The chest x-ray (also known as CXR or Chest Radiograph) is probably the most commonly performed medical imaging procedure. An x-ray can typically discern 4 different densities:
- Air (black)
- Metal – i.e. calcium (in bones) and barium (in contrast) as well as any implants
It is important to remember that is can be difficult to discern 3-dimensional structures from a 2-dimensional image – and typically it is recommended to get two views from 90 degree angles (AP and lateral views).
Like most things in life concept is perhaps best described by a memorable meme:
The list of indications for a chest x-ray is very large! Some common reasons include:
- Chest pain
- Shortness of breath
- Foreign body location
System for describing and interpreting a Chest X-ray
- Explain the type of image you are looking at – e.g. “This is a plain CXR”
- Check it is the right patient, and the right date.
- Check it is also the right orientation (Left and right sides)
- Assess the image for any rotation – usually best visualised by checking if the clavicles appear to be symmetrical. Rotated films cause issues with interpretation as they can falsely suggest:
- Tracheal deviation
- Heart enlargement
- Widened mediastinum
- Asymmetrical lung density
- Check whether it is a PA or AP projection
- PA – Posteroanterior – describes how the x-rays travel through the patient before hitting the x-ray film. This is the standard method by which an x-ray is performed. The patient stands with the chest up against the film, and the x-ray emitter is placed behind them.
- AP – anteroposterior – describes that the x-rays travel through the anterior to the posterior of the patient. This type of film is typically produced when the patient is bed bound and unable to stand for a PA film, and is most commonly seen in acutely unwell patients and those in ICU.
- On an AP CXR the heart often appears larger than it is – so you can’t really comment on the heart size in these films
Check the exposure
- In a normal exposure, you should be able to just see the vertebrae through the heart.
- If it is over exposed, it will be too dark
- If it is underexposed, it will generally be to pale
System – ABCDEF
There are various systems used to help describe a chest x-ray. Some prefer to start from the middle of the film and work outwards to the peripheries. In this example we will describe the ABCD method.
- A – Airway – is it deviated?
- B – Bones – look for fractures, notches, dislocations
- C – Cardiac – is the heart enlarged (greater then ½ the width of the chest; the “cardiothoracic ratio”)
- This can only be interpreted on a PA film (not AP)
- Calcification of the valve may be visible
- Pacemaker and wires may be visible
- Wires in the sternum may be resent (representing previous sternotomy)
- Pericardium – pericardial effusion?
- Mediastinum – is it widened?
- D – Diaphragm – is air under the diaphragm, is the diaphragm raised?
- The phrenic nerve innervates the diaphragm – “C3, C4, C5 keeps the diaphragm alive”
- E – Extrathoracic – air under the diaphragm, shoulder joints, air under the skin (surgical emphysema)
- F – (Lung) Fields
- Any obvious masses?
- Consolidation – usually a sign of pneumonia – patchy ‘opacity’ (white bits) often but not always confined to one lobe. “Loss of the heart border” – if the heart border is indistinguishable from the lung field – this is also an indication of consolidation.
- Vascular markings – are they visible in the whole lung field? If not – suspect pneumothorax
- Air bronchogram – a term that describes visible black circles created by bronchi as they are viewed ‘end on’. Is almost always pathological, and is caused by a sharp contrast in the air in the bronchi, surrounded by fluid (and / or pus) in the surrounding alveoli. Air bronchcograms are usually most visible around the Hila – in the larger bronchioles.
General Terms, hints and tips
- Hila – the left hilum is slightly higher than the right hilum.
- Gas – remember don’t be confused by gas in the stomach! So gas on the left is often normal, but gas under the right hemi diaphragm is not! Also, free gas in the abdomen usually (but not always) will go to the right first, because this is higher than the left.
- Blood vessels – are more extensive to the lower half of the lung than to the upper half. They sort of loop down from the heart like a droopy moustache.
- Free fluids – in the pleural space – i.e. pleural effusion – may or may not have a meniscus. The fluid (as long as it is erect CXR) will be at the bottom of the lung. If the pressure of the air is high, the meniscus may be flattened.
- If there isn’t a meniscus, you should question if the opacity is due to a fluid level or another cause.
- Ribs – remember you can see them anteriorly and posteriorly. Generally the posterior ribs will be more horizontal, and the anterior ribs will be more diagonally.
- Clavicle – to judge if the CXR is rotated or not, look at the distance between the central end of the clavicle, and the centre of the trachea. This distance should be equal on both sides
- Heart – to quickly estimate if the heart is enlarged – the diameter of the heart should be roughly less than ½ the width of the chest
- Aortic knob – this is the blob that the aortic arch makes on an x-ray
- Vascular markings – these should not be visible in the lateral 1/3 of the lung.
- Costophrenic recess – these will not be visible / will be filled in (opaque), in pleural effusion, and less commonly in consolidation.
- Diaphragm – the right is higher on a normal CXR due to the liver.
- Bronchial tree – the right is steeper than the left – due to the differing lobar structure. Inhaled foreigh object will usually fall down the right as it is steeper.
- Lung fissures – these can often be clearly seen if there is fluid – ie in pleural effusion.
- Transverse fissure – separates upper and middle lobes of the lung.
- Oblique fissure – separate the lower from the other two lobes – the upper and middle lobes are generally anterior, the lower lobe is generally posterior.
- In Right upper lobe collapse – the transverse fissure moves upwards
A normal Chest X-ray