The Importance of Electron Density:

1. 1. One of the best uses of CT images in modern radiotherapy is the ability to perform heterogeneity corrections on the patient's anatomy.

   2. This is possible because the Hounsfield units can be related back to electron density through the use of a calibrated phantom (see the writeup on HU vs. Electron Density).

   3. Electron density is important because Compton scattering is proportional to electron density, and Compton interactions are the dominant interaction type in tissue in megavoltage beams below 25 MV.

   4. Most materials (except hydrogen) exhibit a fairly constant number of electrons per gram.

   5. What we are interested in is the number of electrons per cm3.

   6. The number of e/cm3 is directly related then to the physical density of the material.

   7. The physical density is the reason for increased attenuation in bone or decreased attenuation in lung.

   8. The following are a few important physical densities to remember:

      1. air - 0.0012 g/cm3

      2. lung - 0.33 g/cm3

      3. water - 1.0 g/cm3

      4. bone - 1.8 g/cm3

   9. The above discussion is simplified as the hydrogen content of a material may have an effect on the number of electrons per gram necessitating the use of both the number of e/gm and the physical density (for instance fat has a very high hydrogen content).

   10. To further complicate things pair production can become significant (especially in bone in high energy beams) which does not depend on electron density at all and instead is based on the Z of the material.

   11. Suffice to say heterogeneity corrections are very complicated and a major area of research to establish algorithms that are both fast and accurate.

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