Simplified derivation of stopping power ratio in the human body from dual‐energy CT data

Purpose The main objective of this study is to propose an alternative parameterization for the empirical relation between mean excitation energies (I‐value) and effective atomic numbers (Zeff) of human tissues, and to present a simplified formulation (which we called DEEDZ‐SPR) for deriving the stopping power ratio (SPR) from dual‐energy (DE) CT data via electron density (ρe) and Zeff calibration. Methods We performed a numerical analysis of this DEEDZ‐SPR method for the human‐body‐equivalent tissues of ICRU Report 46, as objects of interest with unknown SPR and ρe. The attenuation coefficients of these materials were calculated using the XCOM photon cross‐sections database. We also applied the DEEDZ‐SPR conversion to experimental DECT data available in the literature, which was measured for the tissue‐characterization phantom using a dual‐source CT scanner at 80 kV and 140 kV/Sn. Results It was found that the DEEDZ‐SPR conversion enables the calculation of SPR simply by means of the weighted subtraction of an electron‐density image and a low‐ or high‐kV CT image. The simulated SPRs were in excellent agreement with the reference values over the SPR range from 0.258 (lung) to 3.638 (bone mineral‐hydroxyapatite). The relative deviations from the reference SPR were within ±0.6% for all ICRU‐46 human tissues, except for the thyroid that presented a −1.1% deviation. The overall root‐mean‐square error was 0.21%. Application to experimental DECT data confirmed this agreement within the experimental accuracy, which demonstrates the practical feasibility of the method. Conclusions The DEEDZ‐SPR conversion method could facilitate the construction of SPR images as accurately as a recent DECT‐based calibration procedure of SPR parameterization based directly on the CT numbers in a DECT data set.