CXRs

How to Interpret Chest Radiographs (CXR) – Gautam Babu

An X-ray is a density-gram where “white” is “dense” and “black” is “not dense”.  Determine a systematic method you use every time you interpret a CXR to ensure you don’t miss anything.

  • Start every CXR you interpret by assessing the quality of the film:
    • Penetration: You should be able to see vertebral bodies through the cardiac silhouette but not into the abdomen. If you cannot see them through the heart the film is “under-penetrated” and everything will appear more “white.” If you can see them through the abdomen the film is “over-penetrated” and everything will appear more “black”
    • Rotation: The spinous processes should be in the middle of the clavicular heads, if not then the film is rotated
  • Two Different Systematic Methods: 
    • ABCDE method
      • Airway – Trachea midline and patent
      • Bones – Bone density and obvious fractures
      • Cardiac Silhouette – You should be able to see the L and R heart border, if not there may be an adjacent opacity (Right Middle Lobe, Lingula)
      • Cardiomegaly defined as heart size 1/2 width of the hemithorax on a PA film
      • Diaphragm – Look for pleural effusions at the costo-phrenic angle. If you cannot see the diaphragm along the way there may be an adjacent opacity (Lower Lobe)
      • “Everything else” – Refers to the lung fields but if you wanted to continue the alphabet mnemonic
      • Extra-Thoracic Soft Tissue – Subcutaneous emphysema
      • Fields and Fissures –lung fields should appear symmetric and “black.” Asymmetry suggests there is an issue on one side.
      • Great Vessels – Tortuosity of the aorta and the outlines of the pulmonary vessels
      • Hilum – Hilar masses, LAD and pulmonary arteries, the left hila is higher than the right normally because of the heart
  • Working around the film method:
    • Imagine the entire CXR film as a square and an inner “box” as the pleural lining
    • Outside the box: Looking for lines, tubes, EKG/tele leads, subcutaneous emphysema, stomach bubble, sub diaphragmatic air
    • The edge of the box: Looking for pleural thickening, pleural effusion, pneumothorax, visualization of the diaphragm
    • The middle of the box: aka Mediastinum -> trachea, vascular pedicle, hila, heart borders, great vessels, retrocardiac space
    • The lung fields
       

One important concept to know is the silhouette sign: Two things of different densities will show a clear border on a chest x-ray. In the contrapositive, the loss of a border you expect to see suggests there has been a change in density of one of the structures. For example, the heart and the lung are different densities and as such you have a sharp border. Loss of this border suggests that the adjacent lung “increased” in density from a PNA (in the right clinical scenario).

  • Often try to distinguish PNA from pulmonary edema, as these are two of the most common causes of abnormal opacities seen on CXR
    • PNA 
      • "fluffy" opacities and air bronchograms frequently indicate alveolar filling
      • typically asymmetric
    • Pulmonary Edema
      • linear opacities, fluid in the fissure, Kerley B lines, cephalization, bilateral pleural effusions
      • Typically symmetric
      • atypical or viral PNA can similarly present with linear opacities

 

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