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Interpretation of the Chest X- Ray

A System for looking at the film

1. Name and side marker

2. Film quality

Is an indication of how precise is the information on the film

Penetration A good film will allow you to localise one or two thoracic disc spaces.

Inspiration. Following good inspiration the diaphragm is normally at or near the level of the posterior 10th rib. The right diaphragm is usually higher than the left.

AP or PA, supine or erect The radiographer should mark how the film has been taken. AP films have the scapula and magnified heart projected over the lung fields and heart size cannot be assessed. Most portables and all supine films are AP. Supine films show less of the lung fields and the mediastinum appears wide: there is no point in trying to decide if a patient has an aortic dissection on a supine film. Interpretation of a film of a patient with a marked kyphosis suffers from similar difficulties.

Centring Allow for rotation before evaluating mediastinal and tracheal shift by examining the relationship between the medial ends of the clavicle and the posterior spinous process. The side to which the patient is rotated usually becomes more translucent (black). But there are exceptions and quite marked degrees of rotation are required to produce significant changes in radiographic density.

Fig 1

This shows a patient who is rotated (turned towards) her right. The medial ends of the clavicles and the spinous processes have been outlined in the lower picture with their apparent direction of movement.  The mediastinum and trachea being anterior structures will move in the same direction as the clavicles.

3. Bones

Start from outside i.e. humeri, shoulders and clavicles, any secondaries, fractures or arthritis. Is the spine straight?


a. posterior, remembering to look through the heart to where the ribs meet the spine.

b. lateral chest wall.

c. anterior end of each rib.

All the time compare the two sides. Many prefer to examine each rib as a whole but it is possible to miss one unless care is taken.

4. Soft Tissues

Briefly examine the muscles of the chest wall -any surgical emphysema?

Trachea central? (Can be slightly to the right in patients with a degree of aortic dilatation).

Mediastinum and hila - size and shape and position

Diaphragm and below - any significant discrepancy of height. Exclude pneumoperitoneum


5. lung fields

Compare the two sides by mentally dividing them into 6 parts i.e. rt. and left. upper mid and lower zones. Go back to the difficult areas: apices, costophrenic angles, hila and through the heart. There is a lot of lung behind the heart.

Note the position and thickness of the horizontal fissure which normally extends out from the centre of the right hilum.

6. Mediastnum

Cardiac shadow On a PA film the maximum width of the heart shadow compared with the maximum width of the combined lung fields is a good approximation to the CT ratio, normally below 50% it can be up to 60% in elderly patients. The true CT ratio is the sum of maximum distances from the centre of the heart shadow to each heart edge over the maximum external bony thoracic wall.

Cardiac shadow   fig 2

A 3D CT cardiac angiogram. The second representation shows the approximate outline of the cardiac shadow. The anterior portions of the ra and rv have been removed.

Svc -superior vena cava ra- right atrium. Rv- right ventricle. Vs-interventricular septum. pt- pulmonary trunk. Lpa- left pulmonary artery. Rpa- right pulmonary artery. a- aorta. aa- aortic arch. lv left ventricle. lpv- left pulmonary vein

  RH- right hilum. LH- left hilum. AK - aortic knuckle. PA- pulmonary artery

Assessment of chamber size is best done by echocardiography but the classic CXR signs of left atrial enlargement  are accurate (double right heart border, infilling of the concavity between the aortic knuckle and the pulmonary artery and splaying of the carina).

The Hila are central mediastinal structures and the hilum on the side to which rotation has occurred may become obscured by the heart shadow. Note the anatomy of the pulmonary vessels is such that the left hilum is the higher. If this is reversed or even if they are at the same level there is likely to be some collapse somewhere.


7 Diaphragm

Normally at the level of the 10th. posterior rib but the position is very variable and height is dependant on a number of factors including radiographic technique. It is a thin structure (3mm) and if it appears to be thicker than this then the gas delineating its lower surface is probably in gut, rather than due to a pneumoperitoneum.

8 Lung fields


Should be of similar density bilaterally. The basal arteries should look notably thicker and longer than those to the upper zones.  The horizontal fissure should be at the level of the RT hilum and is normally very thin. Bronchial walls, if seen at all, should be thin and restricted to the perihilar regions. Any measurable thickness indicates peribronchial thickening.

 There are Limitations to the chest X Ray. CT tells us that only gross cases of vascular redistribution, plethora, air trapping and bronchiectasis can be diagnosed with certainty on the chest X-ray. Low flat diaphragms may indicate emphysema but can also be produced by a massive breath in. Patients with pulmonary emboli can have normal chest radiographs. 

The lateral is sometimes helpful in the further localisation of pathology but has been largely superceded by CT.  It may identify small effusions or basal pneumonia not seen on frontal CXRs. Posteriorly the lung is darkest just above the diaphragm. If not then there is probably some basal pathology, either an effusion or posterior consolidation. The shadows of the humeri, glenoids and scapulae can occasionally be misinterpreted. Look at the diaphragms. Note that you can see almost the whole length of the right one but not the anterior end of the left where it comes up against the heart. 

Fig3 Normal lateral

1. oblique fissure 2. humerus   3. soft tissue of arm 4. scapulae (anterior borders) 5. IVC (posterior border)

a Hila  b Confluence of pulmonary veins


Abnormal Lung shadows

Disease in the different anatomical divisions of the lung give rise to specific appearances and distinction can usually be made between: Interstitial shadows, air space or alveolar shadowing (often called consolidation) and pleural abnormalities. All of these frequently co-exist as in; pneumonia, LVF and ARDS but one type of shadowing will usually dominate. Having decided which of these you are looking at it is then necessary to match up the clinical history with the type and distribution of the shadowing.


Lines and dots with variable distribution.   COAD causes peribronchial thickening, LVF tends to thicken up the peripheral interstitium  (Kerley B lines) as well as causing peribronchial thickening. Malignant involvement (lymphangitis cacinomatosa) tends to cause central perihilar lines. Drug reactions are often widespread and may be associated with alveolar shadowing.  A reticular pattern in the lower zones is typical of fibrosing alveolitis. A pathognomic appearance of interstitial disease is a shaggy appearance to the heart or diaphragm due to adjacent interstitial thickening. Honeycombing is rare.


Fig4 Examples of Interstitial disease.

fig   4 Shows Close ups of two patients with interstitial shadowing the first has fibrosing alveolitis and shows the typical shaggy diaphragm. The second has interstitial pulmonary oedema and shows Kerley B lines. Note the relatively sharp diaphragm.

Alveolar shadowing.  'Fluffy' shadows tending towards coalescence and fading away at the edges except where bounded by a fissure -shadowing with a sharp edge at a fissure or with a clear lobar or segmental distribution must be alveolar. It is due to fluid in the alveolar spaces. Soft tissue borders adjacent to the shadowing are lost but any air containing spaces within the shadowing will be outlined and air bronchograms are a characteristic feature. Distribution of the alveolar shadowing is helpful. A lobar distribution is suggestive of pneumonia. Central positioning is suggestive of LVF or fluid overload. Widespread and changing alveolar shadowing may indicate opportunist, including fungal, infection.

 Fig 5

Fig 5 shows consolidation adjacent to the upper left heart border but the heart border remains visible. This means that the consolidation must be posterior. The lateral view shows the consolidation to be very posterior, overlying the vertebral bodies.  The consolidated segment is below the level of the oblique fissure and therefore must be in the apical segment of the lower lobe. If it were above the oblique fissure it would be in the posterior segment of the upper lobe.

Pleural fluid causes loss of the costophrenic angles and a homogenous basal shadow with a concave upper border (on the erect view); there may be thickening of the horizontal fissure. Smaller amounts of fluid may be seen on the lateral in the posterior costophrenic recess.


Note the size and whether they are well or ill defined. Ill-defined, spiculated masses are usually primary tumours or possibly granulomas. Note any other lesions and any adjacent pathology (e.g. collapse or overlying rib destruction). Thick-walled cavitation suggests an abscess or tumour. Multiple cavities suggest abscesses or infarcts. Try to decide whether a lesion is pulmonary or pleural. En face pleural lesions tend to fade away at the edges. Lung and pleural masses up against the chest wall have fundamentally different shapes. Figure 6. Check the bones to exclude secondaries. Emphysema produces bullae, low flat diaphragms (below the anterior 7th rib), lack of peripheral vessels and an increase in the branch angles of the arteries. Other pathology (e.g. left ventricular failure) may be difficult to identify in the presence of emphysema. 



Asbestos related disease is predicted to increase considerably in the next few years. Asbestos exposure is indicated by thin liner pleural calcifications along the chest wall or diaphragm, asbestosis by associated changes similar to fibrosing alveolitis and mesothelioma by local pleural thickening and effusions. Figure 6 Isolated well defined pleural masses are more likely to represent pleural secondary deposits.


produces:  bullae, low flat diaphragms  (below the anterior 7th rib), lack of peripheral vessels and an increase in the branch angles of the arteries. Other pathology e.g. LVF may be difficult to identify in the presence of emphysema.


The ITU film

The problems associated with supine and portable films are exacerbated by the condition of the patients in ITU. They are usually unable to cooperate and their lungs are often poorly compliant causing unusual appearances to familiar pathologies. The information gained from a single ITU film may therefore be limited counterbalanced by the availability of serial, usually daily, films allowing the progression of changes to be recognised.

Abnormally placed gas

Familiarity with the appearances of surgical emphysema and pneumomediastinum - recognised by air outlining the upper mediastinal structures is necessary as they may point to the presence of an unrecognised pneumothorax or possibly a pneumoperitoneum.

Fig 7

This shows a patient with a massive pneumoperitoneum in whom the gas has tracked up behind the crura into the mediastinum and out through the thoracic inlet to cause surgical emphysema.

 Pneumothorax in the supine ITU patient rarely conforms to the classic appearances of a thin apical or upper zone line with no lung markings peripherally. A white line may be seen adjacent to the mediastinum or roughly parallel to the chest wall or parts of the mediastinum may appear sharper than is usual.  There may be no radiological evidence of a pneumothorax  and a high degree of suspicion in appropriate ITU patients is necessary.

Pneumopericardium is recognised by the presence of air completely surrounding the heart shadow. It is usually benign except for the rare cases of tension but may point to the presence of serious pathology such as fistulae or infection.

Pleural fluid in the supine or even semi erect  patient lies over the posterior  chest wall and will not have the familiar clear cut upper edge. Usually a veil of increased density is seen in the lower parts of the lung fields. It may be distinguished from alveolar shadowing because the basal pulmonary vessels may be seen and by the lack of an air bronchogram. However the two pathologies frequently coexist. Ultrasound shows effusions well and can be used to guide drains if necessary.  The sudden appearance of an effusion in a patient without fluid overload may suggest a haemothorax. Loculated effusions may show as well defined masses or thickening of fissures and should raise suspicions of an empyema or bleeding. A fluid level going straight across the lung field on an erect film indicates fluid plus air in the pleural space.

Alveolar shadows can be associated with: Infection, Pulmonary oedema, aspiration, contusion, and ARDS. 

Infections.  A lobar or segmental distribution suggests lobar pneumonia. More diffuse shadowing in an immunocompromised patient will suggest opportunist infection including fungi. Persistent upper zone shadowing in association with cavitation and fibrosis may mean TB.

Cavitation anywhere could mean specific organisms such as: TB, staph. or pneumococcus and multiple 1-2cm cavities raise the possibility of septic emboli.

Lung Oedema - diffuse if associated with: LVF, fluid overload and fresh water drowning. May be localised following aspiration and associated with other changes including atelectasis.

Contusion may be localised to the site of injury and show some progression for some days following the injury. Pulmonary haemorrhage can be diffuse and indistinguishable from pulmonary oedema.

ARDS can mimic almost any other acute condition. Radiologically it is best identified by its relentless progression from central pulmonary alveolar shadowing to more generalised and persistent coverage of the whole lung.

Atelectasis can vary from complete collapse of a lung as evidenced by massive mediastinal shift to the side of the collapse to small often temporary white lines in subsegmental atelectasis. It is only rarely associated with demonstrable mucous plugging. The various appearances of lobar collapse will be dealt with elsewhere.

Lines etc

Radiographers should be made aware of the indication for a chest X-ray done for a specific line or tube as they may be able to vary the technique accordingly and may repeat the film automatically if the line is not shown well. Anaesthetists may like to know that ultrasound measurements have shown that raising the patient's feet produces just as much jugular vein dilatation as tipping the patient head down.

An endotracheal tube should be at least 5cm above the carina as flexion/extension of the head can move it by as much as 4 cms.

A small pneumomediastinum or surgical emphysema is not uncommon following the insertion of a tracheostomy tube.

On the Chest X ray a subclavian line can be expected to be entering the subclavian vein at the lateral edge of the first rib, the brachiocephalic vein at the sternoclavicular joint and the SVC at the first anterior intercostal space. Commonest malplacements are upwards into the jugular vein and across the midline into the opposite brachiocephalic vein. A medially placed deviation at its lower end may indicate placement into t

Beware of the line which shows a small kink or even a gentle deviation at its distal end. It may be up against the vessel wall with the potential of erosion. This particularly applies to a large catheter in the relatively thin pulmonary arteries, particularly in the presence of pulmonary hypertension. The rare complication of catheter breakage should be dealt with as soon as possible, preferably by an interventional radiologist. Such large emboli can drift more peripherally with time and can erode vessels rapidly, particularly in the pulmonary circulation.

Pacemakers. Most single-wire pacemaker wires are inserted into the subclavian vein and directed into the apex of the right ventricle. A lateral view will show the distal end of the correctly placed wire coursing anteriorly. The course of the wires should be smooth on both the AP and lateral views and any localised kinks be viewed with suspicion. The commonest site of wire fracture is between the clavicle and the first rib, usually well seen on the frontal view. 

Nasogastric tubes have holes along their distal 10 cm and need to be well into the stomach to stop gastro oesophageal reflux occurring via these holes. They can enter the bronchial tree but bizarre looking 'malplacements' are usually due to the presence of a hiatus hernia rather than to oesophageal rupture. FIG 8

--> FIG 8

FIG 8 ‘Bizarre malplacement’
This patient had her NG tube replaced 3 times on the ward before being sent down to X-Ray to have the tube insertion under fluroscopy because of ‘repeated aspiration’ of the tube. Note that the left main bronchus is demonstrated well above the tube as it deviates to the left. A little contrast has been injected into the tube to show the incarcerated hiatus hernia.

Further reading

Seminars in Radiology 1997 volXXX11

Diagnostic Radiology, Grainger and Allison, Churchill Livingston Vol1 Sections 2&3

collapse below he Rt ventricle. Very rarely the catheter can enter the azygous vein or the internal mammary vein producing a localised kink near the SVC origin.

Pneumothorax is the commonest complication of line insertion. Much more rare is complete malposition into the mediastinum which will rapidly result in mediastinal widening and loculated pleural effusions.