Because CT numbers bear a linear relationship with the attenuation coefficients it is possible to infer electron density (electrons cm-3). Although CT numbers can be correlated with electron density, the relationship is not linear in the entire range of tissue densities. The nonlinearity is caused by change in atomic number of tissues, which affects the proportion of beam attenuation by Compton versus photoelectric interactions. Khan (see Figure on pg 233)
To ensure accurate dose calculation, the CT numbers must be converted
to electron densities and scattering powers. The conversion of CT numbers to
electron density and scattering power is usually performed with a user defined
look-up table, which in turn is generated using a water equivalent circular
phantom containing various inserts of known densities simulating normal body
tissues such as bone and lung. Pdgorsak.
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