‘Chest x rays are absorbed to different degrees by different tissues.’
Interpretation of the Chest X ray
Examination of the chest x ray is a process which requires a systematic approach. It would be wrong to base a diagnosis on only the most obvious features of any Chest x ray. One needs to ensure that the chest x ray is a technically good one before then going on to interpret any abnormalities one might see.
The ABCDE mnemonic is a useful one when trying to ensure that the examination of a chest x ray is a systematic one.
E&F- equal (lung) fields
G- gastric bubble
H- hilum and mediastinum
I also have to add the T which stands for technicals.
There are a number of technical aspects to consider when looking at the chest x ray. Firstly one needs to be sure that the x ray is not rotated. Ideally the x-ray should be taken from exactly in front of the patient or behind. If the x ray is taken with the patient slightly rotated to the plate then this can give some unusual views which might hide or make a more prominent certain anatomical features of the x ray. This can make it difficult to interpret.
The easiest way to assess the rotation of the x ray is to find the ends of the clavicles and establish whether they are equidistant from the centre of the vertebral bodies. If one or other appears closer then there is a possibility that the x-ray is slightly rotated.
One also needs to be sure that the film is neither under or over penetrated. The radiographer when taking a Chest x ray has to decide on the settings for the x ray machine. In a good Chest x ray one should be able to see the vertebral bodies through the shadow of the heart down to the diaphragm. If one can see the vertebral bodies lowered down then it could be that the Chest x ray is over penetrated. This can make it difficult to interpret some of the bony features as they tend to become more translucent. If one cannot see the vertebral bodies at all well it could be that the Chest x ray is under penetrated and this will make it difficult to assess some of the softer structures.
One also that needs to go on to establish whether the Chest x ray is a pa (posterior-anterior) or an ap (anterior-posterior) Chest x ray. If it is a pa Chest x ray then the x-ray plate will have been placed in front of the patient and the x-ray machine will have taken the shot from behind the patient. If it is an ap Chest x ray then the plate will have been placed behind the patient and the x-ray machine will have taken the shot from in front of the patient.
This type of Chest x ray is more commonly taken for the poorly patient who is unable to get out of bed or off their trolley. The main reason this is important is because the ap Chest x ray will make the heart seem larger. So with an ap x-ray one cannot really comment on the size of the heart.
When looking at the airway, and on a Chest x ray this will mainly be the trachea, one is trying to assess whether it is central. The trachea could be pushed or pulled to one side or the other for a number of different reasons. A pneumothorax, where air is filling up the pleural space, could possibly push the trachea across. A consolidated or collapsed lobe on the other hand could pull the trachea across towards it. One way to assess whether the trachea is central is to estimate its distance from either end of the clavicles. One also needs to assess the trachea for any narrowing which may indicate stenosis or oedema.
One next needs to turn attention to the bony structures on the Chest x ray. One needs to assess the bones for any fractures. Look along the edges of all the bones making sure that there are smooth lines and no interruptions which may indicate a break. Also look for any discrete darker areas or change in bone density which may indicate a lesion. It is worth noting any lateral deviations of the vertebra such as one would encounter in scoliosis.
Now take note of the cardiac site, size, shape, and borders. The borders of the heart and mediastinum should always be very well defined and clear. This is known as the silhouette sign. When the border of the heart and mediastinum become unclear silhouette is starting to be lost, and this may indicate some kind of consolidation. The area in which the silhouette sign is lost around the heart or mediastinum can give an indication as to which a lobe of the lung is involved.
For example if there is an unclear right border this may be suggestive of the middle lobe consolidation.
You also need to establish the size of the heart which should be less than half the transthoracic diameter. When measuring this however it is important to understand that this can only be properly assessed on an x-ray which is posterior-anterior in orientation. Hopefully this is indicated on the x-ray. If it isn’t one needs to look to see if the edges of the scapula can be seen. If they can, then the x-ray is likely to be anterior-posterior in orientation, and therefore the size of the heart cannot be commented on.
The presence or absence of the silhouette sign is one of the most useful tools when assessing a Chest x ray.
The outline of the diaphragm on both sides should be clear and smooth. The right hemi-diaphragm should be slightly higher than the left. This is because the right has the liver under it and left has the heart over it. The cardio phrenic and costo phrenic angles should be well defined. Any haziness in these areas may indicate a pleural effusion all consolidation. If there is a pleural effusion there may be evidence of a meniscus sign all a flat layer of consolidation indicating fluid. Other than the gastric bubble which one may see under the left hemidiaphragm there should be no other air below the diaphragm.
Equal (lung) Fields
When looking at the lung fields one should scan from left to right all the way from top to bottom. You need to look for equal radiolucency on both sides. That is they should not be any obvious areas which look different from the rest of the lung. You may see air bronchograms which are visible air filled bronchi, outlined by surrounding consolidation.
There may be what is often described as bats wing distribution which describes a pattern of consolidation. This is a bilateral opacification spreading from the hilar regions into the lungs. One of the common causes of this is pulmonary oedema in heart failure.
Look for an air bubble under the left-handed hemidiaphragm. This may not always be there but is worth remarking upon when seen. As said before they should not be any other free air under the diaphragm on either side.
Hilum and mediastinum
Finally look at the hilum which consists of the main bronchus and the pulmonary arteries. The left side should be higher than the right and it is useful to compare them both to make sure that one isn’t much larger than the other. It is worth again looking at the mediastinum at this point and making sure that the silhouette sign is clear.
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