Basics of X-ray Physics
Tissue densities

Key points

  • An X-ray image is a map of X-ray attenuation
  • Attenuation of X-rays is variable depending on density and thickness of tissues
  • Describing X-ray abnormalities in terms of density may help in determining the tissue involved

A radiographic image is composed of a 'map' of X-rays that have either passed freely through the body or have been variably attenuated (absorbed or scattered) by anatomical structures. The denser the tissue, the more X-rays are attenuated. For example, X-rays are attenuated more by bone than by lung tissue.

Describing densities

Contrast within the overall image depends on differences in both the density of structures in the body and the thickness of those structures. The greater the difference in either density or thickness of two adjacent structures leads to greater contrast between those structures within the image.

For descriptive purposes there are five different densities that can be useful to determine the nature of an abnormality. If there is an unexpected increase or decrease in the density of a known anatomical structure then this may help determine the tissue structure of the abnormality.

The 5 X-ray densities

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The 5 X-ray densities

  • Low density material such as air is represented as black on the final radiograph. Very dense material such as metal or contrast material is represented as white. Bodily tissues are varying degrees of grey, depending on density, and thickness.

X-ray tissue densities

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X-ray tissue densities

  • Here are the four natural tissue densities seen on a chest radiograph. Note there is a range of greyness, depending on the thickness of each tissue.

Natural tissue densities

  • 1 - Air/Lung
  • 2 - Fat (layer between soft tissues)
  • 3 - Soft tissue
  • 4 - Bones

X-ray tissue densities

  • The greatest contrast is found in areas of greatest difference in density of adjacent structures (red circle).

If you think there is an abnormal structure in an X-ray, try describing it in terms of density. Ask yourself if density is abnormally increased, or decreased. Ask yourself how dense it is in relation to adjacent normal structures.

Abnormal X-ray densities

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Abnormal X-ray densities

  • There is a large area of increased density on the left side of the chest. This has increased the density (increased whiteness) of this area (green circle). This is a soft tissue density mass that has destroyed a rib which is missing. The posterior rib has lost density and therefore this area appears blacker than normal (red circle).