The trachea is normally located centrally or deviating very slightly to the right.
If the trachea appears significantly deviated, inspect for anything that could be pushing or pulling the trachea. Make sure to inspect for any paratracheal masses and/or lymphadenopathy.
Causes of true and apparent tracheal deviation
Pushing of the trachea: large pleural effusion or tension pneumothorax.
Pulling of the trachea: consolidation with associated lobar collapse.
Apparent tracheal deviation:
Rotation of the patient can give the appearance of apparent tracheal deviation, so as mentioned above, inspect the clavicles to rule out the presence of rotation.
Carina and bronchi
The carina is cartilage situated at the point at which the trachea divides into the left and rightmainbronchus.
On appropriately exposed chest X-ray, this division should be clearly visible. The carina is an important landmark when assessing nasogastric (NG) tube placement, as the NG tube should bisect the carina if it is correctly placed in the gastrointestinal tract.
The right main bronchus is generally wider, shorter and morevertical than the leftmainbronchus. As a result of this difference in size and orientation, it is more common for inhaledforeignobjects to become lodged in the rightmainbronchus.
Depending on the quality of the chest X-ray you may be able to see the main bronchi branching into further subdivisions of bronchi.
The hilar consist of the mainpulmonaryvasculature and the majorbronchi.
Each hilar also has a collection of lymphnodes which aren’t usually visible in healthy individuals.
The lefthilum is often positioned slightlyhigher than the right, but there is a wide degree of variability between individuals.
The hilar are usually the samesize, so asymmetry should raise suspicion of pathology.
The hilarpoint is also a very importantlandmark; anatomically it is where the descendingpulmonaryarteryintersects the superiorpulmonaryvein. When this is lost, consider the possibility of a lesion here (e.g. lung tumour or enlarged lymph nodes).
Causes of hilar enlargement or abnormal position
Hilarenlargement can be caused by a number of different pathologies:
Bilateral symmetrical enlargement is typically associated with sarcoidosis.
Unilateral/asymmetrical enlargement may be due to underlying malignancy.
Abnormal hilar position can also be due to a range of different pathologies. You should inspect for evidence of the hilar being pushed (e.g. by an enlarging soft tissue mass) or pulled (e.g. lobar collapse).
Inspect the lungs for abnormalities:
When interpreting a chest X-ray you should divide each of the lungs into three zones, each occupying one-third of the height of the lung.
These zones do not equate to lung lobes (e.g. the left lung has three zones but only two lobes).
Inspect the lung zones ensuring that lung markings are present throughout.
Compare each zone between lungs, noting any asymmetry (some asymmetry is normal and caused by the presence of various anatomical structures e.g. the heart).
Some lung pathology causes symmetrical changes in the lung fields, which can make it more difficult to recognise, so it’s important to keep this in mind (e.g. pulmonary oedema).
Increased airspace shadowing in a given area of a lung field may indicate pathology (e.g. consolidation/malignant lesion).
The complete absence of lung markings should raise suspicion of a pneumothorax.
Inspect the pleura for abnormalities:
The pleura are not usually visible in healthy individuals. If the pleura are visible it indicates the presence of pleural thickening which is typically associated with mesothelioma.
Inspect the borders of each lung to ensure lung markings extend all the way to the edges of the lung fields (the absence of lung markings is suggestive of pneumothorax).
Fluid (hydrothorax) or blood (haemothorax) can accumulate in the pleural space, resulting in an area of increased opacity on a chest X-ray. In some cases, a combination of air and fluid can accumulate in the pleural space (hydropneumothorax), resulting in a mixed pattern of both increased and decreased opacity within the pleural cavity.
A tension pneumothorax is a life-threatening condition which involves an increasing amount of air being trapped within the pleural cavity displacing (pushing away) mediastinal structures (e.g. the trachea) and impairing cardiac function.
If a tension pneumothorax is suspected clinically (shortness of breath and tracheal deviation) then immediate intervention should be performed without waiting for imaging as this condition will result in death if left untreated.
Assess heart size
In a healthy individual, the heart should occupy no more than 50% of the thoracic width (e.g. a cardiothoracic ratio of less than 0.5).
This rule only applies to PAchest X-rays (as AP films exaggerate heart size), so you should not draw any conclusions about heart size from an AP film.
Cardiomegaly is said to be present if the heart occupies more than 50% of the thoracic width on a PA chest X-ray. Cardiomegaly can develop for a wide variety of reasons including valvular heart disease, cardiomyopathy, pulmonary hypertension and pericardial effusion.
Assess the heart’s borders
Inspect the borders of the heart which should be well defined in healthy individuals:
The right atrium makes up most of the right heart border.
The left ventricle makes up most of the left heart border.
The heart borders may become difficult to distinguish from the lungfields as a result of pathology which increases the opacity of overlying lung tissue:
Reduced definition of the right heart border is typically associated with right middle lobe consolidation.
Reduced definition of the left heart border is typically associated with lingular consolidation.
The right hemidiaphragm is, in most cases, higher than the left in healthy individuals (due to the presence of the liver). The stomach underlies the left hemidiaphragm and is best identified by the gastric bubble located within it.
The diaphragm should be indistinguishable from the underlyingliver in healthy individuals on an erect chest X-ray, however, if freegas is present (often as a result of bowel perforation), air accumulates under the diaphragm causing it to lift and become visibly separate from the liver. If you see free gas under the diaphragm you should seek urgent senior review, as further imaging (e.g. CT abdomen) will likely be required to identify the source of free gas.
There are some conditions which can result in the false impression of free gas under the diaphragm, known as pseudo-pneumoperitoneum, including Chilaiditisyndrome. Chilaiditi syndrome involves the abnormalposition of the colon between the liver and the diaphragm resulting in the appearance of free gas under the diaphragm (because the bowel wall and diaphragm become indistinguishable due to their proximity). As a junior doctor, you should always discuss a scan that appears to show freegas with a senior colleague immediately.
The costophrenicangles are formed from the dome of each hemidiaphragm and the lateralchestwall.
In a healthy individual, the costophrenic angles should be clearly visible on a normal chest X-ray as a well defined acuteangle.
Loss of this acute angle, sometimes referred to as costophrenic blunting, can indicate the presence of fluid or consolidation in the area. Costophrenic blunting can also develop secondary to lunghyperinflation as a result of diaphragmatic flattening and subsequent loss of the acute angle (e.g. chronic obstructive pulmonary disease).
The mediastinum contains the heart, greatvessels, lymphoidtissue and a number of potentialspaces where pathology can develop. The exact boundaries of the mediastinum aren’t particularly visible on a chest X-ray, however, there are some important structures that you should assess.
The aorticknuckle is located at the left lateral edge of the aorta as it arches back over the left main bronchus. Reduced definition of the aortic knuckle contours can occur in the context of an aneurysm.
The aortopulmonarywindow is a space located between the arch of the aorta and the pulmonary arteries. This space can be lost as a result of mediastinallymphadenopathy (e.g. malignancy).
Inspect the visibleskeletalstructures looking for abnormalities (e.g. fractures, lytic lesions).
Inspect the softtissues for obvious abnormalities (e.g. large haematoma).
Tubes, valves and pacemakers
Nasogastric tube placement is something you’ll often be asked to assess on a chest X-ray to confirm safe placement for feeding. See our NG tube placement guide for more details.
Various tubes and cables will be visible as radio-opaque lines on the chest X-ray (e.g. central line, ECG cables).
Artificial heart valves
Artificial heart valves typically appear as ring-shaped structures on a chest X-ray within the region of the heart (e.g. aortic valve replacement).
Pacemakers typically appear as a radio-opaque disc or oval in the infraclavicularregion connected to pacemakerwires which are positioned within the heart.
Finally, before completing your assessment of a chest X-ray, make sure you’ve looked at the ‘reviewareas’ where pathology is often missed. These areas include:
the lung apices
the retrocardiac region
behind the diaphragm
the peripheral region of the lungs
the hilar regions
This ensures you’ve comprehensively assessed the X-ray and reduces the risk of missing subtle pathology (e.g. a small nodule).
Dr Kunal Patel
James Heilman, MD. Right-sided pneumonia. Licence: CC BY-SA 3.0.