Chest X-Ray Interpretation in 7 Easy Steps!

chest xray analysis

It is important to understand that when a chest x-ray is taken, light is absorbed according to the density of what it passes. We have different densities within our bodies ranging from bone, to tissue, to fluid, to air. This results in a monochrome display ranging between white to black. There are three distinct things you should look for on a chest x-ray and they all rhyme:

  • White (white) = high absorption = bone
  • Light (whitish-grey) = medium absorption = fluid/tissue
  • Night (black) = low absorption = air

Chest x-rays on the same patient are often compared to assess for lung disease improvement or deterioration. For this reason, it is important that we are not comparing apples with oranges. Therefore, when comparing chest x-rays we need to ensure that both chest x-rays were taken in similar conditions. There is a systematic approach that we should follow when trying to interpret any chest x-ray:

  1. Identify that it is the correct patient
    • This seems like an obvious starting point, but I’ve seen medical staff “umm” and “ahh” about how the chest x-ray doesn’t match how the patient’s clinical picture only to realise they were looking at the wrong patient’s chest x-ray
  2. Determine that the left and the right side is correctly marked on the x-ray
    • Once again, this seems like an obvious starting but you would be surprised…
  3. Verify if the x-ray is an anterior posterior (AP) or posterior anterior (PA) exposure
    • If the patient is able to stand, the PA view is preferable which is when the x-ray machine is behind the person with the board in front of their chest
      • Due to how a patient positions their hands during a PA chest x-ray, their scapulae are usually retracted laterally and do not overlap the lung fields, giving a clearer image of the lungs
    • If the patient is unable to stand, the AP view can be utilised which is when the x-ray machine is in front of the person with the board behind their back 
      • The AP view can make a normal heart appear enlarged due to the short distance between the x-ray machine and the anterior position of the heart
  4. Verify if the x-ray has been taken on inspiration or expiration
    • If the ribs are further apart = inspiration
      • Due to the alveoli being open and filled with air with a higher inspiratory pressure, this can make the lung fields of a chest x-ray appear well recruited
    • If the ribs are closer together = expiration
      • Due to the alveoli only remaining open due to the lower expiratory pressure, this can make the lung fields of a chest x-ray appear de-recruited
  5. Assess for any patient rotation on the x-ray
    • There should be an equal distance between the spine and the clavicle ends
    • If the spine appears closer to one clavicle then the other, there is patient rotation present that may cause the following difficulties with interpretation:
      • It may be hard to determine if the trachea is deviated to one side by the lung condition or the rotation
      • It may also be hard to determine if the heart is enlarged, with a left rotation resulting in a larger looking heart and a right rotation resulting in a smaller looking heart
      • It may also result in incorrect interpretation of lung disease due to the potentially overlying of soft tissue or bone over the lung fields
  6. Assess the costophrenic angles on the x-ray
    • There should be a sharp angle that forms where the diaphragm meets the chest wall
    • The loss of a costophrenic angle usually indicates that there is fluid pooled in the base of the pleural space or that there is collapse of the alveoli at the base of the lungs
  7. Assess the lung fields on the x-ray
    • There should be air all through the lung fields (night)
    • But there should also be accompanying pulmonary vascular filled will blood (light) fanning to right out to the lung edges

      Normal-CXR-Labelled

In short: check that the chest x-ray belongs to who you think it belongs to, compare apples with apples, check for costophrenic angles at both lung bases, make sure your lungs look like zebras in that they are all black with strips of white all the way to the ends. I know some may argue zebras are all white with strips of black all the way to the ends…but on a chest x-ray that would be really bad, so let’s stick with the former!

Step 6 and step 7 are really the important ones when it comes to diagnosing an abnormal chest x-ray. It all comes down to costophrenic angles and if there is air in the lung fields with accompanying pulmonary vasculature right to the edges of these lung fields. So let’s talk what to expect on a chest x-ray when things start to go wrong:

atelectasis

Atelectasis is the collapse of the alveoli, usually  observed in the bases of the lungs but can work it’s way up to include the whole lung. There will be a loss of the costophrenic angle in affected lung, with “light” replacing “night”. This is because the collapse of the alveoli result in thicker tissue at the site resulting in a higher amount of light absorption and therefore a lightening on the chest x-ray. You can observe this in the base of the right lung in the chest x-ray above. This is an oxygenation issue that is usually accompanied with the clinical symptoms of decreased oxygen saturations, decreased PaO2 and decreased breath sounds to the affected region with localised inspiratory crackles.

rul-pneumonia-pa

Pneumonia is an infective consolidation of the alveoli that presents with a “patchy” appearance of “light” replacing “night”. This is because pneumonia results in the collection of exudate within the affected alveoli sacs resulting in a higher amount of absorption and therefore a lightening on the chest x-ray. You can observe this in the middle lobe of the right lung in the chest x-ray above. It is usually isolated to a single lobe of the lung, but can worsen to involve the whole lung. This is an oxygenation issue that is usually accompanied with the clinical symptoms of decreased oxygen saturations, decreased PaO2, localised crackles, moist cough, purulent sputum and increased temperature.

APO

Acute Pulmonary Oedema is the movement of fluid from the pulmonary vasculature into the alveoli. It presents with a bilateral “bat-wing” appearance of “light” replacing “night”, and often (but not always) is accompanied with an enlarged heart. As with the previous lung condition, the collection of fluid within the affected alveoli sacs result in a higher amount of light absorption and therefore a lightening on the chest x-ray. You can observe this bilaterally fanning out from the centre in the chest x-ray above. This is an oxygenation issue that is usually accompanied with the clinical symptoms of decreased oxygen saturations, decreased PaO2, bilateral crackles, pink frothy sputum and increased restlessness.

ards

Acute Respiratory Distress Syndrome is a severe and life threatening condition that is characterised by widespread inflammation in the lungs. It presents wit h a diffuse, bilateral appearance of “light” replacing “night”. And what did we say earlier about all white with a little black? Not good at all because where there is light, there cannot be night! This is an oxygenation issue that is usually accompanied with the clinical symptoms of decreasing oxygen saturations and decreased PaO2 despite increasing intervention, quiet air entry and widespread crackles.

pleural effusion

Pleural effusion is a collection of fluid within the pleural space that unmistakably presents with a “kidney bean” appearance, with smooth rounded “light” replacing “night” that causes a loss of the costophrenic angle in affected lung. This is not fluid within the alveoli like the previous oxygenation issues, this is fluid sitting around the outside of the lung pushing it up and reducing the lung volume capacity. You can observe this in the right lung in the chest x-ray above. This is a ventilation issue that is usually accompanied with the clinical symptoms of no air entry to the affected region, increased respiratory rate and increased work of breathing.

tension pneumothorax

Pneumothorax is a collection of air within the pleural space. Unless the pneumothorax is extensive (like the chest x-ray above), this is the hardest issue to identify on a chest x-ray unless you are looking for it. It will look similar to a normal chest x-ray in that there will be black air fields, however this will be without the accompanying pulmonary vasculature to the edges of the lung fields. This is because the pressure created from the air in the pleural space pushes the lung inwards, therefore reducing the lung volume capacity.You can observe this in the left lung in the chest x-ray above; “night” with no strips of “light” due to the affected lung being so significantly compressed. This is a ventilation issue that is usually accompanied with the clinical symptoms of no air entry to the affected region, increased respiratory rate and increased work of breathing.

If you would like to read a bit more about ventilation and oxygenation problems that was touched upon in this article, check out my article on type 1 versus type 2 respiratory failure. Hopefully this article has made interpreting chest x-rays more black and white for you, pardon the pun! Practice makes perfect, so start having a look at your patient’s chest x-rays each time you have a chance. They are a great adjunctive tool to utilise when performing a respiratory assessment, especially prior to auscultating the lungs as it almost gives you a cheat sheet of what to listen for!

References

Acknowledgements

  • Simon Plapp – ICU Education Consultant, Western Private Hospital

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