An important part, as well as being aware of the different radiological signs, is to have a systematic approach so that things aren’t missed. There are many, but this is a good one:
Indication for imaging
Relevant examination findings
Interpretation (A to E)
Breathing - lungs
Circulation - CVS and mediastinum
Disability - bones
Having a clear idea of the history is essential, as this will in some ways help interpretation. It can help guide us towards looking more closely for certain types of pathology or certain signs. However, even if this is the case, a full review of the image is still important to make sure nothing is missed.
2. Correct Image
Whilst this may sound simple, modern PACS systems may easily allow the wrong image to be accessed, leading to incorrect clinical correlation. In addition to the wrong patient, patients may have multiple CXRs that can easily be mixed up without specific checking.
This step is to assess the quality of the image. Whilst it is unlikely to render an x-ray useless, some changes in the adequacy could potentially lead to misinterpretation if you are not aware of them to start with.
Rotation refers to any movement of the patients from the normal straight AP position. This can move the location of some of the structure towards or away from the x-ray beamin, both affecting their position and relative size on the radiograph. In the normal position, the vertebral spinous processes should be midway between the medial ends of the clavicles.
The optimal CXR is taken with the patient at full inspiration. This should usually allow 10-11 posterior ribs to be visible. This may not be the case, especially on ICU, and can lead to changes on the CXR. Reduced inspiration leads to a higher diaphragm position, leading to greater crowding of the lung bases and a change of the cardiac shadow.
The exposure of the film is more of a problem for non-digital images, but is still important, as over or under penetration can mask abnormalities. An appropriate exposure should allow visualisation of the vertebral bodies through the cardiac shadow, and maintain the ability to visualise peripheral lung markings.
The different positions for CXR can also change interpretation, so knowing whether it is an AP or PA image is important. In an AP film (common on ICU), the heart size can be magnified, thus making assessment of it inaccurate. The supine position can also have changes on lung and vessel characteristics.
Finally, whilst it may be obvious, it is important to have all the desired areas included in the film. Sometimes, for example with difficult positioning on ICU, areas of the lung may be missed.
Whilst the exact order is not important, a structure that you are familiar with is helpful.
Airway The trachea should run centrally and with parallel walls. Deviation should prompt assessment for a cause (assuming it is not rotational). The carina should be at around T5 level, with the angle between the 2 main bronchi between 32 and 90 degrees. A great angle than this (>100) may be due to atrial enlargement or lymph nodes. The location of any endotracheal tube should be examined.
Breathing Lung Parenchyma The x-ray features of the lung parenchyma is primarily made up of the pulmonary vessels. These should form a branching pattern that goes right to the end of the lung fields. The fields are divided into 4 zones to help with description (apical, upper, middle, lower) because of the overlapping nature of the lung lobes when viewed from the front. Comparison of lung symmetry is probably the easiest way to appreciate abnormalities.
Hila This area is primarily made up of the pulmonary vessels and can be difficult to interpret. A useful method is to look at the ‘hilar V’ - the angle made between the upper and lower lobe blood vessels. And opacity here is highly suspicious for lymphadenopathy. The left is usually slightly higher than the right (2.5cm) and should be the other way round.
Diaphragm Their position and clarity should be assessed. Due to the presence of the liver, the right is usually higher. They should have a clear outline forming costophrenic and cardiophrenic angles bilaterally. Loss of clarity is suspicious for collapse or consolidation. Air under the diaphragm is suspicious for GI perforation. Looking for abnormalities ‘behind’ the diaphragm is also important here.
Circulation Heart Key factors to look at are:
The heart will usually be ⅔ to the left of the chest. The right atrium makes up the majority of the right border. The left ventricle makes up the majority of the left border. It should take up no more than 50% of the internal thoracic diameter (when measured on a PA film) - this can be termed the cardiothoracic ratio. The borders should be clear and smooth. Looking for abnormalities behind the heart also needs special consideration.
Mediastinum This has anterior, middle and posterior compartments. This can be difficult to assess with an AP/PA film, but consideration of the anatomy can be useful in interpretation. The border should be smooth and distinct.
Disability and Everything Else A detailed look at the bones and soft tissue is still important. Fractures may be identified following trauma, or bony disease may be present due to metastases. Surgical emphysema may be clearly visible on CXR.
Airspace disease This can be thought of as different types:
Collapse - atelectasis
Consolidation is where the normal airspace is replaced by fluid or cells (including pus and blood). The features that can help identification are:
Ill defined edges
Dense and relatively homogenous
Appearance is hazy, fluffy
Air bronchograms - patent larger airways
Normal vasculature visible
Minimal/no volume loss
Collapse (atelectasis) can be considered portions of lung where the air that normal exists within them has been lost. This is usually because the gas has diffused out of lung that is not being ventilated. Features that support this include:
Some pathology may actually result in decreased opacification of the parenchyma. This is often when normal lung tissue is destroyed and replaced by air containing space. Different types include
Cavity - opacity with air filled centre
Cystic - well described walls
Bullae - thin walled space
Bronchiectasis - primarily bronchial dilatation with wall thickening
Interstitial Disease This can be thought of as the other category of parenchymal features on CXR and involves the connective tissue surrounding the airspaces. Pathology of the interlobular septae are generally responsible for the changes seen in this pathology. These separate the secondary pulmonary lobules and contain much of the supporting connective tissue of the lungs, as well as the veins and lymphatics. In contrast, the arteries and airways run in the centre of the lobules.
The different patterns that may present include:
A reticular pattern is a criss-cross pattern of opacification that may be fine or coarse. It is seen is diseases such as:
Idiopathic pulmonary fibrosis
Rheumatoid lung disease
A reticulonodular pattern involves multiple small dots. These can vary in size and appearance depending on the disease process. Diseases may include:
The distribution of the pathology may also be useful:
Upper zones - sarcoidosis, coal worker’s pneumoconiosis
Lower zones - IPF, asbestosis, rheumatoid lung disease
Peripheries - Kerley B lines in pulmonary oedema
Nodular disease Finally, certain disease processes may appear more discreetly nodular. A useful way of categorising these is relating to their size.
Miliary (<2mm) - e.g. miliary TB
Micronodule - (2-7mm) - e.g. acute hypersensitivity pneumonitis
Nodule (7-30mm) - e.g. lung met
Mass (>30mm) - cancer
Silhouette Sign The silhouette sign describes an interface between highly exposed and poorly exposed areas of the lung. The main ‘normal’ structures relating to a silhouette are the mediastinum and vessels passing through the lung. If the pathology is in contact with the heart/solid structure, the crispness of the border will be disrupted. If it is not, then the crispness of this border will be maintained, thus allowing some idea of the ‘depth’ of the lesion in the thorax. As such, the silhouette sign usually refers to the loss of a silhouette which usually provides the useful information.
Air Bronchograms This is almost the opposite of the silhouette sign. Normally, smaller size airways are difficult/impossible to see on a plain film image. As the normal aerated lung becomes de-aerated (that is, consolidated) the normal visualisation of blood vessels becomes less possible and instead it becomes increasingly possible to visualise the airways. As such, air bronchograms are the cardinal feature of consolidation.