Comparison of equivalent photon energy calibration methods in computed tomography.

A method of specifying the equivalent photon energy as the energy that gives the maximum correlation between linear attenuation coefficient and CT value of six standard materials, including water, was compared with standard method that specified equivalent photon energy as the energy at which water's linear attenuation coefficient is equal to the detected energy fluence averaged coefficient of water. Comparisons were made for various tube potentials, thicknesses of aluminum filtration, and water phantom thicknesses. Using the experimental data, the first method predicted changes in equivalent photon energy equal to 0.3 kvV kVp-1, 2.8 keV g-1 cm2 and 0.75 keV g-1 cm2, respectively, for the specified conditions; the precision was +/- 2.2 keV. Both methods estimated the same equivalent photon energies within 3 keV. This similarity was shown to be a result of the characteristics of water's attenuation coefficient. The effect of uncertainty in measured CT values and material density on the equivalent photon energy was estimated. The equivalent photon energy was used to predict CT values for high atomic number water solutions, 5 mg/ml. The difference between the measured CT values and the predicted was less than 10 CT number of elements of less than 60.