Energy dependence in the spectral factor approach to computed tomography

Calculations are presented of the energy dependence of the photoelectric coefficients for common biological materials such as cortical bone, liver, blood, fat, and also meglumine iothalamate solution. The authors have also calculated the error which arises when the spectral factor approach is implemented in dual energy CT, and have demonstrated the error with measurements on the EMI C5005 General Purpose Scanner. They concluded that the assumption of energy independent photoelectric coefficients is an approximation with validity at the level of only 5-10%. The use of the spectral factor approach may therefore lead to significant errors when applied to dual energy CT. In addition, errors may be introduced in other calculations involving the spectral factor, for example, the analysis of noise and the derivation of optimum energies in dual energy applications. Characterisation of tissue should be based preferably on truly energy independent coefficients. (U.K.)