Technical Note: Relation between dual‐energy subtraction of CT images for electron density calibration and virtual monochromatic imaging

Purpose: For accurate tissue inhomogeneity correction in radiotherapy treatment planning, the author previously proposed a simple conversion of the energy‐subtracted computed tomography (CT) number to an electron density (ΔHU–ρe conversion), which provides a single linear relationship between ΔHU and ρe over a wide ρe range. The purpose of the present study was to reveal the relation between the ΔHU image for ρe calibration and a virtually monochromatic CT image by performing numerical analyses based on the basis material decomposition in dual‐energy CT. Methods: The author determined the weighting factor, α0, of the ΔHU–ρe conversion through numerical analyses of the International Commission on Radiation Units and Measurements Report‐46 human body tissues using their attenuation coefficients and given ρe values. Another weighting factor, α(E), for synthesizing a virtual monochromatic CT image from high‐ and low‐kV CT images, was also calculated in the energy range of 0.03 < E < 5 MeV, assuming that cortical bone and water were the basis materials. The mass attenuation coefficients for these materials were obtained using the xcom photon cross sections database. The effective x‐ray energies used to calculate the attenuation were chosen to imitate a dual‐source CT scanner operated at 80–140 and 100–140 kV/Sn. Results: The determined α0 values were 0.455 for 80–140 kV/Sn and 0.743 for 100–140 kV/Sn. These values coincided almost perfectly with the respective maximal points of the calculated α(E) curves located at approximately 1 MeV, in which the photon‐matter interaction in human body tissues is exclusively the incoherent (Compton) scattering. Conclusions: The ΔHU image could be regarded substantially as a CT image acquired with monoenergetic 1‐MeV photons, which provides a linear relationship between CT numbers and electron densities.