Chest X-Ray
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Introduction

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
  • Fat
  • Water

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 importance of two views when taking an x-ray
The importance of two views when taking an x-ray

Indications

The list of indications for a chest x-ray is very large! Some common reasons include:

System for describing and interpreting a Chest X-ray

Opening

  • 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
PA vs AP projection on Chest X-ray
PA vs AP projection on Chest X-ray – demonstrating the larger projection size of the heart on an AP radiograph.

 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 – Airwayis 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
Fluid and air enclosed in the same space in CXR – e.g. pleural effusion + pneumothorax = hydropneumothorax. Other examples include a fluid filled stomach, and a fluid filled abscess.

Examples

A normal Chest X-ray

Normal Chest X-ray
A normal chest x-ray in a healthy young male. In particular note the normal markings, clear costophrenic angles, and clear lung fields. There is quite a large gastric bubble, but this is not considered pathologic.

Pneumonia

Bilateral pneumonia
Chest x-ray showing bilateral pneumonia in an elderly patient. Note the patch consolidation in the high mid zone, including some poor differentiation of the right heart border. On the left, not the basal consolidation, including loss of the costophrenic angle and loss of the lower part of the heart border.

Pleural Effusion

Pleural effusion
Chest X-ray showing a large left sided pleural effusion.

Heart Failure

Heart Failure Chest X-ray
Heart failure on Chest X-ray. Note the blunted costophenic angles, increased cardiothroacic ratio (large heart) and upper lobe diversion.

 

CXR Signs in Heart Failure
Chest X-ray signs in Heart Failure. Image from wikipedia.

Lung Cancer (mass)

Lung Cancer on Chest X-ray
Lung Cancer on Chest X-ray. Note the prominent left lung mass. In the clinical setting this would raise the strong suspicion of lung cancer which would need to be confirmed by biopsy.

Pneumothorax

Pneumothorax on CXR
Pneumothorax on Chest X-ray – Image from wikimedia commons. Author: Karthik Easvur

References

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Dr Tom Leach

Dr Tom Leach MBChB DCH EMCert(ACEM) currently works as a GP Registrar and an Emergency Department CMO in Australia. He is also a Clinical Associate Lecturer at the Australian National University. After graduating from his medical degree at the University of Manchester in 2011, Tom completed his Foundation Training at Bolton Royal Hospital, before moving to Australia in 2013. He started almostadoctor whilst a third year medical student in 2009. Read full bio

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