Conversion of the energy-subtracted CT number to electron density based on a single linear relationship: an experimental verification using a clinical dual-source CT scanner

Abstract

In radiotherapy treatment planning, the conversion of the computed tomography (CT) number to electron density is one of the main processes that determine the accuracy of patient dose calculations. However, in general, the CT number and electron density of tissues cannot be interrelated using a simple one-to-one correspondence. This study aims to experimentally verify the clinical feasibility of an existing novel conversion method proposed by the author of this note, which converts the energy-subtracted CT number (ΔHU) to the relative electron density (ρe) via a single linear relationship by using a dual-energy CT (DECT). The ΔHU-ρe conversion was performed using a clinical second-generation dual-source CT scanner operated in the dual-energy mode with tube potentials of 80 kV and 140 kV with and without an additional tin filter. The ΔHU-ρe calibration line was obtained from the DECT image acquisition for tissue substitutes in an electron density phantom. In addition, the effect of object size on ΔHU-ρe conversion was also experimentally investigated. The plot of the measured ΔHU versus nominal ρe values exhibited a single linear relationship over a wide ρe range from 0.00 (air) to 2.35 (aluminum). The ΔHU-ρe conversion performed with the tin filter yielded a lower dose and more reliable ρe values that were less affected by the object-size variation when compared to the corresponding values obtained for the case without the tin filter.

A simple formulation for deriving effective atomic numbers via electron density calibration from dual-energy CT data in the human body

“…for Z eff calculation. Saito and his collaborators demonstrated in previous studies that ρ e can be computed by merely performing the weighted subtraction of high‐ and low‐energy CT images using a calibration phantom and a least‐squares fitting procedure to find the optimal weighting factor. This method yielded values of ρ e with an absolute error of less than 1% for calibration phantoms.…”

Dual-Energy CT in Head and Neck Imaging

“…It was demonstrated that a clinical DECT scanner was able to extract Zeff, and density ρ of different tissue substitutes, next to ΔHU and ρe [79–81]. This suggested that when a large quantity of high-density and high atomic number structures are in the planning field, DECT-derived calculations show accurate and reliable inhomogeneity corrections in RT treatment planning [82]. …”

Initial implementation of the conversion from the energy‐subtracted CT number to electron density in tissue inhomogeneity corrections: An anthropomorphic phantom study of radiotherapy treatment planning

“…To extend the usable region of the ρ e calibration, we also included a homemade insert composed of an aluminum (Al) rod (99.999% purity) embedded in a urethane-resin pipe. 17 Figures 1(b) and 1(c) exemplify the SECT images of the body EDP and head EDP, respectively, as measured using the DSCT scanner at 120 kV. In order to investigate the impact of beam hardening on the reliability of the dose calculation, two different LUTs for the ρ e calibration were provided as inputs for a treatment planning system (TPS).…”

“…The reliable linearity of the ∆HU-ρ e plot has been confirmed experimentally using clinical dual-source CT (DSCT) scanners with electron density phantoms (EDPs) by a number of investigators. [17][18][19] The main objective of this study is to present an initial implementation of the ∆HU-ρ e conversion method for the tissue inhomogeneity correction in radiotherapy treatment planning. This paper demonstrates two radiotherapy plans for an anthropomorphic phantom to compare the performance of the ∆HU-ρ e conversion with the conventional HU-ρ e conversion with regard to the reliability of dose calculations.…”