains the oesophagus and descending aorta.

Interstitial opacities include:

• pulmonary masses:
• >3 cm diameter

• pulmonary nodules:
• usually 5-10 mm diameter

• miliary nodules:
• 1-2 mm diameter

• septal lines

• honeycomb shadowing and interstitial patterning

Pulmonary masses are interstitial opacities of greater than 3 cm diameter on the chest radiograph.

The causes of multiple pulmonary masses include:

• neoplasia:
• bronchial carcinoma
• lymphoma
• metastases
• hamartoma

• granuloma:
• tuberculosis
• fungal lung disease
• rheumatoid lung
• Wegener's granulomatosis

• abscess

• congenital:
• sequestered segment
• bronchogenic cyst
• arteriovenous malformation

• miscellaneous:
• haematoma
• infarct
• progressive massive fibrosis
• amyloidosis
• hydatid cyst

Pulmonary nodules are interstitial opacities on the chest radiograph with a diameter of 5-10 mm.

The causes of multiple pulmonary nodules are:

• metastatic cancer
• sarcoidosis
• silicosis
• tuberculosis
• fungal lung infections
• chichenpox pneumonia
• abscesses
• histiocytosis X
• arteriovenous malformations
• mitral stenosis

miliary shadowing

Causes include:

• miliary TB
• sarcoidosis
• pulmonary oedema

• pulmonary haemosiderosis:
• idiopathic
• due to mitral stenosis

• fibrosing lung disease:
• rheumatoid lung
• fibrosing alveolitis
• systemic sclerosis
• coalworker's pneumoconiosis
• silicosis

• hypersensitivity:
• extrinsic allergic alveolitis

• neoplasm:
• disseminated cancer metastases
• alveolar cell carcinoma
• lymphangitis carcinomatosa

• post-viral pneumonia with miliary calcification:
• especially varicella

septal lines

These are abnormal linear opacities seen on the chest radiograph:

• Kerley B lines
• Kerley A lines

Kerley B lines are linear opacities seen on the chest radiograph. They are 1-2 cm long horizontal lines which meet the pleura at right angles. They are typically seen as a ladder up the side of the lungs beginning at the costophrenic angle.

Kerley B lines represent interlobular lymphatics which have been distended by fluid or tissue.

Kerley B lines are usually an indication of raised pulmonary venous pressure due to:

• left ventricular failure
• mitral stenosis

Other causes include:

• lymphangitis carcinomatosa
• pulmonary fibrosis
• parasitic infection

Kerley A lines have the same significance as Kerley B lines but are less commonly seen on the chest radiograph.

Kerley A lines are seen in the middle and upper zones as long unbranching lines passing from the periphery towards the hilum.

honeycomb lung

This term is used to describe a group of conditions that cause widespread cystic or honeycomb appearances on chest radiology.

Causes include:

• commonly, end-stage pulmonary fibrosis of any cause
• neurofibromatosis
• tuberous sclerosis
• Hand-Schuller-Christian disease
• eosinophilic granuloma

There is a tendency for patients with honeycomb lung to develop pneumothoraces.

reticular pattern

Causes include:

• pulmonary fibrosis
• lymphangitis carcinomatosa

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